The heterogeneous expression of some enzymes/proteins and membrane transporters/receptors in small and large ducts from mice, rats and humans is summarized in Table 2. In human liver, large septal bile ducts mainly express the sialylated Lewisa blood group antigen[78]. In normal and diseased human livers, hepatic, segmental, area, and septal bile ducts, and peribiliary glands express pancreatic enzymes such as pancreatic lipase, pancreatic a-amylase, and trypsin[79,80]. By microarray of RNA from small and large immortalized murine cholangiocytes, we have demonstrated the heterogeneous expression of approximately 80 proteins between small and large cholangiocytes[75]. The pathophysiological relevance of the differential expression of these messages remains to be addressed.
Secretory activity
Recent studies have demonstrated that large bile ducts are the major anatomical sites of cAMP-dependent ductal secretion by activation of cAMP/PKA/CFTR/Cl-/HCO3- exchanger (Figure 3)[3,12,13,48,54]. Specifically, studies in isolated small and large cholangiocytes and IBDU from normal and BDL rats have shown that large (but not small) cholangiocytes express the messages for secretin receptor, CFTR and Cl-/HCO3- exchanger and respond to secretin with increases in cAMP levels, Cl- efflux and Cl-/HCO3- exchanger activity and IBDU lumen expansion (Figure 3)[12,13,48,54]. In rat liver, large ducts express alkaline phosphatase and g-glutamyltranspeptidase[81]. The expression of alkaline phosphatase in large ducts is consistent with our previous studies[81] showing that alkaline phosphatase inhibits secretin-stimulated choleresis by blockage of CFTR activity, which is expressed only in large ducts (Figure 3)[54]. Furthermore, large cholangiocytes (which is the only cholangiocyte subpopulation expressing the somatostatin receptor, SSTR2)[48] are the major anatomical sites of somatostatin inhibition of secretin-stimulated ductal secretion (Figure 3)[48,55]. The inhibitory effects of somatostatin on secretin-stimulated secretion in large cholangiocytes are associated with reduced cAMP levels, Cl- efflux and Cl-/HCO3- exchanger activity[48,55,82]. The counter-regulatory effect of somatostatin on the choleretic effect of secretin is important in modulating ductal secretion in pathological conditions associated with cholangiocyte proliferation/loss[3]. Parallel with the findings observed in rat bile ducts[3,12,13,48, 4], in human liver secretin-stimulated duct secretory activity is heterogeneous, since only large bile interlobular ducts express the Cl-/HCO3- exchanger[83].
We have demonstrated the presence of insulin and CCK-B/gastrin receptors in large cholangiocytes from normal and BDL rats and have shown that these two hormones inhibit secretin-stimulated ductal secretion of BDL rats by IP3/Ca2+/PKCa-dependent decrease of cAMP levels[7,72,77]. Similarly, we found that ETA and ETB receptors are expressed by large cholangiocytes and that ET-1 inhibits secretin-stimulated cAMP levels and ductal bile secretion of BDL rats by interaction with ETA but not ETB receptors[59]. Furthermore, recent data have shown that: (1) the D2 dopaminergic receptors are expressed by large BDL cholangiocytes; and (2) the D2 dopaminergic receptor agonist, quinelorane, inhibits secretin-stimulated ductal secretion by activation of the Ca2+-dependent PKCg[25]. The a2-adrenergic receptor agonist, UK14, 304, inhibits secretin-stimulated cAMP-dependent Cl- efflux and Cl-/HCO3- in large cholangiocytes and secretin-stimulated lumen expansion in large IBDU of BDL rats[66]. The a1-adrenergic receptor agonist, phenylephrine, stimulates cAMP levels and secretin-stimulated secretion of large BDL cholangiocytes by IP3/Ca2+-dependent activation of PKCa and PKCbⅡ[65]. We have recently demonstrated[26] that acetylcholine, by interacting with M3 receptor subtypes, potentiates secretin-stimulated cAMP levels and Cl-/HCO3- exchanger activity in IBDU and purified cholangiocytes by a Ca2+-calcineurin mediated but PKC independent modulation of adenylyl cyclase.
Following hepatocyte secretion[84], bile acids are reabsorbed by the biliary epithelium[85], then they return via the PBP to the hepatocytes for secretion into bile (cholehepatic shunting)[86]. As a mechanism for bile acids entry into cholangiocytes, the apical Na+-dependent bile transporter, ASBT (structurally identical to the ileal bile acid transporter) is expressed on the apical membranes of large cholangiocytes[87]. Consistent with functional activity for ASBT in cholangiocytes, studies have shown Na+-dependent and saturable uptake of taurocholate in normal cholangiocyte cultures[88] and large cholangiocytes[89]. These data suggests that after taurocholate and taurolithocholate enter into large cholangiocytes by ABAT, they stimulate secretin-stimulated ductal bile flow in these cholangiocyte subpopulations[89,90]. Other studies have shown that both taurocholate and taurolithocholate increase secretin-stimulated cAMP levels in large but not small cholangiocytes[90]. Chronic feeding of ursodeoxycholate and tauroursodeoxycholate to BDL rats inhibits secretin-stimulated ductal secretion in large cholangiocytes[62].
As evidence against the notion that small cholangio-cytes may be primitive, undifferentiated cells that do not display secretory activity, recent studies have shown that in pathological conditions associated with damage of large cAMP-responsive ducts (e.g., after acute CCl4 administration) (Figure 3)[50,51], small cholangiocytes transiently compensate for large cholangiocyte damage by de novo activation of secretory (including expression of secretin receptor and secretin-stimulated cAMP response)[50,51] and proliferative[50,51] (see below) activities. Following ANIT feeding and partial hepatectomy, small cholangiocytesproliferate and secrete by the de novo expression of secretin receptor and activation of cAMP response[47,52]. Since preliminary data and unpublished observations (Alpini, 2005) show that small rat and mouse cholangiocytes express receptors (ETA, CCK-B/gastrin, a1-adrenergic, D2 dopaminergic, insulin, H1 histamine) signaling by activation of IP3/Ca2+/PKC[59,91], we propose that there is a secretory gradient in the intrahepatic biliary tree with small cholangiocytes secreting water and electrolytes by activation of the IP3/Ca2+/PKC pathway, whereas large cholangiocytes secrete bile by activation of the cAMP/PKA/CFTR/Cl-/HCO3- exchanger[2,5,2,13,48,54].
Answers to all your Biology Questions
