Hepatic alpha - and beta -adrenergic receptors are not essential for the increase in Ra during exercise in diabetes

Robert H. Coker1, D. Brooks Lacy2, Phillip E. Williams3, and David H. Wasserman1

1 Department of Molecular Physiology and Biophysics, 2 Diabetes Research and Training Center, and 3 Department of Surgery, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-0615


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The purpose of this study was to determine the role of direct hepatic adrenergic stimulation in the control of endogenous glucose production (Ra) during moderate exercise in poorly controlled alloxan-diabetic dogs. Chronically catheterized and instrumented (flow probes on hepatic artery and portal vein) dogs were made diabetic by administration of alloxan. Each study consisted of a 120-min equilibration, 30-min basal, 150-min moderate exercise, 30-min recovery, and 30-min blockade test period. Either vehicle (control; n = 6) or alpha  (phentolamine)- and beta  (propranolol)-adrenergic blockers (HAB; n = 6) were infused in the portal vein. In both groups, epinephrine (Epi) and norepinephrine (NE) were infused in the portal vein during the blockade test period to create suprapharmacological levels at the liver. Isotopic ([3-3H]glucose, [U-14C]alanine) and arteriovenous difference methods were used to assess hepatic function. Arterial plasma glucose was similar in controls (345 ± 24 mg/dl) and HAB (336 ± 23 mg/dl) and was unchanged by exercise. Basal arterial insulin was 5 ± 1 mU/ml in controls and 4 ± 1 mU/ml in HAB and fell by ~50% during exercise in both groups. Basal arterial glucagon was similar in controls (56 ± 10 pg/ml) and HAB (55 ± 7 pg/ml) and rose similarly, by ~1.4-fold, with exercise in both groups. Despite greater arterial Epi and NE levels in HAB compared with controls during the basal and exercise periods, exercise-induced increases in catecholamines from basal were similar in both groups. Gluconeogenic conversion from alanine and lactate and the intrahepatic efficiency of this process were increased by twofold during exercise in both groups. Ra rose similarly by 2.9 ± 0.7 and 2.7 ± 1.0 mg · kg-1 · min-1 at time = 150 min during exercise in controls and HAB. During the blockade test period, arterial plasma glucose and Ra rose to 454 ± 43 mg/dl and 11.3 mg · kg-1 · min-1 in controls, respectively, but were essentially unchanged in HAB. The attenuated response to the blockade test in HAB substantiates the effectiveness of the hepatic adrenergic blockade. In conclusion, these results demonstrate that direct hepatic adrenergic stimulation does not play a role in the stimulation of Ra during exercise in poorly controlled diabetes.

catecholamines; glucagon; insulin; liver; glucose production


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

EXERCISE-INDUCED CHANGES in glucagon and insulin stimulate endogenous glucose production (Ra) during moderate exercise in the healthy state. This exercise-induced increment in Ra is matched by an increase in glucose utilization (Rd), which results in the maintenance of glucose homeostasis (26). In contrast, the exercise-induced increment in Ra in the poorly controlled diabetic state occurs despite hyperglycemia which, by itself, is great enough to support the requirement for glucose for ~45 min (25, 27). The needless increase in Ra is often accompanied by an elevation in sympathetic drive that may contribute to the deleterious effects of exercise in the poorly controlled diabetic state (7). Studies with beta -adrenergic blockade in the depancreatized dog model have shown a small reduction in Ra during moderate exercise (2). However, in addition to the reduction of Ra in these previous studies, arterial free fatty acids (FFA) and gluconeogenic precursors were also reduced due to the beta -adrenergic blockade, which in turn, would decrease their hepatic delivery. The limitation of gluconeogenic substrate supply has important implications, since gluconeogenesis is a large component of Ra in the insulin-deficient state (24). Furthermore, the role of sympathetic drive on Ra could not be completely delineated, since alpha -adrenergic receptors were unblocked in these prior studies.

The present study was designed to determine the role of hepatic adrenergic mechanisms in the stimulation of Ra during exercise in the poorly controlled diabetic state. For this purpose, we used an alpha - and beta -hepatic adrenergic receptor blockade technique that is selective to the liver and that produces minimal extrahepatic effects (6) in chronically catheterized alloxan-diabetic dogs.


    METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Animals and surgical procedures. Experiments were performed on a total of 12 overnight-fasted mongrel dogs (mean wt 21.5 ± 0.5 kg) of either sex that had been fed a standard diet (Pedigree beef dinner and Wayne Lab Blox, 51% carbohydrate-31% protein-11% fat-7% fiber based on dry wt). The dogs were housed in a facility that met American Association for the Accreditation of Laboratory Animal Care guidelines, and the protocols were approved by the Vanderbilt University Animal Care Committee.

At least 16 days before each experiment, a laparotomy was performed under general anesthesia (0.04 mg/kg of atropine and 15 mg/kg pentobarbital sodium presurgery and 1.0% isoflurane inhalation anesthetic during surgery). Silastic sampling catheters were inserted in the carotid artery, portal vein, and hepatic vein as previously described (6). Silastic catheters were also inserted for the infusion of indocyanine green, [3-3H]glucose, and [U-14C]alanine according to methods published previously (27). Last, a Silastic catheter was inserted for the intraportal infusions of phentolamine and propranolol and the infusion of catecholamines during the final period of the experiment, as previously described (6). Ultrasonic transit time flow probes were fitted and secured to the portal vein and hepatic artery (Transonic Systems, Ithaca, NY). The knotted catheter ends and flow probe leads were stored in a subcutaneous pocket in the abdominal region. The carotid artery catheter was stored in a pocket under the skin of the neck.

Alloxan (BDH Chemicals, Poole, England) was used to induce diabetes according to methods previously described (7). After glycosuria was detected, the dogs were treated with regular and isophane pork insulin (Eli Lilly, Indianapolis, IN) to maintain glycosuria <1 mg/dl. Pork insulin was used since it does not yield antibody formation for at least 2 mo, thereby allowing accurate insulin measurements (23). The last injection of intermediate-acting insulin was given 48 h before studies, with the last injection of short-acting insulin given 18 h before study. The regimen of insulin doses used ensures that diabetic dogs are free of subcutaneous insulin on the day of the study (27).

Beginning 7 days after surgery, dogs were acclimatized to running on a motorized treadmill. In addition, blood samples were drawn 3 days before the experiment to determine the leukocyte count and the hematocrit of the animal. Only animals with a leukocyte count below 18,000/mm3, a hematocrit above 38%, a good appetite (consumption of daily food ration), and normal stools were used.

All studies were conducted in dogs after an 18-h fast. The free catheter ends and flow probe leads were accessed through small skin incisions made under local subcutaneous anesthesia (2% lidocaine; Astra Pharmaceutical, Worcester, MA) in the abdominal and neck regions the morning of the experiment. The contents of the catheters were then aspirated and flushed with saline. The exposed catheters were connected to Silastic tubing that was secured to the back of the dog with quick-drying glue.

Experimental procedures. Experiments consisted of a tracer and dye equilibration period (-150 to -30 min), basal period (-30 to 0 min), moderate exercise period (0-150 min), recovery period (150-180 min), and catecholamine infusion period (180-210 min). A primed (42 mCi), constant infusion (0.30 mCi/min) of [3-3H]glucose was initiated at -130 min and was continued throughout the study. A constant-rate indocyanine green infusion (0.1 mg · m-2 · min-1) and [U-14C]alanine (0.36 mCi/min) was also started at time (t) = -150 min and was continued throughout the study. Indocyanine green was used as a backup method of blood flow measurement if the flow probes did not provide a clear signal and as confirmation of hepatic vein catheter placement. Two protocols were performed (Fig. 1). In the hepatic adrenergic blockade protocol (HAB, n = 6), the alpha - and beta -adrenergic receptor blockers phentolamine and propranolol were infused intraportally from -50 to 210 min at rates of 2 and 1 µg · kg-1 · min-1, respectively. These infusion rates block hepatic adrenergic receptors while eliciting minimal extrahepatic effects (6). To test the effectiveness of the blockade, norepinephrine and epinephrine were infused at rates of 0.40 and 0.20 ng · kg-1 · min-1, respectively, from t = 180-210 min of the study. In the control protocol (n = 6), animals were handled and prepared identically except that vehicle alone (saline and ascorbate) was infused. Catecholamine and arterial plasma glucose data for four of the six control dogs have been published previously (7). Glucagon and insulin data for five of the six control dogs have also been published previously (8). Heart rates and blood flows were monitored on-line throughout the experiments.


