Osmotic regulation and urea metabolism in the lemon shark J\rep-aprion brevirostris

GOLDSTEIN, LEON, W. WALTER OPPELT, AND THOMAS I-I. MAREN. Osmotic reguhtiun and urea metabolism in the lemon shark Negaprion brenirostris. Am. J. Physiol. 21516) : 1493-1497. 1968. -Lemon sharks Negaprion breuirostris were transferred gradually (within I wk) f rom full. to approximately half-strength seawater. Steady-state conditions (with respect to solute and water balance) were established by the end of the dilution program. The sharks were then maintained in the dilute environment for an additional week. A control group was maintained in seawater. Hematocrits of the fish in dilute seawater were similar to those in the control group, suggesting that the sharks in the dilute environment maintained water balance. Plasma urea, trimethylamine oxide, and chloride concentrations were reduced 55, 60, and 20y0, respectively, in the sharks in diluted seawater compared to the control group. Urea clearance from the body fluids was approximately three times greater in the group in diluted seawater compared to the controls. The rates of total urea excretion in the steady state (equal to urea production) were similar in the two groups. These results suggest that lemon sharks adapt to environmental dilution by reducing the concentrations of the major extracellular solutes-urea, NaCI, and trimethylamine oxide. The reduction in urea concentration is due mainly to an increase in urea clearance and not a change in biosynthesis under these conditions.