Biomedical subjects

P C Mathias

Publications and source records attributed to P C Mathias.

18 recordsLinked to original sources

PubMed1989

Stimulus-secretion coupling of arginine-induced insulin release. Functional response of islets to L-arginine and L-ornithine.

L-Arginine and L-ornithine stimulate insulin release from pancreatic islets exposed to D-glucose. This coincides with an increased outflow of 86Rb and 45Ca from prelabelled islets and an increased net uptake of 45Ca by the islets. In the presence of D-glucose, L-lysine stimulates insulin secretion to the same extent as L-arginine or L-ornithine, but the hormonal release is not further enhanced by combinations of these cationic amino acids. L-Arginine or L-ornithine failed to enhance insulin release evoked by either L-leucine or 2-ketoisocaproate. The inhibitor of ornithine decarboxylase D,L-alpha-difluoromethyl ornithine failed to affect the metabolism and insulinotropic action of D-glucose in pancreatic islets, and only caused a partial inhibition of the secretory response to either L-arginine or L-ornithine. The latter amino acids inhibited modestly but significantly D-glucose utilization and oxidation by pancreatic islets. These and complementary findings suggest that the secretory response to L-arginine and L-ornithine is not attributable to any major change in the overall oxidative catabolism of nutrients, but involves mainly a biophysical component, such as the depolarization of the plasma membrane by these cationic amino acids.

Animals↗

PubMed1986

The coupling of metabolic to secretory events in pancreatic islets: inhibition by 2-cyclohexene-1-one of the secretory response to cyclic AMP and cytochalasin B.

In rat pancreatic islets perifused in the presence of 2-cyclohexene-1-one (CHX; 1.0 mM), the secretory response to either D-glucose or 2-ketoisocaproate, but not that evoked by the association of L-leucine and L-glutamine, was severely decreased. This coincided with a decreased stimulation of [45Ca] efflux from prelabelled islets, whereas the inhibitory action of D-glucose or 2-ketoisocaproate upon both [86Rb] and [45Ca] efflux appeared little or not affected. In the presence of D-glucose, the islets exposed to CHX were virtually unresponsive to either forskolin, theophylline or cytochalasin B. A severe decrease in the secretory response to forskolin was also observed in CHX-treated islets exposed to L-leucine and L-glutamine. Except for a somewhat lower sensitivity to NaF, no major change in adenylate cyclase activity or cyclic AMP production was observed in CHX-treated islets. The activity of protein kinase A was decreased in such islets but its responsiveness to cyclic AMP appeared unaltered. Transglutaminase activity was severely decreased in homogenates derived from CHX-treated islets. These findings suggest that CHX, possibly by lowering the GSH content of islet cells, impairs the functional capacity of the effector system for insulin release, in addition to and independently of any effect that it may exert upon nutrient catabolism and cationic fluxes in the islet cells.

Adenylyl Cyclases↗

PubMed1986

Fasting-induced dissociation of cationic and secretory events in pancreatic islets.

In pancreatic islets removed from 48 h-fasted rats, as distinct from fed animals, the release of insulin evoked by D-glucose is more severely impaired than that evoked by 2-ketoisocaproate. This decreased secretory response to D-glucose contrasts with an unimpaired cationic response to the sugar in terms of the glucose-induced decrease in both 86Rb and 45Ca outflow from pre-labelled islets. Likewise, fasting only causes a modest decrease of the secondary rise in 45Ca outflow evoked by D-glucose in islets perifused at normal Ca2+ concentration. The latter decrease appears more marked, however, if the cationic response to glucose is expressed relative to that evoked by 2-ketoisocaproate in islets removed from rats in the same nutritional state. It is concluded that, in the process of nutrient-stimulated insulin release, neither the decrease in K+ conductance (inhibition of 86Rb outflow) nor the sequestration of Ca2+ by intracellular organelles and/or direct inhibition of Ca2+ outward transport (decrease in 45Ca outflow) represent the sole determinant(s) of the subsequent gating of Ca2+ channels (secondary rise in 45Ca efflux).

Animals↗

PubMed1985

Cholinergic stimulation of ion fluxes in pancreatic islets.

