Inhibition of SERCA2 Ca2+-ATPases by Cs+

Replacement of K+ with Cs+ on the cytoplasmic side of the sarcoplasmic reticulum (SR) membrane reduces the maximum velocity (V-max) of Ca2+ uptake into the SR of saponin-permeabilized rat ventricular myocytes. To compare the sensitivity of the cardiac and smooth muscle/non-muscle forms of the sarcoplasmic/endoplasmic reticulurn Ca2+-ATPase (SERCA2a and -2b respectively) to replacement of K+ with Cs+, SERCA2a and SERCA2b were expressed in HEK-293 cells. Ca2+ uptake into HEK cell microsomes was inhibited by replacement of extravesicular K+ with CS+ (V-max of SERCA2a-mediated Ca2+ uptake in CsCl was 80% of that in KC1; V-max of SERCA2b-mediated uptake was 70% of that in KCl). The Ca2+ sensitivity of uptake was decreased for both SERCA2a- and SERCA2b-mediated uptake and the Hill coefficients were increased in the presence of CsCl. The effects of Cs+ on uptake were associated with direct inhibition of the ATPase activity of SERCA2a and SERCA2b. Our results indicate that cation binding sites are present in both SERCA2 isoforms, although the extent to which SERCA2b is inhibited by K+ replacement is greater than that of SERCA2a or SERCA1 Consideration of these results and the recent molecular modeling work of others suggsts that monovalent cations could interact with the Ca2+ binding region of SERCA.