Ascorbic acid transport by isolated bovine adrenal cortical cells

Viable, ACTH-sensitive, bovine adrenal cortical cells have the ability to accumulate [1-14C]ascorbic acid. Transport of the vitamin displays saturation kinetics (Km = 16.6 microM), requires Ca2+, and is inhibited maximally by ACTH at the same concentrations of the hormone necessary for maximal stimulation of steroidogenesis. Despite the transport of quantities of radioactive ascorbate amounting to approximately 25% of the total intracellular pool, no change in the ascorbate concentration in the cells can be detected by chemical methods. Transport of the vitamin is apparently a membrane carrier-mediated exchange process, since although intracellular ascorbate concentrations are 60- to 90-fold higher than those in the incubation medium, accumulated label can only be displaced when the cells are incubated in the presence of unlabeled ascorbic acid or metabolic inhibitors. Furthermore, ACTH fails to deplete ascorbic acid from [1-14C]ascorbate-loaded cells. It is hypothesized that the vitamin exists in two pools only one of which can be depleted by ACTH. This ACTH-depletable pool is no longer present in the isolated cell. In fact, as much as 66% of the ascorbic acid is lost from rat adrenals incubated for 15 min post adrenalectomy. The remaining pool of adrenal ascorbate is not depleted by ACTH and is also maintained against a concentration gradient.