A comparison of calcium currents in rat and guinea pig single ventricular cells.

The slow inward calcium currents were compared in rat and guinea pig heart using enzymatically dissociated, single ventricular cells. A single electrode voltage clamp was used, in which current and voltage were sampled separately using a time-sharing method. Spatial homogeneity of membrane potential during peak slow inward calcium current was assessed by measuring the potential with two microelectrodes 50 micron apart; the potentials were within 3 mV of each other. Peak current-voltage relations for slow inward calcium currents were similar for the two species, but the individual currents showed a faster time course of inactivation and a slower time course of recovery from inactivation for rat, compared with guinea pig. The potassium current blockers 4-aminopyridine and tetraethylammonium chloride did not produce significant effects on the net membrane currents recorded at the holding potentials (-50 to -40 mV) used in this study. The underlying mechanism for the inactivation of the slow inward calcium currents was explored using a double pulse procedure. In both rat and guinea pig heart cells prepulses to very positive potentials were associated with a partial restoration of the slow inward calcium current in the following test pulse. In addition, internal ethylene glycol-bis N,N,N',N'-tetraacetic acid or substitution of barium for calcium slowed the rate of inactivation of the slow inward calcium current in rat heart cells. Calcium activation of nonspecific currents was thought less likely to have produced these results due to the lack of effect of depolarizing prepulses on hyperpolarizing test pulses. A calcium-dependent component of inactivation may be responsible for the differences observed in both the inactivation and the recovery time courses of the slow inward calcium current in these species.