Blood flow-dependent prostaglandin F2alpha regulates intestinal glucose uptake from the blood

Chao Han, Zhi Ming, and W. Wayne Lautt

Department of Pharmacology and Therapeutics, Faculty of Medicine, University of Manitoba, Winnipeg, Manitoba, Canada R3E 0W3


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Intestinal glucose uptake (GUi) from blood increased when blood flow (BF) was increased. The increase in BF could elevate shear stress. Therefore, we hypothesize that shear stress-induced release of autacoids mediates the increase in GUi. A surgically separated segment of small intestine was perfused in situ with the use of an arterial circuit in anesthetized cats. Arterial and portal blood samples were taken simultaneously for assessment of GUi. Adenosine was used to elevate intestinal BF. The GUi increased by 45.0 ± 18.3 from 25.3 ± 3.8 µmol · min-1 · 100 g tissue-1 when the BF increased about four times. It was not a direct effect of adenosine because GUi was not altered if the flow was held constant. This increase was blocked by a cyclooxygenase inhibitor, indomethacin, but not by nitric oxide synthase blocker NG-nitro-L-arginine methyl ester. Furthermore, prostaglandin F2alpha (PGF2alpha ) but not PGE2 or PGI2 reversed the blockade of the increase in GUi after indomethacin during elevated blood flow, whereas they had no influence on basal uptake. The results suggest that shear stress-induced release of PGF2alpha mediated the increase in GUi when blood flow was elevated.

shear stress; superior mesenteric artery; nitric oxide; indomethacin; prostaglandin E2


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

ALTHOUGH THE GUT SERVES to regulate the absorption of glucose and other nutrients, few studies have addressed how the intestine regulates its own energy metabolism. Substances such as glutamine (26) and some digested products including glucose (21) are important energy sources for intestinal tissues. Increasing evidence suggests that the intestine is not only important for glucose absorption; it may also be important in the disposal of glucose in basal and postingestive states. Abumrad et al. (1) reported that the gut utilized up to 25% of net hepatic glucose output in basal states and up to 15% of the glucose load in the postabsorptive state in conscious dogs. The intestinal mucosa was found to be a quantitatively important site of action of metformin, an antihyperglycemic agent, to increase glucose utilization (2).

The mechanism that controls intestinal glucose uptake and metabolism is not known. Kellett et al. (14) indicated that normal intestinal glucose metabolism is not insulin concentration dependent, although the presence of insulin seems necessary. Xie and Lautt (27) found that insulin has no effect on net glucose balance in extrahepatic splanchnic tissues, mainly the gut, and that a hepatic insulin-sensitizing substance released by the liver similarly did not affect the gut. These results are consistent with a previous report that hyperglycemia and hyperinsulinemia had no effect on extrahepatic splanchnic tissue glucose uptake (9).

We recently reported (13) that the inhibitory metabolic actions of adenosine (Ado) on intestinal oxygen consumption were antagonized by shear stress-induced nitric oxide (NO), an effect that was seen only when blood flow was allowed to increase. As an index of preparation status we also monitored glucose uptake at several time points. Glucose uptake appeared to increase in the presence of vasodilation but only when blood flow was allowed to rise. Based on this preliminary observation, a hypothesis that increased intestinal glucose uptake is mediated by shear stress-induced release of autacoids, such as NO or prostaglandins, during high-flow perfusion was formulated and tested in this study.

The results demonstrated that intestinal glucose uptake increased when blood flow was raised. The increased intestinal glucose uptake was blocked by a cyclooxygenase inhibitor, indomethacin, but not by the NO synthase blocker NG-nitro-L-arginine methyl ester (L-NAME) at a dose (2.5 mg/kg) previously shown to block intestinal shear stress induction of NO release (19). In addition, the blockade of increased intestinal glucose uptake by indomethacin could be reversed by prostaglandin F2alpha (PGF2alpha ), suggesting that shear stress-induced release of PGF2alpha mediated the increase in intestinal glucose uptake when blood flow was elevated.


    METHODS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Surgical preparation. Fifteen cats (either gender, 3.7 ± 0.1 kg) in three groups were fasted for 18 h (overnight) but allowed free access to water. The animals were anesthetized with pentobarbital sodium (32.5 mg/kg ip). Anesthesia was maintained by continuous infusion (~0.5 ml/min, 0.78 mg/ml of pentobarbital sodium in saline) through a brachial venous cannulation. Supplemental anesthetics (6.5 mg) and fluid (10% Dextran 40 intravenous solution mixed with equal volume of saline) were also given when required. Body temperature was monitored and maintained at 37.0 ± 0.5°C with the use of a thermally controlled surgical table (model 72, Yellow Springs Instruments, Yellow Springs, OH). The respiration was mechanically assisted (Harvard Apparatus, Millis, MA). Systemic arterial and central venous pressures were monitored by left carotid arterial and right femoral venous cannulation, respectively. All pressures were monitored by pressure transducers (Gould and Statham, Gould, Oxnard, CA) that had been set to zero at the level of the central vena cava.

