THE RELATIVE ROLES OF EXTERNAL TAURINE CONCENTRATION AND MEDIUM OSMOLALITY IN THE REGULATION OF TAURINE TRANSPORT IN LLC-PK1 AND MDCK CELLS

Taurine is a beta-sulfonic amino acid that serves as a nutrient important for developing brain and retina and as an osmolyte in the medullary collecting duct. The activity of the taurine transport system is regulated by substrate supply and by the external osmolality; these two stimuli induce changes in taurine transport. Increased medium osmolality (500 mosmol) stimulates taurine uptake into MDCK cells but not LLC-PK1 cells. The enhanced taurine uptake that occurs in response to hyperosmolality is localized primarily to the basolateral surface of MDCK cells, whereas the adaptive response to medium taurine concentration is expressed on both the apical and the basolateral surfaces of both cell lines. The response of MDCK cells to medium osmolality requires protein synthesis and RNA transcription and is expressed in the presence of microtubular toxins. When cell monolayers were loaded with taurine by incubation in high-taurine medium before increasing medium osmolality, the expected increase in taurine uptake was blunted. Similarly, increased external beta-alanine (500 mu M) also prevented the anticipated increase in taurine accumulation in response to hypertonicity; aminoisobutyric acid and betaine (500 mu M) partially prevented the increase in taurine transport after hypertonicity, whereas L-alanine had no effect. The concentration of taurine or structurally similar analogs in the external medium might modify the response of taurine accumulation after expose to hypertonic medium, in that taurine-replete cells behave differently than taurine-depleted cells. These studies indicate that there are at least two distinct mechanisms involved in the regulation of taurine transport: external taurine concentration and medium osmolality, with taurine concentration seeming to be the predominant stimulus. Thus, the changes in cell taurine transport depend on the physiologic stimulus as well as the cell studied, a phenomenon that might be related to the renal tubular site of origin.