EXPRESSION OF MADIN-DARBY CANINE KIDNEY-CELL NA+-DEPENDENT AND CL--DEPENDENT TAURINE TRANSPORTER IN XENOPUS-LAEVIS OOCYTES

Expression of a Madin-Darby canine kidney (MDCK) cell taurine transporter was examined in Xenopus oocytes that had been injected with poly(A)+ RNA extracted from MDCK cells. Compared with water-injected oocytes, injection of total poly(A)+ RNA resulted in an increase in Na+-dependent taurine uptake which was directly related to the amount of RNA injected. The magnitude of expression in poly(A)+ RNA-injected oocytes was 5-10-fold higher than that of water-injected oocytes. Since the V(max) of taurine uptake in MDCK cells is increased by culture in hypertonic medium, we compared oocyte taurine uptake after injection with poly(A)+ RNA from MDCK cells cultured in hypertonic medium with uptake in oocytes injected with poly(A)+ RNA from MDCK cells cultured in isotonic medium. Poly(A)+ RNA from hypertonic cells elicited twice the taurine uptake elicited by poly(A)+ RNA from isotonic cells. The transporter expressed in oocytes was like that in MDCK cells: it was completely dependent on external sodium and was also anion dependent (Cl- greater-than-or-equal-to Br- > SCN- >> gluconate-). Other beta-amino acids, beta-alanine and hypotaurine, inhibited taurine uptake, but L-alanine and 2-(methylamino) isobutyric acid did not. The apparent K(m) of the transporter was 7.0-mu-M. After size fractionation on a sucrose density gradient, poly(A)+ RNA encoding for the MDCK taurine transporter was found in the fraction whose average size was 4.4 kilobases.