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

Abstract

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.

Lipid composition, membrane structure relationships in lens and muscle sarcoplasmic reticulum membranes

“…We have shown previously that the Ca 2ϩ -ATPase activity in the lens was correlated to lipid order. 80,81 SR Ca 2ϩ -ATPase, reconstituted into ordered (about 40% trans) lens lipids, were five times less active than when reconstituted with native SR lipids. 81 It has been established that in synthetic lipid systems, the mobility of a probe in the lipid bilayer (except in the case of lipids completely in the gel or liquid crystalline phase) does not correlate with Ca 2ϩ -ATPase activity.…”

Ca2+-ATPase Activity and Lens Lipid Composition in Reconstituted Systems

“…Based on the findings that lipids with highly structurally ordered hydrocarbon chains, completely in the trans conformation, inhibit Ca# + -ATPase activity, and that, in general, the conformation of biomolecules influence their interactions, it is reasonable to speculate that the relatively high hydrocarbon order of bovine lens lipids (Borchman, Cenedella and Lamba, 1996c) contribute to the inhibition of the pump. It has been shown that the hydrocarbon trans rotomer content directly correlated with the Ca# + -ATPase activity and calcium translocation in rabbit lens cortical microsomes (Delamere et al, 1991).…”

“…Calcium homeostasis is essential to the maintenance of lens clarity (Clark, Bagg and Benedek, 1980 ;Hightower and Reddy, 1982) and the Ca# + -ATPase pump is essential for the removal of cytosolic calcium either across the plasma membrane (Delamere et al, 1991 ;Borchman, Delamere and Paterson, 1988 ;Galvin and Louis, 1988) or into internal organelles such as the endoplasmic reticulum (Duncan et al, 1993). The lens contains comparable amounts of two isoforms of Ca# + -ATPase, the SERCA and PM types (Zeng, Borchman and Paterson, 1995).…”

Thermodynamic Phase Transition Parameters of Human Lens Dihydrosphingomyelin

“…More ordered membranes arc less permeable to oxygen and cations [ 13]. The other phospholipids are more 'fluid' [6] and are possibly necessary to provide a fluid matrix that supports pump [7] and enzyme activity. DHS undoubtedly provides structur al order to the hydrocarbon chain region and upholds the integrity of the membrane under oxidative conditions.…”

“…The focus of this study is on hydrocarbon chain structure. We have previously used spectroscopic techniques such as infrared [6][7][8][9] and FTRaman [10] and dispersive Raman [11] techniques to charac terize lens lipid hydrocarbon chain structure in human and rabbit lenses.…”