Competitive inhibition of amino acid transport in human preovulatory ovarian follicles

Abstract

To date we have yet to examine whether amino acid (AA) transport in human ovarian follicles is affected by competitive inhibition. In contrast, transplacental transfer of AAs in late-gestation sheep is characterized by reciprocal competition. This phenomenon has been described by algebraic equations of umbilical uptake of AAs based on maternal arterial concentrations. In the present translational study at a university teaching hospital, we verified whether these equations apply to the transport of AAs from blood to follicular fluid (FF) in human preovulatory follicles. For this purpose we used our data on AA concentrations in blood and FF measured earlier by high-performance liquid chromatography in specimens from 14 patients undergoing oocyte retrieval for in vitro fertilization after controlled ovarian stimulation. The main outcome measure was statistical significance of Spearman correlation coefficients for measured versus calculated concentrations of 8 AAs: isoleucine, leucine, valine, phenylalanine, methionine, threonine, lysine, and arginine. Equations for umbilical uptake provided a highly accurate description of blood-to-FF transport for 7 AAs with the exception of lysine: R ≥ 0.899 (p < 0.0001) for the branched-chain AAs, R = 0.829 (p = 0.0003) for threonine, R = 0.754 (p = 0.0019) for arginine, and R = 0.631 (p = 0.0156) for phenylalanine and methionine. We conclude that these equations indicate competitive inhibition between the AAs studied. Our study strongly suggests that many AA transport systems operating in the placenta should also be active in the cells of the preovulatory follicle. Future studies on AA fluxes in human ovarian follicles must consider possible competitive inhibition. Abbreviations: AA: amino acid; FF: follicular fluid; HPLC: high-performance liquid chromatography.