Role of primary bile salts in the regulation of sinusoidal substrate uptake in rat liver

Stefanie Kluge,Thomas Berlage,Ralf Kubitz,Olga Domanova,Dieter Häussinger

doi:10.1186/2047-783x-19-s1-s23

Abstract

Background Bile salts undergo enterohepatic circulation which is essential for bile salt homeostasis. From the biliary tract, they are excreted into the small intestine, absorbed into the blood and transported back to the liver. The sinusoidal uptake systems of the liver efficiently extract bile salts and other substrates from portal blood. In rat liver, these systems include Ntcp (sodium taurocholate cotransporting polypeptide) and the organic anion transporting polypeptides Oatp1a1, Oatp1a4 and Oatp1b2. While Ntcp represents the major basolateral uptake transporter for conjugated bile salts, the Oatps have broad substrate spectra including estrogenor leukotriene-conjugates. Their transport function is regulated by long-term and short-term mechanisms. Long-term adaptation of bile salt transporters involve changes at the level of gene expression and transporter degradation, while short-term regulation includes covalent transporter modifications, substrate availability and rapid endoand exocytosis of transporter-containing vesicles (reviewed in [1]). Ntcp as well as Oatp1a1 and Oatp1a4 underlie short-term control which has been demonstrated in several in vitro models. Hypoosmolarity or cAMP for instance increase the Ntcp-dependent bile salt uptake (increase of Vmax) within minutes by translocation of intracellular Ntcp to the plasma membrane [2,3]. In contrast, rapid clathrin-dependent endocytosis of Ntcp was demonstrated only recently by activation of PKC with phorbolesters [4,5]. However, to date in vivo-studies on short-term regulation of basolateral transporters are rare. Bile salt inflow to the liver may increase considerably under pathophysiological conditions or postprandial under physiological conditions [6]. To date, the interrelation between substrate load and transport activity is largely unknown. Therefore, the aim of this study was to investigate the role of primary bile salts in short-term feedback regulation of sinusoidal substrate uptake in perfused rat liver.

Highlights

Bile salts undergo enterohepatic circulation which is essential for bile salt homeostasis
Materials and methods Transporter retrieval from the plasma membrane was analyzed by assessing subcellular distribution of sinusoidal and canalicular transporters in immunofluorescence images from tissue sections of perfused rat liver by an automated image analysis method, as described previously [6,7]
Net substrate uptake was monitored in perfused rat liver by a pulse chase technique

Summary

Introduction

Bile salts undergo enterohepatic circulation which is essential for bile salt homeostasis. The sinusoidal uptake systems of the liver efficiently extract bile salts and other substrates from portal blood. In rat liver, these systems include Ntcp (sodium taurocholate cotransporting polypeptide) and the organic anion transporting polypeptides Oatp1a1, Oatp1a4 and Oatp1b2. While Ntcp represents the major basolateral uptake transporter for conjugated bile salts, the Oatps have broad substrate spectra including estrogen- or leukotriene-conjugates. Their transport function is regulated by long-term and short-term mechanisms. Long-term adaptation of bile salt transporters involve changes at the level of gene expression and transporter degradation, while short-term regulation includes covalent transporter modifications, substrate availability and rapid endoand exocytosis of transporter-containing vesicles (reviewed in [1]). Rapid clathrin-dependent endocytosis of Ntcp was demonstrated only recently by activation of PKC with phorbolesters [4,5]

Objectives

Methods

Results