In vitro stimulation of insulin release by SL 84.0418, a new alpha 2-adrenoceptor antagonist

SL 84.0418 (2-(4,5-dihydro-1H-imidazol-2-yl)-1,2,4,5-tetrahydro-2-propyl-pyrrolo[3, 2,1- hi]-indole hydrocholoride) is a novel alpha 2-adrenoceptor antagonist which possesses anti-hyperglycaemic properties in vitro study, we tested its effects on insulin release from isolated mouse islets. In the presence of 15 mM glucose, SL 84.0418 produced a concentration-dependent (0.1-100 microM) increase of insulin release with a slightly higher potency than tolbutamide. SL 84.0418 antagonized the inhibition of glucose-induced insulin release caused either by the alpha 2-adrenoceptor agonist clonidine or by diazoxide, a selective opener of ATP-sensitive K+ channels in the beta-cell membrane. Its potency was greater on the inhibition by clonidine than on that by diazoxide, complete antagonism of the inhibition being achieved by 0.9 microM and 6 microM SL 84.0418 respectively. When alpha 2-adrenoceptors were blocked by the antagonist idazoxan, the low concentrations of SL 84.0418 (0.1-0.3 microM) no longer increased insulin release, whereas the effect of higher concentrations (> or = 1 microM) was not affected. SL 84.0418 (> or = 1 microM) inhibited 86Rb efflux from islets perifused with a medium containing 3 mM glucose, i.e. under conditions where many ATP-sensitive K+ channels are open. It also reduced the acceleration of 86Rb efflux that diazoxide caused in the presence of 6 mM glucose. Moreover, SL 84.0418 directly inhibited ATP-sensitive K+ currents measured in single beta-cells by the whole-cell mode of the patch-clamp technique.(ABSTRACT TRUNCATED AT 250 WORDS)