Different compartments of sarcoplasmic reticulum participate in the excitation-contraction coupling process in human atrial myocytes

The excitation-contraction coupling process of human atrial myocytes was studied in voltage-clamped myocytes isolated from right atrial appendages obtained during cardiac surgery. Intracellular Ca2+ transients (Ca-i transients) were monitored with 0.1 mmol/L indo 1 added to the internal dialyzing solution. Ryanodine receptors (RyRs) and sarcomeric alpha-actinin were stained with specific antibodies and visualized using plane and confocal microscopy. L-Type Ca2+ current (I-Ca) elicited a prolonged Ca-i transient, with an initial rapidly activating phase (slope 1, 23.6+/-1.2 s(-1)) followed by a slowly activating phase (slope 2, 5.8+/-0.4 s(-1); P<.001 versus slope 1), resulting in a dome-shaped Ca-i transient. Ryanodine (100 mu mol/L) inhibited 79+6% of the Ca-i transient, indicating that it was due essentially to sarcoplasmic reticulum Ca2+ release. During step depolarizations, maximal activation of the Ca-i transient or tail current (I-tail) (in cells dialyzed with Ca2+ buffer-free internal solution) preceded that of I-Ca and did not follow its voltage dependence (n=12). Test pulses lasting from 5 to 150 milliseconds elicited a similar time course of both Cai transient and I-tail (n=5). In a given cell, the two components of the Ca-i transient could be dissociated by altering the intracellular Ca2+ load, by increasing the stimulation rate from 0.1 to 1 Hz, or by varying the amplitude of h. Immunostaining of atrial sections and isolated myocytes showed that a large number of RyRs were located not only in a subsarcolemmal position but also deeper inside the cell, in a regularly spaced transverse band pattern at the level of Z lines. Together, our results indicate that, in human atrial myocytes, I-Ca only par tially controls the activation of RyRs, with the prolonged and dome-shaped Ca-i transient of these cells probably reflecting the activation of RyRs not coupled to L-type Ca2+ channels.