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The question whether K+ depolarization is an appropriate experimental substitute for the physiological nutrient-induced depolarization of the ß-cell plasma membrane was investigated using primary mouse ß-cells and islets. At basal glucose 40 mM K+ induced a massive monophasic response, whereas 15 mM K+ had only a minimal insulinotropic effect, even though the increase in the cytosolic Ca2+ concentration ([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 sulfonylurea plus TEA to block compensatory outward K+ currents. Under these conditions, 15 mM K+ induced prolonged action currents and 40 mM K+ transformed the action current pattern into a continuous inward current. Correspondingly, 15 mM K+ led to an oscillatory increase and 40 mM K+ to a plateau of [Ca2+]i superimposed on the [Ca2+]i elevated by sulfonylurea plus TEA. Raising K+ to 15 or 40 mM in the presence of sulfonylurea (±TEA) led to a fast further increase of insulin secretion. This was reduced to basal levels by nifedipine or CoCl2. The effects of 15 mM K+ on depolarization, action currents, and insulin secretion were mimicked by adding 35 mM Cs+ and those of 40 mM K+ by adding 35 mM Rb+, in parallel with their ability to substitute for K+ as permeant cation. In conclusion, the alkali metals K+, Rb+, or Cs+ concentration-dependently transform the pattern of Ca2+ influx into the ß-cell and may thus generate stimuli of supraphysiological strength for insulin secretion. [ABSTRACT FROM AUTHOR]

Depolarization by a high K+ concentration is a widely used experimental tool to stimulate insulin secretion. The effects occurring after the initial rise in secretion were investigated here. After the initial peak a fast decline occurred, which was followed by a slowly progressive decrease in secretion when a strong K+ depolarization was used. At 40 mM KCl, but not at lower concentrations, the decrease continued when the glucose concentration was raised from 5 to 10 mM, suggesting an inhibitory effect of the K+ depolarization. When tolbutamide was added instead of the glucose concentration being raised, a complete inhibition down to prestimulatory values was observed. Equimolar reduction of the NaCl concentration to preserve isoosmolarity enabled an increase in secretion in response to glucose. Unexpectedly, the same was true when the Na+-reduced media were made hyperosmolar by choline chloride or mannitol. The insulinotropic effect of tolbutamide was not rescued by the compensatory reduction of NaCl, suggesting a requirement for activated energy metabolism. These inhibitory effects could not be explained by a lack of depolarizing strength or by a diminished free cytosolic Ca2+ concentration ([Ca2+]i). Rather, the complexation of extracellular Ca2+ concomitant with the K+ depolarization markedly diminished [Ca2+]i and attenuated the inhibitory action of 40 mM KCl. This suggests that a strong but not a moderate depolarization by K+ induces a [Ca2+]idependent, slowly progressive desensitization of the secretory machinery. In contrast, the decline immediately following the initial peak of secretion may result from the inactivation of voltage-dependent Ca2+ channels. [ABSTRACT FROM AUTHOR]