Cholinergic agents are known to stimulate the hydrolysis of polyphosphoinositides in pancreatic islets. The effect of carbamylcholine upon ion fluxes in the islet cells was investigated. Carbamylcholine provoked a rapid but poorly sustained increase in 45Ca and 86Rb outflow from perifused islets. Such a cationic response was observed at different glucose concentrations (zero to 16.7 mM), at three concentrations of carbamylcholine (10 microM, 100 microM and 1.0 mM), and in the absence or presence of extracellular Ca2+. It coincided with a biphasic stimulation of insulin release, both the cationic and secretory responses being abolished in the presence of atropine (10 microM). At variance with nutrient secretagogues, carbamylcholine failed to affect the net production of cyclic AMP and caused a transient decrease in 32P outflow from islets prelabelled with [32P]phosphate. It is proposed that cholinergic agents mobilize Ca2+ from intracellular sites, possibly through generation of inositol, 1,4,5-triphosphate from phosphatidylinositol 4,5-bisphosphate. The intracellular redistribution of Ca2+ does not appear sufficient, however, to account fully for the secretory response, which may also involve activation of protein kinase C by diacylglycerol.

Animals↗

PubMed1985

Stimulation of protein kinase C and insulin release by 1-oleoyl-2-acetyl-glycerol.

The membrane-accessible diacylglycerol 1-oleoyl-2-acetyl-sn-glycerol (OAG, 5-500 microM) caused a dose-related activation of protein kinase C in rat islet homogenates. In islet cell membranes exposed to [gamma-32P]ATP, OAG (100 microM) stimulated the net production of labelled phosphatidate and inhibited that of labelled phosphatidylinositol 4-phosphate. In intact islets exposed to 5.6 mM D-glucose, OAG (100 microM) decreased the outflow of 86Rb, increased that of 45Ca and caused a rapid stimulation of insulin release. The secretory response to OAG was dose-related in the 50-500 microM range, being most marked, in relative terms, at a glucose concentration close to the threshold value for stimulation of insulin release by this hexose. It was decreased but not abolished in the absence of CaCl2 and presence of EGTA. At variance with tumor-promoting phorbol esters, OAG failed to potentiate insulin release stimulated by a hypoglycaemic sulphonylurea. Although these findings support the view that activation of protein kinase C by diacylglycerol represents an efficient modality for stimulation of insulin release, they suggest that the effect of OAG upon islet function may not be solely attributable to such an activation.

Animals↗

PubMed1985

The coupling of metabolic to secretory events in pancreatic islets. The possible role of glutathione reductase.

The participation of glutathione reductase in the process of nutrient-stimulated insulin release was investigated in rat pancreatic islets exposed to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). BCNU caused a time-and dose-related, irreversible inhibition of glutathione reductase activity. This coincided with a fall in both GSH/GSSG ratio and the thiol content of the islets. Pretreatment of the islets with BCNU inhibited the oxidation of glucose and its stimulant action upon both 45Ca net uptake and insulin release. Although BCNU (up to 0.5 mM) failed to affect the oxidation of L-leucine and L-glutamine, it also caused a dose-related inhibition of insulin release evoked by the combination of these two amino acids. The latter inhibition was apparently not fully accounted for by the modest to negligible effects of BCNU upon 45Ca uptake, 45Ca efflux, 86Rb efflux and cyclic AMP production. Since BCNU failed to inhibit insulin release evoked by the association of Ba2+ and theophylline, these results support the view that glutathione reductase participates in the coupling of metabolic to secretory events in the process of nutrient-stimulated insulin release. However, the precise modality of such a participation, for example the control of intracellular Ca2+ distribution, remains to be elucidated.

Animals↗

PubMed1985

Stimulation by glucose and carbamylcholine of phospholipase C in pancreatic islets.

Phosphoinositide hydrolysis in intact pancreatic islet cells was investigated in an indirect but dynamic manner by monitoring the efflux of radioactivity from islets prelabelled with [3H]inositol. A rise in glucose concentration provoked a rapid, modest but sustained increase in effluent radioactivity, this phenomenon being abolished in the absence of extracellular Ca2+ or presence of verapamil. The release of [3H]inositol was also stimulated at high extracellular K+ concentration, but not by gliclazide. Whether in the presence or absence of glucose, carbamylcholine provoked a marked increase in effluent radioactivity. The response to the cholinergic agent was decreased in the presence of verapamil or absence of extracellular Ca2+ and abolished in the presence of atropine or LiCl. These results suggest that an increase in cytosolic Ca activity, as caused by glucose or membrane depolarization, may cause activation of phospholipase C. In response to cholinergic agents, however, the enzymic activation, although modulated by Ca2+ availability, may result directly from the occupation of muscarinic receptors.

