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

The prolactin-binding affinity ( K D ) and number of binding sites N in Rana catesbeiana tadpole, tail fin and kidney tissues were studied during metamorphosis and following administration of oPRL and L-T 3 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 K d was noted. lL-T 3 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-T 3 and oPRL for 7 days resulted in an inhibition of the T 3- 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 either treatment alone. No change in K D was noted in any of these studies. Therefore, oPRL and L-T 3 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 T 3 during metamorphosis. Thus, the variation in the number of oPRL-binding sites may be due to the changes in endogenous PRL levels during development.