The whole‐cell Ca2+ channel current in single smooth muscle cells of the guinea‐pig ureter.

Abstract

SUMMARY1. Calcium channel currents were recorded in single, enzymatically isolated smooth muscle cells of the guinea-pig ureter using a single-electrode whole-cell voltage clamp technique. Calcium and barium currents through voltage-activated Ca2+ channels were recorded in cells dialysed with Cs+-or Na+-containing saline which suppressed K+ currents.2. Inward currents in Ca2+ (1P5-75 mM) or Ba2+ (1P5-75 mM) were recorded at potentials positive to -50 to -30 mV. Inward currents were maximal at 0 mV in 1-5 mM-Ca2+ and at + 10 mV in 7-5 mM-Ba2t. Current flow through Ca2+ channels in Cs+-filled cells (in 1-5 mM-Ca2+ or 7-5 mM-Ba2+) changed from inward to outward at potentials positive to + 70 mV. In Na+-filled cells this reversal potential was between + 50 and + 60 mV.3. Replacing Ca2+ or Ba2+ with Co2+ (1P5 mM) blocked all inward current flow through these Ca2+ channels; outward currents at potentials positive to +40 mV, however, were increased. Cadmium (100 uM) and nifedipine (0-1-10 ftM) reduced both inward and outward current flow.4. Calcium channel activation showed a sigmoidal relationship with membrane potential; the potential of half-maximal activation was -8-4 mV in 1-5 mM-Ca2+ and -10-8 mV in 7-5 mM-Ba2+. The maximum membrane conductance to Ca2+ (in 1-5 mM-Ca2+) was 2-57 nS/cell or approximately 0 05 mS/cm2.5. Evidence for a voltage-dependent inactivation mechanism included (a) the time-dependent relaxation of the outward currents at potentials positive to the reversal potential and (b) a steady-state inactivation (foo(v)) vs. membrane potential relationship (in 7.5 mM-Ba2+) which ranged between -80 and 0 mV, with a halfmaximal availability at -40 5 mV.6. The voltage dependencies of the inward current elicited from -80 and -30 mV were similar, suggesting that depolarization activated only L-type Ca2+ channels.7. It was concluded that the processes controlling the time course of the Ca2+ current in single ureteral cells bathed in physiological concentrations of Ca2+ were mostly voltage-dependent.

Effects of noradrenaline on membrane currents and action potential shape in smooth muscle cells from guinea‐pig ureter.

“…Flash-photolysis of caged IP3 also induces Ca2+ release and subsequent inhibition of Ica (Komori & Bolton, 1991;Beech, 1993 (Pacaud, Loirand, Mironneau & Mironneau, 1987;Loirand et at. 1990 (Imaizumi et al 1989;Lang, 1990). The partial recovery from NA-induced Ica inhibition in the presence of NA (Fig.…”

T-type Ca2+ channels in non-vascular smooth muscles

“…However, in this tissue either only an L-type Ca 2+ -current was identified (guinea-pig cells; [18,19]), or in rat cells a second, slower component, attributed to a Ca 2+ -sensitive Cl -channel [20]. The absence of a T-type current in the above experiments was in spite of a sufficiently negative holding potential (-80 to -100 mV).…”

“…Ca 2+ currents have also been characterised in smooth muscle from the ureter [18][19][20]. However, in this tissue either only an L-type Ca 2+ -current was identified (guinea-pig cells; [18,19]), or in rat cells a second, slower component, attributed to a Ca 2+ -sensitive Cl -channel [20].…”

Depolarization‐mediated intracellular calcium transients in isolated smooth muscle cells of guinea‐pig urinary bladder.

“…Sustained Ca2+ influx has been interpreted as ICa flowing in the potential window where steady-state activation and inactivation and curves overlap. 'Window ICa' should be maximal at about -20 mV (Klockner & Isenberg, 1985b;Imaizumi et al 1989;Lang, 1990). Direct measurements of non-inactivating ICa give controversial results.…”