Activators of protein kinase C depolarize insulin-secreting cells by closing K+ channels.

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Abstract
Carbohydrate stimuli of insulin secretion depolarize the pancreatic B cell and the B‐cell line RINm5F by inhibiting ATP‐sensitive K+ channels. We examined the possibility that this effect is mediated by activation of protein kinase C. In RINm5F cells, the triose D‐glyceraldehyde evoked a rapid increase of the mass of 1,2‐diacylglycerol, the endogenous activator of protein kinase C. This effect is mainly due to de novo synthesis of the lipid from glycolytic intermediates, as glyceraldehyde carbon was incorporated into 1,2‐diacylglycerol within 1 min of exposure to 14C‐labelled glyceraldehyde. The effects of two exogenous activators of kinase C, 4‐beta‐12‐phorbol‐myristate 13‐acetate (PMA) and 1,2‐didecanoylglycerol (DC10) on single K+ channel currents were examined in RINm5F cell‐attached membrane patches. Both PMA and DC10 depolarized the cells and decreased the open‐state probability of the ATP‐sensitive K+ channels. These actions were not due to changes in cellular ATP content, since PMA, like glyceraldehyde, failed to alter cellular ATP. As is the case for glyceraldehyde, PMA and DC10 raised cytosolic free Ca2+ [(Ca2+]i) and stimulated insulin secretion. Both of these effects are inhibited in the absence of external Ca2+. This, and the attenuation of the [Ca2+]i rise by verapamil, suggest that all three stimuli raise [Ca2+]i by promoting Ca2+ influx through voltage‐gated channels in turn leading to insulin secretion. As the exogenous activators of protein kinase C mimic the effects of glyceraldehyde, it is proposed that the carbohydrate‐mediated production of 1,2‐diacylglycerol constitutes the link between metabolism and membrane depolarization.(ABSTRACT TRUNCATED AT 250 WORDS)