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+ chann el genes, including Kv1.4, are expressed in the heart, produce rapidly inactivating currents when heterologously expressed, and may be the m olecular basis of I-to. 2. We engineered mice homozygous for a targete d disruption of the K+ channel gene Kv1.4 and compared I-to in wild-ty pe (Kv1.4(+/+)), heterozygous (Kv1.4(+/-)) and homozygous 'knockout' ( Kv1.4(-/-)) mice. Kv1.4 RNA was truncated in Kv1.4(-/-) mice and prote in 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 un changed in the targeted mice. 4. The voltage dependence and time cours e of inactivation were not changed by targeted disruption of Kv1.4. Th e mean best-fitting V-1/2 (membrane potential at 50% inactivation) val ues 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 s lope 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, 28.2 +/- 5.1 and 19.6 +/- 2.1 ms in Kv1. 4(+/+) Kv1.4(+/-) and Kv1.4(-/-) myocytes, respectively. At +30 mV, th ey were 35.5 +/- 2.8, 30.0 +/- 2.1 and 28.7 +/- 1.6 ms, respectively. The time constants for the rapid phase of recovery from inactivation a t -80 mV were 32.5 +/- 8.2, 23.3 +/- 1.8 and 39.0 +/- 3.7 ms, respecti vely. 5. Nearly the entire inactivating component as well as more than 60% of the steady-state outward current was eliminated by 1 mM 4-amin opyridine in Kv1.4(+/+), Kv1.4(+/-) and Kv1.4(-/-) myocytes. 6. Wester n blot analysis of heart membrane extracts showed no significant upreg ulation 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.