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

The present study was undertaken to investigate the effects of specific inh ibitors of calmodulin-dependent protein kinase II (CamKII) on macroscopic v oltage-dependent K+ current (K-V) recorded from rabbit portal vein smooth m uscle cells. Inhibition of L-type Ca2+ current facilitation by 1 mu M KN-62 , a blocker of CamKII, was first demonstrated and provided evidence for fun ctional CamKII activity in this preparation. KN-93, another specific and mo re 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 wit h both low(0.1 mM) and high (10 mM) EGTA pipette solution. Prolonged dialys is with 10 mu M of a synthetic peptide inhibitor of CamKII (fragment 281-30 1) had little effect on K-V and did not prevent the inhibitory action of KN -93 on the current. The estimated IC50 for inhibiting peak and late current s during 250-ms steps to +60 mV (holding potential = -60 mV) were 2.9 and 0 .27 mu M, respectively. KN-93 also induced slight shifts of the steady-stat e 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 cu rrent clamp experiments, 5 mu M KN-93 depolarized the myocytes from a contr ol resting membrane potential of -42.3 +/- 2.8 mV to -28.5 +/- 1.4 mV, an e ffect 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 experi ments designed to assess the role of CamKII.