Dual mode of action of glucose pentaacetates on hormonal secretion from the isolated perfused rat pancreas

Viviane Leclercq-Meyer and Willy J. Malaisse

Laboratory of Experimental Medicine, Brussels Free University, B-1070 Brussels, Belgium

    ABSTRACT
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

Isolated perfused rat pancreases were exposed, in the presence of 10.0 mM L-leucine, to either alpha -D-glucose pentaacetate, beta -L-glucose pentaacetate, or unesterified D-glucose, all tested at a 1.7 mM concentration. The pentaacetate ester of alpha -D-glucose and, to a lesser extent, that of beta -L-glucose stimulated both insulin and somatostatin release, whereas unesterified D-glucose failed to do so. In the case of insulin output, the two esters differed from one another not solely by the magnitude of the secretory response but also by its time course and reversibility. Compared with these data, the most salient difference found in the case of somatostatin release consisted of the absence of an early secretory peak in response to alpha -D-glucose pentaacetate administration and the higher paired ratio between the secretory responses evoked by the esters of glucose and by unesterified D-glucose (5.5 mM) administered at the end of the experiments. The two esters provoked an initial and short-lived stimulation of glucagon secretion, in sharp contrast to the immediate inhibitory action of unesterified D-glucose. Thereafter, alpha -D-glucose pentaacetate, but not beta -L-glucose pentaacetate, caused inhibition of glucagon release, such an effect being reversed when the administration of the ester was halted. These findings indicate a dual mode of action of glucose pentaacetate esters on hormonal secretion from the endocrine pancreas. The intracellular hydrolysis of alpha -D-glucose pentaacetate and subsequent catabolism of its hexose moiety may contribute to the early peak-shaped insulin response to this ester, to the persistence of a positive secretory effect in B and D cells after cessation of its administration, and to the late inhibition of glucagon release. However, a direct effect of the esters themselves, by some as-of-yet unidentified coupling process, is postulated to account for the stimulation of insulin and somatostatin release by beta -L-glucose pentaacetate and for the initial enhancement of glucagon secretion provoked by both glucose esters.

insulin secretion; somatostatin secretion; glucagon secretion

    INTRODUCTION
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

SEVERAL ESTERS OF MONOSACCHARIDES were recently introduced as tools to increase the nutritional value and/or biological efficiency of the corresponding carbohydrates. This approach is inspired by the consideration that the esters cross the plasma membrane without requiring the intervention of a specific transport system and then undergo intracellular hydrolysis, so that the sugar moiety becomes readily available to act as a nutrient or metabolic inhibitor (13). For instance, alpha -D-glucose pentaacetate is better metabolized and displays a higher insulinotropic action than unesterified D-glucose (18, 20). Likewise, D-mannoheptulose hexaacetate inhibits D-glucose metabolism in cells otherwise resistant to the heptose (5, 15). The tetraacetate ester of 2-deoxy-D-glucose is also more potent than unesterified 2-deoxy-D-glucose as either an inhibitor of D-glucose metabolism and insulinotropic action in pancreatic islets (21) or as a cytostatic agent in several lines of tumoral cells (2, 10, 11, 19).

Surprisingly, however, a few esters of hexoses that are either not metabolized (e.g., beta -L-glucose pentaacetate) or even inhibitors of D-glucose metabolism (e.g., 2-deoxy-D-glucose tetraacetate) were also found to stimulate insulin release under suitable experimental conditions (12, 14, 18). Several findings indicate that such a situation cannot be accounted for by the metabolism of the acetate moiety of these esters. For example, beta -D-galactose pentaacetate, which is as efficiently taken up and hydrolyzed in pancreatic islets as alpha -D-glucose pentaacetate (20, 22), fails to display any insulinotropic action. Moreover, the secretory response to the latter ester is not duplicated by the combination of unesterified D-glucose and either acetic acid or its methyl or ethyl esters (13, 18).

