We used whole cell patch-clamp and microfluorimetric (indo 1) techniques to measure Ca2+ current through L-type Ca2+ channels (I(Ca)) and Ca2+ transients in human atrial myocytes. During 1-s depolarizing pulses, I(Ca) inactivation was biexponential. The rate of rapid inactivation was slowed by ryanodine and was correlated with the rate of rise of cytoplasmic free Ca2+ concentration (r = 0.80, P < 0.01). Slower-phase I(Ca) inactivation was not affected by ryanodine but was accelerated by increasing the availability of Ca2+ to permeate the Ca2+ channel. Thus Ca2+ released from the sarcoplasmic reticulum (SR) was responsible for most I(Ca) inactivation during the first 50 ms of a depolarization to 0 mV, and thereafter inactivation by Ca2+ permeating the channel predominated. Pure voltage-dependent inactivation had a much slower time course of development (tau > 2 s) and played a smaller role than Ca2+-dependent mechanisms over a duration comparable to that of an action potential. We conclude that human atrial myocytes show both voltage- and Ca2+-dependent I(Ca) inactivation, that Ca2+-dependent mechanisms predominate over the time course of an action potential, and that although both Ca2+ released from the SR and Ca2+ permeating Ca2+ channels play a role, SR-released Ca2+ is particularly important in early, rapid I(Ca) inactivation, whereas Ca2+ permeating Ca2+ channels is more important in the slower phase of Ca2+-dependent inactivation.
Impact of Sarcoplasmic Reticulum Calcium Release on Calcium Dynamics and Action Potential Morphology in Human Atrial Myocytes: A Computational Study