COMPARATIVE-ANALYSIS OF THE ONTOGENY OF A SODIUM-DEPENDENT BILE-ACID TRANSPORTER IN RAT-KIDNEY AND ILEUM

An apical sodium-dependent bile acid transporter (ASBT) has recently b een cloned and characterized in the rat ileum. Northern and Western bl otting revealed both the ASBT mRNA and protein in rat kidney. The codi ng sequence of the kidney transcript was found to be identical to the previously cloned ileal ASBT. Indirect immunofluorescence studies loca lized the ASBT protein to the apical membrane of the renal proximal co nvoluted tubule. Kinetic analysis of sodium-dependent taurocholate upt ake using membrane vesicles revealed a similar Michaelis-Menten consta nt value for taurocholate in the kidney and intestine. ASBT protein an d function were present in the kidney but not the ileum from 7-day-old rats. On postnatal day 7, there was a sevenfold increase in ASBT stea dy-state mRNA levels in the kidney relative to the ileum, yet nuclear run-on assays revealed that the nascent transcription rates at this ag e were virtually the same. This suggests that the difference in the ne onatal expression of the ASBT gene in the kidney and ileum may be in p art due to differences in mRNA stability.