Non-stimulatory Concentration Of Glucose Research Articles

The dihydropyridine derivative, Bay K 8644, enhances insulin secretion by isolated pancreatic islets

The effects of the dihydropyridine derivative Bay K 8644 upon insulin secretion by perifused isolated mouse pancreatic islets were examined. At a non-stimulatory glucose concentration (5 mmol/l) Bay K 8644 (1 mumol/l) did not stimulate insulin release. However, the same drug concentration enhanced the insulin secretory responses to an intermediate (15 mmol/l) or high (30 mmol/l) glucose concentration by 80 or 90%, respectively. Bay K 8644 was half maximally effective at 0.1 mumol/l and maximally effective at 1 mumol/l. The results are compatible with the view that voltage-dependent calcium channels are essential for stimulus-secretion coupling in pancreatic B-cells.

The ionic, electrical, and secretory effects of endogenous cyclic adenosine monophosphate in mouse pancreatic B cells: studies with forskolin.

Forskolin, an activator of adenylate cyclase, was used to study the effects of endogenous cAMP on the stimulus-secretion coupling in mouse pancreatic B cells. Forskolin produced a rapid, dose-dependent (0.05-50 microM) increase in islet cAMP, which was not influenced by the prevailing concentration of glucose and did not require extracellular Ca2+. At a nonstimulatory concentration of glucose (3 mM), a high concentration of forskolin (20 microM) barely doubled basal insulin release, marginally decreased 86Rb+ efflux from the islets, was without effect on the B cell membrane potential, and did not affect 45Ca2+ uptake (5 min). High concentrations of endogenously formed cAMP thus failed to mimic these early steps of the B cell response to glucose and other stimuli. At a threshold concentration of the sugar (7 mM), 5 microM forskolin slightly depolarized the B cell membrane, induced electrical activity (slow waves and spikes), stimulated 45Ca2+ uptake, and triggered insulin release. At a stimulatory concentration of glucose (10 mM), forskolin potentiated insulin release; half maximal and maximal effects were observed at 1 and 20 microM, respectively. Forskolin also increased the rate of 45Ca2+ and 86Rb+ efflux from islet cells, augmented 45Ca2+ uptake, and potentiated the electrical activity triggered by glucose in B cells. Its effects on insulin release, 45Ca2+ fluxes, and electrical activity were inhibited by omission of extracellular Ca2+ and/or Ca channels blockers. At a high concentration of glucose (25 mM), forskolin augmented the amplitude and the duration of the spikes occurring in the depolarized B cell membrane, slightly increased 45Ca2+ uptake, and potentiated insulin release. At threshold or stimulatory concentrations of glucose, endogenously formed cAMP can thus induce or enhance the Ca2+-dependent events normally triggered by the sugar. It is suggested that, besides its action on intracellular Ca stores, cAMP facilitates Ca2+ influx in B cells, by modulating the gating properties of the Ca channels.

Difference in calcium dependency of insulin, glucagon and somatostatin secretion in response to glibenclamide in perfused rat pancreas.

The extracellular calcium requirements for insulin, glucagon and somatostatin release induced by 1 microgram/ml of glibenclamide have been compared in the perfused, isolated rat pancreas. In the absence of glucose, the drug evoked insulin release equally well at physiological (2.6 mmol/l) and low (0.25 mmol/l) levels of total calcium. In contrast, glibenclamide evoked somatostatin release at 2.6 but not at 0.25 mmol/l of calcium. At 2.6 mmol/l of calcium, glibenclamide evoked bimodal effects (stimulation followed by inhibition) on glucagon secretion. At 0.25 mmol/l of calcium, basal secretory rates of glucagon were elevated and a small stimulatory effect of glibenclamide was seen. Addition of 0.5 mmol/l of EGTA to media with low calcium concentrations uniformly abolished the A, B and D cell secretory responses to glibenclamide. The possible modulation of calcium dependency by a non-stimulatory concentration of glucose was tested by its addition at 3.3 mmol/l to the perfusion media. Glucose enhanced glibenclamide-induced insulin secretion, both at 0.25 and 2.6 mmol/l of calcium. However, at 0.25 mmol/l of calcium, the enhancing effect of glucose was more pronounced than at 2.6 mmol/l. At 2.6 mmol/l of calcium, glucose diminished the somatostatin and abolished the glucagon response to glibenclamide. At 0.25 mmol/l of calcium, glucose did not influence somatostatin release while the presence of the sugar diminished basal and glibenclamide-induced glucagon secretion. The present data confirm the requirement of extracellular calcium for A, B and D cell secretion, demonstrating different calcium dependencies for the cell types and indicate that this dependency can, in part, be modulated by glucose.

