Inhibition of frog (Rana esculenta) antidiuretic action of vasotocin by some analogues

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Abstract
The inhibition of the frog antidiuretic action of small amounts of arginine- or lysine-vasotocin by 10–700 times higher amounts of oxytocin and of six of its analogues has been investigated. Oxytocin antagonizes the two tubular actions of the vasotocins (stimulation of water and sodium reabsorptions) but not their glomerular action. The decrease of the relative free water clearance was used as an index of the antidiuretic action of vasotocin. Oxytocin, Ileu8-oxytocin, deamino-oxytocin, and Val3-oxytocin almost completely inhibit the antidiuretic action of vasotocin, whereas lysine-vasopressin, oxypressin, and Phe2-Phe3-Lys8-oxytocin do not. Among the inactive analogues tested, it is suggested that those having a cyclic part identical or closely similar to that of vasotocin are inhibitors by competitive fixation on the reactive site, the different side chain explaining the absence of activity, and that those having a ring different from that of vasotocin (even with an identical side chain) are neither inhibitory nor active because they cannot be bound by the hormonal receptor.

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