Stimulation of insulin secretion by imidazoline compounds displays variable characteristics. Phentolamine (10-100 microM) increased secretion of perifused mouse islets at nonstimulatory glucose concentrations (5 mM) and even in the absence of glucose. Idazoxan (20-100 microM) elicited a moderate increase in insulin secretion, which required the presence of a stimulatory glucose concentration (10 mM). Phentolamine is therefore a stimulator of secretion in its own right, whereas idazoxan may be termed an enhancer of secretion. Both compounds inhibited the activity of ATP-dependent K+ channels in inside-out patches from B-cells; however, idazoxan achieved only an incomplete block. Both compounds depolarized the B-cell plasma membrane to an extent that permitted the opening of voltage-dependent Ca2+ channels (-40 to -30 mV). An increase in cytoplasmic Ca2+ concentration was induced by phentolamine and much less so by idazoxan. Activation of protein kinase C, a possible mechanism to amplify Ca(2+)-induced secretion, could not be verified for phentolamine. It thus appears that stimulation of insulin secretion by phentolamine is due to its blocking effect on KATP channels, which may be the correlate of non-adrenergic imidazoline binding sites which were characterized in insulin-secreting HIT cells. Whether incomplete closure of KATP channels by idazoxan or additional effects are responsible for the requirement of high glucose to stimulate secretion remains to be clarified.

The possible role of protein kinase C (PKC) activation in the course of insulin secretion induced by the imidazoline phentolamine was investigated by measuring the insulin secretion of perifused mouse islets and of insulin-secreting HIT cells and by measuring the PKC activity of HIT cells. When normal mouse islets were perifused with the imidazoline phentolamine (32 microM) or the sulfonylurea glibenclamide (1 microM), neither phentolamine nor glibenclamide could produce a stimulation of secretion which was stronger than that elicited by a strong depolarization. Under the same conditions, tetradecanoylphorbolacetate (TPA, 50 nM), a known activator of PKC activity in pancreatic islets, markedly enhanced the secretion induced by K+ depolarization. Phentolamine also stimulated insulin secretion of superfused HIT cells. When PKC activity in HIT cells was down-regulated to 15% of the initial value by overnight exposure to TPA (50 nM), the stimulatory effect of TPA on secretion was virtually abolished, while phentolamine was still able to elicit a monophasic secretion. TPA (50 nM) induced the typical redistribution of PKC activity in HIT cells: within 2 min, the share of membrane-bound PKC activity rose from 26% to 87% of the total PKC activity, which remained unchanged. In contrast, phentolamine (32 microM) had no effect on PKC distribution, did not down-regulate PKC and had no effect on PKC activity once it was down-regulated by TPA. Thus, the recent suggestion that the insulinotropic effect of imidazolines involves an activation of PKC could not be verified for phentolamine.

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