The regulation of insulin secretion

Signals for insulin secretion

Glucose is an effective stimulus for the release of insulin from pancreatic beta-cells but its pre-eminence for the physiological control of insulin secretion is now challenged. The insulin response to oral glucose is much greater than the response to intravenous glucose because an intestinal factor, or factors, has a powerful stimulant effect on insulin secretion. Conversely, the hyperglycaemia which accompanies sympathetic overactivity is accompanied by reduced plasma insulin levels because adrenaline inhibits pancreatic insulin by an alpha-adrenergic mechanism. Other hormonal and chemical factors also modify pancreatic insulin release. Plasma insulin levels will be determined not only by the interplay of signals which stimulate or inhibit insulin release but also by the rate at which insulin leaves the circulation. Because insulin is secreted into the portal circulation the concentration of insulin to which hepatocytes are exposed is normally greater than that affecting peripheral tissues. This ratio will be affected by portal venous diversion and might be altered by hepatic damage. Both factors might also influence the intestinal control of insulin secretion, as the gut 'hormone' would not have to traverse the liver, which might remove some of it, before it arrived at the pancreatic islets. If insulin is the key 'hepatotrophic factor' stimulating hepatic growth and regeneration, as well as preventing atrophy, we might expect it to induce hepatic growth in otherwise normal liver and we would expect it to be secreted in larger amounts when regeneration is active.

Simultaneous elevation of plasma insulin and catecholamines during endotoxicosis in the conscious and anesthetized rat

Plasma levels of glucose, insulin and catecholamines were assessed during the early phase of sub-lethal endotoxicosis in fasted male rats which were either conscious or continuously anesthetized with sodium pentobarbital. Exogenous glucose challenge was administered during endotoxicosis to probe insulin release at a time when plasma catecholamines were elevated. An endogenous hyperglycemia occurred following endotoxin but was moderated by continuous pentobarbital anesthesia. Plasma insulin was elevated in the conscious but not anesthetized rats during endogenous hyperglycemia following endotoxin. Hyperglycemia with exogenous glucose elevated plasma insulin levels in both conscious and anesthetized groups and occurred in the presence of elevated levels of norepinephrine, epinephrine and dopamine. Simultaneous elevation of plasma catecholamine and insulin levels during endotoxicosis suggests that glucose utilization may be promoted at the same time that glucose is mobilized through adrenergic mechanisms. These events may contribute to the rapid depletion of carbohydrate stores leading to the hypoglycemia of the agonal stage of endotoxic shock.

Peripheral and central short-term effects of fusaric acid, a DBH inhibitor, on tryptophan and serotonin metabolism in the rat

Fusaric acid (FA) administration to the rats promoted one hour later a large decrease in plasma total tryptophan (TRP), without affecting either plasma free TRP or lipolysis, as measured by plasma non esterified fatty acid concentration. The previous change was associated with hypoinsulinemia, hyperglycemia and increased plasma corticosterone level. Regression analysis revealed a significant correlation between brain TRP and the percentage of plasma TRP which was free (i.e. unbound to albumin), both increased by FA injection. The increase in brain TRP promoted an increased brain serotonin synthesis, as measured by the enhanced brain and CSF 5-HIAA levels. Valine pretreatment, which blocks TRP entry into the brain, completely prevented FA-induced brain TRP and brain 5-HIAA increases. These results suggest that the increased brain serotonergic turnover following FA treatment was due to a peripheral action of the drug upon TRP disposition. The latter effect may be caused (i) by in vivo peripheral alterations in catecholaminergic metabolism and (ii) by FA chemical structure since in vitro experiments revealed that FA was able to displace TRP binding to albumin, thus increasing the plasma free TRP pool.

Difference in dose-response curves for glucose-induced insulin and somatostatin release in rat pancreas

We studied the glucose dependence of insulin and somatostatin release from rat pancreata, which were perfused in vitro in the presence of 3-isobutyl-1-methylxanthine (IBMX; 0.5 mM). Half-maximal insulin release occurred at approx. 12 mM glucose, and half-maximal somatostatin release at approx. 7 mM glucose.

