Natriferic and hydrosmotic effects of neurohypophysial peptides and their analogues in augmenting fluid uptake by Bufo melanostictus

1. The effect of intramuscular injection of 8‐arginine vasotocin, 8‐arginine vasopressin, 8‐lysine vasopressin, oxytocin, 8‐ornithine oxytocin and 8‐ornithine vasopressin on fluid uptake across the skin was studied in the live toad, Bufo melanostictus , bathed either in distilled water or in NaCl solution (0·1 g/100 ml.). 2. When the bathing solution was distilled water, 8‐arginine vasotocin was the most potent, 0·14 nmole/kg augmenting the rate of fluid uptake by 50%. Compared with it the others had relative potencies of: 8‐arginine vasopressin 0·8, 8‐lysine vasopressin 0·8 × 10 −3 , oxytocin 0·8 × 10 −3 , 8‐ornithine oxytocin 0·8 × 10 −2 , 8‐ornithine vasopressin < 1·4 × 10 −4 . 3. When the bathing solution contained 0·1% NaCl, 8‐arginine vasotocin was again the most potent, 0·06 nmole/kg augmenting the rate of fluid uptake by 50%. Compared with it the others had relative potencies of: 8‐arginine vasopressin 0·3, 8‐lysine vasopressin 0·3 × 10 −3 , oxytocin 0·3 × 10 −2 , 8‐ornithine oxytocin 0·8 × 10 −2 , 8‐ornithine vasopressin < 0·6 × 10 −4 . 4. Dose—response curves for each peptide showed that in the case of 8‐arginine vasopressin, 8‐lysine vasopressin and 8‐ornithine vasopressin the augmentation of rate of fluid uptake did not differ in the absence or in the presence of NaCl in the bathing solution; whereas in the case of 8‐arginine vasotocin, oxytocin, and 8‐ornithine oxytocin the augmentation was greater in the presence of sodium chloride. 5. Support has been found for the postulate of a binary action of some neurohypophysial peptides on amphibian skin, arginine in position 8 being correlated with hydrosmotic effect, and isoleucine in position 3 with natriferic effect.