Iodide and bromide inhibit Ca uptake by cardiac sarcoplasmic reticulum

Kargacin, Gary J., Zenobia Ali, Shi-Jin Zhang, Natashka S. Pollock, and Margaret E. Kargacin. Iodide and bromide inhibit Ca uptake by cardiac sarcoplasmic reticulum. Am J Physiol Heart Circ Physiol 280: H1624–H1634, 2001.—Recent studies indicate that the Ca permeability of the sarcoplasmic reticulum (SR) can be affected by its anionic environment. Additionally, anions could directly modulate the SR Ca pump or the movement of compensatory charge across the SR membrane during Ca uptake or release. To examine the effect of anion substitution on cardiac SR Ca uptake, fluorometric Ca measurements and spectrophotometric ATPase assays were used. Ca uptake into SR vesicles was inhibited in a concentration-dependent manner when Br or I replaced extravesicular Cl (when Br completely replaced Cl, uptake velocity was ; 70% of control; when I completely replaced Cl, uptake velocity was ; 39% of control). Replacement of Cl with SO4 22 had no effect on SR uptake. Although both I and Br inhibited net Ca uptake, neither anion directly inhibited the SR Ca pump nor did they increase the permeability of the SR membrane to Ca. Our results support the hypothesis that an anionic current that occurs during SR Ca uptake is reduced by the substitution of Br or I for Cl.