Preparation and characterization of plasma membrane-enriched fractions from rat pancreatic islets

Methods have been developed for the isolation on a semi-micro scale of a plasma membrane-enriched fraction from rat islets of Langerhans. An important feature of these experiments is the use of 125I-labeled wheat germ agglutinin as a specific probe for plasma membrane-containing fractions. The partly purified plasma membrane fraction had a density in sucrose of about 1.10 and was enriched in the activities of 5'-nucleotidase, alkaline phosphatase, sodium-potassium, and magnesium-dependent ATPase and adenylate cyclase. It contained only very low levels of acid phosphatase, cytochrome c oxidase, insulin, and RNA. Further purification was hampered by the relatively small amounts of fresh plasma membrane material that could be obtained from 16-24 rats in each experiment. When islets were prelabeled with radioactive fucose, the plasma membrane-enriched fraction contained radioactivity at a four- to fivefold higher specific acivity than the whole islet homogenate. Sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis of plasma membrane-enriched fractions pooled from several experiments revealed a distinctive pattern of protein bands as compared with other less pure fractions. With respect to rapidity, apparent specificity, and easy reversibility of the labeling of the plasma membrane fraction, 125I-wheat germ agglutinin provides a highly useful tool for the detection of microgram quantities of plasma membrane components which should be applicable to many other systems as well.

On the possible role of thiol groups in the insulin-releasing action of mercurials, organic disulfides, alkylating agents, and sulfonylureas

The thiol activity of pancreatic islets was spectrophotometrically assayed as the formation of 6-mercaptonicotinic acid from the organic disulfide, 6,6'-dithiodinicotinic acid. Islets containing more than 90% beta-cells were microdissected from non-inbred ob/ob-mice. Comparisons of intact with homogenized islets indicated that the organic disulfide penetrates relatively slowly into the beta-cells. When tested at concentrations know to enhance insulin release, p-chloromercuribenzene-sulfonic acid almost completely blocked the thiol activity of intact islets, whereas no significant effect was observed with iodoacetamide, D-glucose, or glibenclamide. Although glibenclamide had no demonstrable effect on the thiol activity of free L-cysteine, the binding of glibenclamide to serum albumin was decreased by blocking the albumin thiols with azobenzene-2-sulfenyl bromide. The uptake of glibenclamide by pancreatic islets was inhibited by cysteine or reduced glutathione. Cysteine, as well as 6,6'-dithiodinicotinic acid, also seemed to interact negatively with glibenat organic mercurials and disulfides stimulate insulin release by blocking thiol groups in the beta-cell plasma membranes. The thiol groups involved in iodoacetamide-induced secretion may escape detection by the assay employed, or target groups other than thiols may be involved. The data on glibenclamide are compatible with, but do not unequivocally support, the notion that thiol groups may play a role in sulfonylurea-induced insulin release.

Stimulation of insulin release by thiols

The effects of thiol compounds on insulin release were studied in microdissected pancreatic islets of non-inbred ob/ob micemin control experiments the reactivity of thiols against 6,6'-dithiodinicotinic acid and the degradation of mouse insulin were measured. At a concentration of 0.1 mM, 1-thio-D-glucose or reduced glutathione potentiated the insulin-releasing action of 10 mM D-glucose without affecting glucose oxidation. When tested at a concentration equivalent to about 0.1 mM reactive thiol, dextran-linked L-cysteine also potentiated the glucose-induced insulin secretion. In microperifusion experiments the insulin-releasing action of 1-thio-D-glucose was found to exhibit a rapid onset followed by a decline of the secretory rate to values lower than those observed with 10 mMD-glucose alone. No thiol stimulated insulin release in the absence of glucose. It is suggested that thiol compounds stimulate insulin release by splitting membrane disulphides in the beta-cells.

Effects of insulin secretagogues on phospholipid metabolism in pancreatic beta-cells

The effect of insulin secretagogues on the incorporation of [32-P] orthophosphate into phospholipids was studied in microdissected islets from obese-hyperglycemic mice. Increased 32-P-labelling was observed after incubation for 60 min with 10 mM L-leucine, 10 mM L-arginine or 20 mM D-glucose. Most of the label occurred in the phosphatidyl inositol fraction. The effect of L-leucine was additive to that induced by D-glucose while the effect of L-arginine was not. Glibenclamide (0.05 mM) was ineffective whether or not D-glucose was present. The results suggest that there is no direct correlation between the releasing actions of insulin secretagogues and changes in the metabolism of certain phospholipids and that phospholipid metabolism may be stimulated through more than one mechanism.

