H-89 inhibits transient outward and inward rectifier potassium currents in isolated rat ventricular myocytes

1 Voltage clamp was used to investigate the effects of N-2-p-bromo- cinnamylamino)ethyl-5-isoquinolinesulfonamide (H-89), a potent inhibitor of PKA, on transient outward K ⁺ current (I _to) and inward rectifying K ⁺ current (I _K1) in rat cardiac muscle. 2 Initial experiments, performed using descending voltage ramps, showed that H-89 inhibited both the outward and inward ramp currents in a concentration-dependent manner at concentrations between 5 and 60 μmol l ^-1. A similar degree of inhibition was observed when I _to and I _K1 were recorded using square wave depolarising and hyperpolarising voltage steps, respectively. 3 The IC ₅₀ was 35.8 μmol l ^-1 for I _to and 27.8 μmol l ^-1 for I _K1 compared to 5.4 μmol l ^-1 for L-type Ca ²⁺ current (I _Ca). The Hill coefficients for I _to, I _K1 and I _Ca were -1.97, -1.60 and -1.21, respectively. In addition to inhibiting I _to amplitude, H-89 also accelerated the time to peak and the rate of voltage-dependent inactivation so that the time course of I _to was abbreviated. 4 Paired-pulse protocols were performed to study the effects of H-89 on steady-state activation and inactivation as well as recovery from voltage-dependent inactivation. H-89 produced a concentration-dependent rightward shift in voltage-dependent activation but had no significant effect on steady-state inactivation. Recovery from voltage-dependent inactivation was delayed, although this was only visible at the highest concentration (60 μmol l ^-1) used. 5 In experiments investigating the effects of elevated cyclic AMP, the β-adrenergic agonist isoprenaline and the phosphatase inhibitor calyculin A had no major effects on I _to or I _K1. 6 Data suggest that the effects of H-89 on K ⁺ currents are more complex than simple inhibition of PKA-mediated phosphorylation.