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

We studied the receptor-effector coupling mechanism responsible for al pha(1)-adrenergic receptor-induced increases in abnormal automaticity (AA) occurring at low membrane potentials in ''ischemic'' Purkinje fib ers, superfused with Tyrode's solution containing [K+](o) 10 mmol/L, p H 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 recordi ng. We also measured the membrane time constant, T-m, to assess change s in membrane resistance (R(m)). Phenylephrine effects on R(s1) in sim ulated 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 t oxin (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 concorda nt change in longitudinal resistance. In the presence of CEC, phenylep hrine increased R(s1) by 12%. In contrast, WB blocked phenylephrine ef fects on R(s1) (0.3%). In PTX-treated Purkinje fibers, the levels of P TX-sensitive G protein as well as phenylephrine effects on R(s1) (3%) were significantly reduced. In the absence of WB and of CEC, the pheny lephrine effects both on R(s1) and on the incidence of AA were directl y related to the level of PTX-sensitive substrate. BaCl2 10 mu mol/L i ncreased 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 alp ha(1)-adrenergic stimulation of AA during ischemia.