After a midline incision of the abdomen, the spleen and large intestine, including ascending, transverse, and part of the descending colon, were surgically removed. The inferior mesenteric artery and vein were ligated, and the upper end of the intestinal segment was ligated at the duodenum. The superior mesenteric nerve plexus was surgically denervated. The superior mesenteric artery (SMA) was ligated and cannulated. A pump-controlled vascular circuit (Masterflex, Cole Parmer Instrument, Barrington, IL) that acquired blood from the abdominal aorta by a double aortic cannulation perfused the small intestine through the SMA cannula (13). The animals were heparinized (200 IU/kg) before the vascular circuit was established. The pump rate was controlled to generate a circuit pressure that was similar to systemic pressure. Two venous catheter units (24G, Optiva, Johnson & Johnson Medical, Arlington, TX) were placed into the portal vein for monitoring portal venous pressure and for blood sampling. The flow rate was measured with a flow-through electromagnetic flow probe (EP608, Carolina Medical Electronics, King, NC) that was incorporated into the circuit. Calibration of the flow probe was carried out at the end of each experiment, and the SMA pressure was obtained after correction of circuit pressure for resistance in the circuit (20). The pressures and flow rates were recorded on a dynograph recorder (R611, Sensor Medics, Anaheim, CA). After the surgery, the animals were allowed to stabilize for at least 1 h. At the end of each experiment, the defined intestinal segment was removed from the carcass, and the intestinal contents were cleaned out to obtain a wet tissue weight. The Animal Care Committee at the University of Manitoba approved all procedures.

The influence of blood flow on intestinal glucose uptake. To increase intestinal blood flow, the vasodilator Ado was infused into the SMA through an infusion line in the circuit at a dose (0.4 mg · kg body wt-1 · min-1) previously shown (20) to produce maximal dilation (n = 7). A constant SMA perfusion pressure was maintained, with the use of the pump to increase blood flow during the drug infusion. Blood samples for measuring blood glucose, lactate, and oxygen content were taken from the circuit and portal vein simultaneously at 5 min before the infusion, as control, and after 5 min of the infusion when blood flow and oxygen consumption effects are stable (13). Ado was also infused into the SMA while the blood flow was maintained constant to prevent a rise in shear stress.

The involvement of NO in intestinal glucose uptake. The shear rate in the superior mesenteric vascular bed is increased because of the increase in blood flow when a vasodilator is infused during constant-pressure perfusion. To study the involvement of NO, the animals in the first group (n = 7) were treated with L-NAME (2.5 mg/kg iv, infused over 10 min), a potent nonselective NO synthase blocker. Ado infusions were repeated 10 min after the completion of L-NAME administration. The intestinal glucose uptake in basal state and during raised blood flow were compared before and after NO synthase blockade.

The influence of indomethacin on intestinal glucose uptake. To investigate the role of prostanoids in control of intestinal glucose uptake, the uptake during raised blood flow by Ado infusion (0.4 mg · kg body wt-1 · min-1) was compared before and after the inhibition of cyclooxygenase by indomethacin (5 mg/kg iv bolus infusion over 10 min) in the second group of animals (n = 5). Ado infusion during constant-pressure perfusion was performed before and 30 min after indomethacin administration.

The effects of prostaglandins on intestinal glucose uptake. For further confirmation of the results obtained in the previous protocol and to identify the role of different prostaglandins in control of intestinal glucose uptake, the effects of prostaglandins (intra-SMA infusion) were tested in the second group of animals and compared before and after the inhibition of cyclooxygenase. The effects on intestinal glucose uptake of PGF2alpha and PGE2 (and PGI2 in some animals) infusion (41.2 ng · kg body wt-1 · min-1 for all) alone or with Ado during constant-pressure (elevated flow) perfusion were tested before and after the administration of indomethacin. PGF2alpha was also infused with Ado (0.4 mg · kg body wt-1 · min-1) during constant-flow perfusion in two animals. One extra dose (8.2 ng · kg body wt-1 · min-1) of PGF2alpha was also tested with Ado infusion after indomethacin. The order of testing was randomly assigned before and after indomethacin. The two different PGF2alpha doses after indomethacin were tested sequentially but ordered randomly. The infusion lasted for 10 min in this case, and the dose was changed by switching infusion rate after samples were taken at 5 min for the first dose.

The influence of increased shear stress in the liver. To study the influence of the liver on intestinal glucose uptake in response to increased blood flow, a special surgical procedure was performed in two animals to bypass portal blood from the liver and shunt it directly into the central vena cava. A loop with a T connector was placed in the central vena cava by double cannulation. The portal vein was cannulated and connected to the central venous loop. Intestinal glucose uptake in the two animals was studied during increased SMA blood flow by infusion of Ado (0.4 mg · kg body wt-1 · min-1).

Assays and data analysis. For each sample, blood (<0.5 ml) was withdrawn simultaneously from the circuit and portal venous catheter. Blood glucose and lactate concentration were analyzed with glucose and lactate analyzers (Sports Industrial Analyzer, Yellow Springs Instruments). Total CO2 production was calculated from the PCO2 and pH that were measured with a blood gas analyzer (system 1302, Instrumentation Laboratory, Lexington, MA). Glucose uptake by the intestine was assessed with the use of the product of arterial-venous concentration difference and the blood flow, yielding a result in units of micromoles per minute per 100 g tissue. All data in this study are expressed as means ± SE. Paired Student's t-test or ANOVA was used for detection of any difference. P < 0.05 was chosen as the criterion for significance to reject the null hypothesis. The computer programs GraphPad Prism and Microsoft Excel were used to compute the statistics.