View larger version (13K):
[in this window]
[in a new window]
 
Fig. 1.   Moderate exercise protocol using control and hepatic adrenergic blockade groups in poorly controlled alloxan-diabetic dogs. *Epinephrine and norepinephrine were infused into the portal vein at rates of 0.20 and 0.40 mg · kg-1 · min-1 from time (t) = 180 to t = 210 min. **Phentolamine and propranolol were infused at rates of 2 and 1 mg · kg-1 · min-1, respectively, from t = -50 to 210 min.

Blood sample processing. Plasma glucose concentrations were determined by the glucose oxidase method using a Beckman glucose analyzer (Beckman Instruments, Fullerton, CA). Plasma samples were prepared, and radioactivity (3H and 14C) was determined as described previously (27). Whole blood lactate, alanine, and glycerol concentrations and plasma FFA were determined using previously established methods (6). Labeled and unlabeled plasma alanine levels were determined with a short column-ion exchange chromatographic system that has been described previously (3). Immunoreactive insulin was measured using a double-antibody procedure (interassay coefficient of variation of 16%; see Ref. 17). Immunoreactive glucagon (3,500 mol wt) was measured in plasma samples containing 50 ml of 500 KIU/ml Trasylol (FBA Pharmaceuticals) using a double-antibody system modified from the method developed by Morgan and Lazarow (17) for insulin. Plasma samples for norepinephrine and epinephrine were collected as previously described (6) and analyzed using HPLC (12). The interassay coefficients of variation using this method were 4 and 6% for norepinephrine and epinephrine, respectively. Plasma cortisol was measured as previously described (6) with an interassay coefficient of variation of 6%.

Materials. [3-3H]glucose (New England Nuclear, Boston, MA) was used as the glucose tracer (500 Ci/0.005 mg), and [U-14C]alanine (Amersham, Chicago, IL) was used as the labeled gluconeogenic precursor (171 mCi/mmol). Glucagon and 125I-glucagon were obtained from Novo-Nordisk Research Institute (Copenhagen, Denmark). Glucagon and insulin antisera were obtained from Linco Research (St. Louis, MO), as were standard insulin and 125I-insulin. Enzymes and coenzymes for metabolite analyses were obtained from Boehringer Mannheim Biochemicals and Sigma Chemical.

Calculations. Net hepatic lactate uptake (NHLU), alanine uptake (NHAU), glycerol uptake (NHGlyU), and glucose output were determined according to the formula HAF × ([A] - [H]) + PVF × ([P] - [H]), such that [A], [P], and [H] are the arterial, portal vein, and hepatic vein substrate concentrations, and HAF and PVF are the hepatic artery and portal vein blood flows. The sign was reversed for the calculation of NHLU, NHAU, and NHGlyU so that net uptake would be a positive number. Ra and Rd were determined by the equations for isotope dilution during a constant-rate infusion of radioactive glucose ([3-3H]glucose; see Ref. 10).

Hepatic [14C]glucose production was determined using [3H]glucose in a manner analogous to the measurement of Ra, with the difference being that the specific activity was the ratio of plasma [3H]glucose to plasma [14C]glucose. Gluconeogenic conversion and the efficiency of gluconeogenic conversion of alanine and lactate to glucose were then calculated according to methods previously described (27). Due to isotope dilution in the liver, the values obtained for gluconeogenic conversion and efficiency are minimum estimates (13). For this reason, the [U-14C]alanine gluconeogenic method cannot quantify gluconeogenesis. Its value rests in the qualitative assessment of changes relative to basal (4). The gluconeogenic measurements are described in more detail in previous publications (4, 27).

Statistical analysis. Superanova (Abacus Concepts, Berkeley, CA) software installed on a Macintosh Power PC was used to perform statistical analysis. Statistical comparisons between groups and over time were made using ANOVA designed to account for repeated measures. Time points were specifically examined for significance using contrasts solved by univariate repeated measures. Statistics are reported in Tables 1-3 or Figs. 1-8 for each variable. Data are presented as means ± SE. Statistical significance was defined as P < 0.05.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Arterial insulin, glucagon, and cortisol concentrations . Glucagon and insulin data for five of the six control dogs have been published previously (8). The plasma insulin change from basal was similar in both groups during the exercise and recovery periods. Although plasma insulin was decreased (P < 0.05) from basal levels in HAB during the blockade test period, insulin increased (P < 0.05) above baseline in controls (Fig. 2). Basal arterial plasma glucagon was similar in HAB (56 ± 8 pg/ml) and controls (57 ± 9 pg/ml; Fig. 2). The increase (P < 0.05) in plasma glucagon was similar in HAB and controls during the exercise and recovery periods. During the blockade test period, plasma glucagon decreased toward baseline levels in HAB but remained elevated in controls. Basal plasma cortisol was similar in HAB (1.4 ± 0.2 mg/ml) and controls (1.2 ± 0.2 mg/ml). The exercise-induced change in cortisol was not different between groups (Fig. 3).


