COMPETITION OF BILE-ACIDS ON THE SULFOBROMOPHTHALEIN UPTAKE IN BASOLATERAL RAT-LIVER PLASMA-MEMBRANE VESICLES

The effect of different bile acids (BA) on the hepatic uptake of sulfo bromophthalein (BSP) was investigated in liver plasma membrane vesicle s enriched in basolateral fraction. BSP uptake was measured either in the absence (electroneutral component) or in the presence of a membran e potential (electrogenic component) induced by the addition of valino mycin in the presence of an inwardly-directed potassium gradient. BSP uptake was also measured in the presence of different BA [cholate (C), taurocholate (TC), ursodeoxycholate (UDC) and tauroursodeoxycholate ( TUDC)]. Electrogenic BSP uptake was not affected by BA. Conversely, th e electroneutral portion of the BSP uptake was inhibited with an inhib ition constant (K-i, mu M) of 230 +/- 40 for C, 103 +/- 33 for TC, 99 +/- 34 for UDC and 120 +/- 39 for TUDC, respectively (means +/- SD, N = 4). The Dixon and Cornish-Bowden plot of the data revealed an uncomp etitive type of inhibition for each BA. These data indicate that the e lectroneutral, but not the electrogenic, BSP transport system is modul ated by BA.