Cross-signaling between L-type Ca2+ channels and ryanodine receptors in rat ventricular myocytes

Calcium-mediated cross-signaling between the dihydropyridine (DHP) receptor, ryanodine receptor, and Na(+)-Ca2+ exchanger was examined in single rat ventricular myocytes where the diffusion distance of Ca2+ was limited to < 50 nm by dialysis with high concentrations of Ca2+ buffers. Dialysis of the cell with 2 mM Ca(2+)- indicator dye, Fura-2, or 2 mM Fura-2 plus 14 mM EGTA decreased the magnitude of ICa-triggered intracellular Ca2+ transients (Cai-transients) from 500 to 20-100 nM and completely abolished contraction, even though the amount of Ca2+ released from the sarcoplasmic reticulum remained constant (approximately 140 microM). Inactivation kinetics of ICa in highly Ca(2+)-buffered cells was retarded when Ca2+ stores of the sarcoplasmic reticulum (SR) were depleted by caffeine applied 500 ms before activation of ICa, while inactivation was accelerated if caffeine-induced release coincided with the activation of ICa. Quantitative analysis of these data indicate that the rate of inactivation of ICa was linearly related to SR Ca(2+)-release and reduced by > 67% when release was absent. Thapsigargin, abolishing SR release, suppressed the effect of caffeine on the inactivation kinetics of ICa. Caffeine-triggered Ca(2+)-release, in the absence of Ca2+ entry through the Ca2+ channel (using Ba2+ as a charge carrier), caused rapid inactivation of the slowly decaying Ba2+ current. Since Ba2+ does not release Ca2+ but binds to Fura-2, it was possible to calibrate the fluorescence signals in terms of equivalent cation charge. Using this procedure, the amplification factor of ICa-induced Ca2+ release was found to be 17.6 +/- 1.1 (n = 4). The Na(+)-Ca2+ exchange current, activated by caffeine-induced Ca2+ release, was measured consistently in myocytes dialyzed with 0.2 but not with 2 mM Fura-2. Our results quantify Ca2+ signaling in cardiomyocytes and suggest the existence of a Ca2+ microdomain which includes the DHP/ ryanodine receptors complex, but excludes the Na(+)-Ca2+ exchanger. This microdomain appears to be fairly inaccessible to high concentrations of Ca2+ buffers.