View larger version (26K):
[in this window]
[in a new window]
 
Fig. 2.   Exercise-induced changes in arterial insulin and glucagon during the basal, exercise, recovery, and blockade test periods. Basal values for insulin and glucagon were 4.8 ± 0.9 mU/ml and 57 ± 9 pg/ml in controls and 4.0 ± 0.6 mU/ml and 56 ± 7 pg/ml in hepatic adrenergic blockade (HAB), respectively. Delta , Change. Data are means ± SE; n = 6 dogs for controls and n = 6 dogs for HAB. # P < 0.05, difference between basal and exercise period. * P < 0.05, difference between controls and HAB.



View larger version (22K):
[in this window]
[in a new window]
 
Fig. 3.   Exercise-induced changes in arterial cortisol during the basal and exercise periods. Basal values were 1.2 ± 0.2 mg/ml in controls and 1.2 ± 0.3 mg/ml in HAB. Data are means ± SE; n = 6 dogs for controls and n = 6 dogs for HAB. # P < 0.05, difference between basal and exercise period.

Arterial epinephrine and norepinephrine concentrations . Catecholamine values for four of the six control dogs have been published previously (7). Although the basal arterial plasma epinephrine was significantly higher in HAB (314 ± 77 pg/ml) compared with controls (83 ± 17 pg/ml), the exercise-induced increase (P < 0.05) in arterial plasma epinephrine was similar (305 ± 85 pg/ml in HAB and 365 ± 127 pg/ml in controls; Fig. 4). The plasma epinephrine change from basal was also similar during the recovery and blockade test periods. Basal arterial plasma norepinephrine was also significantly higher in HAB (744 ± 134 pg/ml) compared with controls (292 ± 81 pg/ml). However, plasma norepinephrine increased (P < 0.05) similarly in HAB (630 ± 85 pg/ml) and controls (619 ± 91 pg/ml) during exercise (Fig. 4). Likewise, the plasma norepinephrine change from basal was similar during the recovery and blockade test periods (Fig. 4).


View larger version (26K):
[in this window]
[in a new window]
 
Fig. 4.   Exercise-induced changes in arterial epinephrine and norepinephrine during the basal, exercise, recovery, and blockade test periods. Basal values for epinephrine and norepinephrine were 83 ± 17 and 292 ± 81 pg/ml in controls and 314 ± 77 and 744 ± 134 pg/ml in HAB, respectively. Data are means ± SE; n = 6 dogs for controls and n = 6 dogs for HAB. # P < 0.05, difference between basal and exercise period.

Arterial glucose concentrations and kinetics . Although arterial plasma glucose was similar in HAB and controls during the basal, exercise, and recovery periods, arterial plasma glucose was lower (P < 0.05) in HAB compared with controls during the blockade test period (Fig. 5). Ra was similar throughout the basal, exercise, and recovery periods. However, Ra was lower in HAB (8.0 ± 1.0 mg · kg-1 · min-1) compared with controls (11.0 ± 1.8 mg · kg-1 · min-1) during the blockade test period (Fig. 6). Rd was similar in both groups throughout the basal, exercise, recovery, and blockade test periods (Fig. 6).


View larger version (25K):
[in this window]
[in a new window]
 
Fig. 5.   Arterial glucose during the basal, exercise, recovery, and blockade test periods. Data are means ± SE; n = 6 dogs for controls and n = 6 dogs for HAB. * P < 0.05, difference between controls and HAB.



View larger version (28K):
[in this window]
[in a new window]
 
Fig. 6.   Endogenous glucose production and glucose utilization during the basal, exercise, recovery, and blockade test periods. Data are means ± SE; n = 6 dogs for controls and n = 6 dogs for HAB. * P < 0.05, difference between controls and HAB.

