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

The antagonism between KATP channel-blocking insulinotropic imidazolines – phentolamine, alinidine, idazoxan and efaroxan – and KATP channel openers, diazoxide and nucleoside diphosphates, was studied in mouse pancreatic islets and B-cells. In inside-out patches from B-cells, 500 μM MgGDP abolished the inhibitory effect of the imidazolines. 300 μM diazoxide further increased channel activity. The depolarizing effect of all imidazolines (100 μM) on the B-cell membrane potential was practically completely antagonized by 300 μM 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 μM 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 μM), whereas efaroxan proved to be susceptible even at a highly effective concentration (100 μM). 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.