A discrete Na-Ca exchange-dependent Ca compartment in rat ventricular cells: exchange and localization

Application of a new rapid perfusion (up to 4.8 ml/s) technique to 45Ca-labeled ventricular cells from adult rat heart has defined a discrete intracellular calcium (Ca) compartment with the following characteristics: 1) its exchange is absolutely dependent on operation of the Na-Ca exchanger, i.e., its isotopic content remains constant during washout in the absence of Na and Ca and is released only upon addition of Na and Ca to the perfusate. 2) At an extracellular Ca concentration of 1.0 mM it contains 350 mumol/kg dry wt cells and exchanges with half time of 650 ms. Ca flux from the compartment is 385 mumol.kg dry wt-1.s-1 or 20% of the total nonperfusion limited flux from the cells. 3) Its content is decreased 19% by 10 mM caffeine but not diminished by exposure of the cells to 10(-6) M ryanodine and not accessible to lanthanum (La) displacement. 4) Only limited exchange occurs when only Na or Ca is present and exchange is virtually eliminated by substitution of extracellular Li for extracellular Na. 5) Replacement of Na and Ca to the perfusate after various periods of removal produces no contraction (despite immediate Ca release from the cell). The results define a discrete intracellular Ca compartment which exchanges only via the Na-Ca exchanger. It is not La accessible, not in the ryanodine- or caffeine-sensitive portions of the sarcoplasmic reticulum, not in the mitochondria nor at the myofilaments, but may reside at inner sarcolemmal leaflet binding sites.