Voltage-clamp characteristics of ventricular myocytes in rabbit

Action potentials and ionic currents were measured by patch-clamp in single right ventricular myocytes isolated enzymatically from rabbits and guinea pigs at room temperature. Based upon the shape of their action potentials, the rabbit myocytes were divided into two groups. In group 1 (n = 15), a prominent phase of initial rapid repolarization (phase 1) occurred following the upstroke of the action potential. Action potentials recorded in the second group (n = 9) did not have a noticeable phase 1. In group 1, voltage-clamp protocols revealed a transient outward current (Ito) activated upon depolarization from a holding potential of -50 mV to potentials positive to -20 mV. This outward current inactivated rapidly (tau = 50 ms) in a voltage-independent manner. The transient outward current was not apparent in group 2. In all the rabbit myocytes, the inward calcium current (ICa) was activated between -30 and -20 mV, with a peak at 0 mV. The inactivation kinetics of the inward calcium current were dependent on the voltage. In 4 out of 11 cells, inactivation was best described by a single exponential relation, and in the remaining 7 cells by a double exponential relation. The recovery of the inward calcium current from inactivation and the restitution of the duration of the action potential showed similar time-courses of 132 +/- 11.6 ms (n = 8) and 163.7 +/- 23.0 ms (n = 6), respectively. In all the rabbit myocytes the relation between current and voltage, established at the end of 400 ms command pulses, displayed a negative slope conductance. In rabbit myocytes, unlike those of guinea pig, pulses as long as 5 s revealed the delayed rectifier outward current to be weak or absent. Our results suggest that two populations of ventricular myocytes exist in the rabbit; neither has a prominent delayed rectifier, at least at room temperature, while one group has a transient outward current and the other does not. The major currents controlling the duration of the action potential at room temperature in myocytes from the right ventricle of the rabbit, therefore, are the inward calcium current and the inward rectifier potassium current (IK1).