High affinity (Na(+) + Cl-)-dependent taurine transport by lactating mammary tissue.

The transport of taurine by lactating rat mammary tissue has been examined. Taurine uptake was found to be dependent upon the presence of extracellular Na+ and Cl-, which is consistent with (Na(+) + Cl(-) + taurine) cotransport. The Ka and Vmax of taurine influx were respectively 43 microM and 37.5 mumol/kg cell water per 15 min. It is apparent that the mechanism responsible for taurine uptake is highly selective for beta-amino acids. Taurine efflux consisted of a fast extracellular component and a slow membrane-limited component. The slow component was relatively insensitive to temperature, suggesting that it may represent simple diffusion. Mammary tissue was found to contain a high level of intracellular taurine: 7.29-7.44 mmol/kg cell water. We suggest that taurine is taken up and concentrated by the mammary gland by a (Na(+) + Cl(-) + taurine) cotransport mechanism situated in the blood-facing aspect of the secretory epithelium and that a low outward permeability to taurine allows a high intra-to-extracellular concentration gradient to be maintained. Milk taurine may be derived from taurine diffusing from the cell cytosol across the apical membrane.