The unexpected observation that esters of hexoses devoid of nutritional value may stimulate insulin release has led to the proposal that such esters might themselves act directly on pancreatic islet B cells at the intervention of specific receptors in a manner somehow comparable to that involved in the recognition of bitter compounds by taste buds (17).

In light of these considerations, the major aim of the present study was to search for further evidence in support of the postulated dual mode of action of selected hexose pentaacetates on insulin release from the endocrine pancreas. For such a purpose, the effects of alpha -D-glucose pentaacetate and beta -L-glucose pentaacetate on the secretion of insulin, glucagon, and somatostatin were compared with those of unesterified D-glucose in isolated perfused rat pancreases.

    MATERIALS AND METHODS
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

The pentaacetate esters of alpha -D-glucose and beta -L-glucose were purchased from Sigma Chemical (St. Louis, MO).

Fed female Wistar rats (B & K Universal, Hull, UK) were anesthetized with pentobarbital sodium, and the pancreas was perfused free from adjacent organs through both the celiac and superior mesenteric arteries, as described elsewhere (16).

The basal salt-balanced perfusion medium contained L-leucine (10.0 mM), dextran (clinical grade; 40 g/l), and BSA (RIA grade; 5 g/l). It also contained, as required, alpha -D-glucose pentaacetate (1.7 mM), beta -L-glucose pentaacetate (1.7 mM), or unesterified D-glucose (1.7 or 5.5 mM).

The techniques used for the measurement of plasma glucose and insulin concentrations; perfusion pressure; and pancreatic insulin, glucagon, and somatostatin content and release were identical to those mentioned elsewhere (7, 8). The concentration of D-glucose in the effluent was measured by the hexokinase/glucose-6-phosphate dehydrogenase method (1).

The oscillatory pattern of hormonal release was assessed by a procedure previously reported (6). In each individual experiment, the mean hormonal output over a given period of time was calculated from all measurements made over that period. The basal hormonal output was computed from min 20 to min 25, inclusive. The absolute values for such a basal output were, as a rule, not significantly different from one another in the three series of experiments conducted with alpha -D-glucose pentaacetate, beta -L-glucose pentaacetate, and unesterified D-glucose, whether in the case of insulin, somatostatin, or glucagon. The basal output of glucagon was somewhat higher (P < 0.05), however, in the experiments with alpha -D-glucose pentaacetate than with beta -L-glucose pentaacetate. Table 1 lists the characteristics of rats and experimental variables.

                              
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Table 1.   Characteristics of rats and experimental variables of perfusions conducted with alpha -D-glucose pentaacetate, beta -L-glucose pentaacetate, or D-glucose

All results are presented as means ± SE together with the number of individual observations (n). The significance of differences between mean values was assessed by use of Student's t-test and ANOVA.

    RESULTS
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

In the experiments conducted with unesterified D-glucose, the concentration of the hexose in the effluent progressively increased from 0.27 ± 0.03 mM at min 28 to 1.69 ± 0.05 mM at min 34 and decreased from 1.71 ± 0.07 mM at min 53 to 0.34 ± 0.01 mM at min 58. These measurements allowed for correction for the dead space of the perfusion device. Hence, as seen in Figs. 1-6, the vertical dotted lines correspond to the time at which a change in D-glucose concentration was first detected in the effluent medium collected from the perfused pancreas.

In the experiments conducted in the presence of either alpha -D-glucose pentaacetate or beta -L-glucose pentaacetate, no sizable increase (<0.02 mM) in the D-glucose concentration of the effluent was observed in response to the administration of the ester.

The perfusion pressure was not affected by the administration of either D-glucose or its esters (data not shown).

