Interaction of amino acids and cyclic AMP on the release of insulin and glucagon by newborn rat pancreas.

Splenic lobes from the pancreas of newborn rats (48-64) hr. were used for the in vitro investigation of cyclic AMP, glucose and amino acid interaction in hormonal secretion. The slight discrepancy found in glucagon relaease with radioimmunoassay and binding assay to specific receptors in liver does not affect the ratio of stimulated to control values. The insulin release due to gheophylline dibutyrl cyclic AMP (dbcAMP) or to arginine is glucose-dependent as in adult rats and provides an index for the validity of the preparations. Glucose alone is efficient in stimulating insulin release but does not affect glucagon secretion; however simultaneous addition of 10 mM arginine, alanine, and lysine (A.A.) or of arginine alone resulted in a higher glucagon release at 1.6 mM than at 16.7 mM GLUCOSE. Theophylline (5 mM)and dbcAMP (2mM) induced a 2=fold increase in glucagon release at low or hight glucose concentrations .Incubation of theophylline (10 mM) and A.A. or arginine resulted in a considerable increase in glucagon release. Potentation of the 3 A.A.-induced glucagon reby dbcAMP was about 1800% no matter what the glucose concentration; similar observations were made for insulin with a 700% potentiation of the 3 A.A.effect glucagon was released more effectively by dbcAMP than was insulin,whereas the reverse was observed with theophylline. These findings suggest that knowledge of the cyclic AMP content is essential when assessing the influence of substrates on glucagon release. The combination of substrates with cyclic AMP clearly demonstrated that potentiation of glucagon release occurs mainly with amino acids, whereas for insulin occurs mainly with amino acids, whereas for insulin release it is mainly glucose which potentiates release.     

An apparent inhibition of insulin biosynthesis resulting from inhibition of transport of neutral amino acids by arginine

Summary At concentrations higher than 10 mM, the cationic amino acid, arginine, inhibited the incorporation of the neutral amino acids such as alanine, threonine, valine and leucine into insulin in the presence of glucose. This inhibitory effect probably did not result from the stimulatory effect of arginine on insulin release because, in the absence of glucose, arginine failed to stimulate insulin release but nevertheless inhibited the incorporation of leucine into insulin. This inhibitory effect of arginine was shared by another basic amino acid, histidine, but not by lysine. Arginine inhibited the incorporation of leucine not only into insulin but also into other islet proteins. This inhibition was not accompanied by any disturbance of glucose metabolism in the islet cells. Further studies indicated that the inhibition of incorporation resulted primarily from the interference of uptake of the neutral amino acids by arginine.     

In vitro insulin secretion from the penguin (Pygoscelis papua) pancreas.

In experiments performed at the Almirante Brown Scientific Station (64° 53′ South; 62° 53′ West), arginine induced in penguins significant release of insulin from incubated pieces of pancreas, while glucose acted as stimulus only at very high concentrations. Substances active on the adenyl cyclase system, such as glucagon, theophylline, isoproterenol, and even 3′,5′ cyclic AMP, stimulated insulin release.l-Propranolol inhibited isoproterenol-induced insulin release. Phentolamine, in the doses used, had no effect on either basal or isoproterenol-induced insulin secretion.Ouabain and glibenclamide failed to promote any change in the amount of insulin secreted.The results suggest that (a) in the penguin pancreas arginine rather than glucose may be a prominent stimulus for insulin secretion; (b) the penguin beta cell shows decreased sensitivity to glucose; (c) the adenyl cyclase system plays an important role in the mechanism(s) regulating insulin secretion; (d) under our experimental conditions, adrenergic modulation of insulin release is mediated chiefly through beta receptors.     

