ATPases of ciliary epithelium: cellular and subcellular distribution and probable role in secretion of aqueous humor

Abstract

The distribution of ion-stimulated ATPases of the ciliary epithelium has been examined in tissues from bovine and rabbit eyes. In homogenates of tissues from both species, both Na,K- and anion-stimulated enzyme activities were found, but no K,H-stimulated activity was detected. The anion ATPase had a broad specificity for a number of anions, and was strongly inhibited by thiocyanate. Following separation of pigmented (outer) and non-pigmented (inner) layers of the bovine ciliary epithelium and isolation of the two cell types on density gradients, higher activities of both Na,K- and anion ATPases were found in the non-pigmented cells. Subcellular fractionation of a mixed population of cells showed that the anion ATPase was almost exclusively associated with a mitochondrial fraction, rather than with the plasma-membrane fraction containing the Na,K-ATPase. These results confirm histochemical studies of the distribution of Na,K-ATPase in the ciliary epithelium and support the concept that the inner, non-pigmented cell layer is chiefly responsible for the active transport of ions into the posterior chamber. It is concluded that this transepithelial transport can be driven only by the energy derived via the Na,K-ATPase, and that any subsequent anion or proton transport in the formation of aqueous humor is driven by the sodium gradient through exchange mechanisms.