Evidence for Ca-mediated inactivation of ICa in dialysed guinea-pig atrial cardioballs.

Ca current (ICa) was studied in cultured myocytes from right atria of adult guinea-pigs (cardioballs) by means of single low-resistance patch pipettes. K currents were blocked by dialysis of the cells with solutions containing Cs+ as main cation and extracellular TEA (70 mM). Under this condition membrane currents elicited by voltage clamp pulses of 200 ms in duration from -45 mV (holding potential) are net inward for depolarizations up to +55 mV without detectable contamination by outward current components. The peak inward current (ICa) has a maximum between +5 and +10 mV and reverses around +60 mV. Measurements of ICa tail currents obtained after clamp pulses of increasing duration to more and more positive membrane potentials suggest that ICa inactivation is not genuinely voltage-dependent.