Antagonistic effects of prolactin and oxytocin on tail fin regression and acid phosphatase activity in metamorphosing Ambystoma tigrinum.

We have found that the inhibition of thyroxine-induced tail fin regression by prolactin in larval tiger salamanders is antagonized by oxytocin. Other workers have shown that prolactin blocks the rise in activity of several hydrolytic enzymes that occurs in regressing tissue during metamorphosis. Here, we examine the effects of prolactin and oxytocin--given alone and in combination--on tail fin regression and acid phosphatase specific activity in this tissue. Both long-term (12-day) and short-term (48-hr) treatment paradigms using prolactin and oxytocin are investigated. The results show that long-term prolactin treatment of metamorphosing larvae blocks fin regression and the rise in acid phosphatase specific activity seen in metamorphosing controls; short-term prolactin treatment of metamorphosing larvae inhibits fin regression within 48 hr, but does not block the rise in acid phosphatase activity seen in controls; oxytocin antagonizes the effects of prolactin on tail fin regression; and oxytocin treatment (long-term or short-term) of metamorphosing larvae causes an elevation of acid phosphatase activity above that seen in metamorphosing controls. With long-term treatment, this effect of oxytocin is slightly antagonized by prolactin; with short-term treatment, no antagonism is observed even though an effect of prolactin on fin height is still evident. We have interpreted these results as suggesting that the effect of prolactin on hydrolase activity is not a prerequisite for its inhibitory effect on fin regression to occur.