The administration of alpha -D-glucose pentaacetate (1.7 mM) provoked a biphasic increase in insulin output, with slow oscillations during the late period of exposure to the ester (Fig. 1, top). For instance, after the initial peak at min 34-35, a second secretory peak was recorded at min 42-43, i.e., 7.5 ± 0.3 min later. After the first peak, the hormonal output reached, within 3.5 ± 0.3 min, a nadir value, which corresponded to a 66.9 ± 5.0% fractional decrease in secretory rate, whereas the subsequent rise in insulin release after this first nadir corresponded to a 160.9 ± 19.5% relative increment in secretory rate (n = 4 in all cases). A third reascension in insulin output was observed after the second nadir (min 45.0 ± 0.4), the hormonal output again increasing over 4 min by 60.3 ± 15.5% above the paired preceding value. As computed over a period of 22 min (min 29-50), the integrated amount of insulin released during stimulation by the ester averaged 98.8 ± 14.2% (n = 4) of the paired value recorded at the end of the experiments (min 79-100) in response to the administration of 5.5 mM unesterified D-glucose.


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Fig. 1.   Time course for changes in insulin output in experiments involving administration of alpha -D-glucose pentaacetate (PA) (top), beta -L-glucose PA (middle), and unesterified D-glucose (bottom). Values (means ± SE) refer to 4 individual experiments in each case.

The administration of beta -L-glucose pentaacetate (1.7 mM) also resulted in an increase in insulin output. However, the mean output of insulin during exposure to beta -L-glucose (min 29-53, inclusive) averaged no more than 0.43 ± 0.05 ng/min (n = 4), as distinct (P < 0.01) from 6.98 ± 1.54 ng/min (n = 4) in the case of alpha -D-glucose pentaacetate. Likewise, between min 46 and 50, the output of insulin, when expressed relative to the paired basal value (min 20-25), averaged no more than 6.4 ± 1.9 (n = 4) during exposure to beta -L-glucose pentaacetate, as compared (P < 0.05) with 63.2 ± 21.9 (n = 4) in the case of alpha -D-glucose pentaacetate (Fig. 2).


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Fig. 2.   Time course for changes in insulin output evoked by alpha -D-glucose PA (open circle ), beta -L-glucose PA (bullet ), and unesterified D-glucose (star ), hormonal output being expressed as percentage of paired basal value (min 20-25). Values (means ± SE) refer in each case to same 4 individual experiments as those illustrated in Fig. 1.

The secretory responses of the B cells to each of these two esters also differed from one another by their time courses. Thus, whereas an early secretory peak reaching its zenith value at min 34-35 was always recorded during stimulation by alpha -D-glucose pentaacetate, such apparently was not the case in the pancreases exposed to beta -L-glucose pentaacetate. Nevertheless, in the latter experiments, the secretory rates recorded 3 min before and 3 min after the highest value found at min 33-35 averaged, respectively, 46.8 ± 15.2 and 75.8 ± 6.6% of such a value, suggesting the possible occurrence of a modest early phasic response.

The experiments conducted in the presence of alpha -D-glucose pentaacetate and beta -L-glucose pentaacetate also differed from one another by the pattern of reversibility when the administration of the esters was interrupted at min 53. After exposure to beta -L-glucose pentaacetate, the output of insulin, whether expressed in absolute terms or relative to basal value, rapidly declined to a level close to that found after administration of unesterified D-glucose. After exposure to alpha -D-glucose pentaacetate, a rapid and dramatic decrease in insulin output was also observed. Thereafter, however, an oscillatory pattern of insulin release was recorded over the ensuing 15 min, the hormonal output remaining higher than that found at the same time in the two other series of experiments. Thus, between the 58th and 72nd min of perifusion, the secretory rate averaged 0.96 ± 0.24 ng/min after exposure to alpha -D-glucose pentaacetate, as distinct (P < 0.005) from only 0.34 ± 0.05 and 0.26 ± 0.09 ng/min after administration of beta -L-glucose pentaacetate and unesterified D-glucose, respectively. Relative to paired basal value (min 20-25), these secretory rates corresponded to mean respective values of 5.77 ± 2.46 as compared (P < 0.10) with 3.42 ± 0.87 and 2.02 ± 0.68 (n = 4 in all cases).

