An antiserum that recognizes mesotocin and isotocin: development of a homologous radioimmunoassay for plasma mesotocin in chickens (Gallus domesticus).

This report presents characteristics of an antiserum raised in a rabbit immunized with synthetic mesotocin (MT) conjugated to bovine thyroglobulin. Cross-reactivity studies indicate that the antiserum (Kl-II) recognizes the carboxyl-terminal "tail" of MT and isotocin (IT). A homologous, disequilibrium radioimmunoassay (RIA) for MT has been developed that can detect less than 1 pg of peptide. Plasma was extracted with octadecasilyl-silica. Recovery of MT from plasma was correlated with the amount added and averaged 70%. Different volumes of plasma and posterior pituitary extract, when measured in the assay system, yielded inhibition curves that were parallel with standard MT. Immunoreactive MT and AVT of plasma and neural lobe coeluted with synthetic standards after gel filtration. The ED50 of a heterologous, sequential saturation RIA for IT was 17.4 pg, suggesting that the MT antiserum may be useful for measuring the oxytocin-like principle in bony fishes. Immunoreactive MT in plasma of cockerels increased and decreased with iv infusion of hypo- and hyperosmotic saline, respectively. The changes in plasma MT were inversely related to osmolality. Hyperosmotic saline infusion resulted in correlated increases in plasma AVT and osmolality. The data suggest that MT may be released by dilution and/or expansion of extracellular fluid in chickens.