Effect of cholera toxin on insulin release in monolayer cultures of the endocrine pancreas

Summary The effect of cholera toxin on insulin release by monolayer cultures of endocrine pancreas has been studied. The toxin has a marked stimulatory effect upon insulin release at concentrations as low as 10^–10M. The toxin had a small effect at low glucose concentrations, but was strongly stimulatory at high glucose concentrations and in the presence of arginine. Its effect could be detected within 30 min of application and only two minutes' exposure to the toxin was required for it to subsequently stimulate release. In comparative studies on insulin release the toxin was equal to, or slightly more potent than, 1.5 M glucagon and significantly more potent than cyclic AMP (10 mM) or theophylline (5 mM).     

Secretory regulation of endocrine pancreas: Cyclic AMP and glucagon secretion.

Activation of adrenergic beta receptors has been found to stimulate insulin release in vitro that may be mediated through the augmentation of cyclic AMP in the beta cell. The activation of adrenergic alpha receptors in the beta cell inhibits the insulin release. The present studies have shown that isoproterenol (0.62 mug/ml) and sodium dibutyryl cyclic AMP (50 mug/ml) stimulate the insulin secretion and inhibit the glucagon secretion in the presence of 50 mg/100 ml glucose by the isolated pancreatic perfusion of the rat, while norepinephrine (0.5 mug/ml) inhibits the insulin secretion induced by 150 mg/100 ml glucose and stimulates the glucaton secretion. Theophylline (50 mug/ml) does not stimulate the insulin and the glucagon secretion. When norepinephrine is added to theophylline, the output of glucagon does not occur. From these results it can be deduced that the pancreatic alpha cell function may be inhibited by elevation of intracellular cyclic AMP, in contrast to the beta cell function which is stimulated by an increment of cyclic AMP.     

Correction of altered plasma amino acid pattern in cirrhosis of the liver by somatostatin.

The purpose of our study was to evaluate the effect of somatostatin (500 microgram/h intravenously) upon insulin, c-peptide, glucagon and plasma amino acids concentrations in patients with and without cirrhosis of the liver. The typical plasma amino acid pattern in cirrhosis is characterised by increased concentrations of the aromatic amino acids and decreased concentrations of the branched chain amino acids and of alanine and glycine. After administration of somatostatin insulin, c-peptide and glucagon concentrations decreased and those of the branched chain amino acids in both groups increased; in addition in patients with cirrhosis the plasma concentrations of threonine, serine, glycine, alanine, lysine, and arginine increased also. Infusion of somatostatin plus insulin in patients with cirrhosis succeeded in preventing the increase in the branched chain amino acid concentrations, while the infusion of somatostatin plus glucagon decreased threonine, serine, glycine, alinine, phenylalanine, tyrosine, lysine and arginine concentrations. It is therefore suggested that the effect of somatostatin on the plasma amino acids may be because of the reduction of insulin and glucagon concentrations; however, other effects of somatostatin cannot be excluded at present.     

Interplay of nutrients and hormones in the regulation of glucagon release

The role of nutrients and hormones in the regulation of glucagon release is investigated in pancreatic A cells purified by autofluorescence-activated cell sorting. Purified A cells lack secretory activity in 1-h incubation at 1.4 mM glucose. Their release mechanism can be activated by arginine, alanine, and glutamine, alone or in combination. Glucose inhibits amino acid-induced glucagon release through a direct insulin-independent action upon pancreatic A cells. Nutrient-induced glucagon release is suppressed by somatostatin and amplified by (Bu)2cAMP or epinephrine. The epinephrine stimulus is inhibited by 10(-11) M somatostatin and abolished by 10(-10) M of this peptide. The effects of somatostatin and epinephrine are associated with parallel changes in cellular cAMP levels, which is not the case for the variations induced by amino acids or glucose. It is confirmed that calcium is an essential requirement for glucagon release. In contrast to its exquisite sensitivity for somatostatin, the glucagon release process is relatively insensitive to insulin during a 1-h exposure. The hormone affects solely epinephrine-induced glucagon release and its inhibitory action is partial and only observed at 10(-7) M. This suppressive effect of insulin is not attributable to variations in glucose handling but appears associated with the stimulatory effect of epinephrine. It is concluded that a nutrient-induced signal interacts with a hormone-inducible cAMP signal to activate the secretory process in pancreatic A cells.     