In contrast to the results just mentioned, the administration of unesterified D-glucose (also 1.7 mM) failed to affect significantly insulin output. Indeed, in the presence of the unesterified hexose, the output of insulin, when expressed relative to paired basal value, averaged only 1.3 ± 0.5 (n = 4) between min 46 and 50.

The functional integrity of the perfused pancreases was assessed at the end of the experiments by monitoring the secretory response to 5.5 mM unesterified D-glucose. In all cases, the hexose evoked a biphasic increase in insulin release, with typical oscillations during the late phase of stimulation. The functional response to the sugar appeared somewhat higher after prior stimulation with either alpha -D-glucose pentaacetate or beta -L-glucose pentaacetate than after a prior administration of unesterified D-glucose (Fig. 1). Such differences failed, however, to achieve statistical significance, the mean value for insulin release between min 70 and 100 averaging 6.61 ± 1.65, 4.46 ± 0.62, and 3.52 ± 0.90 ng/min after prior exposure to alpha -D-glucose pentaacetate, beta -L-glucose pentaacetate, and unesterified D-glucose, respectively.

The secretory data concerning the release of somatostatin were, in three respects, comparable to those obtained in the case of insulin output. Thus alpha -D-glucose pentaacetate and, to a lesser extent (P < 0.005), beta -L-glucose pentaacetate, but not unesterified D-glucose, augmented somatostatin output (Fig. 3). Second, when the administration of the esters was interrupted at min 53, the output of somatostatin rapidly declined, reaching between min 58 and 72 a mean level somewhat higher, albeit not significantly so, after exposure to alpha -D-glucose pentaacetate (4.6 ± 0.9 pg/min) or beta -L-glucose pentaacetate (6.8 ± 3.2 pg/min) than after administration of unesterified D-glucose (1.7 ± 0.3 pg/min). Expressed relative to paired basal value, such secretory rates averaged, respectively, 1.31 ± 0.19 as distinct (P < 0.001) from 0.55 ± 0.12 and 0.36 ± 0.09 (Fig. 4). Last, the mean value for somatostatin release evoked by D-glucose at the end of the experiments was higher after prior exposure of the pancreas to alpha -D-glucose pentaacetate or beta -L-glucose pentaacetate rather than unesterified D-glucose (Fig. 3). Such a difference remained obvious when the release of somatostatin was expressed relative to its paired basal value. For instance, the secretory peak recorded at min 81 averaged, relative to paired basal output, 2.99 ± 0.49 and 2.02 ± 0.87 after prior administration of alpha -D-glucose pentaacetate and beta -L-glucose pentaacetate, respectively, as distinct (P < 0.05) from only 0.83 ± 0.26 after prior exposure of the pancreas to unesterified D-glucose.


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Fig. 3.   Time course for changes in somatostatin output. Same presentation as in Fig. 1.


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Fig. 4.   Time course for changes in somatostatin output evoked by alpha -D-glucose PA (open circle ), beta -L-glucose PA (bullet ), and unesterified D-glucose (star ), hormonal output being expressed as percentage of paired basal value (min 20-25). Values (means ± SE) refer in each case to same 4 individual experiments as those illustrated in Fig. 3.

The secretory response to the esters largely exceeded, however, that evoked at the end of the experiments by 5.5 mM D-glucose, at variance with the situation found in the case of insulin output. Another marked difference between insulin and somatostatin output consisted of the virtual absence of an early somatostatin secretory peak in the pancreases exposed to alpha -D-glucose pentaacetate. The absence of an early stimulation of somatostatin release by the glucose esters contrasted with the occurrence of an early secretory peak when the pancreas was exposed to unesterified D-glucose at the end of the experiments. Indeed, in the latter situation, an initial peak-shaped response was always observed, even after prior exposure to unesterified D-glucose. Thus the output of somatostatin at min 79 and 83, respectively, averaged, relative to the paired value recorded at min 81, 15.5 ± 8.7 and 12.9 ± 14.2% after prior stimulation by alpha -D-glucose pentaacetate, 32.4 ± 6.1 and 49.2 ± 11.7% after prior delivery of beta -L-glucose pentaacetate, and 36.8 ± 5.1 and 34.2 ± 9.1% after prior administration of unesterified D-glucose (n = 4 in all cases).

