Comparison of sarcolemmal calcium channel current in rabbit and rat ventricular myocytes.

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1. Fundamental properties of Ca2+ channel currents in rat and rabbit ventricular myocytes were measured using whole cell voltage clamp. 2. In rat, as compared with rabbit myocytes, Ca2+ channel current (ICa) was half-activated at about 10 mV more negative potential, decayed slower, 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 suggest that under comparable voltage clamp conditions, including action potential (AP) clamp, more Ca2+ influx would be expected in rat myocytes. 4. Ca2+ channel current carried by Na+ and Cs+ in the absence of divalent ions (Ins) also activated at more negative potential and decayed more slowly in rat. 5. The reversal potential for Ins was 6 mV more positive in rabbit, consistent with a larger permeability ratio (PNa/PCs) in rabbit than in rat. ICa also reversed at slightly more positive potentials in rabbit (such that PCa/PCs might also be higher). 6. Ca2+ influx was calculated by integration of ICa 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 25% greater in rat, as predicted from the fundamental properties of ICa and Ins. 7. However, the longer duration of the AP in rabbit myocytes compensated for the difference in influx, such that the integrated Ca2+ influx via ICa in response to the species-appropriate waveform was about twice as large as that seen in rat.

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