Water uptake byBufo melanostictus, as affected by osmotic gradients, vasopressin and temperature

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Abstract
The rate of water uptake across the skin was studied in the live toad, Bufo melanostictus. When toads were kept in distilled water at 29[degree] C the uptake of water amounted to 16.9 [plus or minus][mu] 1.3 [mu]l/cm2/hr; when bathed in sucrose or urea solutions, the water uptake diminished with increasing osmotic pressure. There was no water uptake observed when toads were kept in 200 m-osmolar sucrose or urea. Intramuscular injections of vasopressin increased the rate of water uptake from distilled water. There was a good relation between doses and responses over various time intervals. A dose of 4 m-u vasopressin/g body wt. doubled the rate of water uptake over a period of 1 hr. The same dose of vasopressin doubled the rate of water uptake when the toads were kept in solutions of sucrose or urea of different osmolarity. The rate of water uptake when the toads were bathed in sodium chloride solutions was consistently 8 [mu]l/cm2/hr greater than when bathed in sucrose or urea solutions of equal osmolarity. There was no water uptake when the sodium chloride solution was 285 m-osmolar. Vasopressin (4 m-u/g) injected intramuscularly doubled the rate of water uptake from sodium chloride solutions of different osmolarity. With solutions of potassium chloride, sodium nitrate, and potassium nitrate, in concentrations up to 150 m-osmoles/1., the rate of water uptake was found to be the same as with solutions of sodium chloride of the same osmolarity. Similarly, it was doubled by injection of vasopressin (4 m-u./g.). The effect of temperature on the rate of water uptake before and after injection of vasopressin was investigated in toads kept in distilled water, sucrose, or sodium chloride solutions. For temperatures between 20 and 37[degree]C, vasopressin (4 m-u/g) reduced the activation energy involved in the process of water uptake by 4000 cal. The results agree with the view that water uptake follows a diffusion process which is facilitated by vasopressin, possibly as a result of increasing the size or number of available pores.