Figure 5 illustrates the effects of hexose esters and unesterified D-glucose on glucagon secretion.


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Fig. 5.   Time course for changes in glucagon output. Same presentation as in Fig. 1.

The first effect of the glucose esters was to provoke a rapid but short-lived stimulation of glucagon release. In terms of the absolute or relative magnitude of this early increment in glucagon secretion and its time course, there was little to distinguish between alpha -D-glucose pentaacetate and beta -L-glucose pentaacetate. For instance, the peak values recorded at min 30-31 were 0.78 ± 0.26 and 0.56 ± 0.22 ng/min higher than the paired reading at min 28 in the case of alpha -D-glucose pentaacetate and beta -L-glucose pentaacetate, respectively. Likewise, when expressed relative to the paired mean basal value (min 20-25), the mean output of glucagon during the early peak-shaped response (min 29-34) averaged 103.4 ± 7.6 and 115.4 ± 14.0% in pancreases exposed to alpha -D-glucose pentaacetate and beta -L-glucose pentaacetate, respectively (Fig. 6).


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Fig. 6.   Time course for changes in glucagon output evoked by alpha -D-glucose PA (open circle ), beta -L-glucose PA (bullet ), and unesterified D-glucose (star ), hormonal output being expressed as percentage of paired basal value (min 20-25). Values (means ± SE) refer in each case to same 4 individual experiments as those illustrated in Fig. 5.

The early stimulation of glucagon output by the glucose esters contrasted with an early inhibitory action of unesterified D-glucose on glucagon secretion. Indeed, as computed over two successive periods of 7 min each, the exponential line relating glucagon release (R) to time (t), according to the equation R = Ro · e-Kt, yielded a higher K value (P < 0.05) after (min 28-34) than before (min 22-28) introduction of the hexose, with a mean paired difference of 2.45 ± 0.67 × 10-2/min (n = 4).

The effects of alpha -D-glucose pentaacetate and beta -L-glucose pentaacetate on glucagon release differed vastly from one another, however, from the 35th min onward. In pancreases exposed to the ester of L-glucose, the rate of glucagon release resumed a value virtually identical to that calculated by exponential extrapolation between the readings recorded at min 28 and 63, i.e., just before administration of the ester and 10 min after such an administration. Thus the mean output of glucagon between the 39th and 53rd min averaged 101.2 ± 3.4% of the paired theoretical value on the basis of such an extrapolation. In contrast, in the pancreases exposed to alpha -D-glucose pentaacetate, the output of glucagon rapidly reached much lower values. As a result of this difference, the release of glucagon during the last 15 min of administration of the ester (min 39 to 53), when expressed relative to the paired basal value, averaged no more than 23.8 ± 5.7% in the case of alpha -D-glucose pentaacetate, as distinct (P < 0.06) from 48.5 ± 8.4% in the case of beta -L-glucose pentaacetate. Moreover, when the administration of the ester was halted and after a transient fall in secretory rate, the output of glucagon remained virtually unchanged after exposure to beta -L-glucose pentaacetate while progressively increasing after delivery of alpha -D-glucose pentaacetate. This indicates that the late inhibitory action of the latter ester on glucagon release was rapidly reversible. The secretory rates reached 15 min after interruption of the administration of the ester (min 69 to 75) were indeed virtually identical in the two series of experiments. Relative to paired basal value (min 20-25), they averaged 42.1 ± 8.3 and 42.8 ± 7.0% in the experiments involving the administration of alpha -D-glucose pentaacetate and beta -L-glucose pentaacetate, respectively.

