The stimulus-secretion coupling of glucose-induced insulin release. XL. Effect of K+ deprivation upon insulin release by the perfused rat pancreas

The effects of cesium chloride on insulin release, ionic fluxes and membrane potential in pancreatic B-cells

Cs+ decreases K+ permeability in nerve and muscle cells. Its effects on the pancreatic B-cell function were studied with mouse islets. In the presence of 3 mM glucose, Cs+ substitution for K+ steadily inhibited 86Rb+ efflux and hyperpolarized the B-cell membrane. Addition of Cs+ to a K+-medium also inhibited 86Rb+ efflux, but depolarized the B-cell membrane. None of these changes altered insulin release. Substitution of Cs+ for K+ in a medium containing 10 mM glucose caused a Ca2+-dependent stimulation of insulin release and 45Ca2+ efflux, produced an initial fall and a secondary rise in 86Rb+ efflux and augmented the electrical activity in B-cells. Reintroduction of K+ to the medium was followed by a marked and transient inhibition of insulin release, that was blocked by ouabain and accompanied by an inhibition of 45Ca2+ and 86Rb+ efflux and by a hyperpolarization of the B-cell membrane. Addition of Cs+ to a K+ medium containing 10 mM glucose stimulated insulin release, 45Ca2+ efflux and 86Rb+ efflux. It also increased the electrical activity in B-cells. In the absence of Ca2+, however, Cs+ addition decreased the rate of 86Rb+ efflux. The effects of Cs+ on the B-cell function may be explained by its ability to decrease K+ permeability of the plasma membrane, by its inability to activate the sodium pump, and by a third unidentified effect likely brought about by the accumulation of intracellular Cs+.

The electrogenic sodium-potassium pump of mouse pancreatic B-cells

1. The contribution of the sodium pump to the membrane potential of mouse pancreatic B-cells was studied with micro-electrodes.2. In 0 or 3 mM-glucose, ouabain rapidly (within 2 min) depolarized the B-cell membrane by an average of 7 mV, whereas K omission hyperpolarized it markedly.3. In 6 or 7 mM-glucose, ouabain still produced depolarization, but K omission had no consistent effect. Both induced electrical activity in certain cells.4. In 10 mM-glucose, withdrawal of ouabain or K re-introduction caused a transient hyperpolarization with suppression of electrical activity. Duration and amplitude of the hyperpolarization increased with the time of pump blockade and with the concentration of ouabain.5. The hyperpolarization following K re-admission was abolished by ouabain and that following ouabain withdrawal was prevented by K omission. Re-admission of various K concentrations showed that the hyperpolarization was not due to depletion of K just outside of the membrane.6. In 10 mM-glucose, the membrane potential of B-cells exhibited repetitive slow waves with bursts of spikes on the plateau. These electrical events were modified by ouabain in a dose-dependent manner. The frequency of the slow waves augmented markedly because of an increase in the slope of the pre-potential and a shortening of the intervals; the slope of their repolarization phase decreased, but their duration was not changed.7. Omission of K increased the slope of the pre-potential and the frequency of the slow waves. It also accelerated their repolarization phase and reduced their duration, likely because of the increase in driving force for K efflux. Increasing K concentration to 8 mM slowed the repolarization phase and lengthened the slow waves without changing their frequency.8. Even when K permeability of the B-cell membrane was increased by high extracellular Ca, ouabain and K omission augmented the frequency of the slow waves.9. In 0 or 10 mM-glucose, ouabain increased (86)Rb(+) efflux from perifused islets, whereas K omission decreased it. In 10 mM-glucose, a marked decrease in (86)Rb(+) efflux accompanied ouabain withdrawal and K re-introduction. The hyperpolarization is thus not due to an increase in K permeability.10. It is concluded that, in pancreatic B-cells, the sodium pump is truly electrogenic, contributes to the resting potential and modulates the slow waves of membrane potential induced by glucose. Rapid changes in insulin release occurring upon inhibition or activation of the sodium pump may thus be due to the changes in B-cell membrane potential.

Stimulus-secretion coupling of amino acid-induced insulin release VII. The B-cell memory for L-glutamine

In the absence of another exogenous nutrient, L-glutamine does not stimulate insulin release from rat pancreatic islets or isolated perfused pancreases. L-glutamine, however, augments insulin release evoked by L-leucine. These two amino acids could interact by providing both the substrate (L-glutamate) and an activator (L-leucine) for the reaction catalyzed by glutamate dehydrogenase. Under suitable experimental conditions, as little as 0.5 mM L-glutamine is sufficient to enhance leucine-stimulated insulin release. When the pancreases or islets are first exposed to L-glutamine and then stimulated with L-leucine, the rate of secretion is much higher than that evoked by L-leucine in tissue not first exposed to L-glutamine. The memory of a prior exposure to L-glutamine persists for at least 25 min after removal of the latter amino acid from the extracellular fluid. This memory phenomenon is not dependent on the presence of Ca2+ in the extracellular fluid during the first exposure to L-glutamine, but is suppressed when such a prior exposure is performed in the absence of extracellular K+. The memory phenomenon could be due, in part at least, to inhibition by L-glutamine of K+ conductance in the B-cell plasma membrane. Moreover, the amount of L-glutamate which accumulates in islets exposed to L-glutamine is sufficient to maintain, for a much longer period than with other nutrient secretagogues, a sustained increase in catabolic fluxes after removal of the amino acid from the extracellular fluid.