Metabolites . Basal arterial lactate was similar in HAB and controls. Although arterial lactate was similar in both groups at 50 min of exercise, lactate levels were lower (P < 0.05) in HAB compared with controls at 150 min of exercise (Table 1). Hepatic fractional lactate extraction was also lower (P < 0.05) in HAB compared with controls at 150 min of exercise (Table 1). Last, lower levels of arterial lactate and hepatic fractional lactate extraction corresponded to a reduced (P < 0.05) NHLU in HAB compared with controls at 150 min of exercise (Table 1). Arterial alanine, hepatic fractional alanine extraction, and NHAU were similar throughout the basal and exercise periods in both groups (Table 1). Arterial glycerol, hepatic fractional glycerol extraction, and NHGlyU were also similar throughout the basal and exercise periods (Table 1). Plasma FFA was not different between groups during the basal and exercise periods (Table 2).

                              
View this table:
[in this window]
[in a new window]
 
Table 1.   Arterial lactate, alanine, and glycerol, hepatic fractional lactate, alanine, and glycerol extraction, and NHLU, NHAU, and NHGlyU during the basal and exercise periods in control and hepatic adrenergic blockade diabetic dogs


                              
View this table:
[in this window]
[in a new window]
 
Table 2.   Plasma free fatty acids during the basal and exercise periods in control and HAB diabetic dogs

Gluconeogenic indexes. Gluconeogenic conversion from alanine and lactate (expressed as percent of basal) increased (P < 0.05) twofold by exercise and was similar in HAB and controls (Fig. 7). The efficiency of gluconeogenic conversion from alanine and lactate (expressed as percent of basal) was increased (P < 0.05) by almost twofold during exercise in both groups (Fig. 7).


View larger version (22K):
[in this window]
[in a new window]
 
Fig. 7.   Exercise-induced changes in gluconeogenic conversion from alanine and lactate and the efficiency of gluconeogenic conversion from alanine and lactate during the basal and exercise periods. Data are means ± SE; n = 6 dogs for controls and n = 6 dogs for HAB. # P < 0.05, difference between basal and exercise period.

Heart rates and hepatic blood flows. Heart rates were similar during the basal period and increased (P < 0.05) similarly with exercise in HAB (204 ± 7 beats/min) and controls (216 ± 13 beats/min; Fig. 8). Portal vein and hepatic artery blood flows were similar in HAB and controls during the basal and moderate exercise periods (Table 3).


View larger version (21K):
[in this window]
[in a new window]
 
Fig. 8.   Heart rates during the basal, exercise, recovery, and blockade test periods. Data are means ± SE; n = 6 dogs for controls and n = 6 dogs for HAB. # P < 0.05, difference between basal and exercise period.


                              
View this table:
[in this window]
[in a new window]
 
Table 3.   Hemodynamic measurements


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

We have previously shown that nonhepatic splanchnic and hepatic norepinephrine spillover (index of hepatic sympathetic drive) are increased in the poorly controlled alloxan-diabetic dog during moderate exercise in excess of the response in normal healthy dogs (7). In addition, other studies have suggested that glucose fluxes may be more sensitive to adrenergic stimulation in diabetes (19-21). Therefore, it seems reasonable to propose that hepatic adrenergic mechanisms are a stimulus for exercise-induced increments in Ra under such conditions. This proposal is consistent with studies which show that peripheral administration of propranolol results in a reduction in Ra during exercise in depancreatized dogs (2). Although these studies were important in demonstrating the effectiveness of beta -adrenergic blockade in the control of Ra during exercise in the diabetic state, the regulatory elements by which adrenergic stimulation can mediate glucoregulation are multifactorial. For example, adrenergic stimulation may influence the rates of hepatic glycogenolysis and gluconeogenic parameters through peripheral and hepatic mechanisms. In addition, it may act through alpha - as well as beta -adrenergic receptors. The methodological approach used in the present study allowed us to selectively determine the importance of direct hepatic adrenergic stimulation (6) in the control of Ra during exercise in the poorly controlled diabetic state.

The selective hepatic adrenergic blockade in the present study prevented the adrenergic stimulation of the liver. The similar increases in Ra in HAB and controls during exercise show that adrenergic mechanisms acting at the liver are not responsible for the exercise-induced increment in Ra in the poorly controlled diabetic state. Although adrenergic mechanisms were not implicated in the direct stimulation of hepatic glycogenolysis or gluconeogenesis in the present study, the local delivery of propranolol and phentolamine largely preserved peripheral stimulatory effects of the catecholamines on the mobilization of gluconeogenic substrate. Thus the peripheral action of circulating catecholamines was most likely sufficient to facilitate exercise-induced changes in gluconeogenic parameters and contribute to equivalent exercise-induced changes in Ra in HAB and controls.