The administration of unesterified D-glucose (5.5 mM) at the end of the experiments always resulted in a rapid and significant decrease in glucagon output, demonstrating the metabolic and functional integrity of the glucagon-producing cells. As judged by paired comparison of the mean secretory rates recorded just before introduction of the unesterified hexose (min 74-78) and at the end of the perfusion period (min 96-100), the relative extent of the inhibitory action of D-glucose on glucagon release averaged 70.0 ± 2.6, 58.2 ± 4.1, and 59.0 ± 6.7% (P < 0.005 or less) after prior exposure to alpha -D-glucose pentaacetate, beta -L-glucose pentaacetate, and unesterified D-glucose, respectively. It was thus somewhat more pronounced (P < 0.05) in the first case than in the latter two.

    DISCUSSION
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

The present experiments were aimed at comparing the effects of alpha -D-glucose pentaacetate, beta -L-glucose pentaacetate, and unesterified D-glucose on insulin, somatostatin, and glucagon release by the isolated perfused rat pancreas. They were conducted in the presence of L-leucine (10.0 mM) to increase the magnitude of the insulin secretory response to the two esters of glucose (18). The esters were tested at a 1.7 mM concentration, which is close to their limit of solubility. No sizable hydrolysis of the esters took place in the perfusate.

The findings illustrated in Figs. 3 and 4 confirm that alpha -D-glucose pentaacetate and, to a much lesser extent, beta -D-glucose pentaacetate stimulate insulin release under experimental conditions in which unesterified D-glucose, tested at the same molar concentration, fails to do so. As already mentioned in the introduction, the insulinotropic action of the hexose esters cannot be attributed to the catabolism of their acetate moieties. Likewise, in the case of beta -L-glucose pentaacetate, the stimulation of insulin release cannot be due to the catabolism of the hexose moiety of the ester, since L-glucose is not phosphorylated by hexokinase isoenzymes (unpublished observation). The secretory response to this ester thus appears unrelated to its nutritional value and, instead, could be due to a direct effect of the ester itself on some as-of-yet unidentified receptor system.

In addition to its vastly lower magnitude, the secretory response of the B cells to beta -L-glucose pentaacetate differed from that evoked by alpha -D-glucose pentaacetate by the virtual absence of an early secretory peak and the absence of a residual insulinotropic action when the administration of the ester was interrupted.

These two differences may well be attributable to the capacity of alpha -D-glucose pentaacetate, as distinct from beta -L-glucose pentaacetate, to generate D-glucose by intracellular hydrolysis of the ester with further phosphorylation and metabolism of this hexose in the islet cells (20). Such a metabolic component of the secretory response could indeed be required to allow for the occurrence of an early secretory peak. It may also account for the existence of a residual stimulation of insulin release after removal of the ester from the perfusate. Indeed, the large amounts of the ester that accumulate in islet cells (20) may be sufficient to maintain for some time a sizable glycolytic flux.

Many of the considerations so far presented may also apply to the secretion of somatostatin. Such is the case, for instance, for the difference between alpha -D-glucose pentaacetate and beta -L-glucose pentaacetate in terms of the magnitude of the secretory response to these esters and their residual effect on hormonal output when removed from the perfusate. Likewise, both the insulin and somatostatin release evoked by unesterified D-glucose at the end of the experiments was somewhat higher after prior exposure to a glucose ester than after prior administration of unesterified D-glucose.

