Metabolic control of potassium permeability in pancreatic islet cells

Stimulus-secretion coupling of glucose-induced insulin release. Timing of early metabolic, ionic, and secretory events

The timing of the early metabolic, ionic, and secretory responses to glucose in rat pancreatic islets was monitored by measuring, at 12 sec intervals, the concentrations of glucose, lactic, and pyruvic acids, 32P, 86Rb, 45ca, and insulin in the effluent of perifused prelabeled islets. The increase in glucose concentration from zero to 16.7 mM was complete within 133 sec. The output of organic acids increased after 24 sec of exposure to glucose and, in the case of lactic acid, fell slightly after the initial elevation. The phosphate flush was initiated only after 96 sec of exposure to glucose, whereas the decreases in 86Rb and 45Ca outflow were both detectable within 72 sec of stimulation. The secondary rise in 45Ca efflux was first seen after 157 sec of stimulation and its time course was not vastly different from that of insulin release. These data indicate that, in the secretory sequence, metabolic changes precede both the remodelling of ionic fluxes and the stimulation of insulin release. The results are compatible with the view that the secondary rise in 45Ca outflow is attributable, in part at least, to the glucose-induced decrease in K conductance (but not to the increase in phosphate outflow), with resulting membrane depolarization and gating of voltage-dependent Ca channels.

Effects of amino acids on membrane potential and 86Rb+ fluxes in pancreatic beta-cells

The membrane potential of beta-cells was studied with microelectrodes in mouse islets and their potassium permeability was evaluated by measuring 86Rb+ fluxes in rat islets. In the absence of glucose, L-leucine, its metabolite ketoisocaproate, and its nonmetabolized analogue 2-aminonorbornane-2-carboxylic acid (BCH) depolarized beta-cells and triggered bursts of electrical activity like glucose. The effect of leucine was weak, but was potentiated by a low concentration of glucose or by theophylline; the effect of ketoisocaproate was stronger and faster than that of an equimolar concentration of glucose. Arginine alone produced only a fast depolarization of beta-cells, insufficient to trigger electrical activity. Leucine and arginine potentiated the activity induced by glucose. In a glucose-free medium, alanine only slightly depolarized beta-cells, whereas isoleucine and phenylalanine had no effect. Leucine, ketoisocaproate, and BCH reversibly decreased 86Rb+ efflux from islets perifused in the absence of glucose and increased 86Rb+ uptake. By contrast, both in the absence or presence of glucose, arginine increased 86Rb+ efflux and decreased 86Rb+ uptake. It is proposed that leucine, ketoisocaproate, and BCH, as glucose, depolarize beta-cells by decreasing their potassium permeability, whereas arginine acts differently. The appearance of bursts of electrical activity with secretagogues unrelated to glucose suggests that they reflect an intrinsic property of the beta-cell membrane.

The effect of pH on 86Rubidium efflux from pancreatic islet cells

The influence of pH on the K+ permeability of pancreatic islet cells was investigated by measuring 86Rb+ fluxes in isolated rat islets perifused or incubated in the absence of glucose. Acidification of the medium (to pH 6.5), by decreasing the concentration of HCO-3 or by increasing pCO2 at constant HCO-3 reversibly reduced the rate of 86Rb+ efflux from perifused islets. Alkalinization of the medium (to pH 7.8), by decreasing the pCO2, reversibly increased 86Rb+ efflux. Similar changes were recorded upon alteration of the pH in a bicarbonate-free, Hepes-buffered medium with or without calcium. Alteration of the CO2 level at constant external pH - in order to modify internal pH - produced only a small and transient increase in efflux rate when CO2 was lowered, and a decrease in efflux rate when CO2 was raised. NH4Cl reversibly augmented 86Rb+ efflux in the presence and in the absence of HCO-3. At low pH (6.5), 86Rb+ uptake by islet cells was reduced after 10 min (25%) and 30 min (11%), but not after 60 min, of incubation; it was not significantly affected by high pH (7.8). Calcium uptake and insulin release were reduced at low pH and increased at high pH. These results show that the K+ permeability of islet cells is affected by changes in extracellular but not in intracellular pH. They suggest that the endogenous production of protons that accompanies glucose stimulation of islet cells is not the mediator of the decrease in K+ permeability induced by the sugar.

The stimulus-secretion coupling of glucose-induced insulin release. XLVI. Physiological role of L-glutamine as a fuel for pancreatic islets

Exogenous L-glutamine is actively metabolized in rat pancreatic islets. The rate of L-glutamine deamidation largely exceeds the rate of glutamate conversion to gamma-aminobutyrate and alpha-ketoglutarate. The latter conversion occurs in part by oxidative deamination, and in part by transamination reactions coupled with the conversion of 2-keto acids (pyruvate, oxaloacetate), themselves derived from the metabolism of glutamine, to their corresponding amino acids (alanine, aspartate). An important fraction of malate formed from alpha-ketoglutarate leaves the Krebs cycle and is converted to pyruvate, the process being apparently associated with the induction of a more reduced state in cytosolic redox couples. L-Glutamine abolishes the oxidation of endogenous nutrients is documented by the fact that the glutamine-induced increase in O2 consumption is much lower than expected from the rate of 14CO2 output from islets exposed to L-[U-14C]glutamine, L-Glutamine, although decreasing K+ conductance, fails to stimulate insulin release both in the absence and presence of D-glucose. It is proposed that L-glutamine represents a major fuel for pancreatic islets under physiological conditions.