Sodium-retaining bioassay of prolactin in the intact teleost Tilapia mossambica acclimated to sea water

Abstract

A bioassay method for fish prolactin was developed based on the ability of prolactin to increase the plasma sodium concentration of intact Tilapia mossambica acclimated to sea water. The plasma sodium concentration is linearly related to log dose of ovine prolactin. Homogenates of whole pituitary gland from fresh water T. mossambica give a parallel dose response. A significant elevation of plasma sodium is elicited by 4 μg/g body wt of ovine prolactin. The same amount (wet wt) of whole pituitary gland from freshwater T. mossambica has equal potency, indicating that the assay is much more sensitive to at least some teleost prolactins than to mammalian prolactins. The assay is unaffected by ACTH, dexamethasone, bovine growth hormone, porcine prolactin, or human chorionic somatomammo-from Cichlasoma labiatum, Roccus saxatilis, Mugil cephalus, and Poecilia latipinna, In addition to Tilapia, sodium-retaining activity was detected in pituitaries from Cichlasoma labiatum, Roccuss saxatilis, Mugil cephalus, and Poecilia latipinna, but not in any of 13 other species of fish examined. This activity was restricted to the rostral pars distalis of Tilapia and Cichlasoma. It was localized in a single prominent band with a R f of .47 after electrophoresis of Tilapia pituitaries. The sodium-retaining potency of seawater Tilapia pituitaries was about one-eighth that of freshwater Tilapia pituitaries.