Iodide and bromide inhibit Ca2+ uptake by cardiac sarcoplasmic reticulum

Recent studies indicate that the Ca2+ permeability of the sarcoplasmic reti culum (SR) can be affected by its anionic environment. Additionally, anions could directly modulate the SR Ca2+ pump or the movement of compensatory c harge across the SR membrane during Ca2+ uptake or release. To examine the effect of anion substitution on cardiac SR Ca2+ uptake, fluorometric Ca2+ m easurements and spectrophotometric ATPase assays were used. Ca2+ uptake int o SR vesicles was inhibited in a concentration-dependent manner when Br- or I- replaced extravesicular Cl- (when Br- completely replaced Cl-, uptake v elocity was; 70% of control; when I- completely replaced Cl-, uptake veloci ty was similar to 39% of control). Replacement of Cl- with SO42- had no eff ect on SR uptake. Although both I- and Br- inhibited net Ca2+ uptake, neith er anion directly inhibited the SR Ca2+ pump nor did they increase the perm eability of the SR membrane to Ca2+. Our results support the hypothesis tha t an anionic current that occurs during SR Ca2+ uptake is reduced by the su bstitution of Br- or I- for Cl-.