Ultrastructural Localization of Na⁺+K⁺-ATPase in Specialized Membranes of Salt Transporting Cells in Marine Vertebrates

Abstract

All marine vertebrates regulate the salt concentration of their body fluids to levels far below those found in the marine environment. In addition to renal mechanisms, osmotic homeostasis is maintained by the active excretion of NaCl from specialized organs, the location and structure of which varies considerably among the classes of vertebrates. Thus in marine teleosts, the gill is the primary site of extra-renal salt secretion, while in elasmobranch fish the rectal gland fulfills this role. In marine birds, paired glands located just above the orbits of the eyes are responsible for salt secretion. Considering the phylogenetic diversity and varied anatomical location of these organs, one might expect that the secretory cells that comprise each of these glands would exhibit a comparable diversity of structure. This, however, is not the case. Rather, the commonality of function shared by these cells is mirrored in the underlying similarity of their cytoarchitectural design. The most salient features shared by these epithelial cells are an extensive elaboration of basolateral plasma membranes and an abundant supply of mitochondria, the mitochondria generally in close proximity to the amplified basal and lateral cell surfaces. Other structural features, such as specific modifications of the occluding junctions that unite adjacent secretory cells, also exhibit marked similarities in these salt secretory tissues.