Low molecular weight pineal and cerebral fractions affecting insulin secretion in vitro

Studies were performed to examine the effects of various fractions of pineal gland origin upon rates of immunoreactive insulin release during short-term incubations of pancreatic islets from pinealectomized rats. Fractions of cerebral cortex were employed in control incubations. An ultrafiltrate of pineal extracts containing materials of molecular weight less than or equal to 1000 daltons stimulated insulin release while fractions of greater molecular weight were without effect. The stimulation of insulin release observed with the lower molecular weight pineal fraction was seen with both nonstimulatory (2 mM) and stimulatory (10 and 30 mM) medium glucose concentrations, but was abolished in the presence of 2,4-dinitrophenol (200 microM). Upon further fractionation of the low molecular weight pineal extract, fractions I (estimated molecular weight range 700-1000 daltons) and 11 (estimated molecular weight range 180-700 daltons) exhibited comparable stimulatory effects upon islet insulin release: similar effects were observed with cerebral cortical fractions in these molecular weight ranges. However, pineal fraction III (estimated molecular weight range less than 180 daltons) exhibited a striking inhibitory effect upon rates of insulin release compared to cerebral cortical fraction III. Potassium, dopamine, norepinephrine and epinephrine concentrations in both pineal and cortical tissue fractions were similarly low. We conclude that insulinotropic constituent(s) of similar molecular weight occur in pineal gland and cerebral cortex, and that the pineal insulinotropic activity stimulates active insulin secretion by the islets independently of concomitant stimulation by glucose; however, lower molecular weight insulinostatic constituent(s) are notable only in the pineal. It is suggested that mild hyperinsulinemia seen in pinealectomized rats under some circumstances may occur as a result of loss of the pineal insulinostatic factor(s).

Insulin secretion by a transplantable rat islet cell tumour

Investigation of the subcellular and molecular components of insulin secretion has been made difficult by the small quantities of material available. The recent development of a transplantable rat islet cell tumour of high insulin content and state of differentiation suggested a system more amenable to analysis. To validate the tumour as a model of secretion we have studied its release of insulin. In acute experiments in vitro immunoreactive insulin release was increased by leucine, glucagon, theophylline and dibutyryl cyclic AMP, though not by glucose. Leucine (20 mmol/l) plus theophylline (5 mmol/l) caused an abrupt, sustained and rapidly reversible stimulation of two- to fivefold. The response was inhibited by antagonists of cellular oxidative phosphorylation (cyanide, 2,4-dinitrophenol, antimycin A), calcium flux (EGTA, verapamil, Mg2+), calmodulin (trifluoperazine), microtubules (vinblastine, colchicine) and by adrenaline and somatostatin. These findings suggest that the tumour secretes insulin by an exocytotic mechanism similar to that of normal islet tissue.

Insulin release: reconciliation of the receptor and metabolic hypotheses. Nutrient receptors in islet cells

Nutrients which stimulate insulin secretion are currently thought to initiate the series of cellular events eventually leading to insulin release either by interacting with a stereospecific receptor system (the regulatory site hypothesis) or by acting as a fuel (the substrate site hypothesis) in the pancreatic B-cell. The latter hypothesis is supported by a number of observations indicating that the capacity of nutrients to stimulate insulin release is indeed highly dependent on their capacity to increase catabolic fluxes in isolated pancreatic islets. However, these observations do not rule out the existence of nutrient receptors in islet cells. For instance, a nonmetabolized analog of L-leucine stimulates insulin release by causing allosteric activation of glutamate dehydrogenase, which should be considered, therefore, as a receptor for certain amino acids. Likewise, the increase in glycolytic flux, which is associated with the process of glucose-stimulated insulin release, is attributable not solely to a mass action phenomenon but also to the activation of phosphofructokinase by fructose 2.6-bisphosphate. The biosynthesis of this activator may involve a glucose receptor system. The fact that certain nutrient secretagogues (e.g. D-glucose and L-leucine) act in the B-cell both as substrates and enzyme activators permits reconciliation of the substrate site and regulatory site hypotheses for insulin release.

