Melanotropin bioassays: In vitro and in vivo comparisons

A reflectance method was utilized to compare the in vitro responses in three species of frogs (Rana pipiens, R. berlandieri forrei, and R. catesbeiana) and a lizard (Anolis carolinensis) to α- and β-melanotropins (α- and β_p-MSH). The integumental chromatic response of the three ranid species was identical, in that α-MSH was about 2 times more potent than β_p-MSH. The melanotropins were equipotent in the lizard skin bioassay. A remarkable feature of the Anolis skin assay is that skins from this lizard can be utilized repeatedly many times in one day with an extremely high degree of precision. The reflectance method was also used to determine the in vivo potencies of α-MSH and β_p-MSH in the frog, R. pipiens. Surprisingly, the melanotropins were more active in the in vivo assay than in the in vitro bioassay. The darkening response of the frogs to α-MSH was reversed by 6 hr, but the response to β_p-MSH persisted for more than 8 hr. When α-MSH was incubated in frog serum, the melanotropic activity was almost totally abolished by 30 min, whereas the melanotropic activity of β_p-MSH was evident much longer (4 hr) in the presence of the serum. In light of the observation that the melanotropic activity of α-MSH is rapidly lost by incubation in frog serum, it is unclear why the hormone was more active as measured in vivo and why the darkening response in vivo persisted so long.