Ca²⁺-dependent ATPase activity of bovine receptor cell outer segment

Abstract

THE bleaching of as little as one molecule of rhodopsin can trigger hyperpolarisation of the vertebrate visual receptor cell¹. A light modulated Ca²⁺ flux may function in the receptor cell outer segments (OS) as an intracellular messenger amplifying and transducing the photochemical signal from its disk location to the external plasma membrane^2,3. In support of this hypothesis, Ca²⁺ has been shown to be concentrated in dark adapted disks⁴ and to be released on bleaching^5–7. This suggests the presence of a Ca²⁺-dependent ATPase, an enzyme often associated with cellular Ca²⁺ pumps. The report that either the bleaching of a few per cent of the rhodopsin molecules within a frog rod suspension or the intra-OS elevation of Ca²⁺ to 10⁻⁷ M or greater, by means of ionophore A23187, can induce a rapid and very large decrease in ATP content⁸ provides indirect evidence for the presence of a Ca²⁺-activated ATPase. Interpretation of this finding is difficult, however, due to other known light-activated, ATP-dependent OS functions⁹. We present here direct evidence that purified bovine OS exhibit significant Ca²⁺-dependent ATPase activity, which seems to be independent of contributions from subcellular contamination.