It is well recognized that glucagon is critical for the stimulation of Ra during exercise under healthy (28) and diabetic (30) conditions. Furthermore, recent studies from our laboratory have demonstrated an approximately fourfold greater exercise-induced increment in portal vein glucagon in the poorly controlled diabetic state compared with normal dogs (8). Because sympathetic drive to nonhepatic splanchnic tissue (including pancreas) is elevated during exercise in the poorly controlled diabetic state (7), increased sympathetic drive could be important in mediating pancreatic hormone secretion via adrenergic mechanisms (16, 18). A causal relationship between exaggerated sympathetic drive and glucagon secretion in the alloxan-diabetic dogs during exercise are important since glucagon is probably a major controller of gluconeogenic precursor extraction, intrahepatic conversion to glucose, and hepatic glycogenolysis under these conditions (31). Therefore, exaggerated glucagon secretion in the diabetic state probably contributes to the increase in hepatic glycogenolysis and the increase in gluconeogenic conversion. This is supported by studies that show that suppression of glucagon below basal levels reduces arterial glucose in exercising alloxan-diabetic dogs (29).

The increased arterial lactate levels in controls resulted in a greater rate of NHLU in this group. Even though NHLU and hepatic fractional lactate extraction were greater in controls, gluconeogenic parameters (alanine and lactate conversion to glucose) increased similarly during exercise in both groups. Thus the disparity in NHLU in controls and HAB was insufficient to cause a detectable difference in the gluconeogenic conversion from alanine and lactate. The difference in NHLU between HAB and controls would have resulted in a maximum contribution of lactate to glucose of ~0.25 mg · kg-1 · min-1, if all of the lactate consumed by the liver was channeled into glucose. These results are consistent with studies in lactate-infused (4-fold basal) dogs in which NHLU but not gluconeogenic parameters were measured (9).

It is interesting to note that basal arterial epinephrine and norepinephrine concentrations were elevated in HAB compared with controls. These results are in agreement with other studies using "global" alpha - and/or beta -adrenergic blockade in healthy (11) and diabetic (22) humans. However, there were no differences in catecholamines in our previous study using the intraportal blockade technique in nondiabetic dogs (6). This discrepancy is probably attributable to increased sympathetic nerve activity in diabetes (7). Despite the differences in catecholamines during the basal period, the exercise-induced increments in the catecholamines were similar in both groups. Therefore, even though the absolute catecholamine levels were higher in HAB, the hepatic adrenergic blockade has been shown to inhibit hepatic Ra even under conditions in which portal epinephrine and norepinephrine values are ~10-fold higher than those in HAB (5). Furthermore, the inhibition of changes in Ra during the blockade test period in HAB also substantiates the effectiveness of the blockade, even at high catecholamine levels.

The completeness of the blockade is supported by the attenuation of Ra during the portal vein infusion of catecholamines in HAB, whereas Ra was increased in controls. The selectivity of the blockade is exemplified by similar glycerol and FFA responses in both groups, since systemic beta -adrenergic blockade of adipose tissue normally attenuates glycerol and FFA responses to exercise (14). In addition, similar heart rate responses in both groups also provide evidence for the selectivity of the hepatic adrenergic blockade. Studies show that peripheral beta -adrenergic blockade can reduce the exercise-induced heart rate response by 60% (1). The dramatic reduction of the systemic effects of the adrenergic blockers due to portal vein infusion is attributable to the marked reduction in the infusion rate needed to block hepatic adrenergic receptors compared with the infusion rate needed when using peripheral infusion, as well as their efficient extraction by the liver (5).

In summary, the novel approach of the present study, whereby the proposed direct effects of the catecholamines could be distinguished from their indirect effects on the liver (mobilization of gluconeogenic precursors and FFA), shows that direct adrenergic stimulation of Ra is not an important part of the response to exercise in the diabetic state. These results are consistent with other studies from our laboratory in which HAB did not attenuate Ra during heavy exercise (6), another condition in which adrenergic drive to the liver is high.


    ACKNOWLEDGEMENTS

We are grateful to Deanna Bracy, Eric Allen, Pamela Venson, and Wanda Snead for excellent technical assistance.


    FOOTNOTES

This research was supported by National Institute of Diabetes and Digestive and Kidney Diseases Grant R01-DK-50277 and Diabetes Research and Training Grant 5-P60-DK-20593. R. H. Coker was a recipient of a Postdoctoral Fellowship Award from the Juvenile Diabetes Foundation International.