Stimulatory and inhibitory effects of cyclic AMP on pancreatic glucagon release from monolayer cultures and the controlling role of calcium

Summary When glucagon release from monolayer cultures of newborn rat pancreas was measured over four hours in media containing 2.5 mM Ca⁺⁺, a significant cyclic AMP-related inhibition of release was observed. This was noted whether intracellular cyclic AMP levels were raised by the addition of exogenous cyclic AMP or dibutyryl cyclic AMP, by phosphodiesterase inhibition with theophylline, or by the stimulation of adenylate cyclase with cholera toxin. The inhibition was concentration dependent for cyclic AMP and could not be reproduced by the addition of AMP, ADP or ATP. Adenosine also inhibited glucagon release while ATP was stimulatory. From time course studies it appeared that the inhibitory effects of cyclic AMP and cholera toxin were progressive after two hours of incubation. With cholera toxin an early stimulation of glucagon release was observed. The effects of cyclic AMP and cholera toxin on argininestimulated glucagon release were to stimulate further the glucagon release during the first hour of the incubation. Thus, the effects of raising intracellular cyclic AMP levels were biphasic in that both an early stimulation and a late inhibition of glucagon release were observed. In examining the nature of these responses a remarkable controlling role for Ca⁺⁺ was uncovered: at Ca⁺⁺ concentrations of 0.3 mM and lower no effect of cyclic AMP on glucagon release was found. With 1 mM Ca⁺⁺ in the medium cyclic AMP stimulated glucagon release early (30 min) and thereafter had no further effect. In the presence of 2.5 mM Ca⁺⁺ cyclic AMP did not stimulate early but did cause the delayed inhibition of release. It is concluded that the effect of cyclic AMP on glucagon release can be either stimulatory or inhibitory depending upon the Ca⁺⁺ concentration of the medium and the duration of exposure to raised cyclic AMP levels.     

Beta cell desensitization to glucose induced by hyperglycemia is augmented by increased calcium

Chronic hyperglycemia has been shown to induce a decrease in beta cell sensitivity to a subsequent glucose challenge. Calcium is a necessary cofactor in the insulin secretory process and glucose elevates cytoplasmic levels. This study was designed to study whether chronic exposure to different extracellular calcium and glucose concentrations would affect the islets' subsequent response to regulatory stimuli. Islets were isolated and cultured in TC 199 plus 10% beta calf serum, glucose (5.5 or 27.5 mM) and calcium (0.5, 2.5 or 4.0 mM) for 48 h. Following culture, the islets were harvested and incubated a second time in the presence of glucose and/or arginine, theophylline, and trifluoperazine (TFP). Some islets were used for insulin content, protein synthesis studies and/or CO2 production from labelled glucose. Islets cultured in a normal glucose environment with low or normal calcium concentration maintained the capacity to respond to a subsequent glucose or arginine challenge. However, islets cultured in a high glucose or high calcium medium failed to respond to a second glucose or arginine stimulus. Theophylline stimulated insulin secretion from both glucose-sensitive and non-sensitive islets, while trifluoperazine inhibited glucose-stimulated insulin secretion in previously sensitive islets and increased insulin secretion in previously non-sensitive islets. The different culture conditions did not alter insulin content, protein synthesis or glucose conversion to labelled CO2. We conclude that chronic exposure to high glucose decreases beta cell responsiveness to glucose and amino acids. Increased extracellular calcium augmented this response. However, the beta cell remained sensitive to theophylline-induced insulin secretion, while TFP paradoxically increased insulin secretion in the glucose-insensitive beta cells. Protein synthesis and glucose oxidation were not affected by culture conditions. Thus we suggest that the glucose-induced desensitization of the beta cells may be due to alterations in the calcium-dependent release mechanism.     

