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

摘要

class="enumerated" style="list-style-type:decimal">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 (Ito) and inward rectifying K⁺ current (IK1) in rat cardiac muscle.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 Ito and IK1 were recorded using square wave depolarising and hyperpolarising voltage steps, respectively.The IC50 was 35.8 μmol l⁻¹ for Ito and 27.8 μmol l⁻¹ for IK1 compared to 5.4 μmol l⁻¹ for L-type Ca²⁺ current (ICa). The Hill coefficients for Ito, IK1 and ICa were −1.97, −1.60 and −1.21, respectively. In addition to inhibiting Ito amplitude, H-89 also accelerated the time to peak and the rate of voltage-dependent inactivation so that the time course of Ito was abbreviated.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.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 Ito or IK1.Data suggest that the effects of H-89 on K⁺ currents are more complex than simple inhibition of PKA-mediated phosphorylation.