Part of this work was presented at the 59th Annual Meeting of the American Diabetes Association, San Diego, CA, in June, 1999.

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: R. H. Coker, Dept. of Exercise Science, Univ. of Mississippi, University, MS 38677 (E-mail address: rhcoker{at}olemiss.edu).

Received 13 July 1999; accepted in final form 12 October 1999.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

1.   Ahlborg, G., and A. Juhlin-Dannfelt. Effect of beta -receptor blockade on splanchnic and muscle metabolism during prolonged exercise in men. J. Appl. Physiol. 76: 1037-1042, 1994[Abstract/Free Full Text].

2.   Bjorkman, O., P. Miles, D. H. Wasserman, L. Lickley, and M. Vranic. Muscle glucose uptake during exercise in total insulin deficiency: No effect of beta -adrenergic blockade. J. Clin. Invest. 81: 1759-1767, 1988[ISI][Medline].

3.   Cherrington, A. D., W. W. Lacy, and J. L. Chiasson. Effect of glucagon on glucose production during insulin deficiency in the dog. J. Clin. Invest. 62: 664-677, 1978[ISI][Medline].

4.   Chiasson, J. L., J. E. Liljenquist, A. S. Jennings, W. W. Lacy, and A. D. Cherrington. Gluconeogenesis: methodological approaches in vivo. Federation Proc. 36: 230-235, 1977.

5.   Chu, C. A., D. K. Sindelar, D. W. Neal, and A. D. Cherrington. Portal adrenergic blockade does not inhibit the gluconeogenic effects of circulating catecholamines on the liver. Metabolism 46: 458-465, 1997[ISI][Medline].

6.   Coker, R. H., M. G. Krishna, D. B. Lacy, D. P. Bracy, and D. H. Wasserman. Role of hepatic alpha - and beta - adrenergic receptor stimulation on hepatic glucose production during heavy exercise. Am. J. Physiol. Endocrinol. Metab. 273: E831-E838, 1997[Abstract/Free Full Text].

7.   Coker, R. H., M. G. Krishna, D. B. Lacy, B. Zinker, and D. H. Wasserman. Sympathetic drive to liver and nonhepatic splanchnic tissue during prolonged exercise is increased in diabetes. Metabolism 46: 1327-1332, 1997[ISI][Medline].

8.   Coker, R. H., D. B. Lacy, M. G. Krishna, and D. H. Wasserman. Splanchnic glucagon kinetics in exercising alloxan-diabetic dogs. J. Appl. Physiol. 86: 1626-1631, 1999[Abstract/Free Full Text].

9.   Connolly, C. C., R. W. Stevenson, D. W. Neal, D. H. Wasserman, and A. D. Cherrington. The effects of lactate loading on alanine and glucose metabolism in the conscious dog. Metabolism 42: 154-161, 1993[ISI][Medline].

10.   Debodo, R. D., R. Steele, N. Altzuler, A. Dunn, and J. S. Bishop. On the hormonal regulation of carbohydrate metabolism: studies with [14C]glucose. Recent Prog. Horm. Res. 19: 445-448, 1963[ISI].

11.   Galbo, H., N. J. Christensen, and J. J. Holst. Catecholamines and pancreatic hormones during autonomic blockade in exercising man. Acta Physiol. Scand. 101: 428-437, 1977[ISI][Medline].

12.   Golstein, D. S., G. Feurstein, and J. L. Izzo. Validity and reliability of liquid chromatography with electrochemical detection for measuring plasma norepinphrine and epinephrine in man. Life Sci. 28: 467-475, 1981[ISI][Medline].

13.   Hetenyi, G. Calculation of the rate of gluconeogenesis in vivo. In: Carbohydrate Metabolism, edited by C. Cobelli, and R. N. Bergman. New York: Wiley, 1981, p. 2091-2109.

14.   Kjaer, M., K. Engfred, A. Fernandez, N. Secher, and H. Galbo. Regulation of hepatic glucose production during exercise in humans: role of sympathoadrenergic activity. Am. J. Physiol. Endocrinol. Metab. 265: E275-E283, 1993[Abstract/Free Full Text].

15.   Lloyd, B., J. Burrin, P. Smythe, and K. G. M. M. Alberti. Enzymatic fluorometric continuous-flow assays for blood glucose, lactate, pyruvate, alanine, glycerol, and 3-hydroxybutyrate. Clin. Chem. 24: 1724-1729, 1978[Abstract/Free Full Text].

