RECEPTOR-EFFECTOR COUPLING PATHWAY FOR ALPHA(1)-ADRENERGIC MODULATION OF ABNORMAL AUTOMATICITY IN ISCHEMIC CANINE PURKINJE-FIBERS

We studied the receptor-effector coupling mechanism responsible for alpha(1)-adrenergic receptor-induced increases in abnormal automaticity (AA) occurring at low membrane potentials in ''ischemic'' Purkinje fibers, superfused with Tyrode's solution containing [K+](o) 10 mmol/L, pH 6.8, PO2 <25 mm Hg. To exclude beta-adrenergic actions, propranolol was added to all solutions. We derived membrane slope resistance (R(s1)) from the current-voltage relation obtained with two microelectrodes for intracellular current injection and transmembrane voltage recording. We also measured the membrane time constant, T-m, to assess changes in membrane resistance (R(m)). Phenylephrine effects on R(s1) in simulated ischemia were studied in the absence or presence of the alpha(1)-subtype blockers WB 4101 (WB) or chloroethylclonidine (CEC), both 0.1 mu mol/L, and in Purkinje fibers from dogs injected with pertussis toxin (PTX) (30 mu g/kg IV, 60 to 72 hours before study). There were no significant differences in mean values of R(s1) before phenylephrine superfusion among all groups of Purkinje fibers. T-m increased by 23% during phenylephrine 0.1 mu mol/L superfusion, and R(s1) increased by 11%. These two results suggest a 23% increase in R(m) with no concordant change in longitudinal resistance. In the presence of CEC, phenylephrine increased R(s1) by 12%. In contrast, WB blocked phenylephrine effects on R(s1) (0.3%). In PTX-treated Purkinje fibers, the levels of PTX-sensitive G protein as well as phenylephrine effects on R(s1) (3%) were significantly reduced. In the absence of WB and of CEC, the phenylephrine effects both on R(s1) and on the incidence of AA were directly related to the level of PTX-sensitive substrate. BaCl2 10 mu mol/L increased R(s1) by 22% and augmented phenylephrine effects on AA. Hence, an alpha(1)-receptor subtype that is blocked specifically by WB and inhibits K conductance via a PTX-sensitive G protein underlies the alpha(1)-adrenergic stimulation of AA during ischemia.