CA2-INDUCED INACTIVATION OF CA2+ CURRENT IN RAT VENTRICULAR MYOCYTES - EVIDENCE FOR LOCAL CA2+ SIGNALING( RELEASE)

1. Inactivation of Ca2+ current (I-Ca) induced by Ca2+ release from sa rcoplasmic reticulum (SR) was studied in single rat ventricular myocyt es using whole-cell patch-clamp and indo-1 fluorescence measurement te chniques. 2. Depolarizing pulses to 0 mV elicited large Ca2+ transient s and I-Ca with biexponential inactivation kinetics. Varying SR Ca2+ l oading by a 20 s pulse of caffeine showed that the fast component of I -Ca inactivation was dependent on the magnitude of Ca2+ release. 3. In activation of I-Ca induced by Ca2+ release was quantified, independent ly of voltage and Ca2+ entry, using a function termed fractional inhib ition of I-Ca (FICa). The voltage relation of FICa had a negative slop e, resembling that of single-channel Ca2+ current (i(Ca)) rather than the bell-shaped current-voltage (I-V) relation of macroscopic I-Ca and Ca2+ transients. 4. Intracellular dialysis of myocytes with 10 mM EGT A (150 nM free [Ca2+]) had no effect on I-Ca inactivation induced by C a2+ release, despite abolition of Ca2+ transients and cell contraction . Dialysis with 3 or 10 mM BAPTA (180 nM free [Ca2+]) attenuated FICa in a concentration-dependent manner, with greater inhibition at positi ve than at negative potentials, consistent With more effective bufferi ng of Ca2+ microdomains of smaller i(Ca). 5. Spatial profiles of [Ca2] near an opened Ca2+ channel were simulated. [Ca2+] reached submillim olar levels at the mouth of the channel, and dropped steeply as radial distance increased. At any given distance from the channel, [Ca2+] wa s higher at negative than at positive potentials. The radii of Ca2+ mi crodomains were significantly reduced by 3 or 10 mM BAPTA, but not by 10 mM EGTA. 6. In conclusion, the distinctive voltage dependence and s usceptibility of Ca2+ release-induced I-Ca inactivation to fast and sl ow Ca2+ buffers suggests that the process is mediated through local ch anges of [Ca2+] in the vicinity of closely associated Ca2+ channels an d ryanodine receptors.