Synthetic alpha-melanotropin stimulated the release of immunoreactive adrenocorticotropin from primary cultures of rat anterior pituitary cells. The effect of the alpha-melanotropin was dose-dependent. Cells incubated with synthetic arginine-vasopressin and alpha-melanotropin simultaneously produced an amount of adrenocorticotropin that was greater than the sum of the amount that the cells produced in response to each peptide added separately. Other peptides structurally similar to alpha-melanotropin, such as, beta-, gamma 1-, gamma 2-, and gamma 3-melanotropin, were also tested for adrenocorticotropin-releasing activity. Only the gamma 3-melanotropin demonstrated a statistically significant effect. A vasopressin preparation (Pitressin, Parke-Davis) purified from posterior pituitaries and previously shown to contain some alpha-melanotropin was much more potent in releasing adrenocorticotropin than the synthetic vasopressin.

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