Studies of corticotropin receptors on rat adipocytes

Abstract

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 K_D=170±11.9 pM. Competition experiments with unlabeled ACTH also yielded a comparable value for the apparent K_D (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 K_m being 145-177 pM . At a concentration of 4% BSA in the incubation medium, the binding curve was shifted significantly to the right (apparent K_D=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 ).