CALCIUM-PUMP IN THE DISK MEMBRANES ISOLATED FROM BOVINE RETINAL ROD OUTER SEGMENTS

The existence of a Ca2+ pump in rod outer segment disks of bovine reti na is strongly suggested by the isolation on sodium dodecyl sulfate po lyacrylamide gel electrophoresis of a hydroxylamine-sensitive phosphor ylated intermediate (E-P) of molecular mass of about 100 kDa as well a s by measurements of active calcium transport and adenosine 5'-triphos phate (ATP) hydrolysis. Active Ca2+ uptake by disks was dependent on t he presence of Mg2+-ATP, was inhibited by vanadate or lanthanum and ap peared poorly sensitive to calmodulin. ATP hydrolysis by disk membrane s was a function of free Ca2+ concentration in the absence of exogenou s Mg2+ The presence of a Ca2+ pump on disk membranes is discussed in t erms of its possible role in Ca2+ ion buffering during photoreceptor c ell functioning.