Calcium transport, Ca2(+)-ATPase, and lipid order in rabbit ocular lens membranes

Calcium transport was monitored by measuring ATP-dependent 45Ca uptake into membrane vesicles prepared from rabbit lens cortex. Calcium-stimulated adenosinetriphosphatase (Ca2(+)-ATPase) activity was also measured in the same membrane preparation. Both uptake and Ca2(+)-ATPase activity were inhibited by vanadate. Calcium activation of the uptake process was similar to that of the Ca2(+)-ATPase. Calcium uptake was prevented by calcium ionophore A23187, suggesting that the calcium transported into the vesicles remains diffusible. The ATP-dependent calcium uptake probably represents the transport of calcium into "inside-out" membrane vesicles by the Ca2(+)-ATPase mechanism that normally shifts calcium outward from the lens cytoplasm. The temperature dependence of the Ca2(+)-ATPase and the calcium uptake process was determined. Because lipid order can influence Ca2(+)-ATPase function, we attempted to correlate calcium transport with the physical state of the membrane lipids. Infrared spectroscopy was used to determine the temperature dependence of the CH2 symmetric stretching frequency (an order parameter) in the lipids. A similarity was noted between the temperature-dependence curves for lipid order, Ca2(+)-ATPase, and calcium uptake rate. Entropy, enthalpy, and transition temperature calculated for the Ca2(+)-ATPase and calcium uptake process were in the same range as those parameters calculated for the lipid-phase transition.