The secretory behavior of the D cells differed, however, from that of B cells in two major respects. First, in response to alpha -D-glucose pentaacetate administration, no early peak-shaped release of somatostatin could be detected, at variance with the secretory pattern found, within the same experiments for insulin release. Second, as computed over periods of 22 min, the paired ratio between insulin output in the presence of the glucose esters (min 29-50) and unesterified D-glucose (min 79-100) was about one order of magnitude higher in the case of somatostatin than insulin release. Thus such ratios averaged, in the experiments conducted in the presence of alpha -D-glucose pentaacetate and beta -L-glucose pentaacetate, respectively, 98.8 ± 14.2 and 8.4 ± 2.0% in the case of insulin release, as distinct (P < 0.001) from 548.5 ± 110.6 and 161.6 ± 42.3% in the case of somatostatin secretion. These findings suggest that the relative contribution of the two postulated components of the secretory effects of glucose esters, i.e., the metabolic component linked to the catabolism of their hexose moieties and the direct effect of the esters themselves, is not identical in B and D cells. This view is further supported by the fact that, relative to the mean value found with alpha -D-glucose pentaacetate, the steady-state secretory rate found between the 46th and 50th min of exposure to beta -L-glucose pentaacetate was lower, whether expressed in absolute terms or relative to basal output, in the case of insulin than somatostatin release, with overall mean values of 8.0 ± 1.6 and 41.2 ± 12.4% (P < 0.02).

The most dramatic evidence for the postulated dual mode of action of the glucose esters in the endocrine pancreas was provided by the measurements of glucagon release. Thus the two esters first stimulated glucagon secretion in a comparable manner, whether in terms of the magnitude or time course of this early secretory response, suggesting that such a glucagonotropic action is unrelated to the nutritional value of the esters. However, the two series of experiments differed from one another by the fact that alpha -D-glucose pentaacetate, but not beta -L-glucose pentaacetate, inhibited glucagon release during the late period of exposure to the esters, this effect being reversed after removal of alpha -D-glucose pentaacetate from the perfusate. Likewise, alpha -D-glucose pentaacetate, but not beta -L-glucose pentaacetate, apparently primed the glucagon-producing cells to the inhibitory action of unesterified D-glucose administered at the end of the experiments. The latter two phenomena are likely to result from the metabolism of the hexose moiety of alpha -D-glucose pentaacetate, since they indeed duplicate the effect of unesterified D-glucose on the secretory behavior of A cells. The inhibitory action of the hexose on glucagon release is indeed well known. Likewise, several reports have already documented that prior exposure of the pancreas to D-glucose decreases the secretory response of alpha 2-cells to the subsequent administration of arginine (3, 4). To our knowledge, however, the results obtained in the experiments conducted with alpha -D-glucose pentaacetate provide the first evidence for A cell priming in terms of the inhibitory action of D-glucose on glucagon output.

It should also be stressed that in the present experiments, unesterified D-glucose, when tested at the low concentration of 1.7 mM, obviously inhibited glucagon release despite failing to cause any sizable increase in insulin release. This finding is compatible with the view that the hexose can inhibit glucagon secretion independently of any stimulation of insulin secretion (9).

In conclusion, the present findings convincingly document that glucose esters, such as alpha -D-glucose pentaacetate and beta -L-glucose pentaacetate, affect insulin release through a dual mode of action and extend such a knowledge to the secretion of both somatostatin and glucagon by the endocrine pancreas. In our opinion, advantage could be taken of the fact that esters of nonmetabolizable hexoses stimulate insulin release by an as-of-yet unknown mechanism, to develop new insulinotropic tools for the treatment of non-insulin-dependent diabetes (12, 14).

    ACKNOWLEDGEMENTS

We are grateful to J. Marchand for technical assistance and to C. Demesmaeker for administrative help.

    FOOTNOTES

This study was supported by a Concerted Research Action (94-99/183) of the French Community of Belgium and a grant (3.4513.94) from the Belgian Foundation for Scientific Medical Research.

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: W. J. Malaisse, Laboratory of Experimental Medicine, Brussels Free Univ. (CP 618), 808 Route de Lennik, B-1070 Brussels, Belgium.

Received 10 February 1998; accepted in final form 24 June 1998.