Drugs and chemicals. Ado, L-NAME, indomethacin, and PGF2alpha were purchased from Sigma (St. Louis, MO). PGE2 was purchased from Upjohn (Kalamazoo, MI). Ado and L-NAME were dissolved in saline before each experiment. The prostaglandins were dissolved in 95% ethanol at 1 mg/ml concentration as a stock solution, which was stored as small aliquots at -20°C. Fresh solutions were made by dilution of the stock solution to desired concentrations with saline in each experiment. One tube of stock solution was used at a time, and all prostaglandin solutions were protected from light with aluminum foil.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Basal metabolic and hemodynamic conditions. Basal conditions of the animals were stabilized within 2 h after the surgery. On stabilization, for the first and second group of animals basal arterial glucose concentrations were 98.1 ± 6.5 and 101.9 ± 2.7 mg/dl (5.44 ± 0.36 and 5.65 ± 0.15 mmol/l), respectively, and the SMA perfusion pressures were 114.9 ± 7.6 and 105.3 ± 3.1 mmHg at flow rates of 37.6 ± 8.1 and 35.5 ± 2.0 ml · min-1 · 100 g tissue-1. The basal conditions of these two groups were not significantly different. The third group of two animals had similar basal hemodynamic and metabolic conditions.

Arterial glucose levels had a tendency to rise slightly following Ado infusion in constant-pressure condition (101.9 ± 2.7 to 111.6 ± 2.6 mg/dl). The glucose levels were not different before and after L-NAME or indomethacin or with any prostaglandin infusion whether glucose uptake was increased or not. The arterial glucose levels before and after all procedures ranged from 94.2 ± 6.6 to 118.1 ± 8.5 mg/dl. Arterial lactate levels ranged from 2.2 ± 0.4 to 2.5 ± 0.5 µmol/l with no significant change associated with any drug. PO2 did not change consistently or significantly throughout the experiments.

The influence of SMA blood flow on intestinal glucose uptake. The infusion of Ado increased the SMA blood flow by 111.9 ± 14.3 from 31.6 ± 7.5 ml · min-1 · 100 g tissue-1 (P < 0.0005, n = 7, Fig. 1), whereas the perfusion pressure was maintained [123.9 ± 5.5 vs. 120.0 ± 3.8 mmHg for control, not significant (NS)]. The glucose uptake across the intestine increased by 45.0 ± 18.3 from 25.3 ± 3.8 µmol · min-1 · 100 g tissue-1 (P < 0.05, Fig. 1). The total production of CO2 and lactate balance across the intestine were not changed (CO2: 45.1 ± 25.4 vs. 37.4 ± 12.3 µmol · min-1 · 100 g tissue-1 for control, NS; lactate: 19.0 ± 8.1 vs. 18.0 ± 4.9 µmol · min-1 · 100 g tissue-1 for control, NS). Ado was also infused while holding the SMA blood flow constant. The SMA perfusion pressure was reduced by 68.5 ± 11.3 from 114.9 ± 7.6 mmHg (P < 0.001, n = 7) during Ado infusion, whereas the flow rate was unchanged (36.4 ± 7.7 vs. 37.0 ± 7.5 ml · min-1 · 100 g tissue-1, NS). Glucose uptake in the constant-flow situation, however, was not altered (22.0 ± 3.9 vs. 19.3 ± 4.3 µmol · min-1 · 100 g tissue-1 for control, NS, Fig. 1).


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Fig. 1.   Superior mesenteric artery (SMA) blood flow (A) and intestinal glucose uptake (B) in control and during adenosine (Ado, 0.4 mg · kg body wt-1 · min-1) infusion during constant-flow (CF) and constant-pressure (CP) perfusion. The responses were similar before (left) and after (right) nitric oxide (NO) synthase antagonism by NG-nitro-L-arginine methyl ester (L-NAME) (2.5 mg/kg); n = 7.

Thus the intestinal glucose uptake increased dramatically when blood flow was increased by Ado (constant pressure); the glucose uptake was not altered by Ado infusion during constant-flow perfusion.

The influence of NO synthase blockade on intestinal glucose uptake. To assess the possible involvement of NO in the regulation of intestinal glucose uptake during constant-pressure perfusion in which shear stress was elevated, L-NAME (2.5 mg/kg) was used to block endogenous NO production. L-NAME significantly increased the SMA perfusion pressure from 116.8 ± 3.6 to 178.1 ± 7.8 mmHg (P < 0.0005, n = 7), whereas the blood flow was not changed (32.8 ± 8.0 before vs. 30.7 ± 8.0 ml · min-1 · 100 g tissue-1 after L-NAME, NS). The basal glucose uptake across the intestine was not altered by L-NAME. After the administration of L-NAME, the intestinal glucose uptake was increased by 52.0 ± 10.2 from 20.2 ± 4.5 µmol · min-1 · 100 g tissue-1 (P < 0.005, n = 7, Fig. 1) when the blood flow was increased by 112.8 ± 15.2 ml · min-1 · 100 g tissue-1 (P < 0.0005) during Ado infusion. The increase in blood flow was controlled as close as possible before L-NAME. The increases in the intestinal glucose uptake during increased blood flow were the same before and after L-NAME (increased by 45.0 ± 18.3 before vs. 52 ± 10.2 µmol · min-1 · 100 g tissue-1 after L-NAME, NS). Similar to the response before L-NAME, Ado infusion during constant-flow perfusion did not change intestinal glucose uptake (22.8 ± 5.3 vs. 28.8 ± 5.3 µmol · min-1 · 100 g tissue-1 for control after L-NAME, NS). Thus the blockade of NO formation had no influence on the basal or blood flow-induced increase in intestinal glucose uptake.