Insulin and glucagon secretory responses to arginine, glucagon, and theophylline during perifusion of human fetal islet-like cell clusters

The secretory responsiveness of human fetal pancreatic endocrine cells was studied by perifusion of cultured islet-like cell clusters (ICC). ICC were obtained from 7 fetuses at 13-15 weeks gestation and 21 fetuses at 17-22 weeks gestation. The ICC were challenged with glucose (20 mmol/L), arginine (10 mmol/L), glucagon (1.4 mumol/L), and theophylline (10 mmol/L) combined with zero, low (2 mmol/L), or high (20 mmol/L) glucose. At 13-15 weeks, glucose and arginine enhanced insulin release in some experiments, whereas glucagon and theophylline were always potent stimuli (mean response, 4-fold regardless of the glucose concentration). At 17-22 weeks, both glucose alone (20 mmol/L) and arginine (10 mmol/L, with 2 mmol/L glucose) induced a small (1.4- to 1.5-fold) increase in insulin release. When arginine was combined with 20 mmol/L glucose, the response was potentiated to become a 2.3-fold increase. In contrast, glucagon was equally effective in 2 and 20 mmol/L glucose (2.9- and 2.6-fold responses, respectively) and produced a half-maximal response even in the absence of glucose. In this age range the most potent stimulus for insulin release was clearly theophylline. The effect of theophylline was also remarkably independent of the glucose concentration of the perifusate (5.6-, 8.1-, and 8.6-fold responses at 0, 2, and 20 mmol/L glucose, respectively). Glucagon release from the ICC of the 17- to 22-week-old fetuses was low (mean basal glucagon release, 2.9; insulin, 24.8 fmol/100 ICC/min). Glucagon release was not affected by 20 mmol/L glucose, but was stimulated by arginine and theophylline. These findings suggest that in the fetal pancreas, in contrast to the adult organ, insulin release results from elevation of intracellular cAMP concentrations (by glucagon or theophylline) relatively independent of the exogenous glucose concentration. Therefore, glucagon may have an important role in regulating insulin release during the early development of human fetal B-cells.     

Selective impairment of pancreatic A cell suppression by glucose during acute alloxan-induced insulinopenia: in vitro study on isolated perfused rat pancreas

This study was performed in order to investigate the role of insulin in the modulation of pancreatic A cell response to glucose. The isolated perfused rat pancreas model was used: intraislet insulinopenia was induced in vitro by 0.56 mM alloxan infusion over 15 min. Alloxan caused a transitory insulin release but did not affect glucagon secretion. Exposure to alloxan completely abolished insulin response to 20 mM arginine, 1.6 mM glucose, and 11.1 mM glucose. Glucagon response to 20 mM arginine and 1.6 mM glucose was unchanged by alloxan pretreatment compared to control pancreata not treated with alloxan. However, the suppression of glucagon release by 11.1 mM glucose was abolished in the alloxan experiments. Twenty milliunits per ml of insulin infused during 11.1 mM glucose infusion restored glycemic suppression of glucagon release, but it produced only a slight inhibitory effect on A cell function in the presence of 3.9 mM glucose. Our study indicates that glucose is the physiological suppressor of the pancreatic A cell and that, in this regard, insulin exerts only a permissive effect.     

Characterization of the inhibitory effect of somatostatin upon insulin and glucagon release in the isolated perfused canine pancreas: evidence for interaction with calcium

Somatostatin is a potent inhibitor of insulin and glucagon release from the isolated perfused canine pancreas. The present investigation was undertaken to characterize the pancreatic effects of somatostatin by studying its ability to influence insulin and glucagon release from the same perfusion preparation in response to various well-known stimuli and modulators. Somatostatin inhibited insulin and glucagon release in all test situations chosen but one. Thus, somatostatin inhibits pancreatic hormone secretion irrespective of whether it is modulated by (1) a primary initiator of insulin release--glucose (1.3 or 8.3 mM), leucine (4.1 mM), tolbutamide (2.6 mM); (2) a potentiator of insulin release, i.e., a substance that requires the presence of glucose--arginine (1 mM); (3) substances known to increase the level of cyclic AMP (cAMP) in the islets--glucagon (2 ng/ml), cAMP (1 mM), theophylline (1 mM); (4) an autonomic agent--epinephrine (2 ng/ml), acetylcholine (10 microM); or (5) alpha and beta adrenergic antagonists--phentolamine (1 microM), propranolol (1 microM). In contrast, high Ca++ concentrations (4.8 and 8.2 mM) abolished the inhibitory action of somatostatin on both insulin and glucagon release. These findings lend support to the hypothesis that somatostatin acts at a stage of secretory processes, possibly related to Ca++ inactivation, late in the chain of events leading to hormone release.     

