9613738 Petzel Antarctic fish have the highest serum osmolality of any seawater teleost. Maintenance of fluid balance is crucial for survival. Upon warm acclimation from -1.5o to 4oC the fish lmse 20% of their serum osmolality through extrusion of NaC1 across the gill. NaC1 extrusion in fish is primarily performed by chloride-secreting cells located on the gill arches and gill opercula. The driving force for NaC1 transport is the Na/K-ATPase. To date, no information is available concerning role and regulation of the elevated serum osmolarity in Antarctic fish. Questions which arise include: What role does the chloride cell play in mediating salt extrusion? Which hormones regulate chloride cell activity? The chloride cell physiology and regulation in Antarctic fish will be compared with a New Zealand fish which is eurythermal. The goals of the proposed research are to determine the plasticity of Antarctic and New Zealand fish gill function at the physiological level (through studies of ion transport activity) and molecular level (through studies of the Na/K-ATPase enzyme), specifically this research will: 1) determine the gill extrusion mechanisms underlying the increase in gill Na/K-ATPase activity upon warm acclimation in Antarctic fish and 2) determine the hormonal regulation of the gill extrusion mechanisms. Previously it was shown that upon warming of Antarctic fish there is an increase in NaC1 extrusion as a result of increases in gill total Na/K-ATPase enzyme levels. The proposed experiments will measure chloride cell morphology, metabolism, physiolmgy and regulation in parallel with effects of warming and cooling on the whole fish. Measurements of chloride cell morphology will include changes in diameter and number of cells at different temperatures. In order to ascertain the role of Na/K-ATPase in mediating the enhanced salt extrusion upon warming, we will measure total enzyme activity and total number of enzymes. The role of the chlor ide cell in transport of C1 and Na will assayed by placing the gill opercula sheet epithelia in Using chambers to determine the influence of fish warming on short-circuit current, which is a direct measurement of chloride transport through chloride cells. Finally, regulation of chloride cell function will be determined by altering palsma levels cortisol, prolactin and endogenous endocrines. The results from the proposed experiments will, for the first time, describe in detail the underlying mechanism(s) mediating the enhance hypo-osmoregulation observed in Antarctic fish and will allow the comparison of these results to those observed in a eurythermal New Zealand fish.
