LVA and HVA Ca Currents in Ventricular Muscle Cells of the Lymnaea Heart

Yeoman, M. S., B. L. Brezden, and P. R. Benjamin. LVA and HVA Ca currents in ventricular muscle cells of the Lymnaea heart. J. Neurophysiol. 82: 2428–2440, 1999. The single-electrode voltageclamp technique was used to characterize voltage-gated Ca currents in dissociated Lymnaea heart ventricular cells. In the presence of 30 mM tetraethylammonium (TEA), two distinct Ca currents could be identified. The first current activated between 270 and 260 mV. It was fully available for activation at potentials more negative than 280 mV. The current was fast to activate and inactivate. The inactivation of the current was voltage dependent. The current was larger when it was carried by Ca compared with Ba, although changing the permeant ion had no observable effect on the kinetics of the evoked currents. The current was blocked by Co and La (1 mM) but was particularly sensitive to Ni ions ('50% block with 100 mM Ni) and insensitive to low doses of the dihydropyridine Ca channel antagonist, nifedipine. All these properties classify this current as a member of the low-voltage–activated (LVA) T-type family of Ca currents. The activation threshold of the current (270 mV) suggests that it has a role in pacemaking and action potential generation. Muscle contractions were first seen at 250 mV, indicating that this current might supply some of the Ca necessary for excitationcontraction coupling. The second, a high-voltage–activated (HVA) current, activated at potentials between 240 and 230 mV and was fully available for activation at potentials more negative than 260 mV. This current was also fast to activate and with Ca as the permeant ion, inactivated completely during the 200-ms voltage step. Substitution of Ba for Ca increased the amplitude of the current and significantly slowed the rate of inactivation. The inactivation of this current appeared to be current rather than voltage dependent. This current was blocked by Co and La ions (1 mM) but was sensitive to micromolar concentrations of nifedipine ('50% block 10 mM nifedipine) that were ineffective at blocking the LVA current. These properties characterize this current as a L-type Ca current. The voltage sensitivity of this current suggests that it is also important in generating the spontaneous action potentials, and in providing some of the Ca necessary for excitation-contraction coupling. These data provide the first detailed description of the voltage-dependent Ca currents present in the heart muscle cells of an invertebrate and indicate that pacemaking in the molluscan heart has some similarities with that of the mammalian heart.