Endocrine regulation of ascorbic acid transport and secretion in luteal cells

Luteal ascorbic acid depletion by LH and prostaglandin (PG) F(2α) is well known, but how such depletion occurs is not. We therefore investigated the nature and regulation of ascorbic acid uptake and depletion in the rat CL and luteal cells. In vivo studies showed that blockade of steroidogenesis by aminoglutethimide prevented ascorbate depletion by LH, but not PGF(2α). Also, the time course for half-maximal depletion of ascorbic acid in vivo in response to PGF(2α) was extremely rapid {2-3 rain) compared to that known for LH (60 min). Thus, ascorbate depletion by LH and PGF(2α) appears to occur by different mechanisms. In luteal cells, ascorbate uptake was energy- , sodium-, and microfilament-dependent with a Michaelis constant (K(m)) of 33 μM, similar to that reported for other cells. In contrast to findings for other cells, PGF(2α) was found to be a potent and rapid inhibitor of ascorbate uptake with a half-maximal inhibition (IC₅₀) of about 5 nM in luteal cells. Ascorbate uptake was unaffected by LH, PGE₂, glucose, bromo- cAMP, progesterone, phorbol ester, ionomycin, hydrogen peroxide (H₂O₂), or aminoglutethimide. Also novel was the finding that luteal cell secretion of ascorbic acid was rapidly and potently stimulated by PGF(2α) (IC₅₀ about 5 nM), an effect mimicked by LH, H₂O₂, generators of reactive oxygen, calcium ionophore, and cytochalasin B. Basal release of ascorbic acid was energy- dependent, as secretion was blocked by a mitochondrial uncoupler and lowered temperature. Phorbol ester, bromo-cAMP, progesterone, aminoglutethimide, and ouabain had no effect on ascorbic acid secretion in luteal cells. These findings indicate that the secretion of ascorbic acid induced by PGF(2α), and possibly LH, may be mediated by calcium, reactive oxygen, and cytoskeletal changes. The ability of PGF(2α) to inhibit ascorbate transport and to stimulate secretion implicates these processes as the basis for the rapid depletion of ascorbic acid in the CL. Ascorbate depletion by LH is associated with stimulation of steroidogenesis and an increase in ascorbic acid secretion.