Quantitative Assessment of the SR Ca Leak-Load Relationship

Increased diastolic SR Ca leak (Jleak) could depress contractility in heart failure, but there are conflicting reports regarding the Jleak magnitude even in normal, intact myocytes. We have developed a novel approach to measure SR Ca leak in intact, isolated ventricular myocytes. After stimulation, myocytes were exposed to 0 Na , 0 Ca solution 1 mmol/L tetracaine (to block resting leak). Total cell [Ca ] does not change under these conditions with Na -Ca exchange inhibited. Resting [Ca ]i declined 25% after tetracaine addition (126 6 versus 94 6 nmol/L; P 0.05). At the same time, SR [Ca ] ([Ca ]SRT) increased 20% (93 8 versus 108 6 mol/L). From this Ca 2 shift, we calculate Jleak to be 12 mol/L per second or 30% of the SR diastolic efflux. The remaining 70% is SR pump unidirectional reverse flux (backflux). The sum of these Ca effluxes is counterbalanced by unidirectional forward Ca pump flux. Jleak also increased nonlinearly with [Ca 2 ]SRT with a steeper increase at higher load. We conclude that Jleak is 4 to 15 mol/L cytosol per second at physiological [Ca ]SRT. The data suggest that the leak is steeply [Ca ]SRT-dependent, perhaps because of increased [Ca 2 ]i sensitivity of the ryanodine receptor at higher [Ca 2 ]SRT. Key factors that determine [Ca ]SRT in intact ventricular myocytes include (1) the thermodynamically limited Ca 2 gradient that the SR can develop (which depends on forward flux and backflux through the SR Ca ATPase) and (2) diastolic SR Ca leak (ryanodine receptor mediated). (Circ Res. 2002;91:594-600.)