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

Abstract

Rabbit urethral smooth muscle cells were studied at 37 degrees 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 microM), and mibefradil (IC(50) = 2.6 +/- 1.1 microM) but were enhanced when external Ca(2+) was substituted with Ba(2+). The T current was little affected by nifedipine at concentrations <300 nM but was increased in amplitude when external Ca(2+) was substituted with Ba(2+). Both Ni(2+) and mibefradil reduced the T current with an IC(50) = 7 +/- 1 microM and approximately 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.

The physiological function of lower urinary tract smooth muscle

“…Both L-type and T-type Ca 2+ -currents have been recorded in isolated myocytes. Blockade of the former type reduced the number of APs in each burst; the frequency within bursts was attenuated by blockade of T-type current (Bradley et al, 2004). However, both channels represent targets that may modulate spontaneous activity.…”

Calcium Signaling in Smooth Muscle

“…Because of these gating properties, the importance of T-type currents in the regulation of smooth muscle membrane potential is a matter of controversy. However, there is evidence from the rabbit urethra that, at least in this tissue, T-type channels regulate action potential frequency (Bradley et al 2004). …”

Ion channels of the mammalian urethra

“…Most current knowledge regarding the role of ion channels in the urethra has come from isometric tension recordings, [25][26][27][28][29][30][31] sharp microelectrode recordings of intact preparations, 25,27,32,33 and the patch clamp technique. [29][30][31][33][34][35][36][37][38][39][40] Earlier studies measuring electrical and mechanical (i.e., force) activity in rabbit urethral smooth muscle reported that the spontaneous activity was in fact myogenic in nature, 25,41 resembling the activity seen in the GI tract. 42 Thus, the excitability appeared to be driven by a form of pacemaker originating within the muscle itself.…”

“…76,77 This has certainly proved to be consistent with action potential spike events observed in urethral smooth muscle cells. 30,31,33,39,78 It is well-established that the L-type (long-lasting) VGCCs activate at more depolarized potentials than their T-type (transient-lasting) counterpart. 76 It was also previously demonstrated that L-type VGCC current was crucial for generating and maintaining urethral tone in intact preparations.…”

“…In the early 2000s, urethral smooth muscle cell VGCC currents were examined and it was reported that human and rabbit urethral smooth muscle cells functionally contained both L-and T-type VGCC conductances. 33,39,78,80 This was followed up by studies demonstrating that the L-type VGCC current was essential not only for the upstroke in the action potential, but for the transient nature of the BK Ca channel current. 30,31 While detailed biophysical data are available for the L-and T-type VGCCs, the channel subunit isoforms and regulatory subunits have not yet been identified or described.…”