The influence of indomethacin on intestinal glucose uptake. In another group of animals (n = 5), a cyclooxygenase inhibitor, indomethacin, was used to block the production of prostanoids during increased blood flow. Before indomethacin, the intestinal glucose uptake was increased by 41.0 ± 4.4 from 18.8 ± 4.1 µmol · min-1 · 100 g tissue-1 (P < 0.001) by infusion of Ado during constant-pressure perfusion, which increased SMA blood flow by 145.9 ± 9.8 from 35.5 ± 2.0 ml · min-1 · 100 g tissue-1 (P < 0.0001, Fig. 2). The blockade of cyclooxygenase by indomethacin did not change basal intestinal glucose uptake (14.1 ± 5.1 before vs. 17.5 ± 6.8 µmol · min-1 · 100 g tissue-1 after indomethacin, NS), whereas the basal SMA blood flow was slightly increased after indomethacin (P < 0.05, by ANOVA). Ado infusion during constant-pressure perfusion increased SMA blood flow by 152.5 ± 23.5 from 52.0 ± 8.3 ml · min-1 · 100 g tissue-1 (P < 0.005). However, the intestinal glucose uptake after indomethacin was not increased (23.6 ± 11.3 vs. 17.5 ± 6.8 µmol · min-1 · 100 g tissue-1 for control, NS, Fig. 2).


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Fig. 2.   SMA blood flow (A) and intestinal glucose uptake (B) in control and during increased blood flow (high flow) by Ado (0.4 mg · kg body wt-1 · min-1) infusion during constant-pressure perfusion. The blood flow elevation was not altered, but the flow-induced increase in glucose uptake was blocked after cyclooxygenase inhibition by indomethacin (5.0 mg/kg; right) vs. before indomethacin (left); n = 5. * Compared with control before indomethacin.

Therefore, the increase in intestinal glucose uptake caused by the increase in SMA blood flow was inhibited by the blockade of cyclooxygenase metabolite production.

The role of different prostaglandins in control of intestinal glucose uptake. The infusion of PGE2 into the SMA decreased the SMA perfusion pressure by 29.0 ± 4.3 from 112.0 ± 4.2 mmHg (P < 0.005, n = 5) while the blood flow was held steady (40.1 ± 6.0 vs. 35.3 ± 5.1 ml · min-1 · 100 g tissue-1 for control, NS). Infusion of PGF2alpha at a dose of 41.2 ng · kg body wt-1 · min-1 caused a slight and transient increase in the SMA perfusion pressure followed by a sustained small decrease in the perfusion pressure by 15.5 ± 3.8 from 104.3 ± 2.5 mmHg (P < 0.05), whereas the blood flow was not changed (32.6 ± 5.9 vs. 32.6 ± 5.8 ml · min-1 · 100 g tissue-1 for control, NS). After the administration of indomethacin, PGE2 decreased perfusion pressure by 40.9 ± 3.6 from 100.1 ± 4.1 mmHg (P < 0.0005, n = 5), whereas the blood flow was not changed (60.1 ± 9.4 vs. 59.3 ± 9.2 ml · min-1 · 100 g tissue-1 for control, NS). The infusion of PGF2alpha at the same dose after indomethacin did not change perfusion pressure (90.4 ± 3.4 vs. 95.3 ± 1.0 mmHg for control, NS). However, neither the infusion of PGE2 nor of PGF2alpha , before and after the administration of indomethacin, changed intestinal glucose uptake in the constant-flow condition, as shown in Fig. 3.


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Fig. 3.   Intestinal glucose uptake in control and during the infusion of different prostaglandins under normal blood flow. PGF2alpha and PGE2 were both at a dose of 41.2 ng · kg body wt-1 · min-1 (intra-SMA infusion). There was no effect of prostaglandin infusion, and the effects were not different before (left) and after (right) cyclooxygenase inhibition by indomethacin (5.0 mg/kg); n = 5.

The infusion of prostaglandins during increased SMA blood flow by Ado during constant-pressure perfusion did not change the hemodynamics compared with the control (Fig. 4). The intestinal glucose uptake under such conditions increased by 44.9 ± 10.3 and 43.0 ± 10.9 µmol · min-1 · 100 g tissue-1 during PGF2alpha and PGE2 infusion, respectively. The increases were not different from that in the situation of Ado alone (41.0 ± 4.4 µmol · min-1 · 100 g tissue-1, NS). Thus the infusion of prostaglandins during normal SMA blood flow affected perfusion pressure but did not alter intestinal glucose uptake; the infusion of prostaglandins during increased SMA blood flow did not have any additional effect on increased glucose uptake.


