UREA AND WATER PERMEABILITY IN DOGFISH (SQUALUS-ACANTHIAS) GILLS

We used a perfused gill preparation from dogfish to investigate the or igin of low branchial permeability to urea. Urea permeability (C-14 ur ea) was measured simultaneously with diffusional water permeability (( H2O)-H-3). Permeability coefficients for urea and ammonia in the perfu sed preparation were almost identical to in vivo values. The permeabil ity coefficient of urea was 0.032 x 10(-6) cm/sec and of (H2O)-H-3 6.5 5 x 10(-6) cm/sec. Adrenalin (1 x 10(-6) M) increased water and ammoni a effluxes by a factor of 1.5 and urea efflux by a factor of 3.1. Urea efflux was almost independent of the urea concentration in the perfus ion medium. The urea analogue thiourea in the perfusate had no effect on urea efflux, whereas the non-competitive inhibitor of urea transpor t, phloretin, increased efflux markedly. The basolateral membrane is a pproximately 14 times more permeable to urea than the apical membrane. We conclude that the dogfish apical membrane is extremely tight to ur ea, bur the low apparent branchial permeability may also relate to the presence of an active urea transporter on the basolateral membrane th at returns urea to the blood and hence reduces the apical urea gradien t. COMP BIOCHEM PHYSIOL 119A;1:117-123, 1998. (C) 1998 Elsevier Scienc e Inc.