Characterization of T-type calcium current and its contribution to electrical activity in rabbit urethra

Rabbit urethral smooth muscle cells were studied at 37degreesC by using the amphotericin B perforated-patch configuration of the patch-clamp technique, using Cs+-rich pipette solutions. Two components of current, with electrophysiological and pharmacological properties typical of T- and L-type Ca2+ currents, were recorded. Fitting steady-state inactivation curves for the L current with a Boltzmann equation yielded a V-1/2 of -41 +/- 3 mV. In contrast, the T current inactivated with a V-1/2 of -76 +/- 2 mV. The L currents were reduced by nifedipine (IC50 = 225 +/- 84 nM), Ni2+ (IC50 = 324 +/- 74 muM), and mibefradil (IC50 = 2.6 +/- 1.1 muM) but were enhanced when external Ca2+ was substituted with Ba2+. The T current was little affected by nifedipine at concentrations < 300 nM but was increased in amplitude when external Ca2+ was substituted with Ba2+. Both Ni2+ and mibefradil reduced the T current with an IC50 = 7 +/- 1 muM and similar to40 nM, respectively. Spontaneous electrical activity recorded with intracellular electrodes from strips of rabbit urethra consisted of complexes comprising a series of spikes superimposed on a slow spontaneous depolarization (SD). Inhibition of T current reduced the frequency of these SDs but had no effect on either the number of spikes per complex or the amplitude of the spikes. In contrast, application of nifedipine failed to significantly alter the frequency of the SD but reduced the number and amplitude of the spikes in each complex.