Insulin release: the fuel hypothesis

The immediate and direct regulation of insulin release by circulating nutrients, especially glucose, is thought to be mediated in the pancreatic B-cell by a sequence of metabolic, ionic, and motile events. On the basis of previous work, it is assumed that the process by which glucose is recognized as an insulinotropic agent entirely depends on the metabolic changes evoked by the sugar in the islet cells. Several factors are considered as possible candidates for the coupling between these metabolic changes and subsequent ionic events such as altered phosphate, chloride, sodium, potassium, and calcium handling. It is acknowledged that changes in the concentrations of glycolytic intermediates and cyclic nucleotides (adenosine- or guanosine-3', 5'-cyclic monophosphate), or both, could play a modulatory role upon stimulated insulin release. However, the initiation of insulin release seems to depend on the generation of two essential coupling factors: H+ and reduced pyridine nucleotides. The changes in H+ fluxes may account for the glucose-induced decrease in K+ and Ca2+ fractional outflow rate, all three parameters displaying hyperbolic-like dose-response curves with half-maximal values at noninsulinotropic glucose concentrations. The changes in NAD(P)H concentration may account for a glucose-induced Ca2+--Ca2+ exchange process due to a change in affinity of a native ionophoretic system. The dose-response curves for these parameters yield a sigmoidal pattern analogous to that which depicts the rate of insulin release at increasing glucose concentrations. It is proposed that such a coupling between metabolic and cationic events is operative in response to other insulinotropic nutrients and that its time course may be relevant to the phasic aspect of insulin release. Thus, the nutrient-induced release of insulin (and possibly other pancreatic hormones), which is essential for the regulation of fuel homeostasis, would depend on the capacity of circulating nutrients to act as a fuel in the islet cells. This concept raises a question as to the existence and nature of feedback mechanisms regulating the metabolic fluxes in the islet cells as a function of their energy expenditure.

Effect of Na+, K+ and Mg2+ on 45Ca+ uptake by pancreatic islets

Microdissected pancreatic islets of noninbred ob/ob-mice were used to study ionic effects on the lanthanum-nondisplaceable 45Ca2+ uptake by islet cells. Omission of Mg2+ from the incubation medium had no effect, but the 45Ca2+ uptake was increased by omission of Na+ and decreased by omission of K+. Excess Mg2+ (1.2--15 mM) inhibited and excess K+ (4.7--25 mM) stimulated the 45Ca2+ uptake in a concentration-dependent manner. Stimulation of 45Ca2+ uptake in Na+-deficient islets was associated with an enhancement of the basal insulin release. Total abolishment of glucose-stimulated 45Ca2+ uptake in K+-deficient islets did not preclude a significant secretory reponse to glucose. It is concluded that the lanthanum-nondisplaceable 45Ca2+ uptake shows a partial correlation to insulin release.

The stimulus-secretion coupling of glucose-induced insulin release. XXVIII. Effect of glucose on Na+ fluxes in isolated islets

The effect of glucose upon the handling of 22Na+ by pancreatic islets was investigated. Using a triple-isotope technique, the apparent concentration of Na+ in islet cells was estimated at 50--75 mM. The pattern of 22Na+ efflux from perifused islets indicates that this intracellular Na+ load is compartmentalized among a small, possibly organelle-bound pool characterized by a low fractional turnover rate (5%/min) and a large, presumably cystosolic pool displaying a much higher fractional turnover rate (20--34%/min). Glucose provokes a rapid, pronounced and sustained increase in the fractional outflow rate of Na+ across the plasma membrane and, under steady-state conditions, moderately reduces the concentration of Na+ inside the islet cells. The glucose-induced increase in Na+ outflow rate, which is also observed in response to glyceraldehyde and dose not require the presence of extracellular Ca2+, might be mediated, in part at least, by an ouabain-resistant ionophoretic system. The experimental data suggest that glucose also increases the inward transport of Na+ in islet cells by a veratridine-sensitive channel.