Effects of priming with D-glucose on insulin secretion from rat pancreatic islets: increased responsiveness to other secretagogues.

Short term exposure to a high concentration of glucose enhances the insulin response to a second stimulation with the sugar. This priming effect of glucose has been further investigated by comparing the insulin-releasing effects of various secretagogues in the perfused rat pancreas before and after previous exposure to 27.7 mM glucose. Priming markedly increased the insulin release to agents whether metabolizable (leucine) or nonmetabolizable (tolbutamide, Ba++, 3-isobutyl-lmethylxanthine, and alloxan). The priming effect was seen both in cases where secretagogues were administered in the complete absence of metabolic substrate and when tested in the simultaneous presence of a low nonstimulatory glucose concentration (3.9 mM). The insulinotropic effects of tolbutamide were examined at concentrations of 50, 200, and 400 μg/ml of the drug, with and without priming. In the absence of priming, 200 and 400 μg/ml were maximally effective. Previous exposure to 27.7 mM glucose enhanced the secretory responses to ...

Interacting Effects of Sulfonylureas and Glucose on Cyclic AMP Metabolism and Insulin Release in Pancreatic Islets of the Rat

The effects of tolbutamide and glibenclamide on the metabolism of cyclic AMP were investigated in pancreatic islets of the rat. Changes in cyclic AMP were assessed by measuring [(3)H]cyclic AMP after labeling of the islets with [2-(3)H]adenine. In the presence of a nonstimulatory concentration of glucose (3.3 mM), both sulfonylureas caused a rapid increase in islet [(3)H]cyclic AMP, which declined within 5 (tolbutamide) or 10 min (glibenclamide). In the absence of glucose, the glibenclamide effect was shortened, but the initial (1 min) response of [(3)H]-cyclic AMP was unaffected. Glucose could be substituted with d-glyceraldehyde but not pyruvate for prolongation of the glibenclamide response. The effect of glucose withdrawal on the glibenclamide response was reproduced by the addition of d-mannoheptulose to glucose containing media. The [(3)H]cyclic AMP response to glibenclamide was influenced by prior exposure of the islets to glucose, a 30-min preincubation with 27.7 mM glucose, enhancing the response to the sulfonylurea over a subsequent 5-min stimulation period. Sulfonylureas exerted their effects at low but not at high glucose concentrations, i.e., shifted the glucose dose-response curve to the left both for [(3)H]cyclic AMP accumulation and insulin release. On the other hand, increasing concentrations of the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine, progressively augmented the effects of the drugs. Omission of Ca(++) from the incubation media inhibited both the glucose and the sulfonylurea [(3)H]-cyclic AMP and insulin responses. Epinephrine (1 muM) partially inhibited the [(3)H]cyclic AMP response to both glucose and sulfonylurea, whereas insulin release was completely abolished. It is concluded that the sulfonylurea effects on islet cyclic AMP are intimately related to those of glucose. It is suggested that sulfonylureas exert a major part of their action by facilitating the effect of glucose on the beta-cell adenylate cyclase; the increased cyclic AMP level, in its turn, enhances the secretion rate of insulin.

