AJP - Regulatory, Integrative and Comparative Physiology, Vol 258, Issue 4 883-R888, Copyright © 1990 by American Physiological Society
Urea transport across urinary bladder and salt acclimation in toad (Bufo viridis)
S. Shpun and U. Katz
Dept. of Biology, Technion, Haifa, Israel.
The fluxes of urea across the urinary bladder of the toad Bufo viridis have
been studied under conditions of acclimation to tap water or 500 mosM NaCl
solution. The [14C]urea fluxes were measured simultaneously with [3H]inulin
to test for nonspecific leakage. The fluxes are quite high (Ktrans = 75 x
10(-7) cm/s at 5 mmol/l urea) and are similar in either the
mucosal-to-serosal or the opposite direction. "Summer" rates were five to
six times higher than the "winter" rates. Antidiuretic hormone (ADH),
theophylline, and forskolin increased the fluxes to variable degrees (two
to five times), similar in the two acclimation conditions. Phloretin
inhibited the urea fluxes by nearly 50%. 1,3-Dimethylurea and thiourea, but
not acetamide, competed with the urea fluxes effectively. The fluxes of
urea were not affected by the osmotic water flow, although both responded
to ADH. It is concluded that urea transport across the urinary bladder of
B. viridis is by facilitated diffusion through a specific pathway
independent of water flux. The inhibitory effect of the structural
analogues on the urea flux was affected by salt acclimation, whereas most
other characteristics did not differ significantly at 5 mmol/l external
urea under the two conditions of acclimation.
