EFFECTS OF PROTEIN-KINASE INHIBITORS ON CANINE PURKINJE-FIBER PACEMAKER DEPOLARIZATION AND THE PACEMAKER CURRENT-IF

1. The effects of the protein kinase inhibitors H-7 and H-8 were investigated on diastolic depolarization of the action potential with microelectrodes and on the pacemaker current i(f) with the two-microelectrode voltage clamp in canine cardiac Purkinje fibres. 2. Both 200-mu-M-H-7 and 100-mu-M-H-8 had no significant effect on the slope of diastolic depolarization but eliminated the actions of isoprenaline (1-mu-M). 3. We examined the actions of H-7 and H-8 on i(f) in the presence and absence of isoprenaline. H-7 (200-mu-M) shifted the pacemaker current i(f) in the negative direction on the voltage axis, whereas 100-mu-M-H-8 had no significant effect by itself. Both 200-mu-M-H-7 and 100-mu-M-H-8 can reverse or prevent the actions of isoprenaline (1-5-mu-M) on i(f). 4. We applied activators of the cyclic AMP cascade down-stream to the beta-receptor, to further evaluate where H-7 and H-8 might be exerting their effects. When exposing Purkinje fibres to an adenylyl cyclase activator (forskolin, 10-50-mu-M), a phosphodiesterase inhibitor (IBMX, 100-mu-M) and a permeable cyclic AMP analogue (8-chlorophenylthio-cyclic AMP, 200-mu-M-1 mM), the amplitude of i(f) was increased. H-7 and H-8 at 100-200-mu-M eliminated each of these actions. 5. These results suggest that a phosphorylation process is involved in the modulation of the pacemaker current, i(f), in Purkinje fibres. The different actions of H-7 and H-8 on basal i(f) suggest the hypothesis that other protein kinases, possibly protein kinase C, might also be involved in regulating basal phosphorylation of i(f) in Purkinje fibres.