Effect of epinephrine and mannoheptulose on early and late phases of glucose-stimulated insulin release

Divergent effects of epinephrine and prostaglandin E2 on glucose-induced insulin secretion from perifused rat islets

The impact of the catecholamine epinephrine and the postulated inhibitory second messenger prostaglandin E(2) (PGE(2)) on the kinetics and magnitude of glucose-induced insulin secretion were compared and contrasted. In agreement with a number of studies, epinephrine was a most effective antagonist of glucose-induced insulin secretion. Dose-response studies using 8 to 10 mmol/L glucose as stimulant established that levels as low as 1 to 10 nmol/L of the catecholamine were effective at inhibiting release. Glucose (20 mmol/L) caused an approximately 25-fold increase in insulin secretion, an effect that was completely abolished by 1 micromol/L epinephrine. Under conditions where it completely abolished 20 mmol/L glucose-induced insulin release, epinephrine (1 micromol/L) reduced, but did not abolish, the stimulatory effect of glucose on phospholipase C activation. Chronic 3-hour exposure to 10 mmol/L glucose alone desensitized the islet to subsequent stimulation by glucose. Despite its ability to completely suppress secretion to 10 mmol/L glucose, epinephrine failed to protect the islet from hyperglycemia-induced desensitization. In sharp contrast to epinephrine, PGE(2) at levels ranging from 1 to 10 micromol/L had no discernible adverse effect on 10 mmol/L glucose-induced secretion. These findings suggest that multiple mechanisms contribute to the inhibitory impact of epinephrine on release and, in conjunction with other studies, cast serious doubt on the concept that PGE(2) plays any significant inhibitory role in the regulation of glucose-induced secretion.

The dynamics of insulin release from mouse pancreatic islet cells in suspension

The overall dynamics of glucose-induced insulin release was strikingly similar in dispersed cells and intact islets perifused in parallel. Both preparations exhibited a latency of 1-2 min, after which period there was a brisk rise of insulin release followed by a sustained second phase. During the second phase, insulin release from dispersed cells attained a stable plateau rate, whereas the release from intact islets continued to rise. Epinephrine (1 muM) inhibited the release in both preparations, but the return to basal rate was faster in the dispersed cells than in the intact islets. The dispersed cells oxidized glucose at a constant rate for at least 60 min; the glucose oxidation was markedly sensitive to changes of the glucose concentration in the range of 3-20 mM.

The pancreatic beta-cell recognition of insulin secretagogues. Effects of calcium and sodium on glucose metabolism and insulin release

The transport and oxidation of glucose, the content of fructose 1,6-diphosphate, and the release of insulin were studied in microdissected pancreatic islets of ob/ob mice incubated in Krebs-Ringer bicarbonate medium. Under control conditions glucose oxidation and insulin release showed a similar dependence on glucose concentration with the steepest slope in the range 5-12mm. The omission of Ca(2+), or the substitution of choline ions for Na(+), or the addition of diazoxide had little if any effect on glucose transport. However, Ca(2+) or Na(+) deficiency as well as diazoxide (7-chloro-3-methyl-1,2,4-benzothiadiazine 1,1-dioxide) or ouabain partially inhibited glucose oxidation. These alterations of medium composition also increased the islet content of fructose 1,6-diphosphate, as did the addition of adrenaline. Phentolamine [2-N-(3-hydroxyphenyl)-p-toluidinomethyl-2-imidazoline] counteracted the effects of adrenaline and Ca(2+) deficiency on islet fructose 1,6-diphosphate. After equilibration in Na(+)-deficient medium, the islets exhibited an increase in basal insulin release whereas the secretory response to glucose was inhibited. The inhibitory effects of Na(+) deficiency on the secretory responses to different concentrations of glucose correlated with those on (14)CO(2) production. When islets were incubated with 17mm-glucose, the sudden replacement of Na(+) by choline ions resulted in a marked but transient stimulation of insulin release that was not accompanied by a demonstrable increase of glucose oxidation. Galactose and 3-O-methylglucose had no effect on glucose oxidation or on insulin release. The results are consistent with a metabolic model of the beta-cell recognition of glucose as insulin secretagogue and with the assumption that Ca(2+) or Na(+) deficiency, or the addition of adrenaline or diazoxide, inhibit insulin release at some step distal to stimulus recognition. In addition the results suggest that these conditions create a partial metabolic block of glycolysis in the beta-cells. Hence the interrelationship between the processes of stimulus recognition and insulin discharge may involve a positive feedback of secretion on glucose metabolism.

