Multiple types of Ca2+ currents in single canine Purkinje cells

Whole-cell Ca2+ channel currents were recorded from isolated single canine Purkinje and ventricular cells to determine whether there were multiple types of Ca2+ channels in these two cell types, as in many other excitable tissues. The experimental conditions were such that currents other than Ca2+ channel currents were largely suppressed. The charge carrier was either Ca2+ or Ba2+ (5mM). In every canine Purkinje cell studied (n = 36), we saw T and L Ca2+ channel currents that are similar to their counterparts in other tissues. Neither current was affected by tetrodotoxin (30 microM), but both were reduced by Mn2+ (5mM). Ni2+ (50 microM) blocked T more than L current. Nisoldipine (1 microM) apparently abolished the L current but also decreased the T current by 50%. Substitution of Ba2+ for Ca2+ augmented and prolonged L current but did not affect T current significantly. At 36 degrees C and with 5 mM [Ca2+]o, T current inactivated over a voltage range from -70 to -30 mV whereas L current inactivated between -30 and +20 mV. T current was detectable in only some of the ventricular cells studied (8 out of 12). In these cells the ratio of maximal T current to maximal L current (0.2 +/- 0.1, n = 8) was lower than the T/L ratio in Purkinje cells (0.6 +/- 0.2, n = 6). The density of peak L current in ventricular cells (7.5 +/- 1.7 pA/pF, n = 8) was higher than that in Purkinje cells (4.4 +/- 3.4 pA/pF, n = 6). Therefore, in ventricular cells the L current is the main Ca2+ current whereas in Purkinje cells, the T current also contributes significantly to membrane electrical activity. In Purkinje cells, beta-adrenoceptor stimulation by isoproterenol (1 microM) increased L current but did not affect T current. On the other hand, in 70% (7 out of 10) of the Purkinje cells, alpha-adrenoceptor stimulation by 10 microM norepinephrine (in the presence of 2 microM propranolol) increased the T current. Our observations show that the distribution of the two types of Ca2+ channels in canine ventricle is heterogeneous and that the two types of Ca2+ channels are modulated by catecholamines by different receptors.