Animals↗

PubMed1985

Stimulation of insulin release by an organic calcium agonist.

The calcium-agonist 4-[2-(difluoromethoxy)phenyl]-1,4,5,7-tetrahydro-2-methyl-5-oxo-fu ro[ 3,4-b]pyridine-3-carboxylic acid ethylester provoked, in the 1.0-100 mumol/l range, a dose-related increase of glucose-stimulated insulin release by rat pancreatic islets. A fixed concentration of the drug (50 mumol/l) caused a shift to the left of the sigmoidal curve relating insulin output to glucose concentration. The drug failed to affect insulin release evoked, in the absence of Ca2+, by the combination of Ba2+ and theophylline. The enhancing action of the calcium-agonist upon insulin release was rapid and sustained, and coincided with stimulation of both 45Ca net uptake and 45Ca efflux, the latter phenomenon being abolished in the absence of extracellular Ca2+. It is concluded that the gating of Ca-channels, as presumably provoked by the calcium-agonist, simulates the stimulant action of glucose upon both Ca influx into and insulin release from the pancreatic islets.

Animals↗

PubMed1985

Role of transglutaminase in insulin release. Study with glycine and sarcosine methylesters.

The Ca2+-responsive enzyme transglutaminase, which catalyzes the cross-bridging of proteins, is present in pancreatic islet cells, but its participation in the process of insulin release remains to be documented. Glycine methylester (1.0-10.0 mM) inhibited, in a dose-related manner, transglutaminase activity in rat pancreatic islet homogenates, decreased [14C]methylamine incorporation into endogenous proteins of intact islets, and caused a rapid and reversible inhibition of insulin release evoked by D-glucose, while failing to affect D-[U-14C]glucose oxidation. Glycine methylester also inhibited insulin release induced by other nutrient or nonnutrient secretagogues. Sarcosine methylester failed to affect transglutaminase activity, [14C]methylamine incorporation, and insulin release. Both methylesters mobilized 45Ca from prelabeled intact islets, from membranes of islet cells, liver or brain, and from artificial lipid multilayers, this Ca mobilization being apparently unrelated to changes in transglutaminase activity. It is proposed that, in the pancreatic B cell, transglutaminase participates in the machinery controlling the access of secretory granules to the exocytotic sites.

Acyltransferases↗

PubMed1985

Suppression by 2-ketoisocaproate of the insulinotropic action of hypoglycemic sulfonylureas.

Tolbutamide (370 microM), gliclazide (62 microM) and glibenclamide (1 microM) failed to enhance insulin release evoked by 2-ketoisocaproate (10 mM) in rat pancreatic islets. Gliclazide also little affected insulin release evoked by 2-ketoisocaproate, whereas the hypoglycemic sulfonylurea stimulated insulin release from islets incubated in the absence of exogenous nutrient or presence of either L-glutamine, D-glucose, D-mannose, D-glyceraldehyde, L-leucine or the combination of D-glucose and pyruvate. In the presence of 2-ketoisocaproate, a modest secretory response to gliclazide was observed when the concentration of the 2-keto acid was decreased to 5 mM, or in perifused islets in which case gliclazide caused a transient increase in both 45Ca outflow and insulin output from prelabelled islets exposed to 10 mM 2-ketoisocaproate. Gliclazide and other hypoglycemic sulfonylureas failed to affect the oxidation of 2-[U-14 c]-ketoisocaproate and the latter 2-keto acid failed to affect the ionophoretic action of gliclazide in an artificial membrane model. Gliclazide increased 45Ca net uptake by islets exposed to 2-ketoisocaproate, but this effect of the sulfonylurea was much less marked than that seen in the presence of D-glucose used at a concentration of equal insulinotropic efficiency. These findings indicate that 2-ketoisocaproate impairs the cationic and secretory responses of islets to hypoglycemic sulfonylureas. It is proposed that such an impairment is compatible with the view that a remodelling of ionic fluxes in the islet cells represents a primary event in the process of sulfonylurea-stimulated insulin release.

Animals↗

PubMed1985

Stimulation by glucose and carbamylcholine of phospholipase A2 in pancreatic islets.