Glucagon and insulin release from the allografted canine pancreas

Six previously pancreatectomized dogs were transplanted with duct-ligated, pancreatic allografts. Glucagon and insulin levels in the venous outflow from the graft and in the systemic circulation were determined during the first 60 minutes after transplantation. Two of the dogs were subjected to L-arginine stimulation 5 days after transplantation and the glucagon levels in the venous outflow from the graft were determined. The glucagon and insulin concentrations increased rapidly after transplantation, describing a biphasic curve. In response to L-arginine stimulation, an increased glucagon release was observed.

The pancreatic beta-cell recognition of insulin secretagogues: does cyclic AMP mediate the effect of glucose?

Insulin release and the content of cAMP were studied in microdissected pancreatic islets of noninbred ob/ob (obese) mice. In the absence of 3-isobutyl-1-methylxanthine, a phosphodiesterase inhibitor, 20 mM glucose had no effect on cAMP save a very small initial rise detectable by a freeze-stop perifusion technique only. However, combined with this methylxanthine, 20 mM glucose produced significant increases of cAMP both in perifused islets and in islets conventionally incubated in closed vials. Glucose shared this capacity to raise the cAMP level with D-glyceraldehyde and 1,3-dihydroxyacetone. Isobutylmethylxanthine (0.05-1.0 mM) or 5 mug/ml of cholera toxin, an activator of adenylate cyclase, also increased the islet cAMP level; the effects of the methylxanthine, whether or not combined with cholera toxin, were potentiated by glucose. Isobutylmethylxanthine (0.05-1.0 mM) or 5 mug/ml of cholera toxin potentiated insulin release in response to 20 mM glucose. However, only 0.5-1.0 mM isobutylmethylxanthine stimulated insulin release in the presence of 3 mM glucose, whereas 0.05-0.1 mM isobutylmethylxanthine or 5 mug/ml of cholera toxin had no effect on secretion at the low glucose concentration. These discrepancies between cAMP-promoting and insulin-releasing activities suggest that glucose does not initiate insulin release by activating the beta-cell adenylate cyclase. By being metabolized in the beta-cells, glucose may both create a release-initiating signal not identical with cAMP and enhance cAMP formation, leading to potentiation of the effect of the initiator signal.

Influence of the mutation "diabetes" on insulin release and islet morphology in mice of different genetic backgrounds

Mice, 7-8-mo old, of the C57BL/KsJ-db strain and homozygotic for the mutant gene db, exhibited marked hyperglycemia and moderately elevated serum insulin levels. Light and electron microscopy provided evidence of a slightly decreased proportion of beta cells in the pancreatic islets, irregular islet architecture with intraislet ducts, and degenerative as well as hypertrophic changes in the individual beta cells. As a rule, islets microdissected from these mice did not release insulin in response to glucose, theophylline, iodoacetamide, or chloromercuribenzene-p-sulphonic acid. The absence of secretory responses was not simply due to lack of insulin. Although the islet content of insulin was decreased in C57BL/KsJ-db/db mice, the remaining amount was severalfold larger than that released from stimulated islets of normal controls. Another mutation, db(2J), an allele of db with identical phenotypic expressions in the C57BL/KsJ strain, was studied on the genetic background C57BL/6J. In contrast to the severely diabetic C57BL/KsJ-db/db animals, the C57BL/6J-db(2J)/db(2J) mice were characterized by highly elevated serum insulin levels and only moderate hyperglycemia. Their endocrine pancreas was enlarged and showed an increased proportion of beta cells. Like the islets of normal mice, those of C57BL/6J-db(2J)/db(2J) mice responded to glucose and chloromercuribenzene-p-sulphonic acid, the glucose-induced responses being potentiated by theophylline or iodoacetamide. C57BL/KsJ-db/db mice should provide a valuable model for studying defects in insulin secretion in relation to diabetes mellitus. Mice of the C57BL/6J strain offer a control material that may help to elucidate the dependence of the insulin secretory defect on the background genome.

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.