Intravital Dynamic and Correlative Imaging of Mouse Livers Reveals Diffusion‐Dominated Canalicular and Flow‐Augmented Ductular Bile Flux

Abstract

Background and Aims Small‐molecule flux in tissue microdomains is essential for organ function, but knowledge of this process is scant due to the lack of suitable methods. We developed two independent techniques that allow the quantification of advection (flow) and diffusion in individual bile canaliculi and in interlobular bile ducts of intact livers in living mice, namely fluorescence loss after photoactivation and intravital arbitrary region image correlation spectroscopy. Approach and Results The results challenge the prevailing “mechano‐osmotic” theory of canalicular bile flow. After active transport across hepatocyte membranes, bile acids are transported in the canaliculi primarily by diffusion. Only in the interlobular ducts is diffusion augmented by regulatable advection. Photoactivation of fluorescein bis‐(5‐carboxymethoxy‐2‐nitrobenzyl)‐ether in entire lobules demonstrated the establishment of diffusive gradients in the bile canalicular network and the sink function of interlobular ducts. In contrast to the bile canalicular network, vectorial transport was detected and quantified in the mesh of interlobular bile ducts. Conclusions The liver consists of a diffusion‐dominated canalicular domain, where hepatocytes secrete small molecules and generate a concentration gradient and a flow‐augmented ductular domain, where regulated water influx creates unidirectional advection that augments the diffusive flux.

Highlight report: New insights in liver physiology: Canalicular bile flux is diffusion dominated

“…Therefore, drugs should be identified that reduce the assumed flow, which would also prevent the increase of damaging pressure. However, based on the present results (Vartak et al, 2020[ 20 ]), this flow-pressure concept should be questioned. It will be interesting to observe the further discussion about the correct model of bile flux and its pathophysiological as well as clinical consequences.…”

“…In contrast to bile canaliculi, the bile ducts are lined by epithelial cells, the cholangiocytes. In the present study, the authors show that it is important to differentiate bile canaliculi and bile ducts as functionally distinct domains (Vartak et al, 2020[ 20 ]): transport of small molecules in the bile canaliculi is diffusion dominated and only in the ducts diffusion is augmented by flow due to water influx (Vartak et al, 2020[ 20 ]). According to this model, bile canaliculi can be compared to a lake with standing water that is connected to a river, the bile duct.…”

“…In the current issue of Hepatology a novel concept has been published how the liver transports bile through canaliculi and ducts to the intestine (Vartak et al, 2020[ 20 ]). Hepatocytes are known to excrete bile acids and xenobiotics into a 'canal system' that finally drains into the intestine (Godoy et al, 2013[ 9 ]).…”

“…When a compound is given into the lake, it will also reach the river - by diffusion - and will only then be carried away by flow. This new model (Vartak et al, 2020[ 20 ]) contradicts the prevailing osmotic model of bile flow that is present in medical text books since decades (Boyer, 2013[ 2 ]; Boyer and Bloomer, 1974[ 3 ]; Boyer et al, 1970[ 4 ]; Layden et al, 1978[ 14 ]; Sperber, 1959[ 18 ]; Wheeler and Ramos, 1960[ 21 ]; Wheeler et al, 1968[ 22 ]; Wood et al, 1977[ 23 ]). According to the osmotic model, bile acids are excreted by hepatocytes into the bile canaliculi.…”

“…Because of the small canalicular diameter flow analysis cannot be accomplished by conventional methods. Vartak and colleagues now established a method that allows the quantification of flow and diffusion in bile canaliculi and ducts in intact livers of living mice (Vartak et al, 2020[ 20 ]). For this purpose, they used a photoactivatable compound, CMNB-fluorescein, which only upon UV irradiation releases fluorescein that then can be detected.…”