ENDOCRINE REGULATION OF ASCORBIC-ACID TRANSPORT AND SECRETION IN LUTEAL CELLS

Luteal ascorbic acid depletion by LH and prostaglandin (PG) F-2 alpha is well known, but how such depletion occurs is not. We therefore inve stigated the nature and regulation of ascorbic acid uptake and depleti on in the rat CL and luteal cells. In vivo studies showed that blockad e of steroidogenesis by aminoglutethimide prevented ascorbate depletio n by LH, but not PGF(2 alpha). Also, the time course for half-maximal depletion of ascorbic acid in vivo in response to PGF(2 alpha) was ext remely rapid (2-3 min) compared to that known for LH (60 min). Thus, a scorbate depletion by LH and PGF(2 alpha) appears to occur by differen t mechanisms. In luteal cells, ascorbate uptake was energy-, sodium-, and microfilament-dependent with a Michaelis constant (K-m) of 33 mu M , similar to that reported for other cells. In contrast to findings fo r other cells, PGF(2 alpha) was found to be a potent and rapid inhibit or of ascorbate uptake with a half-maximal inhibition (IC50) of about 5 nM in luteal cells. Ascorbate uptake was unaffected by LH, PGE(2 alp ha) glucose, bromo-cAMP, progesterone, phorbol ester, ionomycin, hydro gen peroxide (H2O2), or aminoglutethimide. Also novel was the finding that luteal cell secretion of ascorbic acid was rapidly and potently s timulated by PGF(2 alpha) (IC50 about 5 nM), an effect mimicked by LH, H2O2, generators of reactive oxygen, calcium ionophore, and cytochala sin B. Basal release of ascorbic acid was energy-dependent, as secreti on was blocked by a mitochondrial uncoupler and lowered temperature. P horbol ester, bromo-cAMP, progesterone, aminoglutethimide, and ouabain had no effect on ascorbic acid secretion in luteal cells. These findi ngs indicate that the secretion of ascorbic acid induced by PGF(2 alph a), and possibly LH, may be mediated by calcium, reactive oxygen, and cytoskeletal changes. The ability of PGF(2 alpha) to inhibit ascorbate transport and to stimulate secretion implicates these processes as th e 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.