Antagonism of the insulinotropic action of first generation imidazolines by openers of KATP channels

The antagonism between K-ATP channel-blocking insulmotropic imidazolines - phentolamine, alinidine, idazoxan and efaroxan - and K-ATP channel openers, diazoxide and nucleoside diphosphates, was studied in mouse pancreatic islets and B-cells. In inside-out patches from B-cells, 500 mu M MgGDP abolished the inhibitory effect of the imidazolines. 300 mu M diazoxide further increased channel activity. The depolarizing effect of all imidazolines (100 mu M) on the B-cell membrane potential was practically completely antagonized by 300 mu M diazoxide. In contrast, diazoxide was unable to decrease the cytosolic Ca2+ concentration ([Ca2+](i)) which was elevated by phentolamine, whereas the [Ca2+](i) increases induced by the other imidazolines were promptly antagonized. The effects on [Ca2+](i) were reflected by the secretory activity in that the stimulatory effects of alinidine, idazoxan and efaroxan, but not that of phentolamine were antagonized by diazoxide. Metabolic inhibition of intact B-cells by 250 mu M NaCN, most likely by a decrease of the ATP/ADP ratio, significantly diminished the KATP channel-blocking effect of a low concentration of alinidine (10 mu M), whereas efaroxan proved to be susceptible even at a highly effective concentration (100 mu M). This may explain the oscillatory pattern of the [Ca2+](i) increase typically produced by efaroxan in pancreatic B-cells. In conclusion, the inhibitory effect of imidazolines on KATP channels, which is exerted at the pore-forming subunit, Kir6.2, is susceptible to the action of endogenous and exogenous KATP channel openers acting at the regulatory subunit SUR, which confers tissue specificity. With intact cells this antagonism can be obscured, possibly by intracellular accumulation of some imidazolines. (c) 2006 Elsevier Inc. All rights reserved.