Effects of glucosamine on insulin and glucagon secretion in dogs and ducks.

The effects of infusion of glucosamine on immunoreactive glucagon (IRG) and insulin (IRI) secretion were studied in dogs and ducks. During systemic infusion of glucosamine, hyperglycemia developed and insulin secretion was inhibited in both species. An immediate and sustained elevation of peripheral IRG levels was induced in ducks but a transient rise, detectable only in the pancreatic vein blood, was provoked in dogs. Suppression of insulin release and stimulation of glucagon release may be mediated by the inhibition of glucose utilization in beta- and alpha-cells. The very prompt response of IRG in ducks may imply that glucosamine has a specific stimulating effect on the alpha-cells of ducks. Intrapancreatic administration of glucosamine in dogs, however, failed to elicit the rise of IRG, although insulin secretion was inhibited. Thus, it is suggested that the systemic administration of glucosamine in dogs may stimulate IRG secretion by some indirect effect. In one dog, however, a sustained rise of the pancreatic vein IRG was observed. Thus, the possibility cannot be ruled out that the difference in IRG response to glucosamine in dogs and ducks is quantitative rather than qualitative. Glucagon release by glucosamine may provide an additional factor to the hyperglycemic effect of glucosamine, in addition to its effect to suppress insulin release as well as its direct inhibitory effect on glucose utilization in tissues.     

Effects of salicylate on insulin and glucagon secretion by the isolated and perfused rat pancreas

The effects of sodium salicylate, a prostaglandin synthesis inhibitor, on glucose-induced secretion of insulin and glucagon by the isolated perfused rat pancreas have been studied. Sodium salicylate inhibited both basal (2.8 mM glucose) and stimulated (16.7 mM glucose) insulin release in a dose dependent manner (1, 5 and 10 mM). This inhibition is not interpretable in terms of a simple inhibition of cyclooxygenase by sodium salicylate. Basal glucagon release was not changed by 1 mM sodium salicylate but the latter partially blocked its inhibition by 16.7 mM glucose. Higher doses of sodium salicylate (5 and 10 mM) inhibited basal glucagon secretion without affecting its response to 16.7 mM glucose. These findings suggest a predominant stimulatory action of endogenous prostaglandins on glucagon release.     

Effects of pancreastatin on insulin, glucagon and somatostatin secretion by the perfused rat pancreas

Pancreastatin is a novel peptide, isolated from porcine pancreatic extracts, which has been shown to inhibit glucose-induced insulin release "in vitro". To achieve further insight into the influence of pancreastatin on pancreatic hormone secretion, we have studied the effects of this peptide on unstimulated insulin, glucagon and somatostatin output, as well as on the responses of these hormones to glucose and to tolbutamide in the perfused rat pancreas. Pancreastatin strongly inhibited unstimulated insulin release as well as the insulin responses to glucose and to tolbutamide. It did not significantly affect glucagon or somatostatin output under any of the above-mentioned conditions. These findings suggest that pancreastatin inhibits B-cell secretory activity directly, and not through an A-cell or D-cell paracrine effect.     

Effects of porcine diazepam-binding inhibitor on insulin and glucagon secretion in vitro from the rat endocrine pancreas.

Porcine diazepam-binding inhibitor (pDBI) is a novel peptide that has been isolated from the small bowel of the pig, and that occurs also in the islet D-cells. We have studied its effects on hormone release in vitro from the endocrine pancreas of the rat. In isolated islets, pDBI (10(-9)-10(-6)M) did not affect basal insulin release at 3.3 mM glucose, whereas stimulated release at 8.3 mM glucose was dose-dependently suppressed by 32-69% (P less than 0.01). Furthermore, insulin secretion stimulated by either 16.7 mM glucose or 1 mM IBMX (3-isobutyl-1-methylxanthine) or 1 micrograms/ml glibenclamide was suppressed by pDBI at 10(-8) M (by 28-30%, P less than 0.05) and 10(-7) M (by 43-47%, P less than 0.01). In contrast, islet insulin secretion induced by 20 mM arginine was unaffected by these concentrations of pDBI. In the perfused rat pancreas, pDBI (10(-8) M) enhanced by 30% (P less than 0.05) the first phase (0-5 min) of arginine-stimulated insulin release, whereas the second phase (5-20 min) was unchanged. Moreover, pDBI suppressed by 28% (P less than 0.05) the second phase of arginine-induced glucagon release. Arginine-induced somatostatin release was not significantly affected by the peptide. Since pDBI immunoreactivity has been localized also to islet D-cells, the present results suggest that pDBI may act as a local modulator of islet hormone release.     

Adrenergic modulation of pancreatic glucagon and insulin secretion in goats

Five goats were used to investigate adrenergic influences on the secretion of both glucagon and insulin. The secretion of glucagon was augmented via alpha-adrenergic stimulation. The secretion of insulin was enhanced by stimulation of beta-adrenergic receptors and inhibited by alpha-adrenergic stimulation.     

