Studies of corticotropin receptors on rat adipocytes

Oelofsen, Willem ; Ramachandran, J. (1983) Studies of corticotropin receptors on rat adipocytes Archives of Biochemistry and Biophysics, 225 (2). pp. 414-421. ISSN 0003-9861

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Synthetic [125I]-Tyr23, Phe2, Nle4-adrenocorticotropin (ACTH)-(1-38) ([125I]-ACTH analog) with full biological potency and near theoretical specific radioactivity (1800±75 Ci/mmol) was used to investigate ACTH receptors on isolated rat adipocytes derived from 42-day-old rats. Binding to adipocytes was studied in the presence of 1% bovine serum albumin (BSA) as well as 4% BSA. The interaction of the [125I]-ACTH analog with adipocytes was highly specific, rapid, saturable, and reversible. Scatchard analysis of the binding data obtained in medium containing 1% BSA revealed a single class of binding sites with an apparent KD=170±11.9 pM. Competition experiments with unlabeled ACTH also yielded a comparable value for the apparent KD (143±16.5 pM ). The number of receptors per adipocyte was quite low (521-841/cell). The stimulation of lipolysis by ACTH was closely correlated with the binding, the apparent Km being 145-177 pM . At a concentration of 4% BSA in the incubation medium, the binding curve was shifted significantly to the right (apparent KD=446±77 pM) and the binding capacity was also significantly enhanced (1663±208/cell) without any change in the apparent Km for glycerol release (187±7.1 pM ).

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