COMPARISON OF SARCOLEMMAL CALCIUM-CHANNEL CURRENT IN RABBIT AND RAT VENTRICULAR MYOCYTES

1. Fundamental properties of Ca2+ channel currents in rat and rabbit v entricular myocytes were measured using whole cell voltage clamp. 2. I n rat, as compared with rabbit myocytes, Ca2+ channel current (I-Ca) w as half-activated at about 10 mV more negative potential, decayed slow er, was half-inactivated (in steady state) at about 5 mV more positive potential, and recovered faster from inactivation. 3. These features result in a larger steady-state window current in rat, and also sugges t that under comparable voltage clamp conditions, including action pot ential (AP) clamp, more Ca2+ influx would be expected in rat myocytes. 4. Ca2+ channel current carried by Na2+ and Cs+ in the absence of div alent ions (I-ns) also activated at more negative potential and decaye d more slowly in rat. 5. The reversal potential for I-ns was 6 mV more positive in rabbit, consistent with a larger permeability ratio (P-Na /P-Cs) in rabbit than in rat. I-Ca also reversed at slightly more posi tive potentials in rabbit (such that P-Ca/P-Cs might also be higher). 6. Ca2+ influx was calculated by integration of I-Ca evoked by voltage clamp pulses (either square pulses or pulses based on recorded rabbit or rat APs). For a given clamp, waveform, the Ca2+ influx was up to 2 5% greater in rat, as predicted from the fundamental properties of I-C a and I-ns. 7. However, the longer duration of the AP in rabbit myocyt es compensated for the difference in influx, such that the integrated Ca2+ influx via I-Ca in response to the species-appropriate waveform w as about twice as large as that seen in rat.