Effects of insulin, leptin, and glucagon on ghrelin secretion from isolated perfused rat stomach

Ghrelin, an endogenous ligand for the growth hormone secretagogue receptor, was originally purified from the rat stomach. Although ghrelin has been recognized as an important regulator of energy metabolism, the regulation of the ghrelin secretion is largely unknown. Here, we examined the direct effects of insulin, leptin, and glucagon on the release of ghrelin from the isolated rat stomach. The isolated pancreas-spleen-duodenum deprived preparation of rat stomach was used. After a baseline control infusion into the left gastric artery, insulin, leptin, or glucagon were infused for 15 min at concentrations of 0.1, 1, and 10 nM. The levels of immunoreactive ghrelin in the venous effluents were measured with a radioimmunoassay. Insulin and leptin inhibited ghrelin secretion dose-dependently (total amount of ghrelin release: insulin at 1 nM, 73.5+/-7.3% of the control infusion; leptin at 1 nM, 81.8+/-2.5% of the control infusion; n=5, P<0.05), while glucagon increased it dose-dependently (total amount of ghrelin released at 10 nM was 143.9+/-19.3% of the control infusion; n=5, P<0.01). These results indicate that the ghrelin responses observed in vivo could be due to direct effects of multiple hormonal signals on the stomach.     

Effects of glucagon and insulin on liver lysosomes

Recent experimental evidence has been obtained, principally in the laboratory of Glenn Mortimore, that hepatic lysosomes can act as a pool of amino acids during fasting. This pool is generated through $\text{autophagy}$, whereby intracellular proteins are somehow captured by the lysosomes and then rapidly hydrolyzed to free amino acids by the lysosomal proteinases. Two important metabolic fates of these lysosomal digestive products can be: 1) conversion of the glucogenic amino acids into glucose, and 2) conversion of trimethyl-lysine into carnitine. The latter metabolite is required to transfer fatty acids to the mitochondrial site of β-oxidation. Most interesting is the observation that glucagon appears to induce lysosomal autophagy and the resulting degradation of intracellular proteins by decreasing the size of amino acid pools in the perfused liver. This effect of the hormone may be directed at the single amino acid glutamine, since adding it alone to the perfusate can prevent the increase in autophagy caused by glucagon. Insulin also rapidly inactivates hepatic autophagy and its ensuing proteolysis. The $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ for the rate of los of autophagic vocuoles from the insulin-treated liver (or animal) is approximately 8 min. Thus, glucagon and insulin actively control intracellular protein catabolism that takes place within hepatic lysosomes, and this regulation by the two hormones may be one of their major molecular effects on gluconegenesis in the liver.     

Effect of alrestatin on arginine-induced secretion of glucagon and insulin in the rat.

Alrestatin, a lens aldose reductase inhibitor, decreased i.v. arginine-induced glucagon levels and augmented arginine-stimulated insulin release in the ether anesthetized rat. Alrestatin may then be useful in the treatment of diabetes mellitus, due to its actions on insulin and glucagon, and its capacity to delay the onset of sugar-induced cataracts in the rat.     

Glucose-regulated glucagon secretion requires insulin receptor expression in pancreatic alpha-cells

The insulin receptor (IR) and its signaling appear to be essential for insulin secretion from pancreatic beta-cells. However, much less is known about the role of the IR in alpha-cells. To assess the role of the IR in glucagon and insulin secretion, we engineered adeno-viruses for high efficiency small interference RNA (siRNA)-IR expression in isolated mouse pancreatic islets and lentiviruses for siRNA-IR expression in pancreatic alpha- and beta-cell lines (alpha-TC6 and MIN6) with specific, long term stable IR knockdown. Western blot analysis showed that these strategies resulted in 60-80% reduction of IR protein in islets and alpha- and beta-cell lines. Cell growth was reduced by 35-50% in alpha-TC and MIN6 cells stably expressing siRNA-IR, respectively. Importantly, glucagon secretion, in response to glucose (25 to 2.8 mm), was completely abolished in islets expressing siRNA-IR, whereas secretion increased 1.7-fold in islets expressing control siRNA. In contrast, there was no difference in glucose-stimulated insulin secretion when comparing siRNA-IR and siRNA control, with both groups showing a 1.7-fold increase. Islet glucagon and insulin content were also unaffected by IR knockdown. To further explore the role of the IR, siRNA-IR was stably expressed in pancreatic cell lines, which dramatically suppressed glucose-regulated glucagon secretion in alpha-TC6 cells (3.4-fold) but did not affect GSIS in MIN6 cells. Defects in siRNA-IR-expressing alpha-cells were associated with an alteration in the activity of Akt and p70S6K where insulin-induced phosphorylation of protein kinase B/AKt was greatly reduced while p70S6K activation was enhanced, suggesting that the related pathways play important roles in alpha cell function. This study provides direct evidence that appropriate expression of the IR in alpha-cells is required for glucose-dependent glucagon secretion.     

Homotypic cell contact enhances insulin but not glucagon secretion

Intra-islet interactions influence beta-cell function, and disruption of islet architecture results in a reduction in glucose-induced insulin secretion, whereas re-aggregation improves secretory responsiveness. Our studies on MIN6 cells have shown that by configuring beta-cells as three-dimensional islet-like structures there is a marked improvement in glucose-induced insulin secretion compared to that of their monolayer equivalents. In the present study, we have used the mouse glucagon-secreting alphaTC1 cell line to see whether homotypic interactions are important in the regulation of glucagon secretion from alpha-cells. We found no significant difference in the secretory responses of alphaTC1 cells maintained as monolayers or as cell clusters. We also found that different cell adhesion molecules are involved in cell interactions between alpha- and beta-cells; MIN6 cells express ECAD, whereas alphaTC1 cells express NCAM. ECAD is necessary for cell cluster formation by MIN6 cells but not by alphaTC1 cells, whereas NCAM is not needed for the formation of cell clusters in either cell line.