Comparison of alpha-ketoisocaproic acid and glucose in rats: effects on insulin and somatostatin release and on islet cAMP content

The insulinotropic effects of alpha-ketoisocaproic acid and glucose reveal many common characteristics in vivo and in vitro. They qualify as initiators of insulin release, their action is amplified by potentiators of insulin release, and they have a similar potency at equimolar concentrations. The dynamics of insulin release evoked by alpha-ketoisocaproic acid and glucose are similar. Epinephrine completely inhibits the insulinotropic effect of glucose and alpha-ketoisocaproic acid. Mannoheptulose exhibits a complete, immediate and reversible blockade of glucose-induced insulin release. In contrast, inhibition of alpha-ketoisocaproic acid-induced insulin release occurs after a lag period and is not reversed by removal of the inhibitor. alpha-ketoisocaproic acid, at equimolar concentrations, is several-fold more effective than glucose in elevating cAMP content in islet. alpha ketoisocaproic acid and glucose are about equally effective in stimulating somatostatin release from isolated rat pancreatic islets. This stimulation is inhibited by epinephrine. Mannoheptulose inhibits only somatostatin release induced by glucose but not by alpha-ketoisocaproic acid. It suggested that the insulinotropic characteristics of glucose and alpha-ketoisocaproic acid reveal many common features, while their mode of action appears to be different.

The stimulus secretion coupling of glucose-induced insulin release. XXVII. Effect of glucose on K+ fluxes in isolated islets

The effect of glucose upon the handling of K+ by islets of Langerhans removed from normal rats was investigated by measuring both the net uptake of 86Rb+ and its efflux from prelabelled islets. The inflow of K+ into islet cells is mediated, in part at least, by an ouabain-sensitive pump. Glucose fails to affect the inflow rate of K+, but it apparently decreases the permeability of islet cells plasma membrane to effuent K+. The glucose-induced change in permeability is a rapid and rapidly reversible phenomenon. Under steady-state conditions, it leads to an increase in the islet cells K+ pool and a decrease of its fractional turnover rate.

Investigations on isolated islets of langerhans in vitro. 16.Modification of the glucose-dependent inhibition of glucagon secretion

Investigation of glucagon secretion in isolated Wistar rat islets was carried out to elucidate further the regulatory function of glucose and arginine on pancreatic A-cells. The suppressive effect of D-glucose could also be demonstrated with L-glucose, D-mannose, D-fructose, D-galactose, D-glyceraldehyde and DL-dihydroxyacetone, but not in the presence of 3-O-methylglucose or mannitol. Sugars other than D-glucose inhibited glucagon secretion only at much higher concentrations than those at which D-glucose was effective. Furthermore, although 7.5 mM D-glucose up to 80% inhibition, the effects of other sugars appeared to level off at only 50--60% inhibition. The inhibitory action of D-glucose or D-glyceraldedyde on glucagon secretion could not be overcome by L-arginine, but 3-O-methylglucose, mannoheptulose, 2-deoxy-D-glucose, iodoacetamide, theophylline, epinephrine and acetylcholine were effective. The insulin secretion in response to glucose was inhibited by the metabolic inhibitors used, whereas the B-cell response in the presence of glyceraldehyde was diminished by iodoacetamide only. Like D-glucose, a variety of other sugars markedly reduced the stimulatory effect of L-arginine in glucagon release. The results show that the suppression of glucagon secretion is not specific for D-glucose and not strongly connected on a stimulated insulin secretion.