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Fig. 4.   Changes in SMA blood flow (A) and intestinal glucose uptake (B) from control when SMA blood flow was increased (high flow). The effects of different prostaglandins were compared before (left) and after (right) cyclooxygenase inhibition by indomethacin (5.0 mg/kg). PGF2alpha and PGE2 were given at a dose of 41.2 ng · kg body wt-1 · min-1 (intra-SMA infusion) before indomethacin, and 8.2 ng · kg body wt-1 · min-1 for PGF2alpha (low dose) and 41.2 ng · kg body wt-1 · min-1 for PGF2alpha (high dose) and PGE2 after indomethacin. PGF2alpha but not PGE2 (or PGI2; data not shown) restored the glucose uptake in the presence of elevated blood flow to similar levels to those produced before indomethacin; n = 5.

The infusion of prostaglandins during increased flow by Ado did not alter blood flow before and after indomethacin (Fig. 4). However, after indomethacin, the infusion of PGF2alpha during increased blood flow increased intestinal glucose uptake from control by 39.3 ± 5.4 µmol · min-1 · 100 g tissue-1 (P < 0.05) and 68.2 ± 14.0 µmol · min-1 · 100 g tissue-1 (P < 0.01, n = 5) at doses of 8.2 and 41.2 ng · kg body wt-1 · min-1, respectively, whereas the infusion of Ado alone during constant-pressure perfusion after indomethacin did not change the glucose uptake. The changes in glucose uptake in the presence of PGF2alpha were dose dependent and were significantly different (P < 0.01, Fig. 4) at a dose of 41.2 ng · kg body wt-1 · min-1 compared with that during Ado infusion alone. The infusion of another prostaglandin, PGE2, during raised blood flow was not able to increase the glucose uptake (12.4 ± 3.4 vs. 18.9 ± 4.2 µmol · min-1 · 100 g tissue-1 for control, NS, Fig. 4) in the presence of indomethacin.

PGF2alpha was also infused with Ado during constant-flow perfusion before indomethacin in two animals. However, under normal blood flow (27.0 and 22.8 ml · min-1 · 100 g tissue-1), the intestinal glucose uptake was not altered (18.8 vs. 20.7 µmol · min-1 · 100 g tissue-1 for control, and 6.2 vs. 6.9 µmol · min-1 · 100 g tissue-1 for control) by the infusion. The infusion of PGI2 with Ado during constant-pressure perfusion after indomethacin was not able to increase the glucose uptake in pilot experiments (data not shown).

Therefore, PGF2alpha , but not PGE2 and PGI2, dose-dependently reversed the blockade of increase in intestinal glucose uptake by indomethacin during raised blood flow, although the infusion of PGF2alpha before indomethacin showed no additional effect, and PGF2alpha under normal blood flow did not affect basal glucose uptake before and after indomethacin.

Intestinal glucose uptake in liver-bypassed animals. In two animals, the portal venous blood flow was surgically shunted to the central vena cava. The intestinal glucose uptake increased by 24.0 and 89.0 µmol · min-1 · 100 g tissue-1 when the blood flow was increased by 80.1 and 172.3 ml · min-1 · 100 g tissue-1 in response to Ado in the two animals, respectively.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Intestinal glucose uptake from blood increased dramatically when the blood flow increased. Since shear rate was elevated during increased blood flow, we further tested the hypothesis that shear stress-induced release of NO or prostaglandins mediated the increase of intestinal glucose uptake. Indomethacin, a cyclooxygenase inhibitor that blocks the production of prostanoids, eliminated the increase in glucose uptake that was elicited by increased blood flow, suggesting that shear stress-induced release of prostaglandins was involved in mediating increased glucose uptake in the intestine. PGF2alpha , at a dose that produced only a small vascular response, reversed the inhibitory effect of indomethacin on increased intestinal glucose uptake induced by increased blood flow; in contrast, PGE2 and PGI2 had no effect on the glucose uptake. The data suggest that PGF2alpha was responsible for mediating the increase in intestinal glucose uptake when shear stress was elevated. The involvement of another shear stress-induced autacoid, NO, was ruled out because the blockade of NO synthase did not affect the glucose uptake. In addition, the prostaglandins and NO were not involved in the control of basal intestinal glucose uptake because the blockade of cyclooxygenase or NO synthase did not change the basal glucose uptake. An unusual aspect of the results was that PGF2alpha only increased glucose uptake when blood flow was increased. To our knowledge, this is the first time that a blood flow-elicited increase in intestinal glucose uptake has been reported and that the mechanism of shear stress-induced release of PGF2alpha was suggested.

Methodology consideration. In this study, we surgically separated a segment of small intestine in situ, which was perfused by an arterial circuit through the SMA. This setup provided a model that would be least affected by changes in systemic and hemodynamic reflexes. The surgical procedures, including the removal of the spleen and large intestine and ligation at the duodenum, were to ensure that the portal blood was exclusively from the defined portion of intestine. A sample line in the circuit and a portal catheter provided routes to access arterial and portal blood samples simultaneously.

A pump was incorporated into the circuit to allow the SMA blood flow to be adjusted. However, it was necessary to use a vasodilator to increase the SMA blood flow substantially without the need to produce a massive increase in the perfusion pressure. Ado is a potent vasodilator and has been suggested to mediate postprandial and reactive hyperemia in the intestine (24). To assess the direct effect of Ado on glucose uptake in the intestine, we administered the same dose of Ado during both constant-flow and constant-pressure perfusion. In contrast to constant-pressure perfusion, Ado did not change basal glucose uptake during constant-flow perfusion, suggesting that there was no direct effect of Ado on intestinal glucose balance.