Glucose, in high concentrations (16.7-27.8 mM), caused a modest increase in effluent radioactivity from rat pancreatic islets prelabelled with [U-14C] arachidonate. The response to glucose was abolished in the absence of extracellular Ca2+. At a low glucose concentration (5.6 mM), carbamylcholine (1.0 mM) provoked a more marked and sustained increase in effluent radioactivity. The response to the cholinergic agent was abolished in the presence of atropine or absence of extracellular Ca2+. These results suggest that carbamylcholine and, to a lesser extent, glucose cause a Ca2+-dependent activation of phospholipase A2 in intact islet cells.

Animals↗

PubMed1984

Gating and blocking of calcium channels by dihydropyridines in the pancreatic B-cell.

The organic calcium-antagonist nifedipine inhibits glucose-stimulated 45Ca net uptake and insulin release by rat pancreatic islets. However, the chemically related dihydropyridine derivative BAY K 8644 (methyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyridine-5- carboxylate) enhances 45Ca net uptake and insulin release and protects the B-cell against the inhibitory action of nifedipine. It is proposed that a regulatory site exists in or near the calcium channels, in the B-cell plasma membrane, and that occupation of this site by selected dihydropyridines may either facilitate or inhibit Ca2+ influx into the B-cell.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗

PubMed1984

Inhibition by mepacrine and p-bromophenacylbromide of phosphoinositide hydrolysis, glucose oxidation, calcium uptake and insulin release in rat pancreatic islets.

Mepacrine and p-bromophenacylbromide were both found to impair 3H-inositol phosphate production in response to both nutrient and hormone-neurotransmitter stimuli in islets prelabelled with 3H-inositol. Both drugs also inhibited net 45Ca uptake in response to glucose or glibenclamide and considerably modified the patterns of 45Ca and 86Rb efflux from perifused islets under both basal and glucose-stimulated conditions. In addition, the oxidation of [U-14C] glucose in islets was impaired by either mepacrine or p-bromophenacylbromide. These inhibitory effects were found to be concentration-related for both mepacrine (0.01-1.0 mM) and p-bromophenacylbromide (0.03-0.3 mM) and were accompanied, in general, by a similar degree of inhibition of insulin secretion. These results suggest that both mepacrine and p-bromophenacylbromide can inhibit phospholipase C activity in intact islets, but also impair 45Ca and 86Rb fluxes and oxidation of nutrients. The diversity of these drugs' inhibitory actions makes them unsuitable tools for examining the role of specific cellular processes in the regulation of islet function.

Acetophenones↗

PubMed1984

Methylamines and islet function: cationic aspects.

Methylamine (2 to 10 mM) caused a dose-related inhibition of insulin release evoked in rat pancreatic islets by nutrient or non nutrient secretagogues. Trimethylamine exerted comparable effects upon insulin release. Methylamine (2 mM) inhibited insulin secretion but failed to affect 45Ca uptake and efflux in response to a rise in extracellular K+ concentration, suggesting that methylamine acts, to a certain extent at least, at a distal site in the secretory sequence. Methylamine, however, also exerted untoward ionic effects. First, methylamine (2 to 10 mM) apparently caused a dose-related increase in cellular pH. Second, methylamine (2mM) augmented 86Rb outflow from islets perifused either in the absence or presence of glucose or gliclazide, and inhibited Ca2+ inflow (as judged from the net uptake or efflux of 45Ca) in islets stimulated by D-glucose, L-leucine or 2-ketoisocaproate. This multiplicity of ionic and other effects may account for the fact that, in the presence of distinct secretagogues, the secretory process appeared more or less sensitive towards methylamine, depending on the relative importance of changes in cellular pH, K+ permeability and intracellular Ca2+ distribution as determinants of the secretory response.

Acyltransferases↗

PubMed1984

Inhibition by corticosterone of calcium inflow and insulin release in rat pancreatic islets.

