Influence of monovalent cations on the Ca 2+-ATPase of sarcoplasmic reticulum isolated from rabbit skeletal and dog cardiac muscles. An interpretation of transient-state kinetic data

Abstract

Transient-state kinetics of phosphorylation and dephosphorylation of the Ca 2+-ATPase of sarcoplasmic reticulum vesicles from rabbit skeletal and dog cardiac muscles were studied in the presence of varying concentrations of monovalent and divalent cations. Monovalent cations affect the two types of sarcoplasmic reticulum differently. When the rabbit skeletal sarcoplasmic reticulum was Ca 2+ deficient, preincubation with K + (as compared with preincubation with choline chloride) did not affect initial phosphorylation at various concentrations of Ca 2+, added with ATP to phosphorylate the enzyme. This is in contrast to preincubation with K + of the Ca 2+-deficient dog cardiac sarcoplasmic reticulum, which resulted in an increase in the phosphoenzyme level. When Ca 2+ was bound to the rabbit skeletal sarcoplasmic reticulum, K + inhibited E ~ P formation; but under the same conditions, E ~ P formation of dog cardiac sarcoplasmic reticulum was activated by K + at 12 μM Ca 2+ and inhibited at 0.33 and 1.3 μM Ca 2+. Li +, Na + and K + also have different effects on E ~ P decomposition of skeletal and cardiac sarcoplasmic reticulum. The latter responded less to these cations than the former. Studies with ADP revealed differences between the two types of sarcoplasmic reticulum. For rabbit skeletal sarcoplasmic reticulum, 40% of the phosphoenzyme formed was ‘ADP sensitive’, and the decay of the remaining E ~ P was enhanced by K + and ADP. Dog cardiac sarcoplasmic reticulum yielded about 40–48% ADP-sensitive E ~ P, but the decomposition rate of the remaining E ~ P was close to the rate measured in the absence of ADP. Thus, these studies showed certain qualitative differences in the transformation and decomposition of phosphoenzymes between skeletal and cardiac muscle which may have bearing on physiological differences between the two muscle types.