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MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

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 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 µM), and mibefradil (IC₅₀ = 2.6 ± 1.1 µM) 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 µM and 40 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.

spontaneous activity; pacemaking; smooth muscle

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