The relative contributions of different intracellular and sarcolemmal systems to relaxation in rat ventricular myocytes

OBJECTIVE: The aim was to estimate the relative contributions of the various intracellular and sarcolemmal systems to the relaxation of the systolic calcium transient. METHODS: The experiments were performed on isolated rat ventricular myocytes. The cells were loaded with the fluorescent indicator indo-1 in order to measure Ca2+i. RESULTS: The application of caffeine to release calcium from the sarcoplasmic reticulum produced a rise of Ca2+i which decayed about 7-8 times more slowly than the electrically stimulated calcium transient. This suggests that the sarcoplasmic reticulum accounts for about 87% of the calcium removal. The rate of decay of the caffeine response was decreased to about 33% of the control by inhibiting the Na-Ca exchange with Ni2+. In the presence of Ni2+ the rate could be inhibited further by inhibiting either the sarcolemmal Ca-ATPase (by increasing extracellular calcium concentration, Ca2+o) or the mitochondria (with FCCP and oligomycin). The relative contributions of the various processes were estimated to be: sarcoplasmic reticulum 87%, mitochondria 1.7%, Na-Ca 8.7%, sarcolemmal Ca-ATPase 2.6%. CONCLUSIONS: These experiments show that the Na-Ca exchange accounts for 67% of the calcium removal not mediated by the sarcoplasmic reticulum. This is a smaller fraction than in rabbit cardiac cells and highlights the importance of the Ca-ATPase in the rat heart.