The role of cyclic AMP in the pathogenesis of glucose desensitization in rat pancreatic islets

Adenosine-3',5'-cyclic monophosphate (cyclic AMP) promotes exocytosis of insulin in pancreatic beta cells. This study was performed to investigate the role of cyclic AMP in the pathogenesis of glucose desensitization in rat pancreatic islets. In islets cultured with high glucose for 48 hours, 27 mmol/L glucose-induced insulin release was markedly impaired, while 3.3 mmol/L glucose-or arginine-induced insulin release was enhanced, indicating glucose desensitization. Islet cyclic AMP content was 190% enhanced in high glucose-culture islets for 48 hours. In islets cultured with dibutyryl-cyclic AMP (dbcAMP) or 3-isobutyl methy-xanthine (IBMX), islet insulin content or 27 mmol/L glucose-induced insulin release was deteriorated. In contrast, 3.3 mmol/L glucose- or arginine-induced insulin release was increased, which was similar to glucose-desensitized islets. Wash-out of dbc AMP for the last 24 hours of the 48-hour culture period restored impaired high glucose-induced insulin release in the same manner as wash-out of high glucose. Diazoxide, the KATP channel opener, also restored impaired high glucose-induced insulin release from dbcAMP-cultured islets. The data suggest that enhancement of cyclic AMP in high glucose-culture islets may be one of the pathogenesis of glucose desensitization.     

Defective early phase insulin release in perifused isolated pancreatic islets of spiny mice (acomys cahirinus)

In order to characterize pancreatic beta cell function in Geneva bred spiny mice (acomys cahirinus), the dynamics of immunoreactive insulin release were examined during perifusion of pancreatic islets isolated from normoglycemic acomys. The initial insulin response of acomys was slow: no clear-cut early (1 to 10 min) peak of insulin release was observed when glucose in the perifusion medium was abruptly raised from 2.8 mM to concentrations as high as 56 mM. This was true for islets of either young, or older more obese acomys. However, after 20 to 30 min of perifusion at the high glucose concentrations, the rate of insulin release from acomysislets became similar to that from islets of rats or mice. By contrast, glucose-induced insulin release responses observed with islets of Wistar-derived rats, Swiss albino mice, and inbred C57BL/6J lean or obese (ob/ob) mice, were clearly biphasic. Tolbutamide 1.5 mM, arginine 16 mM, and theophylline 10 mM were ineffective in stimulating insulin release from acomys islets in the presence of a substimulatory glucose concentration (2.8 mM), whereas these agents were effective in rat islets at the same substimulatory concentration of glucose. On the other hand, when these agents, as well as cyclic AMP 10 mM or cytochalasin B 10 mug/ml were applied in the presence of a stimulating concentration of glucose (16.8 mM), the glucose-stimulated insulin release from acomys islets was increased to the same or to a greater extent than from rat islets. It is suggested that the failure of all the agents tested to stimulate an early rapid phase of insulin release from acomys islets may be secondary to the observed initial insensitivity to glucose, which insensitivity may in turn reflect a selective impairment in the recognition of glucose as an insulinogenic signal in this species.     

