Inhibitors of calmodulin-dependent protein kinase are nonspecific blockers of voltage-dependent K+ channels in vascular myocytes.

The present study was undertaken to investigate the effects of specific inhibitors of calmodulin-dependent protein kinase II (CamKII) on macroscopic voltage-dependent K(+) current (K(V)) recorded from rabbit portal vein smooth muscle cells. Inhibition of L-type Ca(2+) current facilitation by 1 microM KN-62, a blocker of CamKII, was first demonstrated and provided evidence for functional CamKII activity in this preparation. KN-93, another specific and more potent inhibitor of CamKII in the rat brain, suppressed K(V) and enhanced the rate of inactivation in a dose-dependent manner, in cells dialyzed with both low (0.1 mM) and high (10 mM) EGTA pipette solution. Prolonged dialysis with 10 microM of a synthetic peptide inhibitor of CamKII (fragment 281-301) had little effect on K(V) and did not prevent the inhibitory action of KN-93 on the current. The estimated IC(50) for inhibiting peak and late currents during 250-ms steps to +60 mV (holding potential = -60 mV) were 2.9 and 0.27 microM, respectively. KN-93 also induced slight shifts of the steady-state activation (-7 mV) and inactivation (-6 mV) curves. KN-62, and KN-92, an inactive analog of KN-93, produced effects similar to those of KN-93. In current clamp experiments, 5 microM KN-93 depolarized the myocytes from a control resting membrane potential of -42.3 +/- 2.8 mV to -28.5 +/- 1.4 mV, an effect that was partially reversible after washout (-34.4 +/- 1.3 mV, n = 6). In conclusion, blockers of CamKII produce nonspecific inhibitory effects on K(V) that warrant cautious use of these compounds in physiological experiments designed to assess the role of CamKII.