Iodoacetamide-induced sensitization of the pancreatic beta-cells to glucose stimulation

At a glucose concentration of 3mm or less, iodoacetamide had no effect on the release of insulin from microdissected pancreatic islets of ob/ob-mice. At higher glucose concentrations, iodoacetamide exerted both an initial stimulatory and a subsequent inhibitory action. When islets were perifused with 1mm-iodoacetamide and 17mm-glucose the inhibitory action predominated after about 15min of transient stimulation. With decreasing concentrations of iodoacetamide the stimulatory phase was gradually prolonged, and with 0.003-0.1mm-iodoacetamide stimulation only was observed for 75min. Prolonged stimulation was also noted after a short pulse of iodoacetamide. Similar responses to 0.1mm-iodoacetamide were observed with islets from normal mice. With islets from ob/ob-mice the effect of 0.1mm-iodoacetamide was reproduced with 0.1mm-iodoacetate, whereas 0.1mm-acetamide had no apparent effect. Iodoacetamide increased the V(max.) of glucose-stimulated insulin release without altering the apparent K(m) for glucose. Leucine, glibenclamide or theophylline could not replace glucose in this synergistic action with iodoacetamide. Iodoacetamide rather inhibited the insulin-releasing action of theophylline. Iodoacetamide-induced potentiation of the glucose-stimulated insulin release was rapidly and reversibly inhibited by mannoheptulose, adrenaline, or calcium deficiency. The potentiating effect on insulin release was not paralleled by effects on glucose oxidation or on islet fructose 1,6-diphosphate. However, the inhibitory action of iodoacetamide might be explained by inhibition of glycolysis as evidenced by an inhibition of glucose oxidation and a rise of fructose 1,6-diphosphate. The results support our previous hypothesis that thiol reagents can stimulate insulin release by acting on relatively superficial thiol groups in the beta-cell plasma membrane. Glycolysis seems to be necessary in order for iodoacetamide to stimulate in this way.

Effects of organic mercurials on mammalian pancreatic -cells. Insulin release, glucose transport, glucose oxidation, membrane permeability and ultrastructure

The effects of p-chloromercuribenzoic acid and chloromercuribenzene-p-sulphonic acid on pancreatic islets were studied in vitro. Obese-hyperglycaemic mice were used as the source of microdissected islets containing more than 90% beta-cells. p-Chloromercuribenzoic acid and chloromercuribenzene-p-sulphonic acid stimulated insulin release at concentrations of 0.01mm or above. This stimulation was significantly inhibited by the omission of Ca(2+) or the addition of adrenaline, diazoxide or 2,4-dinitrophenol. p-Chloromercuribenzoic acid or chloromercuribenzene-p-sulphonic acid did not interfere with the insulin-releasing ability of glucose. Micro-perifusion experiments revealed that the release of insulin in response to organic mercurial occurred almost instantaneously, was reversible, and was biphasic. The two mercurials inhibited glucose transport as well as glucose oxidation, and increased the mannitol and sucrose spaces of isolated islets. Compared with the effects on insulin release, those on glucose transport and membrane permeability were characterized by a longer latency and/or required higher concentrations of organic mercurial. Apart from a seemingly higher proportion of beta-cells exhibiting certain degenerative features, in islets exposed to 0.1mm-chloromercuribenzene-p-sulphonic acid for 60min, no significant differences with respect to beta-cell fine structure were noted between non-incubated islets and islets incubated with chloromercuribenzene-p-sulphonic acid or glucose or both. It is suggested that insulin release may be regulated by relatively superficial thiol groups in the beta-cell plasma membrane.