Melanocyte-Stimulating Activity in a Transplantable Mouse Pituitary Tumor1

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Abstract
Levels of melanocyte-stimulating activity were determined in the circulation and in the tumors of mice bearing a transplantable pituitary tumor which produced darkening of the coat color. An average concentration of 5.1 ([plus or minus] 0. 6) x 104 U/mg dry weight was found for 8 tumors. The level of activity in the circulation (CLA) was followed as a function of time following transplantation of the tumor and was correlated with the change in the reflectance of the coat and the growth of the tumor. An average CLA of approximately 6 U/ml/mg tumor wet weight was found. Changes in coat color became apparent while the tumor was still quite small and the CLA was low (less than 100 U/ml); however, maximal darkening occurred at a time when the CLA was approximately 1000 U/ml or more. The half-lives of the endogenous circulating activity in the mouse and of exogenous a-MSH (melanocyte stimulating hormone) in the rat were both found to be approximately 1. 7 min. A secretion rate of approximately 1. 5 x 104 U/day/mg tumor wet weight was calculated, indicating that the tumor activity turned over an average of 1. 65 times/day. In spite of secretion rates as high as 3. 7 x 107 U/day, less than 75 U of activity was found in the 24-hr urine. Preliminary separation experiments suggest that the tumor contains and secretes a-MSH.