The T-type Ca channel in guinea-pig ventricular myocytes is insensitive to isoproterenol

Whole cell clamp experiments and single channel measurements were used to study whether the T-type Ca channel in guinea-pig ventricular myocytes is sensitive to beta-adrenergic stimulation. In whole cell clamp experiments we determined the T-type Ca current by measuring the difference between the currents at a test potential of -30 mV in Na-free solution, starting from a holding potential either at -90 or at -50 mV. Application of 1 microM isoproterenol (ISO) did not change the difference current, while the L-type Ca current was enhanced during application of the drug. In cell attached patch experiments with 110 mM CaCl2 in the pipette solution, single channel currents were observed which were due to opening of the T-type Ca channel. Application of 4 microM ISO to the bath solution did neither change the appearance of the channel openings, nor the ensemble averaged currents. It is concluded that the T-type Ca channel in guinea-pig ventricular cells is insensitive to ISO.