L- and T-type Calcium Currents Differ in Finch and Rat Ventricular Cardiomyocytes

We compared -type Ca current (ICaL) and -type Ca current (ICaT) in finch and rat myocytes, using whole-cell patch clamp techniques. Cell capacitance averaged 50±4 pF in finch (n=25)v145±8 pF in rat (n=38) cells,P<0.001. In cells bathed in 1 m Caoat 22°C, peakICaLamplitude, during a voltage clamp step (10 m EGTA in pipette) from −45 mV to −5 mV, averaged 10.5±0.3 pA/pF in finchv6.9±0.6 pA/pF,P<0.001 in rat cells.ICaLinactivation kinetics were faster in finch (4.6±0.3 ms) than in rat (13.4±1.3 ms) cells,P<0.001.ICaTwas not detectable in rat cells (2 m bathing [Ca]); but in finch cells, a largeICaTwhich averaged 6.8±1.4 pA/pF was activated at −30 mV and was relatively insensitive to nitrendipine (0.1μ ). The distinctive features ofICaLandICaTin 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.