Phosphorylation of purified bovine cardiac sarcolemma and potassium‐stimulated calcium uptake

Sarcolemmal vesicles were prepared from bovine cardiac muscle by differential and discontinuous sucrose density gradient centrifugation. Na+/K+-ATPase was purified 33-fold to a specific activity of 53 ± 0.5 (12) μmol Piμmg±1×h−1, binding sites for strophantin 20-fold to a density of 56.3 ± 5.3 (14) pmol/mg and that for the calcium antagonist nitredipine 5.5-fold to a density of 0.72 ± 0.07 (6) pmol/mg. The specific activity of the Na+/Ca2+ exchanger was 61.1 ± 3.7 (6) nmol/mg. The vesicles had an intravesicular volume of 20 ± 4 (4) μl/mg and 56.9 ± 6 (4)% of the vesicles were right-side-out oriented. Several peptides of the purified membranes were phosphorylated in the presence of Mg · ATP and EGTA. Most of the radioactive phosphate was incorporated into a peptide with an apparent molecular mass of 22 kDa. Denaturation of the membranes at 100°C changed the mobility of this peptide to 15 kDa and 11 kDa. This peptide could not be distinguished from a sarcoplasmic reticulum peptide of similar molecular mass. The phosphorylation of the sarcolemmal peptide was stimulated by Ca2+/calmodulin, cAMP and the catalytic subunit of cAMP-dependent protein kinase. A comparison of the phosphorylation of sarcolemmal membranes with that of sarcoplasmic reticulum showed that Ca2+/calmodulin stimulated in each membrane, the phosphorylation of the 22-kDa peptide and a 44-kDa peptide, and in the sarcoplasmic reticulum the phosphorylation of an additional peptide of 55-kDa. Ca2+/calmodulin-dependent phosphorylation of a 55-kDa peptide could not be demonstrated in sarcolemma, regardless if sarcolemmal membranes were incubated together with sarcoplasmic reticulum or if the phosphorylation was carried out in the presence of purified cardiac myosin light chain kinase or phosphorylase kinase. ‘Depolarization’ induced Ca2+ uptake which was measured according to Bartschat, D. K., Cyr, D. L. and Lindenmayer, G. E. [(1980) J. Biol. Chem. 255, 10044–10047] was 5 nmol/mg protein. This uptake was not enhanced after preincubation of the vesicles with Mg · ATP or Mg · ATP and cAMP-dependent protein kinase. The value of 5 nmol/mg protein is in agreement with the theoretical amount of Ca2+ which can be accumulated by the bovine cardiac sarcolemma in the absence of a driving force other than the Ca2+ gradient. The potassiumstimulated Ca2+ uptake was not blocked by the organic Ca2+ channel blockers. Prolonged incubation of Mg · ATP with sarcolemmal vesicles in the presence of various ATPase inhibitors led to the hydrolysis of ATP. The liberated phosphate precipitated with Ca2+ in the presence of LaCl3. These precipitates amounted to an apparent Ca2+ uptake ranging from 50 to over 1000 nmol/mg. The results suggest that potassium-stimulated Ca2+ uptake of bovine cardiac sarcolemmal vesicles is not enhanced in the presence of ATP or by phosphorylation of a 22-kDa peptide.