16.   Luyckx, A. S., and P. J. Lefebvre. Mechanisms involved in the exercise-induced increase in glucagon secretion in rats. Diabetes 23: 81-93, 1974[ISI][Medline].

17.   Morgan, C. R., and A. L. Lazarow. Immunoassay of insulin: two antibody system. Plasma insulin of normal, subdiabetic, and diabetic rats. Am. J. Med. Sci. 257: 415-419, 1963.

18.   Samols, E., and G. C. Weir. Adrenergic modulation of pancreatic A, B, and D cells. J. Clin. Invest. 63: 230-238, 1979[ISI][Medline].

19.   Shamoon, H., R. Hendler, and R. S. Sherwin. Altered responsiveness to cortisol, epinephrine, and glucagon in insulin-infused juvenile-onset diabetics: a mechanism for diabetic instability. Diabetes 29: 284-291, 1980[ISI][Medline].

20.   Shamoon, H., and R. S. Sherwin. beta -Adrenergic blockade is more effective in suppressing adrenaline-induced glucose production in Type 1 (insulin-dependent) diabetes. Diabetologia 26: 183-189, 1984[ISI][Medline].

21.   Sherwin, R. S., H. Shamoon, R. Hendler, L. Sacca, N. Eigler, and M. Walesky. Epinephrine and the regulation of glucose metabolism: effect of diabetes and hormonal interactions. Metab. Clin. Exp. 29: 1146-1154, 1980[ISI][Medline].

22.   Simonson, D. C., V. A. Koivisto, R. S. Sherwin, E. Ferrannini, R. Hendler, and R. A. Defronzo. Adrenergic blockade alters glucose kinetics during exercise in insulin-dependent diabetics. J. Clin. Invest. 73: 1648-1658, 1984[ISI][Medline].

23.   Vranic, M., R. Kawamori, S. Pek, N. Kovacevic, and G. Wrenshall. The essentiality of insulin and the role of glucagon in regulating glucose utilization and production during strenuous exercise in dogs. J. Clin. Invest. 57: 245-255, 1976[ISI][Medline].

24.   Wahren, J., L. Hagenfeldt, and P. Felig. Splanchnic and leg exchange of glucose, amino acids, and free fatty acids during exercise in diabetes mellitus. J. Clin. Invest. 55: 1303-1314, 1975[ISI][Medline].

25.   Wasserman, D. H., J. L. Bupp, J. L. Johnson, D. Bracy, and D. B. Lacy. Glucoregulation during rest and exercise in depancreatized dogs: role of the acute presence of insulin. Am. J. Physiol. Endocrinol. Metab. 262: E574-E582, 1992[Abstract/Free Full Text].

26.   Wasserman, D. H., and A. D. Cherrington. Regulation of extrahepatic fuel sources during exercise. In: Handbook of Physiology. Bethesda, MD: Am. Physiol. Soc, 1996.

27.   Wasserman, D. H., J. L. Johnson, J. L. Bupp, D. B. Lacy, and D. P. Bracy. Regulation of gluconeogenesis during rest and exercise in the depancreatized dog. Am. J. Physiol. Endocrinol. Metab. 265: E51-E60, 1993[Abstract/Free Full Text].

28.   Wasserman, D. H., H. L. A. Lickley, and M. Vranic. Interactions between glucagon and other counterregulatory hormones during normoglycemic and hypoglycemic exercise in dogs. J. Clin. Invest. 74: 1404-1413, 1984[ISI][Medline].

29.   Wasserman, D. H., H. L. A. Lickely, and M. Vranic. Important role of glucagon during exercise in diabetic dogs. J. Appl. Physiol. 59: 1272-1281, 1985[Abstract/Free Full Text].

30.   Wasserman, D. H., H. L. A. Lickley, and M. Vranic. The role of beta -adrenergic mechanisms during exercise in poorly-controlled diabetes. J. Appl. Physiol. 59: 1282-1289, 1985[Abstract/Free Full Text].

31.   Wasserman, D. H., J. S. Spalding, D. B. Lacy, C. A. Colburn, R. E. Goldstein, and A. D. Cherrington. Glucagon is a primary controller of the increments in hepatic glycogenolysis and gluconeogenesis during exercise. Am. J. Physiol. Endocrinol. Metab. 257: E108-E117, 1989[Abstract/Free Full Text].


Am J Physiol Endocrinol Metab 278(3):E444-E451
0193-1849/00 $5.00 Copyright © 2000 the American Physiological Society