Corticosterone (0.6 mumol/l) inhibited both 45Ca outflow and insulin release evoked by glucose, the combination of leucine and glutamine, 2-ketoisocaproate, gliclazide or the association of gliclazide and a tumour-promoting phorbol ester in rat pancreatic islets perifused at normal extracellular Ca2+ concentration (1.0 mmol/l). In all cases, the inhibitory action of corticosterone reached statistical significance within 10-22 min of exposure to this steroid and failed to be rapidly reversible. Corticosterone failed to affect basal 45Ca outflow and insulin release. The steroid also failed to affect the inhibitory action of glucose upon 45Ca outflow, as judged from either the glucose-induced early fall in effluent radioactivity from islets maintained at normal extracellular Ca2+ concentration or the steady-state values for 45Ca outflow from glucose-stimulated but Ca2+-deprived islets. Corticosterone caused a modest increase in 86Rb outflow from islets perifused in the presence of glucose (16.7 mmol/l). It is concluded that corticosterone impairs Ca2+ inflow into the islet cells and, by doing so, causes a progressive inhibition of insulin release. The pancreatic B cell might thus serve as a further model for the study of the rapid biological response to steroids, as presumably mediated by alteration in the biophysical properties of the plasma membrane.

Animals↗

PubMed1984

Inhibition of transglutaminase by hypoglycaemic sulphonylureas in pancreatic islets and its possible relevance to insulin release.

Pancreatic islet homogenates display calcium-sensitive transglutaminase activity, but the role of this enzyme in the process of insulin release remains to be elucidated. Tolbutamide, gliclazide, glisoxepide, glipizide and glibenclamide inhibited transglutaminase activity in islet homogenates. When the cationic response of islet cells to hypoglycaemic sulphonylureas was suppressed by exposing intact islets to quinine, tolbutamide, gliclazide and glibenclamide caused a rapid, sustained, reversible and dose-related inhibition of insulin release. The relative efficiency of distinct hypoglycaemic sulphonylureas as inhibitor of transglutaminase activity was in mirror image of their relative potency as insulin secretagogue. However, the dose-action relationship for the inhibitory action of these agents upon insulin release from quinine-treated islets was similar in response to either tolbutamide, gliclazide or glibenclamide. These results indicate that hypoglycaemic sulphonylureas may exert an inhibitory action upon insulin release, but suggest that such an effect is not tightly related to inhibition of transglutaminase.

Acyltransferases↗

PubMed1984

Influence of extracellular pH upon the ionic and secretory response to gliclazide in pancreatic islets.

The influence of extracellular pH upon the ionic and secretory response to gliclazide was examined in perifused rat islets. Gliclazide usually decreased 86Rb outflow from the islets except in the presence of glucose (7.0 mM) and Ca2+ (1.0 mM) and at low (7.0) or normal (7.4) pH, in which cases it caused a rapid increase in 86Rb output. Gliclazide failed to affect 45Ca outflow in the absence of extracellular Ca2+, whatever the extracellular pH. However, in the presence of Ca2+ and glucose (7.0 mM), gliclazide enhanced 45Ca outflow and insulin release. Under the latter experimental conditions, the gliclazide-induced increment in both 45Ca and insulin output was progressively increased as the pH was raised from 7.0 to 7.4 and 7.8, despite the fact that glucose-induced insulin release was progressively decreased over the same pH range. The gliclazide-induced facilitation of Ca2+ inflow into the islet cells and the subsequent stimulation of insulin release, whatever their precise molecular determinants, thus displayed the same dependency towards extracellular pH as that characterizing the ionophoretic action of the drug. The influence of extracellular pH upon the cationic and secretory response to gliclazide is compatible, therefore, with the view that the insulinotropic action of hypoglycemic sulfonylureas is somehow related to their ionophoretic capacity.

Animals↗

PubMed1983

Comparison of the cationic and secretory response of pancreatic islets to gliclazide and/or potassium.

The concept that hypoglycemic sulfonylureas stimulate Ca2+ inflow and insulin release in the pancreatic B-cell by causing the gating of voltage-sensitive Ca2+ channels was tested by comparing the cationic and secretory response of perifused pancreatic islets to gliclazide and/or an increase in extracellular K+ concentration. In the presence of glucose (2.8 mM), both procedures resulted in an immediate and sustained stimulation of 45Ca and insulin release from prelabelled islets. The capacity of gliclazide to stimulate 45Ca and insulin release persisted, to a limited extent, in islets exposed to 20 mM K+, but was abolished in islets exposed to 50 mM K+. At the latter concentration, however, K+ was still able to augment 45Ca outflow and insulin secretion from islets first exposed to gliclazide. These findings support the view that the depolarization of the B-cell membrane plays a critical role in the stimulus-secretion coupling of sulfonylurea-induced insulin release.

Animals↗