REGULATION OF CALCIUM-PUMP FUNCTION IN BACK INHIBITED VESICLES BY CALCIUM-ATPASE LIGANDS

Objective: The aim was to evaluate the effect of changes in substrate-product concentrations of the overall sarcoplasmic reticular Ca2+-ATPase reactions on calcium pump function with an emphasis on those factors that modify back inhibition of the pump by intrareticular free calcium. Methods: Sarcoplasmic reticular vesicles were isolated from the rabbit heart. Calcium uptake by the vesicles was measured with a calcium sensitive minielectrode and was related to ATPase activity, which was measured by the rate of inorganic phosphate (Pi) production or NADH oxidation. Back inhibition was varied by changing [oxalate]. Results: At the high level of back inhibition and low calcium transport efficiency, calcium uptake by sarcoplasmic reticulum was stimulated by pH decrease, hydrophobic compounds, dimethyl sulphoxide, and ATP regeneration. These factors apparently modify either calcium binding to the low affinity binding site of Ca2+-ATPase or the effect derived from the binding of calcium to these sites. Under conditions where back inhibition was avoided and calcium transport efficiency was high, the same factors had a depressive effect on calcium uptake. Inorganic phosphate had a dual effect on the rate of calcium uptake supported by low [oxalate]: Pi < 2-3 mM strongly inhibited calcium uptake while further increase in [Pi] reversed this inhibition. At the lower level of back inhibition higher [Pi] was required to inhibit calcium transport. Conclusions: Changes in [H+], [ADP], and [Pi] can significantly affect calcium pump function, but the effect is dependent on the extent of back inhibition of calcium transport. Changes in the sarcoplasmic reticular Ca2+-ATPase environment which mimic those expected to take place at the start of reperfusion (pH increase to 7.0-7.1, high myoplasmic [Ca2+]) may have a depressive effect on the efficiency of calcium transport, provided that intrareticular free calcium is increased. Under those conditions factors able to decrease the inhibitory effect derived from the calcium binding to the low affinity binding sites are expected to improve the efficiency of calcium transport.