L- and T-type calcium currents differ in finch and rat ventricular cardiomyocytes

We compared L-type Ca current (I(CaL)) and T-type Ca current (I(CaT)) in finch and rat myocytes, using whole-cell patch clamp techniques. Cell capacitance averaged 50 ± 4 pF in finch (n = 25) v 145 ± 8 pF in rat (n = 38) cells, P < 0.001. In cells bathed in 1 mM Ca(o) at 22^°C, peak I(CaL) amplitude, during a voltage clamp step (10 mM EGTA in pipette) from -45 mV to -5 mV, averaged 10.5 ± 0.3 pA/pF in finch v 6.9 ± 0.6 pA/pF, P < 0.001 in rat cells. I(CaL) inactivation kinetics were faster in finch (4.6 ± 0.3 ms) than in rat (13.4 ± 1.3 ms) cells, P < 0.001. I(CaT) was not detectable in rat cells (2 mM bathing [Ca]); but in finch cells, a large I(CaT) which averaged 6.8 ± 1.4 pA/pF was activated at -30 mV and was relatively insensitive to nitrendipine (0.1 μM). The distinctive features of I(CaL) and I(CaT) in finch cells may have a role in the ability of the finch to achieve a very rapid heart rate. They may also facilitate excitation-Ca²⁺ release coupling in finch ventricular cells which are devoid of T tubules and have relatively few junctions between the sarcolemma and the sarcoplasmic reticulum.