INHIBITION OF INSULIN RELEASE BY A FORMAMIDINE PESTICIDE AMITRAZ AND ITS METABOLITES IN A RAT BETA-CELL LINE - AN ACTION MEDIATED BY ALPHA-2-ADRENOCEPTORS, A GTP-BINDING PROTEIN AND A DECREASE IN CYCLIC-AMP

We tested the hypothesis that the formamidine pesticide amitraz and it s metabolites inhibit insulin release from a rat beta-cell line(RINm5F ) by decreasing intracellular cyclic AMP levels and examined whether G TP-binding proteins were involved in the process. Amitraz and its acti ve metabolite BTS27271 (0.1-10 mu M) inhibited insulin release that wa s induced by 100 mu M of IBMX in a dose-dependent manner. Other metabo lites of amitraz tested failed to inhibit insulin release. A potent an d specific alpha-2 adrenoceptor agonist, medetomidine (0.1 mu M), abol ished the IBMX-induced insulin release. Amitraz, BTS27271 and medetomi dine also decreased intracellular cyclic AMP concentrations that were elevated by IBMX administration. Moreover, all three drugs inhibited t he insulin release stimulated by forskolin (1 mu M), an adenylyl cycla se activator. Yohimbine (0.01, 0.1 and 1 mu M), an alpha-2 adrenocepto r antagonist, prevented the inhibitory effect of amitraz and BTS27271 on insulin release, whereas prazosin (1 mu M), an alpha-1 adrenoceptor antagonist, did not. Yohimbine, but not prazosin, also prevented the effect of amitraz, BTS27271 and medetomidine on IBMX-stimulated accumu lation of cyclic AMP. Pretreatment of cells with PTX (0.1 mu g/ml) for 16 h, antagonized the effects of amitraz and BTS27271 on IBMX-induced increase in insulin release. Thus, one mechanism by which amitraz and its metabolite BTS27271 decrease insulin release is by inhibiting ade nylyl cyclase, an action mediated through a PTX-sensitive G-protein.