Effects of monovalent cations on Ca2+ uptake by skeletal and cardiac muscle sarcoplasmic reticulum

Ca2+ transport by the sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA) is sensitive to monovalent cations. Possible K+ binding sites have been identified in both the cytoplasmic P-domain and the transmembrane transport-domain of the protein. We measured Ca2+ transport into SR vesicles and SERCA ATPase activity in the presence of different monovalent cations. We found that the effects of monovalent cations on Ca2+ transport correlated in most cases with their direct effects on SERCA. Choline+, however, inhibited uptake to a greater extent than could be accounted for by its direct effect on SERCA suggesting a possible effect of choline on compensatory charge movement during Ca2+ transport. Of the monovalent cations tested, only Cs+ significantly affected the Hill coefficient of Ca2+ transport (nH). An increase in nH from ∼2 in K+ to ∼3 in Cs+ was seen in all of the forms of SERCA examined. The effects of Cs+ on the maximum velocity of Ca2+ uptake were also different for different forms of SERCA but these differences could not be attributed to differences in the putative K+ binding sites of the different forms of the protein.