The transient outward current in mice lacking the potassium channel gene Kv1.4

1 The transient outward current (I_to) plays a prominent role in the repolarization phase of the cardiac action potential. Several K⁺ channel genes, including Kv1.4, are expressed in the heart, produce rapidly inactivating currents when heterologously expressed, and may be the molecular basis of I_to. 2 We engineered mice homozygous for a targeted disruption of the K⁺ channel gene Kv1.4 and compared I_to in wild-type (Kv1.4^+/+), heterozygous (Kv1.4^+/-) and homozygous 'knockout' (Kv1.4^-/-) mice. Kv1.4 RNA was truncated in Kv1.4^-/- mice and protein expression was absent. 3 Adult myocytes isolated from Kv1.4^+/+, Kv1.4^+/- and Kv1.4^-/- mice had large rapidly inactivating outward currents. The peak current densities at 60 mV (normalized by cellular capacitance, in pA pF^-1; means ± s.e.m.) were 53.8 ± 5.3, 45.3 ± 2.2 and 44.4 ± 2.8 in cells from Kv1.4^+/+, Kv1.4^+/- and Kv1.4^-/- mice, respectively (P < 0.02 for Kv1.4^+/+vs. Kv1.4^-/-). The steady-state values (800 ms after the voltage clamp step) were 30.9 ± 2.9, 26.9 ± 3.8 and 23.5 ± 2.2, respectively (P < 0.02 for Kv1.4^+/+vs. Kv1.4^-/-). The inactivating portion of the current was unchanged in the targeted mice. 4 The voltage dependence and time course of inactivation were not changed by targeted disruption of Kv1.4. The mean best-fitting V_1/2 (membrane potential at 50 % inactivation) values for myocytes from Kv1.4 ^+/+, Kv1.4^+/- and Kv1.4^-/- mice were -53.5 ± 3.7, -51.1 ± 2.6 and -54.2 ± 2.4 mV, respectively. The slope factors (k) were -10.1 ± 1.4, -8.8 ± 1.4 and -9.5 ± 1.2 mV, respectively. The fast time constants for development of inactivation at -30 mV were 27.8 ± 2.2, 26.2 ± 5.1 and 19.6 ± 2.1 ms in Kv1.4^+/+, Kv1.4^+/- and Kv1.4^-/- myocytes, respectively. At +30 mV, they were 35.5 ± 2.6, 30.0 ± 2.1 and 28.7 ± 1.6 ms, respectively. The time constants for the rapid phase of recovery from inactivation at -80 mV were 32.5 ± 8.2, 23.3 ± 1.8 and 39.0 ± 3.7 ms, respectively. 5 Nearly the entire inactivating component as well as more than 60 % of the steady-state outward current was eliminated by 1 mm 4-aminopyridine in Kv1.4^+/+, Kv1.4^+/- and Kv1.4^-/- myocytes. 6 Western blot analysis of heart membrane extracts showed no significant upregulation of the Kv4 subfamily of channels in the targeted mice. 7 Thus, Kv1.4 is not the molecular basis of I_to in adult murine ventricular myocytes.