Changes in specific prolactin binding in Rana catesbeiana Tadpole tissues during metamorphosis and following prolactin and thyroid-hormone treatment.

The prolactin-binding affinity (KD) and number of binding sites *N) in Rana catesbeiana tadpole liver, tail fin and kidney tissues were studied during metamorphosis and following administration of oPRL and L-T3 to premetamorphic tadpoles. With increasing developmental stage there was an increase in N; a maximum was found at stage XVIII followed by a gradual decrease in N through metamorphic climax for all 3 tissues. No change in KD was noted. L-T3 treatment of premetamorphic tadpoles for 7 days caused a significant decrease in tail length and height and body length and an increase in hindlimb length with a concurrent increase in N of approximately 3-fold while treatment for 1 or 3 days was without effect on tadpole morphology or oPRL binding. OPRL treatment for 7 days caused a significant increase in tail length and height and body length with no significant changes in hindlimb length and a 3-5-fold increase in N. Treatment with both L-T3 and oPRL for 7 days resulted in an inhibition of the T3-induced decrease in tail length and height and body length and no inhibition of the hindlimb length increase. N increased in all tissues similar to that found with eight treatment alone. No change in KD was noted in any of these studies. Therefore, oPRL and L-T3 are able to regulate the numbers of specific oPRL-binding sites in amphibian tissues. The change in N with development parallels the reported change in tadpole pituitary capacity to stimulate growth but occurs prior to the reported surge of endogenous T3 during metamorphosis. Thus, the variation in the number of oPRL-binding sites may be due to the changes in endogenous PRL levels during development.