Glucose regulates proinsulin and prosomatostatin but not proglucagon messenger ribonucleic acid levels in rat pancreatic islets

Insulin and glucagon are the major hormones involved in the control of fuel metabolism and particularly of glucose homeostasis; in turn, nutrients tightly regulate insulin and glucagon secretion from the islets of Langerhans. Nutrients have clearly been shown to affect insulin secretion, as well as insulin biosynthesis and proinsulin gene expression; by contrast, the effects of nutrients on proglucagon gene expression have not been studied. We have investigated the effect of glucose, arginine, and palmitate on glucagon release, glucagon cell content, and proglucagon messenger RNA (mRNA) levels from isolated rat islets in 24-h incubations. We report here that concentrations of glucose that clearly regulate insulin and somatostatin release as well as proinsulin and prosomatostatin mRNA levels, do not significantly affect glucagon release, glucagon cell content or proglucagon mRNA levels. In addition, though both 10 mM arginine and 1 mM palmitate strongly stimulated glucagon release, they did not affect proglucagon mRNA levels. We conclude that, in contrast to insulin and somatostatin, glucose does not affect glucagon release and proglucagon mRNA levels, and arginine and palmitate do not coordinately regulate glucagon release and proglucagon mRNA levels.     

Iodoacetate and iodoacetamide-induced alterations of pancreatic alpha- and beta-cell responses.

Iodoacetate and iodoacetamide were compared as to their capacity to block islet glycolysis and interfere with glucose inhibition of glucagon release and glucose stimulation of insulin release. Glycolysis was measured in isolated rat islet by the rate of lactate formation from 27 mM glucose. Hormone release was investigated by perfusing isolated rat pancreas with a 10 mM mixture of 19 amino acids, with and without 5 mM glucose. In perfusion experiments, lactate (2.5 mM) and pyruvate (0.5 mM) were present to provide alternate source of energy independent of glycolysis. Iodoacetate was about twice as potent as iodoacetamide in blocking glycolysis in islets, 0.2 and 0.5 mM, respectively being needed for complete inhibition of lactate production. Levels of either agent lower than 0.05 mM did not affect lactate accumulation. Iodoacetate, at the level which completely inhibited glycolysis did not interfere with the permissive action of glucose for insulin release. In contrast, iodoacetamide at a level (0.05 mM) which had no effect on lactate production, changed the response of the beta-cell dramatically: amino acids now released insulin even in the absence of glucose and insulin release by 5 mM glucose alone was greatly augmented. Both thiol reagents at 0.025 mM concentration completely prevented glucose from suppressing amino acid stimulated glucagon release, iodoacetamide being more potent than iodoacetate. These data indicate that the opposite physiological actions of glucose in alpha and beta-cells are in each case dissociable from the fuel function of the sugar molecule, and the results best support the concept that glucose and thiol reagents effect insulin and glucagon secretion by acting on sulfhydryl groups related to receptor sites in the alpha-and beta-cell membrane.     

Effect of neurotensin on insulin, glucagon, and somatostatin release from isolated pancreatic islets.

The effects of neurotensin on the release of insulin, glucagon, and somatostatin were investigated in isolated pancreatic islets prepared from 3- to 4-day-old rats and maintained in culture for 48 h before use. Islets were incubated for 20 and 60 min in the presence of 3 or 23 mM glucose with or without neurotensin. In 20-min incubations at 3 mM glucose, neurotensin (10-100 nM) increased the release of insulin, glucagon, and somatostatin by 60%, 90%, and 110%, respectively. These increases were not detected in 60-min incubations. Neurotensin (100 nM) inhibited the release of both insulin (by 60-90%) and somatostatin (by 100%) which was induced by 23 mM glucose in 60-min incubations; this inhibitory effect could be detected with neurotensin at a concentration of 1 nM. Neurotensin also significantly inhibited the elevations in glucagon, insulin, and somatostatin release induced by 20 mM arginine. It is concluded that neurotensin exerts a dual effect on the endocrine pancreas in vitro: 1) at low glucose concentration and over short term (20 min) incubations, the peptide stimulates insulin, glucagon, and somatostatin release; and 2) under stimulated conditions (high glucose or arginine), neurotensin inhibits insulin, glucagon, and somatostatin release.     

