Regulation of junctional and non-junctional sarcoplasmic reticulum calcium release in excitation-contraction coupling in cat atrial myocytes

We have characterized the dependence on membrane potential (V-m) and calcium current (I-Ca) of calcium-induced calcium release (CICR) from the junctional-SR (j-SR, in the subsarcolemmal (SS) space) and non-junctional-SR (nj-SR, in the central (CT) region of the cell) of cat atrial myocytes using whole-cell voltage-clamp together with spatially resolved laser-scanning confocal microscopy. Subsarcolemmal and central [Ca2+](i) transient amplitudes and I-Ca had a bell-shaped dependence on V-m, but [Ca2+](i) reached a maximum at more negative V-m (-10 to 0 mV) than I-Ca (+10 mV). Termination of I-Ca after a brief depolarization (2.5 to 22.5 ms) immediately interrupted only the SS [Ca2+](i) transient, leaving the development of the CT [Ca2+](i) transient unaffected. Block of SR function with 20 mum ryanodine and 2 mum thapsigargin, revealed that > 90% of the control [Ca2+](i) transient amplitude was attributable to active SR Ca2+ release through ryanodine receptors (RyRs). The gain of SR Ca2+ release was highest in the SS space at negative test potentials and was less pronounced in the CT region. Inhibition of Na+-Ca2+ exchange resulted in prolonged and higher amplitude [Ca2+](i) transients, elevated resting [Ca2+](i), accelerated propagation of CICR, decreased extrusion of Ca2+ and an increase in j-SR Ca2+ load. Increasing the cytosolic Ca2+ buffer capacity by internal perfusion with 1 mm EGTA limited SR Ca2+ release to the SS region, indicating that Ca2+ release from nj-SR is initiated by diffusion of Ca2+ from the cell periphery and propagating CICR. Junctional-SR Ca2+ release occurred at discrete sites whose order of activation and amplitude of release varied from beat to beat. In conclusion, during normal excitation-contraction coupling in cat atrial myocytes, only Ca2+ release from the j-SR is directly activated by Ca2+ entering via I-Ca. Elevation of SS [Ca2+](i) is required to provide the cytosolic Ca2+ gradient needed to initiate regenerative and propagating CICR from nj-SR.