The uptake of 45Ca by the rabbit lens reached equilibrium within 20 hr; the steady state lens/medium ratio was 0.08. This ratio is similar to that found for the lens/medium distribution of extracellular markers. Efflux of 45Ca from the lens was rapid and insensitive to iodoacetate. Mathematical analysis of 45Ca efflux curves revealed that calcium efflux from the lens could be described solely upon the basis of passive movement from the extracellular space. It is concluded that the exchangeable calcium in the normal rabbit lens is predominantly located in the extracellular space.
Membrane-rich vesicle preparations of rabbit and bovine lenses were prepared in such a manner as to preserve ATPase activity. The lipid:protein ratio of these preparations was increased 22- to 33-fold with a 94% recovery of total phospholipid. Using this preparation, calcium stimulated ATPase was routinely determined in both individual lenses and in pooled specimens. The pattern of stimulation of ATPase activity by a range of calcium concentrations was found to be similar in membrane preparations of epithelium and cortex, from rabbit and bovine lenses. The concentration of calcium necessary for half-maximal stimulation of ATPase activity was approximately 10(-6) M. Calcium concentrations in excess of 10(-4) M reduced the ATPase activity. Calcium-ATPase was undetectable in the lens nuclear region of both species. The regional distribution of sodium-potassium ATPase was also measured.
