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

Smooth Muscle Group, Department of Physiology, The Queen's University of Belfast, Belfast, BT9 7BL, North Ireland Submitted 23 October 2003 ; accepted in final form 28 November 2003 Rabbit urethral smooth muscle cells were studied at 37°C 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 Ca 2+ 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 (IC 50 = 225 ± 84 nM), Ni 2+ (IC 50 = 324 ± 74 µM), and mibefradil (IC 50 = 2.6 ± 1.1 µM) but were enhanced when external Ca 2+ was substituted with Ba 2+ . The T current was little affected by nifedipine at concentrations