EFFECT OF ALPHA(2)-ADRENOCEPTOR STIMULATION ON ISOLATED CANINE PURKINJE-FIBER CONTRACTION

We have recently identified the presence of postjunctional alpha(2)-ad renoceptors in canine Purkinje fibers. In this study, we examined the effects of alpha(2)-adrenoceptor stimulation on the contraction streng th of isolated Purkinje fibers. Exposure to the alpha(2)-adrenoceptor specific agonist and antagonist, UK 14,304 -(4,5-dihydro-1H-imidazol-2 -yl)-6-quinoxalinamine) and yohimbine (17-hydroxyyohimban-16-carboxyli c acid methyl ester hydrochloride) alone at 0.1 mu M respectively, did not produce any significant effect on Purkinje contraction strength. Purkinje contraction strength was augmented by isoproterenol(0.1 mu M) , forskolin (0.1 mu M), or 8-bromo-adenosine cyclic 2',3'-monophosphat e (8-bromo-cAMP, 10 mu M). UK 14,304 significantly reversed the effect s of isoproterenol and forskolin but not those of 8-bromo-cAMP on Purk inje contraction strength. After incubation with pertussis toxin, the positive inotropic effect of forskolin on Purkinje contraction strengt h remained intact, but the forskolin effect could no longer be reverse d by UK 14,304. These results suggest that the postjunctional alpha(2) -adrenoceptors in canine Purkinje fibers are coupled to a pertussis to xin-sensitive G protein,probably G(i). Stimulation of the alpha(2)-adr enoceptor antagonizes the effect of beta-adrenoceptor stimulation on P urkinje contraction strength in an accentuated antagonism manner. (C) 1998 Elsevier Science B.V.