Differentiation between Ca2+ transport and ATP-induced Ca2+ binding by sarcoplasmic reticulum

Effects of compound 48/80 on the Ca2+ release by reversal of the Ca2+ pump and by the Ca2+ channel of sarcoplasmic reticulum membranes

The effect of the calmodulin antagonist, compound 48/80, on the Ca2+ release from skeletal muscle sarcoplasmic reticulum was investigated. Both the Ca2+ release by reversal of the Ca2+ pump and the Ca2+ release by the Mg2(+)-controlled Ca2+ channel were studied. It was observed that, when reversal of the pump is inoperative and Mg2+ is not present in the reaction medium, 48/80 stimulates Ca2+ release from the vesicles. In contrast, in the presence of Mg2+, which blocks the Ca2+ channel, 48/80 inhibits Ca2+ release induced by ADP and Pi. This effect is strong at low concentrations of Pi (approximately 1 mM), whereas high concentrations (approximately 15 mM) protect the system against the drug. Furthermore, it was observed that 48/80 has a maximum effect on the channel-mediated Ca2+ release at concentrations of about 20 micrograms/ml, whereas maximal inhibition of the pump-mediated Ca2+ release occurs at concentrations of about 60-80 micrograms/ml. The results indicate that both the Ca2+ channel complex and the Ca2(+)-ATPase may be target systems for the effects of 48/80 on the Ca2+ transport activity of sarcoplasmic reticulum. However, the Ca2+ channel is more sensitive to the drug, suggesting an involvement of calmodulin on this mechanism of Ca2+ release.

The ADP- and Mg2+-reactive calcium complex of the phosphoenzyme in skeletal sarcoplasmic reticulum Ca2+-ATPase

The effects of ATP on Ca2+ binding in the absence of added Mg2+ to the purified sarcoplasmic reticulum Ca2+-ATPase were studied at pH 7.0 and 0 degrees C. ATP increased the number of Ca2+-binding sites of the enzyme from 2 to 3 mol per mol of phosphorylatable enzyme. The association constant for the ATP-induced Ca2+ binding was 4 X 10(5) M-1, which was not significantly different from that obtained in the absence of ATP. AdoP[CH2]PP had little effect on the Ca2+-binding process. The amount of phosphoenzyme formed was equivalent to the level of ATP-induced Ca2+ binding. ADP decreased the level of ATP-induced Ca2+ binding and phosphoenzyme by the same amount. These results suggest that ATP-induced Ca2+ binding exists in the form of an ADP-reactive phosphoenzyme . Ca complex. In addition, the Ca2+ bound to the enzyme in the presence of ATP was released on the addition of 1 mM MgCl2; after the release of Ca2+, the phosphoenzyme decayed. These observations suggest that Mg2+, added after the ATP-induced Ca2+-binding process, may replace the Ca2+ on the phosphoenzyme and initiate phosphoenzyme decomposition.

Determinants of calcium loading at steady state in sarcoplasmic reticulum

The determinants of steady-state calcium loading by sarcoplasmic reticulum vesicles were evaluated by measuring the contribution of different pathways of calcium flux to the total calcium flux at steady state. The diffusional passive pathway was least significant at all calcium loads studied. Diffusional passive calcium flux was evaluated by a number of methods which gave comparable results and support its designation as passive and diffusional. These methods included (a) flux measurements with the simple pump-leak system which pertains when acetyl phosphate is used to load the vesicles; (b) flux measurements made after quenching the pump with EGTA; (c) flux measurements made after quenching the pump with glucose plus hexokinase; and (d) evaluation of the effect of pump activity on the efflux of mannitol. The calcium efflux not accounted for by the diffusional pathway was assigned to non-diffusional pathways. Efflux through the non-diffusional pathways required ATP, ADP and extravesicular Ca2+. The ADP-dependent, phosphoenzyme-independent pathway described by Beirao and DeMeis (Biochim. Biophys. Acta (1976) 433, 520-530) was not significantly involved in efflux. We propose that the level of calcium loading achieved at steady state is determined by the levels of the intermediates of the calcium pump which are established at this pseudo-equilibrium condition, these levels being determined by the concentrations of intravesicular and extravesicular calcium ([Ca2+]i and [Ca2+]), ATP and ADP. The different levels of calcium loading achieved by skeletal and cardiac sarcoplasmic reticulum are attributed to different nucleotide and calcium kinetics in these two types of sarcoplasmic reticulum and possibly to different intravesicular volumes. Differences in diffusional permeability are not responsible for differences in calcium loading.