Undirectional calcium and nucleotide fluxes in cardiac sarcoplasmic reticulum. II. Experimental results

Unidirectional calcium influx and efflux were evaluated in cardiac sarcoplasmic reticulum (SR) by 45Ca-40Ca exchange at steady state calcium uptake in the absence of calcium precipitating anions. Calcium efflux was partitioned into a pump-mediated efflux and a parallel passive efflux by separately measuring passive efflux referable to the steady state. Unidirectional and net ATP-ADP fluxes were measured using [3H]-ATP----ADP and [3H]-ADP----ATP exchanges. Methods are presented that take into account changing specific activities and sizes of the nucleotide pools during the measurement of nucleotide fluxes. The contribution of competent and incompetent vesicles to the unidirectional and net nucleotide fluxes was evaluated from the specific activity of these fluxes in incompetent vesicles and from the fraction of vesicles that were incompetent. The results indicate that, in cardiac SR, unidirectional calcium fluxes are larger than the unidirectional nucleotide fluxes contributed by competent vesicles. Because the net ATPase rate of competent vesicles is similar to the parallel passive efflux, it appears that cardiac SR Ca-ATPase tightly couples ATP hydrolysis to calcium transport even at static head, with a coupling ratio near 1.0.