Tubular system membranes of teleost chloride cells: osmotic response and transport sites

Abstract

Chloride cells represent the sites of branchial osmoregulatory activity of teleosts. The cells undergo characteristic changes with osmotic challenge and the response is mediated by hormones. Either increased salinity or cortisol treatment will elicit well-known changes in chloride cells; these changes are clearly interrelated and may be collectively referred to as the "hyperosmotic response" of chloride cells. Fundamental features of the "hyperosmotic response" include proliferation and hypertrophy of chloride cells, an amplification of the cell's extensive tubular network or tubular system (TS), an increase in the specific activity of the transport-associated enzyme, Na-K-ATPase, and a concomitant increase in electrolyte transport. The TS displays frequent examples of continuity with the basolateral plasmalemma and the Na-K-ATPase enzyme complex resides in the membranes of the TS. The enzyme complex maintains its conventional polarity with regard to internal substrate and sodium binding sites and external or TS luminal surface, potassium, and ouabain binding sites. The luminal surface of the TS is anionic at pH 1.8 and above. The TS membranes display in situ, in isolation, and by freeze fracture, a characteristic asymmetric structure consisting of repeating particles; larger particles are associated with the cytoplasmic surface of the membrane and smaller particles with the external or luminal surface of tubules. The dimensions of the particles, and their disposition with respect to the cytoplasmic and external surfaces of the TS membrane, support the hypothesis that they are the visual manifestation of the transport-associated Na-K-ATPase complex. The TS and its associated transport activity represent essential components of a recently proposed paracellular transport model for chloride cells. The TS of the pseudobranch cell is also discussed in this review.