PREJUNCTIONAL ACTIONS OF N-ETHYL-MALEIMIDE AND PHENOXYBENZAMINE IN RAT VAS-DEFERENS

In studies of electrically evoked isometric contractions of rat vas deferens, N-ethyl-maleimide (30 mu M) pretreatment significantly reduced the prejunctional inhibitory potencies of xylazine and 5-hydroxytryptamine but failed to affect the potency of the alpha(1)-adrenoceptor agonist amidephrine. Phenoxybenzamine (1 mu M) or N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) (10 mu M) produced significant shifts in the potency of xylazine and significantly reduced the maximum inhibition, but the combination of phenoxybenzamine or EEDQ and N-ethyl-maleimide (30 mu M) produced no further alteration in the effects of xylazine. In displacement studies, N-ethyl-maleimide displaced the binding of [H-3]MK 912 ((2S,12bS)1',3'-dimethylspiro-(1,3,4,5',6,6',7,12b-octahydro-2H-benzo[b]furo[2,3-a]quinazoline)-2,4'-pyrimidin-2' one) to rat renal cortex membranes with a K-i of 466 +/- 133 mu M (n = 5), and so does not bind to alpha(2)-adrenoceptors in the concentration range in which it affects prejunctional receptor mediated responses. This may suggest that N-ethyl-maleimide has actions other than inactivation of G-proteins or that the irreversible alpha(2)-adrenoceptor antagonists phenoxybenzamine and EEDQ inactivate G-proteins sensitive to N-ethyl-maleimide in concentrations at which they bind to alpha(2)-adrenoceptors.