Routes of Calcium Flux in Cardiac Sarcoplasmic Reticulum

Abstract

Net release of Ca²⁺ from the sarcoplasmic reticulum (SR) plays an important role in coupling excitation to contraction in cardiac muscle cells, and the uptake of Ca²⁺ by SR plays a major role in effecting relaxation of the contractile apparatus [9,20,26]. There are several possible routes of Ca^{2 +} influx and efflux in SR that could account for the net uptake and release of Ca²⁺. These putative pathways include passive diffusion, pump-mediated Ca²⁺ influx and efflux, carrier-mediated facilitated diffusion, and Ca²⁺ efflux through a gated channel. One goal of research in this field is to identify the routes of Ca²⁺ flux in SR and to determine their magnitude, time course, and roles in physiological regulation of Ca²⁺ flux. In this presentation, the routes of Ca²⁺ flux in SR vesicles isolated from dog hearts are examined. The results suggest that Ca²⁺ fluxes in cardiac SR occur through only three routes: (1) forward pump-mediated Ca²⁺ influx; (2) reverse pump-mediated Ca²⁺ efflux; and (3) passive efflux.