Insulinotropic effect of high potassium concentration beyond plasma membrane depolarization

Background and aims: In experiments designed to dissect the signalling pathways in beta cells, the depolarizing effect of glucose metabolism is often replaced by a strong K⁺ depolarization. Recent observations raised doubt that high K⁺ is an appropriate experimental substitute for the physiologically induced depolarization.

Methods: Using mouse islets insulin secretion was measured by batch perifusion and ELISA and the free cytosolic Ca²⁺ concentration ([Ca²⁺]_i) by the Fura technique. Primary mouse beta cells were used for electrophysiological experiments.

Results: At basal glucose 40 mM K⁺ induced a massive monophasic response, whereas 15 mM K⁺ had only a minimal insulinotropic effect, although the increase in [Ca²⁺]_i was not inferior to that by 20 mM glucose. In voltage-clamp experiments Ca²⁺ influx appeared as nifedipine-inhibitable inward action currents in the presence of sulphonylurea plus TEA to block compensatory outward K⁺ currents. Under such conditions 15 mM K⁺ prolonged the action currents and 40 mM K⁺ transformed them into a continuous inward current. Correspondingly, 15 mM K⁺ led to an oscillatory increase, 40 mM K⁺ to a plateau of [Ca²⁺]_i superimposed on the [Ca²⁺]_i elevated by sulphonylurea plus TEA. Raising K⁺ to 15 or 40 mM in the presence of sulphonylurea (+/- TEA) led to a fast further increase of insulin secretion, which could be antagonised by nifedipine or CoCl₂.

Conclusion: Raising extracellular K⁺ does not simply activate voltage-dependent Ca²⁺ channels, but concentration-dependently transforms the pattern of Ca²⁺ influx into the beta cell and may thus generate stimuli of supraphysiological strength for insulin secretion.