Properties of human atrial I(Ca) at physiological temperatures and relevance to action potential

Abstract

There are no published characterizations of Ca2+ current ((ICa)) at physiological temperatures in human atrium. Depolarization of human atrial myocytes at 36°C elicited I(Ca) that peaked at +10 mV, with a mean maximum current density of 10.8 ± 1.1 pA/pF and no evidence for T-type current. Overlap between activation and inactivation curves and incomplete inactivation during pulses comparable to normal action potential duration (APD) were compatible with the observed role of I(Ca) in maintaining the plateau. I(Ca) was frequency dependent between 0.1 and 2 Hz, and I(Ca) blockade with 0.2 mM Cd2+ reduced rate-dependent changes in APD: under control, APD at 90% repolarization was 230 ± 15 ms at 0.1 Hz and 178 ± 14 ms at 2 Hz (decrease of 52 ± 5 ms); with Cd2+, values were 121 ± 7 ms at 0.1 Hz and 115 ± 6 ms at 2 Hz (decrease of 6 ± 3 ms, P < 0.01). Isoproterenol (1 μM) increased I(Ca) and prolonged APD from 138 ± 13 to 199 ± 15 ms (P < 0.01). These results indicate that, in human atrial cells at 36°C, the properties of L-type I(Ca) contribute importantly to the rate- dependent and autonomic control of APD.