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Summary Arrhenius plots of Ca ++ -stimulated ATP hydrolysis by sarcoplasmic reticulum (SR) show breaks (T t ) at 16.7°C and 11.5°C for rabbit and lobster preparations, respectively. The energies of activation (Eact) are about 10 and 19.5 Kcal/mole above and below T t , respectively, and are similar for both lobster and rabbit SR. The antibiotic filipin increases T t by about 7°C for both preparations, but the Eact above and below the new T t values remain similar to those of the controls. Desintegrated membranes do not show breaks in Arrhenius plots and the Eact assume relatively high values. The Ca ++ ionophore X-537A does not affect either the T t values nor the Eact.

Sarcolemma tubes isolated from rabbit skeletal muscle exhibit a predominantly radial contraction which can be induced by ATP, Zn2+, and Cd2+ and to a smaller extent by ADP, SCN−, Ca2+ and Mg2+. The contraction is an intrinsic property of the sarcolemma, since it cannot be attributed to osmotic effects or to the activity of adhering myofibrils. Parallel studies on the surface ATPase activity of isolated semitendinosus muscles of the frog show that this enzymatic activity is inhibited reversibly by Zn2+ or Cd2+. The results are related to the twitch potentiating effects of some of the substances tested.

1 The Ca2+ bound actively in the presence of ATP by sarcoplasmic reticulum isolated from rabbit skeletal muscle is released by Zn2+ (or Cd2+) as well as by phospholipases A or C and by trypsin. Zinc ions also release Mg2+ and are taken up passively by the reticulum membranes, whereas phospholipases A and C and trypsin release only the actively bound Ca2+. 2 Phospholipase A, and to a smaller extent phospholipase C and trypsin, increase the passive cation-binding capacity of reticulum. Thus, concurrently with the loss of selectively bound Ca2+ caused by these agents, there is an increase in the passively bound Mg2+. The increase in the passive cation-binding capacity of reticulum induced by phospholipase depends on the presence of the splitting products of phospholipids, since their removal when digested reticulum is washed with a 2.5% albumin solution decreases the binding capacity to about 50% of the original value. 3 When Ca2+, Mg2+ or Sr2+ is bound passively by reticulum, the ratio of H+ released to either of these divalent cations bound, expressed in terms of nequiv H+ released per nequiv divalent cation bound is about 1.0, but when Zn2+ is taken up, the ratio H+/Zn2+ is only about 0.5, which suggests that some of the Zn2+ is bound at sites that do not release H+ or bind Ca2+. 4 After lipid extraction with 90% acetone, the cation-binding capacity and H+ release are decreased to about 40% of the original values. This effect is particularly evident between pH values of about 6.0 and 8.0. The apparent passive Ca2+-binding affinity is not affected by the lipid removal.

Ruthenium red, a powerful inhibitor of Ca 2+ transport by mitochondria, does not inhibit the active Ca 2+ uptake by sarcoplasmic reticulum isolated from rabbit skeletal muscle promoted by 5 mM ATP-Mg in the presence or absence of potassium oxalate. Although concentrations of ruthenium red up to 100 μM do not affect the active uptake of Ca 2+ , 25 μM of the inorganic dye inhibit the passive binding of Ca 2+ by about 50%. This inhibitory effect is observed in sarcoplasmic reticulum even after its lipid fraction is extracted with acetone. Although active Ca 2+ uptake by sarcoplasmic reticulum is not inhibited by ruthenium red, in the absence of oxalate it inhibits significantly the Ca 2+ -dependent ATPase activity but not the Mg 2+ -ATPase. However, if potassium oxalate is present, the Ca 2+ -stimulated ATPase is not sensitive to the dye. It is not clear how oxalate functions to protect the Ca 2+ -ATPase against the inhibitor effect of ruthenium red. The high sensitivity to ruthenium red of the Ca 2+ transport mechanism in mitochondria as compared to the Ca 2+ transport in sarcoplasmic reticulum may be useful in determining the extent to which each organelle functions in the cell to regulate intracellular free Ca 2+ .