Glucagon secretion in rats with non-insulin-dependent diabetes: an in vivo and in vitro study

Non-insulin-dependent diabetes ( NIDD ) was obtained in adult rats following a neonatal streptozotocin injection. Rats with NIDD exhibited a chronic low-insulin response to glucose in vivo, slightly elevated basal plasma glucose values (less than 2 g/l) and low pancreatic insulin stores (50% of the controls). Glucagon secretion was studied in this model, in vivo and in vitro using the isolated perfused pancreas technique. Normal basal plasma glucagon levels were observed in the fed state and were in accordance with normal basal glucagon release in vitro. The pancreatic glucagon stores were normal in the diabetics. In experiments with the perfused pancreas, the increased glucose concentration suppressed glucagon release as readily in the diabetics as in the controls. Moreover 5.5 mM glucose suppressed glucagon release stimulated by 19 mM arginine to the same extent in both groups. These data indicate that the suppression of A cell function by glucose is normal in rats with NIDD . Theophylline and isoproterenol also produced normal glucagon release in diabetics. By contrast, the glucagon secretion in response to arginine was lower in the diabetics. This was observed either in vivo (arginine infusion) or in vitro in the presence or the absence of glucose in the perfusate. But in the presence of theophylline the response to arginine was normalized in the diabetics. Impairment of A cell function of the diabetics is not limited to recognition of amino-acids, since acetylcholine evoked a lower glucagon response in the diabetics than in the controls. These defects are different from those described in their B cells.     

Acute effects of alloxan- and streptozotocin-induced insulin deficiency on somatostatin and glucagon secretion by the perfused isolated rat pancreatico-duodenal preparation

Summary The secretion of somatostatin and glucagon by the perfused rat pancreatico-duodenal preparation was examined in situ under control conditions and after the induction of acute insulin deficiency by alloxan or streptozotocin. A 10 min 0.625 mmol/l alloxan perfusion resulted in an immediate and transient increase in basal insulin and glucagon release and a slightly delayed and persistent increase in basal somatostatin secretion. The insulin responses to 16.7 mmol/l glucose, 1 mmol/l theophylline, and 19 mmol/l arginine alone or in combination were virtually eliminated by alloxan treatment, Somatostatin secretion in response to the stimuli was completely inhibited or markedly attenuated. The glucagon-suppressive effect of glucose was unaltered by alloxan and the stimulatory effect of arginine was enhanced. Addition of 1 g/ml porcine insulin to the perfusion medium did not modify the alterations in somatostatin and glucagon responses to arginine. Streptozotocin treatment 90 min prior to the onset of perfusion resulted in changes in somatostatin, glucagon, and insulin responses to glucose and arginine similar to those of alloxan. The present results are consistent with an effect of alloxan and streptozotocin on the D cell similar to that on the B cell, namely, interference with a glucose-mediated effect on hormone secretion.     

Secretagogues for pancreatic hormone release in the channel catfish (Ictalurus punctatus)

We investigated the effect of several potential carbohydrate secretagogues, amino acids, a ketoacid, and potassium chloride on insulin, glucagon, and somatostatin release from the in vitro perfused Brockmann body of channel catfish (Ictalurus punctatus). Mannose (15 mM) stimulated the release of insulin and somatostatin. Fructose (30 mM) induced only a small and transient release of somatostatin. Galactose (15 mM) was not a secretagogue. Likewise, glyceraldehyde failed to stimulate hormone release. Among the amino acids newly tested, alanine and leucine, and also alpha-ketoisocaproic acid were without effect. A high concentration of potassium (25 mEq/liter) induced a pronounced release of insulin and glucagon and a moderate release of somatostatin. In conclusion, a striking similarity exists between catfish and higher vertebrates in their pancreatic endocrine response to hexoses; on the other hand, the catfish Brockmann body appears to respond only to a few of the common stimuli of pancreatic hormone release in mammals.