Evaluation of T- and L-type Ca²⁺ currents in shark ventricular myocytes

Two types of Ca²⁺ currents with characteristics of T- and L-type Ca²⁺ currents were recorded in ventricular myocytes of dogfish (Squalus acanthias). The T-type Ca²⁺ current activated near -70 mV and had a peak current density of 9.8 pA/pF at -34 mV. The L-type Ca²⁺ current activated near -50 mV and had a peak current density of 10.6 pA/pF near 0 mV. The threshold for activation of the T-type Ca²⁺ current was 20 mV negative to that of the tetrodotoxin-sensitive Na⁺ current. Inactivation of the T-type Ca²⁺ current was rapid with a limiting time constant of 5 ms at positive potentials. The T-type Ca²⁺ current was not modulated by isoproterenol or acetylcholine. In dogfish the T-type Ca²⁺ channel has current densities equivalent to the L-type channel and is likely to activate before the Na²⁺ channel, contributing significantly to generation of the foot of the action potential.