EFFECT OF PROLACTIN-INHIBITING SUBSTANCES BROMOCRIPTON AND LERGOTRILE ON HYDROMINERAL REGULATION IN RAINBOW-TROUT SALMO-GAIRDNERI

Plasma Na+, K+ and osmotic pressure were measured in rainbow trout (S. gairdneri) following the administration of the prolactin-inhibiting substances Lergotrile and Bromocripton. Both drugs elicited a significant fall in plasma Na+ concentrations although a significant response to Bromocripton was apparent only in trout acclimated to distilled water. The changes in plasma Na+ levels and in some cases of plasma osmotic pressure following administration of the prolactin-inhibiting substances were consistent with the hypothesis that prolactin acts to maintain plasma Na+ levels in this salmonid species in the same manner as in other teleosts. Since the changes in plasma ions were small (albeit significant), prolactin may play a less important role in osmotic and/or ionic regulation in this species than it does in other teleostean species. The drugs effect a less complete blockage of prolactin secretion than they appear to do in mammals. Ovine prolactin, administered with the drugs, effected a partial retention of plasma of Na+ in Lergotrile-injected fish but did not significantly modify the effect of Bromocripton. These findings were discussed in light of the proposed action of the drugs, namely that of inhibiting the release of endogenous prolactin. Both Bromocripton and Lergotrile caused a significant fall in hematocrit values. Since plasma osmotic pressure values and plasma K+ concentrations were not markedly affected by the drugs (except for a significant (P < 0.01) reduction in plasma osmotic pressure in the Bromocripton-injected groups maintained in distilled water) it was thought that these changes were due to a reduction in the number of blood cells in the peripheral circulation rather than to an influx of water in response to the inhibition of prolactin.