The influence of blood flow on intestinal glucose uptake. We demonstrated that the intestinal glucose uptake from blood increased dramatically when the SMA blood flow was increased. The active role of the gut in glucose disposal in basal and absorptive states reported by Abumrad et al. (1) suggested that, in addition to basal utilization of up to 25% of hepatic glucose output by the gut in dogs, the glucose uptake was increased after ingestion; the change in intestinal blood flow was, unfortunately, not reported. The gastrointestinal blood flow is normally increased during ingestion. However, the absorption of glucose from the intestinal lumen makes it difficult to assess the net glucose utilization by the intestine. Direct glucose absorption from the lumen in roughage-fed ruminants is negligible. Bergman et al. (4) demonstrated that the gut of sheep used about 16% of hepatic output of glucose in a "continuously fed" model. Blood flow was not reported.

Data from other studies also implied that there is a relationship between increased intestinal blood flow and glucose uptake. Fernández-López et al. (10) reported that the gut consumes a considerable portion of glucose load in rats, in which the portal blood flow increased two- to threefold. In that study, the portal glucose concentration was actually decreased, whereas the portal blood flow increased more than twofold when a small amount of glucose (0.1 mmol) was gavaged. Intestinal glucose uptake was elevated when blood flow increased during septic shock (15), although the authors concluded that the elevated glucose uptake was not dependent on increased blood flow because somatostatin reduced the blood flow but did not reduce glucose uptake in the gut. However, somatostatin did not bring the intestinal blood flow to its control level, and somatostatin-induced vasoconstriction could possibly increase shear stress in that situation.

The influence of shear stress on intestinal glucose uptake. We previously discussed (13) that when blood flow increases during constant-pressure perfusion the shear rate increases at areas not actively dilated, including the capillary endothelium. Shear stress stimulates the release from the endothelial cells of autacoids, including NO, prostaglandins, and endothelium-derived hyperpolarizing factor (5). These substances may have direct or indirect effects on the intestinal glucose uptake and metabolism. We showed that the release of NO was increased when blood flow was increased (13). In the present study, we used L-NAME to block NO synthase to reduce shear stress-induced release of NO. The dose (2.5 mg/kg) used in the study was previously demonstrated to be sufficient to block NO production (13, 20). The increase in SMA pressure after L-NAME in this study further confirmed the successful blockade. However, neither the basal nor increased intestinal glucose uptake was altered by NO synthase blockade, suggesting that NO was not primarily involved in the control of intestinal glucose uptake.

Prostaglandins are arachidonic acid metabolites of cyclooxygenase. Increase in shear stress may stimulate the endothelium to release prostaglandins (5, 6, 22), which in turn may have direct or indirect effects on intestinal glucose uptake. In order to reduce the release of prostaglandins when shear stress was elevated, indomethacin, a cyclooxygenase inhibitor, was used. The slight increase in SMA blood flow after indomethacin suggested that the basal prostaglandin(s) produced mainly vasoconstriction in cats. The basal intestinal glucose uptake was not affected by indomethacin, suggesting that the prostaglandin(s) was not involved in the control of basal glucose uptake in the intestine. The increased glucose uptake during raised blood flow was totally blocked in the presence of indomethacin. These results supported our hypothesis that elevated shear stress plays a key role in the control of intestinal glucose uptake, and further suggested that it was shear stress-induced release of prostaglandin that mediated this event.

The increase in SMA blood flow directly resulted in an increase in the portal blood flow, which may have elevated shear stress in the liver as well. Elevated shear stress in the liver could trigger a hepatic event that had influence on intestinal glucose uptake through hormone circulation or nerve reflexes. Although the superior mesenteric plexus was denervated, an enterohepatic reflex could pass through the posterior hepatic plexus (16), which distributes along the portal vein and bile duct, or hepatic vagal branches (17). In two animals, we surgically diverted portal venous blood from the liver to the central vena cava, which avoided exposure of the liver to raised portal blood flow. The intestinal glucose uptake was stimulated as well when the SMA flow was increased in this group. The data ruled out the possibility that the liver regulated intestinal glucose uptake in response to elevated portal flow, and provided further evidence that the site of increased shear stress responsible for the regulation of glucose uptake was in the intestine.

The effect of PGF2alpha . PGI2, PGE2 and PGF2alpha are released from the endothelial cells in response to shear stress (5, 6, 22). PGE2 and PGI2 had no influence on glucose uptake in our study. PGF2alpha stimulated glucose metabolism in rat uterus (12). PGF2alpha is believed to be a vasoconstrictor that causes increase in the SMA pressure (18) or decrease in the blood flow (23). However, PGF2alpha at the dose used in the present study, which was about 12 times lower than that used in dogs (23) and much lower than that in the cat study (18), produced only a minor and transient increase in the SMA pressure followed by a sustained small vasodilation. In our study, the indomethacin-inhibited increase in intestinal glucose uptake was reversed by PGF2alpha in a dose-dependent manner but not by PGE2 and PGI2 at a similar molar dose. Our data suggest that PGF2alpha is the specific prostaglandin that is responsible for mediating shear stress-induced increase in intestinal glucose uptake.

