2. Light, as a determining factor in ‘urea pore’ efficiency, may prevent urea excretion
during the lighted phase. This factor may be under hormonal control. Vasopressin,
for example, has been reported to stimulate urea permeability in mammalian kidney
(Knepper and Star, 1990, cited by Hasegawa and Verkman, 1993).
In marine fish, plasma urea levels were seven times higher than those in salmonids, in