Urea and water permeability in the ureotelic gulf toadfish (Opsanus beta)

The permeability of toadfish gills and skin to urea and water has been meas ured in order to investigate the mechanisms behind the pulsatile excretion of urea previously described in this species. A perfused gill preparation w as used in the all studies and isolated pieces of skin mounted in an Ussing chamber in the skin studies. Simultaneously, urea and water permeability w as measured in vivo in free swimming fish. In vivo the nonpulsing urea perm eability was exceptionally low compared to other teleosts, while the tritia ted water permeability was similar to that of other teleosts. The urea perm eability increased 30-fold during a pulse while water permeability stayed u naffected. Compared to in vivo, tritiated water permeability was approximat ely 50% lower in the gills and the skin when measured directly in the isola ted preparations. The urea permeability was almost identical between the th ree preparations. Four out of 20 perfused gill preparation showed a spontan eous urea pulse during perfusion. Several treatments were tested to elicit the pulse artificially but without success. Hormones and drugs tested were: arginine-vasotocin (AVT), 10(-10) M; adrenaline, 10(-7) M; isoprenaline, 1 0(-5) M; acetylcholine, 10(-7) and 10(-6) M; serotonin, 10(-7) and 10(-6) M ; adenosine, 10(-6) M; cortisol, 10(-7) M; and combinations of AVT, adrenal ine, and cortisol. Adrenaline and isoprenaline increased tritiated water pe rmeability without affecting urea permeability. Gradually increasing the am monia levels in the perfusate from 0.1 mM to 1.6 mM caused a slight increas e in water permeability but a marked and progressive increase in urea perme ability. No indications of an ammonia trapping mechanism in the gills were found. There was no effect of AVT (10(-10) mol l(-1)) in the urea permeabil ity of the skin preparation while cortisol (10(-7) M) led to a modest incre ase in urea permeability. Based on a comparison between the in vivo and in vitro preparations used here, we conclude that the urea pulse in a urea-pul sing toadfish occurs through the gills and not the skin. We still do not kn ow which internal mechanism or signal triggers the urea pulse in the toadfi sh. J. Exp. Zool. 283:1-12, 1999. (C) 1999 Wiley-Liss, Inc.