Unresolved issues. It seems that the increased shear stress or blood flow per se was a necessary or permissive factor, because the infusion of PGF2alpha under normal blood flow did not alter the glucose uptake. An example of permissive control of prostaglandin effects has been previously reported. 17-beta Estradiol was necessary for PGF2alpha and PGE1 to have their effect on uterus glucose metabolism, although the hormone per se did not have any effect on glucose metabolism (12). Ado did not serve in this role because the infusion of PGF2alpha with Ado during constant-flow perfusion did not change the glucose uptake. Shear stress-induced release of other endothelium-derived factor(s) could possibly serve as a permissive factor for PGF2alpha action. It was reported that NO had a permissive effect on endothelial arachidonic acid metabolism and the arteriole dilation caused by the metabolite(s) (3). This possibility, however, was ruled out by our observation that L-NAME did not block the increased intestinal glucose uptake.

Rather than shear stress, the blood flow itself could possibly be the important factor because of the complex structure of intestinal villus microcirculation. A shunt between the central arteriole and the venule that drains the blood in the opposite direction in the intestinal villus was modeled by Shepherd and Kiel (25), who suggested that the tip of the microvilli could be in a hypoxic or subhypoxic state during normal blood flow. Raised blood flow might be necessary for oxygenation and activation of the tissue to respond to PGF2alpha . Further studies are required to elucidate the factor behind the permissive effect of blood flow on the prostaglandin action in control of intestinal glucose uptake.

During raised SMA blood flow, the glucose uptake increased about threefold, but there was no evidence of increase in glucose oxidation because the CO2 production and lactate balance were not different from the baseline. Whether the glucose taken up into the intestinal tissue was used for glycogen synthesis, for mucus synthesis (polysaccharide as a major component), or for other purposes is another question that needs to be addressed in further studies.

It is interesting that the intestinal tissue is not sensitive to insulin. Even though the intestine could use other substrates, glucose is still one of the most important energy sources for basal intestinal metabolism and activities such as absorption and motility. The results from the present study suggest that raised SMA blood flow increases intestinal glucose uptake secondary to the release of PGF2alpha . Prostaglandins may play a role in the initiation and maintenance of postprandial intestinal hyperemia (11). Increased blood glucose concentration due to postingestive absorption normally occurs at the same time as the hyperemia, and this is also the time that the intestine needs energy for absorption and increased motility. The extent of low-flow-induced intestinal mucosal lesions, including epithelial disruption and damage of the villi, is flow dependent (7). This could be because of the deprivation of oxygen and energy supply to the intestinal villi. Protection against the low-flow-induced mucosal lesion by intraluminal glucose (8) strongly suggested that the deprivation of energy source was the main cause of the damage. The absence of PGF2alpha and other essential factor(s) in the low-flow state, however, could be the primary mechanism of the cause. Our findings may also explain, in part, the side effect on the gut of indomethacin and other nonsteroidal anti-inflammatory drugs. In conclusion, the intestinal glucose uptake is regulated by blood flow-dependent release of PGF2alpha and other factors, which may be important for the prostaglandin action; however, the character of this factor needs to be identified.


    ACKNOWLEDGEMENTS

We thank Dallas J. Legare for excellent technical assistance and Karen Sanders for preparation of the manuscript.


    FOOTNOTES

This project was supported by the Manitoba Heart and Stroke Foundation and The Medical Research Council of Canada. C. Han received a Canadian Hypertension Society-Pfizer-Medical Research Council of Canada studentship.

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Address for reprint requests and other correspondence: W. W. Lautt, 753 McDermot Ave., Dept. of Pharmacology and Therapeutics, Faculty of Medicine, Univ. of Manitoba, Winnipeg, Manitoba, Canada R3E 0W3 (E-mail: wlautt{at}cc.umanitoba.ca).

Received 30 November 1998; accepted in final form 21 April 1999.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

1.   Abumrad, N. N., A. D. Cherrington, P. E. Williams, W. W. Lacy, and D. Rabin. Absorption and disposition of a glucose load in the conscious dog. Am. J. Physiol. 242 (Endocrinol. Metab. 5): E398-E406, 1982[Abstract/Free Full Text].

2.   Bailey, C. J., K. J. Mynett, and T. Page. Importance of the intestine as a site of metformin-stimulated glucose utilization. Br. J. Pharmacol. 112: 671-675, 1994[Abstract].

3.   Bakker, E. N., and P. Sipkema. Permissive effect of nitric oxide in arachidonic acid induced dilation in isolated rat arterioles. Cardiovasc. Res. 38: 782-787, 1998[Medline].

4.   Bergman, E. N., R. P. Brockman, and C. F. Kaufman. Glucose metabolism in ruminants: comparison of whole-body turnover with production by gut, liver, and kidneys. Federation Proc. 33: 1849-1854, 1974[Medline].

5.   Busse, R., and I. Fleming. Pulsatile stretch and shear stress: physical stimuli determining the production of endothelium-derived relaxing factors. J. Vasc. Res. 35: 73-84, 1998[Medline].

6.   Charo, I. F., S. Shak, M. A. Karasek, P. M. Davison, and I. M. Goldstein. Prostaglandin I2 is not a major metabolite of arachidonic acid in cultured endothelial cells from human foreskin microvessels. J. Clin. Invest. 74: 914-919, 1984[Medline].

7.   Chiu, C. J., A. H. McArdle, R. Brown, H. J. Scott, and F. N. Gurd. Intestinal mucosal lesion in low-flow states. I. A morphological, hemodynamic, and metabolic reappraisal. Arch. Surg. 101: 478-483, 1970[Medline].

