Cytochemical study of K+-p-nitrophenyl phosphatase (K+-NPPase) in planum semilunatum and dark cells of the squirrel monkey.

We examined the ultrastructure of the planum semilunatum, and also cytochemically demonstrated K+-dependent p-nitrophenyl phosphatase (K+-NPPase) activity in planum semilunatum and ampullary dark cells of the squirrel monkey. K+-NPPase activity was found in the basal part of the planum semilunatum. The reaction products were limited to the cytoplasmic side of the plasma membranes. A more intense reaction was found in the basal infoldings and the lateral interdigitations of the dark cells. On the other hand, the apical cell surface of both cell types showed no K+-NPPase activity. The present results show that both the planum semilunatum and the dark cells may both be involved in maintaining endolymph homeostasis.