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  μ mol 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  μ mol l −1 for I to and 27.8  μ mol l −1 for I K1 compared to 5.4  μ mol 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  μ mol 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.British Journal of Pharmacology (2006) 148 , 1091–1098. doi: 10.1038/sj.bjp.0706810