8.   Chiu, C. J., H. J. Scott, and F. N. Gurd. Intestinal mucosal lesion in low-flow states. II. The protective effect of intraluminal glucose as energy substrate. Arch. Surg. 101: 484-488, 1970[Medline].

9.   DeFronzo, R. A., E. Ferrannini, R. Hendler, P. Felig, and J. Wahren. Regulation of splanchnic and peripheral glucose uptake by insulin and hyperglycemia in man. Diabetes 32: 35-45, 1983[Medline].

10.   Fernández-López, J. A., J. Casado, M. J. Argilés, and M. Alemany. Intestinal handling of a glucose gavage by the rat. Mol. Cell. Biochem. 113: 43-53, 1992[Medline].

11.   Gallavan, R. H., Jr., and C. C. Chou. Possible mechanisms for the initiation and maintenance of postprandial intestinal hyperemia. Am. J. Physiol. 249 (Gastrointest. Liver Physiol. 12): G301-G308, 1985[Abstract/Free Full Text].

12.   Gonzalez, E. T., M. A. F. Gimeno, and A. L. Gimeno. Prostaglandin E2 alters the metabolism of labeled glucose in uteri isolated from ovariectomized rats. Effects of 17-beta estradiol and indomethacin. Prost. Leukot. Essent. Fatty Acids 35 (1): 31-35, 1989.

13.   Han, C., Z. Ming, and W. W. Lautt. Shear stress-induced nitric oxide antagonizes adenosine effects on intestinal metabolism. Am. J. Physiol. 276 (Gastrointest. Liver Physiol. 39): G1227-G1234, 1999[Abstract/Free Full Text].

14.   Kellett, G. L., A. Jamal, J. P. Robertson, and N. Wollen. The acute regulation of glucose absorption, transport and metabolism in rat small intestine by insulin in vivo. Biochem. J. 219: 1027-1035, 1984[Medline].

15.   Lang, C. H., J.-C. A. Obih, G. J. Bagby, J. N. Bagwell, and J. J. Spitzer. Increased glucose uptake by intestinal mucosa and muscularis in hypermetabolic sepsis. Am. J. Physiol. 261 (Gastrointest. Liver Physiol. 24): G287-G294, 1991[Abstract/Free Full Text].

16.   Lautt, W. W. Hepatic nerves: a review of their function and effects. Can. J. Physiol. Pharmacol. 58: 105-123, 1980[Medline].

17.   Lee, K. C. Reflex suppression and initiation of gastric contractions by electrical simulation of the hepatic vagus nerve. Neurosci. Lett. 53: 57-62, 1985[Medline].

18.   Lippton, H. L., W. M. Armstead, A. L. Hyman, and P. J. Kadowitz. Characterization of the vasoconstrictor activity of indomethacin in the mesenteric vascular bed of the cat. Prostaglandins Leukot. Med. 27: 81-91, 1987[Medline].

19.   Macedo, M. P., and W. W. Lautt. Shear-induced modulation by nitric oxide of sympathetic nerves in the superior mesenteric artery. Can. J. Physiol. Pharmacol. 74: 692-700, 1996[Medline].

20.   Macedo, M. P., and W. W. Lautt. Potentiation to vasodilators by nitric oxide synthase blockade in superior mesenteric but not hepatic artery. Am. J. Physiol. 272 (Gastrointest. Liver Physiol. 35): G507-G541, 1997[Abstract/Free Full Text].

21.   Nicholls, T. J., H. J. Leese, and J. R. Bronk. Transport and metabolism of glucose by rat small intestine. Biochem. J. 212: 183-187, 1983[Medline].

22.   Nollert, M. U., E. R. Hall, S. G. Eskin, and L. V. McIntire. The effect of shear stress on the uptake and metabolism of arachidonic acid by human endothelial cells. Biochim. Biophys. Acta 1005: 72-78, 1989[Medline].

23.   Pawlik, W., A. P. Shepherd, and E. D. Jacobson. Effects of vasoactive agents on intestinal oxygen consumption and blood flow in dogs. J. Clin. Invest. 56: 484-490, 1975[Medline].

24.   Sawmiller, D. R., and C. C. Chou. Role of adenosine in postprandial and reactive hyperemia in canine jejunum. Am. J. Physiol. 263 (Gastrointest. Liver Physiol. 26): G487-G493, 1992[Abstract/Free Full Text].

25.   Shepherd, A. P., and J. W. Kiel. A model of countercurrent shunting of oxygen in the intestinal villus. Am. J. Physiol. 262 (Heart Circ. Physiol. 31): H1136-H1142, 1992[Abstract/Free Full Text].

26.   Windmueller, H. G., and A. E. Spaeth. Respiratory fuels and nitrogen metabolism in vivo in small intestine of fed rats. J. Biol. Chem. 225: 107-112, 1980.

27.   Xie, H., and W. W. Lautt. Insulin resistance of skeletal muscle produced by hepatic parasympathetic interruption. Am. J. Physiol. 270 (Endocrinol. Metab. 33): E858-E863, 1996[Abstract/Free Full Text].


Am J Physiol Gastroint Liver Physiol 277(2):G367-G374
0002-9513/99 $5.00 Copyright © 1999 the American Physiological Society




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