    REFERENCES
Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References

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9.   Leclercq-Meyer, V., J. Marchand, and W. J. Malaisse. Role of glucose and insulin in the dynamic regulation of glucagon release by the perfused rat pancreas. Diabetologia 24: 191-195, 1983[Medline].

10.   Malaisse, W. J., and A. Delvaux. Cytostatic effect of 2-deoxy-D-glucose and its tetraacetate ester in transformed mouse fibroblasts. Med. Sci. Res. 25: 727-728, 1997.

11.   Malaisse, W. J., A. Delvaux, J. Rasschaert, and M. M. Kadiata. Cytotoxic action of 2-deoxy-D-glucose tetraacetate in tumoral pancreatic islet cells. Cancer Lett. 125: 45-49, 1998[Medline].

12.   Malaisse, W. J., L. E. Flores, and M. M. Kadiata. Dual effect of 2-deoxy-D-glucose tetraacetate upon glucose-induced insulin release. Biochem. Mol. Biol. Int. 45: 429-434, 1998[Medline].

13.   Malaisse, W. J., H. Jijakli, M. M. Kadiata, A. Sener, and O. Kirk. Stimulation of insulin release by alpha -D-glucose pentaacetate. Biochem. Biophys. Res. Commun. 231: 435-436, 1997[Medline].

14.   Malaisse, W. J., and M. M. Kadiata. Insulinotropic action of the polyacetate esters of two non-nutrient monosaccharides in normal and diabetic rats. Int. J. Mol. Med. 2: 95-98, 1998.[Medline]

15.   Malaisse, W. J., M. M. Kadiata, O. Scruel, and A. Sener. Esterification of D-mannoheptulose confers to the heptose inhibitory action on D-glucose metabolism in parotid cells. Biochem. Mol. Biol. Int. 44: 625-633, 1998[Medline].

16.   Malaisse, W. J., V. Leclercq-Meyer, and F. Malaisse-Lagae. Methods for the measurement of insulin secretion. In: Peptide Hormones: A Practical Approach, edited by J. C. Hutton, and K. Siddle. Oxford, UK: IRL, 1990, p. 211-231.

17.   Malaisse, W. J., and F. Malaisse-Lagae. Bitter taste of monosaccharide pentaacetate esters. Biochem. Mol. Biol. Int. 43: 1367-1371, 1997[Medline].

18.   Malaisse, W. J., C. Sánchez-Soto, M. E. Larrieta, M. Hiriart, H. Jijakli, C. Viñambres, M. L. Villanueva-Peñacarrillo, I. Valverde, O. Kirk, M. M. Kadiata, and A. Sener. Insulinotropic action of alpha -D-glucose pentaacetate: functional aspects. Am. J. Physiol. 273 (Endocrinol. Metab. 36): E1090-E1101, 1997[Abstract/Free Full Text].

19.   Reinhold, U., and W. J. Malaisse. Cytotoxic action of 2-deoxy-D-glucose tetraacetate upon human lymphocytes, fibroblasts and melanoma cells. Int. J. Mol. Med. 1: 427-430, 1998.[Medline]

20.  Sener, A., N. Welsh, F. Malaisse-Lagae, M. M. Kadiata, and W. J. Malaisse. Insulinotropic action of alpha -D-glucose pentaacetate: metabolic aspects. Mol. Gen. Metab. In press.

21.   Vanhoutte, C., M. M. Kadiata, A. Sener, and W. J. Malaisse. Potentiation by its esterification of the inhibitory action of 2-deoxy-D-glucose on D-glucose metabolism and insulinotropic action. Biochem. Mol. Biol. Int. 43: 189-195, 1997[Medline].

22.  Vanhoutte, C., A. Sener, and W. J. Malaisse. Hydrolysis of hexose pentaacetate esters in rat pancreatic islets. Biochem. Biophys. Acta. In press.


Am J Physiol Endocrinol Metab 275(4):E610-E617
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