Glucose-induced Proinsulin Biosynthesis Role of Islet Cyclic AMP

The effects of glucose on cAMP accumulation, insulin (IRI) release, and proinsulin synthesis were simultaneously measured in isolated rat islets. Proinsulin synthesis was assayed by measuring the incorporation of [3H] leudne into islet proteins reacting with Sepharose-bound anti-insulin gamma globulin. The accumulation of cAMP in islets and incubation medium was measured by the incorporation of [3H]-adenine into [3H] CAMP and purified by ion-exchange chromatography and barium sulfate precipitation. Glucose dose-response studies were performed in the absence and presence of 0.1 and 1.0 mM isobutylmethylxanthine (IBMX). The IRI and cAMP responses to glucose demonstrated similar stimulation thresholds and Km values at all IBMX concentrations. With increasing amounts of IBMX, the Vmax was increased and the Km decreased for both responses. There was parallelism between IRI secretion and accumulation of [3H] CAMP in the medium, except at nonstimulatory glucose concentrations, where, especially in the presence of 1.0 mM IBMX, a marked stimulation of cAMP accumulation was accompanied by only modest IRI release. Proinsulin biosynthesis showed a much greater sensitivity to glucose than the cAMP and IRI responses. The threshold for stimulation by glucose was around 1.4 mM, and the response already saturated at 7.0 mM. Except when 1.0 mM IBMX was present, no IRI or cAMP responses were registered in this glucose dose range. These findings indicate that IRI release and proinsulin synthesis have distinct glucose recognition mechanisms and that ghicoseinduced cAMP stimulation does not mediate the effect of the hexose on hormone synthesis in the islet cell. When islet cAMP was elevated with IBMX, a decrease in the Km of the proinsulin response to glucose was observed, while the Vmax of the reaction was only slightly modified. This suggests that the level of cAMP in the islet may modulate the sensitivity of the B-cell for the effect of glucose on insulin biosynthesis and thus provide a mechanism for the regulation of insulin biosynthesis by such factors as gut hormones and glucagon under in-vivo conditions.

Characterization of acetylcholine-induced insulin secretion in the isolated perfused dog pancreas.

In the presence of a nonstimulatory concentration of glucose, a 60-min perfusion with 50 muM acetylcholine was shown to elicit a monophasic release of insulin in the isolated dog pancreas preparation. A decline in secretory response, which may be due to desensitization of the beta-cell to acetylcholine, was noted during the latter part of the perfusion interval. The potent insulin secretory response elicited by acetylcholine during the 60-min period was abolished 0y 25 muM atropine. Inhibition of the insulinotropic action of acetylcholine was also noted with administration of the mitotic spindle inhibitor, colchicine. When compared to 20-min control perfusions, addition of 1 mM colchicine resulted in a 50% reduction in acetylcholine-induced insulin release. These results suggest that insulin secretion stimulated by acetylcholine can be considered to be due to a muscarinic action of this agent which is dependent, at least in part, upon the microtubular system of the beta-cell.

Release of growth hormone from ox pituitary slices after pronase treatment.

Proteolytic enzymes have been used both to modify properties of the cell membrane and to dissociate cells from many tissues including pituitary (4, 5, 12). Exposure of secretory tissues to pronase can alter their secretory response. Thus incubation of pancreatic islets of Langerhans in the presence of low concentrations of pronase increased the subsequent release of insulin in the presence of stimulatory and nonstimulatory glucose concentrations (7). The purpose of the present investigation was to determine whether low concentrations of pronase have the same stimulatory effect on the release of a pituitary hormone, growth hormone. Such an effect on hormone release could be of some importance in view of the development of dissociated cell systems as models for the study of the control of hormone release (4, 5).

Insulin Release from Pancreatic Islets of a Lizard Following Leucine or Arginine

The effect of leucine or arginine on insulin release from pancreatic islets of Anolis carolinensis was studied in an in vitro perifusion system. A biphasic pattern of insulin secretion resulted when 10 mM leucine or arginine was present in the incubation medium containing a non-stimulatory concentration of glucose. The present results suggest that insulin secretagogues in mammals, in addition to glucose, are also effective in elicting insulin release in reptiles.

Diazoxide effects on biphasic insulin release: "adrenergic" suppression and enhancement in the perifused rat pancreas.

An in vitro system for perifusion of rat pancreas has been used to investigate the effects of diazoxide on glucose-induced insulin release. Administration of diazoxide with a stimulating concentration of glucose produced a dose-dependent suppression of insulin release. This effect was partly reversed by phentolamine. In the presence of nonstimulatory concentrations of glucose, diazoxide plus phentolamine, but neither alone, stimulated a biphasic release of insulin similar to that observed with 1-isopropyl norepinephrine. A prior period of perifusion with a low concentration of diazoxide enhanced the primary component of subsequent glucose-stimulated insulin release, an effect inhibited by addition of either phentolamine or propranolol to the diazoxide during this "prestimulation" period. These effects are similar to those observed with epinephrine. By contrast with epinephrine however, increasing the concentration of diazoxide during the period before glucose stimulation enhanced both the primary and secondary components of subsequent glucose-induced insulin release. These data suggest that at least some of the direct effects of diazoxide on the pancreas are mediated through alpha- and beta-adrenergic receptor mechanisms.