New Insights Into Selectivity of Potassium Channels Using Small Cation Blockers

Abstract

Objective: The transient outward K þ current (I to ) contributes to repolarization in ventricular muscle. The functional effects of a novel I to activator, NS5806, were determined in canine left ventricular myocytes. Methods: Epicardial (epi), midmyocardial (mid) and endocardial (endo) cells were isolated by enzymatic dissociation. Whole cell patch clamp techniques were used to identify I to in the 3 cell types. Oocytes injected with Kv4.3 and KChiP2 were also used to evaluate the effect of NS5806. Results: Application of NS5806 (10 mM) to oocytes containing only Kv4.3 decreased I to charge by 57% whereas in oocytes containing Kv4.3:KChiP2 (1:1 ratio), NS5806 increased I to charge by 45% (at þ40 mV). In ventricular myocytes, NS5806 increased the magnitude of I to by 80%, 82%, and 16% in epi, mid, and endo myocytes, respectively (at þ40 mV). Similarly, the decay of I to was slowed in the presence of NS5806, resulting in increased total charge of 227%, 192% and 83% compared to control in epi, mid, and endo cells respectively (at þ40 mV). Steady-state inactivation of I to was more negative in the presence of NS5806 in all 3 cell types. Patch clamp results suggest the larger increase in I to by NS5806 in epi and mid cells may be due to the presence of a transmural gradient of KChiP2. To confirm, Kv4.3 and KChiP2 mRNA were measured in the 3 cell types. Kv4.3 message was uniform in the 3 cell types, whereas KChiP2 levels were significantly greater in epi and mid cells. Conclusions: The activation of I to by NS5806 is dependent on the presence of KChiP2 to increase the magnitude and total charge of I to . Our results suggest a transmural gradient of KChiP2 underlies the transmural gradient of I to in the canine left ventricle.