Urea efflux from the Squalus acanthias pup: the effect of stress.

The extremely high concentration of urea found in elasmobranch blood (approx. 400 mmol I") is secondary to nearly complete reabsorption (90—95 %) of this solute in the renal tubules (Forster, 1967), and a branchial urea permeability which is approximately 1 % that of the toad bladder (<10 X 10~cm s", Payan, Goldstein & Forster, 1973). Evans, Oikari, Kormanik & Mansberger (1982) have recently shown that the premature spiny dogfish (Squalus acanthias) pup is capable of osmoregulation in sea water, despite an initial reduction in plasma urea concentration (13 % in 3 days). This fall could have been secondary to a reduced synthetic rate, or caused by an increased efflux across surfaces such as the gills, renal and/or rectal gland epithelia, or yolk sac. The present study was therefore undertaken to measure the rate of urea efflux from intact spiny dogfish pups, and to determine the effect of stress on that efflux. Stress was induced by the experimentally common procedure of handling, MS-222 anaesthesia, and weighing. Pups were removed from sacrificed females and used in experiments within 24 h. Individual pups were placed into 200 ml of sea water in a plastic container, maintained at 15 ± 2 °C in a running-seawater table. At hourly intervals for 3-5 h thereafter, 5 ml samples of the efflux baths were removed and frozen for subsequent analysis for urea using the diacetyl monoxime reaction (Sigma Kit no. 531). At the end of the control efflux period the pups were anaesthetized by pipetting appropriate volumes of MS222 (to reach a concentration of 0-01 %) into the bath. Anaesthetized pups were weighed and then placed into new efflux baths. In some experiments the yolk sac was Hgated and removed after weighing in order to examine the role of this structure in urea efflux. In other experiments the cloacal opening was cannulated (under anaesthesia, at the start of the control efflux period) as described previously (Evans et al. 1982), in order to assess the role of renal + rectal gland loss in the response to stress. Effluxes were calculated from the change in bath urea concentration with time, using the initial weights of the pups (including yolk sac), even in experiments where the yolk sac was removed, to avoid a weight bias of the calculated efflux. A possible