Communication via gap junctions modulates bile secretion in the isolated perfused rat liver.
ABSTRACT Bile secretion is regulated in part by adenosine 3',5'-cyclic monophosphate (cAMP) and cytosolic Ca2+ (Ca2+i). Hormone receptors that link to these second messengers are not uniformly distributed across the hepatic lobule, but both cAMP and Ca2+i cross gap junctions, so we tested whether gap junctional communication plays a role in changes in bile flow induced by the activation of these receptors.
cAMP levels in isolated perfused rat livers were increased by using glucagon, because glucagon receptors are predominantly on pericentral hepatocytes, or by using dibutyryl cAMP, which acts on hepatocytes throughout the hepatic lobule. Ca2+i concentration was increased by using vasopressin, because V1a receptors are most heavily expressed on pericentral hepatocytes, or by using 2,5-di(tert-butyl)-1, 4-benzo-hydroquinone (t-BuBHQ), which increases the Ca2+i concentration in hepatocytes throughout the hepatic lobule. We used 18alpha-glycyrrhetinic acid (alphaGA) to block gap junction conductance, which was assessed by fluorescence recovery after photobleaching.
alphaGA blocked fluorescence recovery after photobleaching without altering the basal rate of bile flow. Glucagon and dibutyryl cAMP increased bile flow; alphaGA blocked the glucagon-induced increase but not that induced by dibutyryl cAMP. Vasopressin and t-BuBHQ decreased bile flow; alphaGA exacerbated the decrease induced by vasopressin but not by t-BuBHQ.
Glucagon and vasopressin modulate bile flow in a manner that depends in part on gap junctional communication, even though the two hormones activate second messengers with opposing effects on bile flow. The organization of second messenger signals across the hepatic lobule may be an important component of hormonal regulation of bile secretion.
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ABSTRACT: Members of the connexin gene family are integral membrane proteins that form hexamers called connexons. Most cells express two or more connexins. Open connexons found at the nonjunctional plasma membrane connect the cell interior with the extracellular milieu. They have been implicated in physiological functions including paracrine intercellular signaling and in induction of cell death under pathological conditions. Gap junction channels are formed by docking of two connexons and are found at cell-cell appositions. Gap junction channels are responsible for direct intercellular transfer of ions and small molecules including propagation of inositol trisphosphate-dependent calcium waves. They are involved in coordinating the electrical and metabolic responses of heterogeneous cells. New approaches have expanded our knowledge of channel structure and connexin biochemistry (e.g., protein trafficking/assembly, phosphorylation, and interactions with other connexins or other proteins). The physiological role of gap junctions in several tissues has been elucidated by the discovery of mutant connexins associated with genetic diseases and by the generation of mice with targeted ablation of specific connexin genes. The observed phenotypes range from specific tissue dysfunction to embryonic lethality.Physiological Reviews 11/2003; 83(4):1359-400. · 26.87 Impact Factor
Article: Intercellular calcium waves mediate preferential cell growth toward the wound edge in polarized hepatic cells.[show abstract] [hide abstract]
ABSTRACT: During liver regeneration, hepatocytes sense the damage and initiate proliferation of the quiescent cells through poorly understood mechanisms. Here, we have used cultured hepatic cells to study the roles played by intercellular calcium in mediating wound-healing processes. Well-differentiated and polarized Hep-G2 cells repaired an experimentally induced wound by induction of cell divisions. The resulting cellular growth did not occur evenly across the healing cell lawn; instead, proliferations were three times more active within 150-200 microm from the wound edge than further away; this periwound preferential cell growth was not observed in the poorly differentiated and/or nonpolarized cells. We have provided experimental evidence demonstrating that the wounding procedure itself could elicit a propagating calcium wave, and interestingly, blocking this injury-associated intercellular calcium communication could effectively inhibit the biased cell growth along the margin of the wound. A photolithography-based patterned cell culture system was employed to help delineate the mechanisms underlying this type of calcium signaling. In conclusion, our results suggested that intercellular communications via propagating calcium waves coordinate regenerative cell proliferations in response to hepatic tissue losses.Experimental Cell Research 08/2003; 287(2):209-18. · 3.58 Impact Factor
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ABSTRACT: Hepatocytes and other digestive epithelia exchange second messengers and coordinate their functions by communicating through gap junctions. However, little is known about intercellular communication in cholangiocytes. The aim of this study was to examine expression and regulation of gap junctions in cholangiocytes. Connexin expression was determined by confocal immunofluorescence in rat bile ducts and in normal rat cholangiocyte (NRC) cells, a polarized cholangiocyte cell line. Intercellular Ca(2+) signaling was monitored by fluorescent microscopy. Microinjection studies assessed regulation of gap junction permeability in NRC cells and in SKHep1 cells, a liver-derived cell line engineered to express connexin 43. Immunochemistry showed that cholangiocytes from normal rat liver as well as the NRC cells express connexin 43. Localization of apical, basolateral, and tight junction proteins confirmed that NRC cells are well polarized. Apical exposure to ATP induced Ca(2+) oscillations that were coordinated among neighboring NRC cells, and inhibition of gap junction conductance desynchronized the Ca(2+) oscillations. NRC cells transfected with a connexin 43 antisense were significantly less coupled. Transcellular dye spreading was inhibited by activation of protein kinase A or protein kinase C. The same was observed in transfected SKHep1 cells, which expressed only connexin 43. Rat cholangiocytes and NRC cells express connexin 43, which permits synchronization of Ca(2+) signals among cells. Permeability of connexin 43-gap junctions is negatively regulated by protein kinases A and C. In conclusion, cholangiocytes have the capacity for intercellular communication of second messenger signals via gap junctions in a fashion that is under hormonal control.Hepatology 10/2002; 36(3):631-40. · 11.66 Impact Factor