Postnatal expression of the canalicular bile acid transport system of rat liver

Canalicular plasma membrane (CPM) vesicles prepared by a Ca2+ precipitation method from developing (7 and 14 days old) and adult rat liver were used to directly examine the postnatal ontogenesis of taurocholate (TC) transport. The initial rate of 50 microM TC uptake by vesicles derived from 14-day-old and adult but not 7-day-old animals was markedly inhibited by the anion transport inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). DIDS-sensitive TC uptake was 21.6 +/- 5.6 (SE) at 14 days compared with 58.1 +/- 8.1 pmol.mg protein-1.5 s-1 in adults (P less than or equal to 0.01). Kinetic studies were performed by preloading these predominantly "right-side out" vesicles with TC (25-800 microM) and measuring the initial rate (5 s) of efflux into bile salt-free medium. Computer analysis of the DIDS-sensitive portion of efflux revealed saturable kinetics with a similar Vmax (2.72 +/- 0.36 vs. 1.97 +/- 0.17 nmol.mg protein-1.min-1; P = NS) but a threefold higher Km (0.35 +/- 0.09 vs. 0.11 +/- 0.02 mM; P less than or equal to 0.05) in 14 day vs. adult CPM vesicles. In contrast, efflux from 7 day CPM vesicles increased linearly with increasing concentrations of TC and was not inhibited by DIDS. Immunoblots of canalicular membranes, probed with an antibody against the 100-kDa bile acid transport protein, showed that the amount of immunoreactive carrier protein in the membranes of 14-day-old and adult rats was similar but was only 37% of the adult level at 7 days of age.(ABSTRACT TRUNCATED AT 250 WORDS)