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 sup -1) followed by a slowly activating phase (slope 2, 5.8 +/- 0.4 s sup -1; P<.001 versus slope 1), resulting in a dome-shaped Ca_i transient. Ryanodine (100 micro 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 sub 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 Ca_i 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 I_Ca. 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 partially 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. (Circ Res. 1997;80:345-353.)