['In the present study we examined the effects of diltiazem, an L-type Ca(2+) channel blocker widely used for the control of the ventricular rate in patients with supraventricular arrhythmias, on hKv1.5 and Kv4.3 channels that generate the cardiac ultrarapid delayed rectifier (I(Kur)) and the 4-aminopyridine sensitive transient outward (I(to)) K(+) currents, respectively.', 'hKv1.5 and Kv4.3 channels were stably and transiently expressed in mouse fibroblast and Chinese hamster ovary cells, respectively. Currents were recorded using the whole-cell patch clamp.', 'Diltiazem (0.01 nM-500 muM) blocked hKv1.5 channels, in a frequency-dependent manner exhibiting a biphasic dose-response curve (IC(50)=4.8+/-1.5 nM and 42.3+/-3.6 muM). Diltiazem delayed the initial phase of the tail current decline and shifted the midpoint of the activation (Vh=-16.5+/-2.1 mV vs -20.4+/-2.6 mV, P<0.001) and inactivation (Vh=-22.4+/-0.7 mV vs. -28.2+/-1.9 mV, P<0.001) curves to more negative potentials. The analysis of the development of the diltiazem-induced block yielded apparent association (k) and dissociation (P) rate constants of (1.6+/-0.2) x 10(6) M(-1)s(-1) and 46.8+/-4.8 s(-1), respectively. Diltiazem (0.1 nM-100 muM) also blocked Kv4.3 channels in a frequency-dependent manner exhibiting a biphasic dose-response curve (IC(50)=62.6+/-11.1 nM and 109.9+/-12.8 muM). Diltiazem decreased the peak current and, at concentrations > or =0.1 microM, accelerated the inactivation time course. The apparent association and dissociation rate constants resulted (1.7+/-0.2) x 10(6) M(-1)s(-1) and 258.6+/-38.1 s(-1), respectively. Diltiazem, 10 nM, shifted to more negative potentials the voltage-dependence of Kv4.3 channel inactivation (Vh=-33.1+/-2.3 mV vs -38.2+/-3.5 mV, n=6, Plt;0.05) the blockade increasing at potentials at which the amount of inactivated channels increased.'] Does diltiazem inhibit hKv1.5 and Kv4.3 currents at therapeutic concentrations?