An osmometric method for the bioassay of vasotocin and related peptides in the toad bladder

A supramaximal dose of AVT reduced disc fluid osmolality by 188 mosmol/kg H₂O within 15 min. Similar maximal responses were observed with 8-arginine vasopressin (AVP), oxytocin, and 8-lysine vasopressin, although the potencies of these hormones diminished in the order listed above. AVT was 112-fold more potent than AVP, and AVP, in turn, was 329-fold more potent than 8-lysine vasopressin. The lower limit at which AVT was detected in this assay was 0.25 pg/disc (3 ± 10^-11 m). The intra- and interassay variabilities for AVT were 14% and 28% (±SD), respectively. This assay is suitable for measuring the biological activity of hormone analogs lacking vasopressor activity, such as desmopressin, which was found to have a hydroosmotic activity of 8.3 ± 2.4 U/mg. After osmotic stimulation, AVT was detected in toad plasma at a concentration of 1.4 ± 10^-10 m. Therefore, this method has the requisite sensitivity for measuring this hormone in biological fluids of amphibia, reptiles, fish, and birds.