The cation receptor subsite of the choline transporter in preimplantation mouse conceptuses resembles a cation receptor subsite of several amino acid transporters.

Mediated choline transport in preimplantation mouse conceptuses was inhibited competitively by Na+ and other cationic osmolites. Uptake of choline by conceptuses was also inhibited relatively strongly by ethanolamine, hemicholinium-3, harmaline, harmalol and harmine. The Ki values for inhibition of choline transport by most of the latter inhibitors were of the same order of magnitude as the Km value for choline transport (approximately 100 microM). To our knowledge, we are the first to show that mediated 'Na(+)-independent' choline transport is, nevertheless, inhibited strongly by the Na(+)-site inhibitor, harmaline. Inhibitions by harmaline, Na+ and other cations have been used to draw a parallel between the substrate receptor sites of amino acid transport systems y+ and bo.+. We suggest that the latter parallel should be extended to include the Na(+)-independent mammalian choline transporter. In addition, the choline transport activity in conceptuses increased by more than 100-fold between the 2-cell and blastocyst stages of development. Mouse blastocysts probably utilize choline for the synthesis of membrane phospholipids during cellular differentiation and when they begin to grow about ten hours prior to implantation. Since we show here that mouse conceptuses develop the capacity to transport choline prior to the onset of growth, some of the choline utilized for growth could come from an exogenous source.