OSMOTIC REGULATION OF TAURINE TRANSPORT VIA SYSTEM-BETA AND NOVEL PROCESSES IN MOUSE PREIMPLANTATION CONCEPTUSES

Taurine was shown recently to increase the frequency at which 2-cell m ouse conceptuses develop into blastocysts in vitro. For this reason an d because taurine helps cells adapt to external stresses, we studied t ransport of this and related amino acids by preimplantation mouse conc eptuses. The most conspicuous component of taurine transport in concep tuses at the 1-cell through blastocyst stages of development was both Na+- and Cl--dependent. This Na+- and Cl--dependent transport system i nteracted relatively strongly with beta- but not alpha-amino acids. By these criteria, transport system beta is responsible for Na+-dependen t taurine transport in preimplantation mouse conceptuses. Moreover, de tection of mRNA encoding the taurine transport protein (TAUT) in early conceptuses supports the theory that TAUT is a major component of sys tem beta. Transport of taurine by system beta in 1-cell conceptuses wa s slower in hypotonic than in hypertonic media, whereas the reverse wa s true for system beta in blastocysts. In contrast, hypotonically stim ulated Na+-independent taurine transport was, of course, more rapid in hypotonic than in hypertonic media in both 1-cell conceptuses and bla stocysts. Transport via this hypotonically stimulated process also sho wed no sign of saturation by up to 10 mM taurine. Hypotonically stimul ated taurine transport appeared transiently in 1-cell conceptuses unde r hypotonic conditions until they had recovered their initial volumes. Hence, we suggest that a decrease in taurine uptake via system beta a nd an increase in taurine exodus via the Na+-independent, nonsaturable transport process could contribute to the regulatory volume decrease in 1-cell conceptuses in hypotonic medium. Since taurine uptake by sys tem beta in blastocysts is, however, higher in hypotonic than in hyper tonic media, taurine uptake by system beta in blastocysts might intens ify a tendency to increase cell volume in hypotonic medium. Such an in crease in taurine uptake could further favor anabolic changes associat ed with cell swelling. In addition to contributing to regulation of ce llular volume and perhaps metabolism, the hypotonically stimulated Na-independent transport processes in early embryos have novel character istics. Hypotonically stimulated Na+-independent taurine transport was inhibited by niflumate, N-ethylmaleimide and NaN3 but not by furosemi de, iodoacetate, KCN, ouabain or alpha- or beta-amino acids. Furthermo re, 4,4'-diisothiocyanostilbene-2,2'-disulfo inhibited this transport in 1-cell conceptuses but not in blastocysts. Hence, different hypoton ically stimulated Na+-independent taurine transport processes appear t o be present in 1-cell conceptuses vs. blastocysts. The functions of t hese and other instances of developmental regulation of expression of transport processes in preimplantation conceptuses remain largely to b e elucidated. Moreover, neither of the hypotonically stimulated Na+-in dependent taurine transport processes in conceptuses appears to have b een detected in other types of cells. Instead, these processes may be unique to preimplantation conceptuses.