Quantitative assessment of canalicular bile formation in isolated hepatocyte couplets using microscopic optical planimetry

Isolated rat hepatocyte couplets (IRHC) are primary units of bile secretion that accumulate fluid in an enclosed canalicular space with time in culture. We have quantitated the rate of canalicular secretion in IRHC cultured for 4-8 h by measuring the change in canalicular space volume by video-microscopic optical planimetry using high resolution Nomarski optics. Electron microscopic morphometric studies revealed signifi-cant increases in canalicular membrane area after 4-6 h in culture. Canalicular secretion in basal L-15 medium (3.8±1.3 fl/min) increased significantly with the choleretic bile salts (10 uMM), taurocholate, and ursodeoxycholate (14±7 fl/min each). Secretion rates after exposure to bile acids correlated directly with the canalicular surface area before stimulation. In contrast, expansion times after stimulation varied inversely with initial canalicular volumes. Ursodeoxycholic acid failed to produce a hypercholeresis at 10-, 100-, or 200-,MM concentra-tions compared with taurocholate, either in normal or taurine-depleted IRHC. The present findings establish that rates of canalicular bile secretion can be quantitated in IRHC by serial optical planim-etry, both in the basal state and after stimulation with bile acids. Furthermore, ursodeoxycholate does not acutely induce hypercholeresis at the canalicular level in this model. Rather, both taurocholic and ursodeoxycholic acids induced secretion in proportion to the surface area of the canalicular membrane. The IRHC are a useful model to identify canalicular cholere-tics and for studies of canalicular bile formation