Altered Na+ and Cl- flux during diet-induced mixed gallstone formation in the prairie dog

Leptin regulates gallbladder genes related to absorption and secretion

Dysregulation of gallbladder ion and water absorption and/or secretion has been linked to cholesterol crystal and gallstone formation. We have recently demonstrated that obese, leptin-deficient (Lep(ob)) mice have enlarged gallbladder volumes and decreased gallbladder contractility and that leptin administration to these mice normalizes gallbladder function. However, the effect of leptin on gallbladder absorption/secretion is not known. Therefore, we sought to determine whether leptin would alter the expression of genes involved in water and ion transport across the gallbladder epithelium. Affymetrix oligonucleotide microarrays representing 39,000 transcripts were used to compare gallbladder gene-expression profiles from 12-wk-old control saline-treated Lep(ob) and from leptin-treated Lep(ob) female mice. Leptin administration to Lep(ob) mice decreased gallbladder volume, bile sodium concentration, and pH. Leptin repletion upregulated the expression of aquaporin 1 water channel by 1.3-fold and downregulated aquaporin 4 by 2.3-fold. A number of genes involved in sodium transport were also influenced by leptin replacement. Epithelial sodium channel-alpha and sodium hydrogen exchangers 1 and 3 were moderately downregulated by 2.0-, 1.6-, and 1.3-fold, respectively. Carbonic anhydrase-IV, which plays a role in the acidification of bile, was upregulated 3.7-fold. In addition, a number of inflammatory cytokines that are known to influence gallbladder epithelial cell absorption and secretion were upregulated. Thus leptin, an adipocyte-derived cytokine involved with satiety and energy balance, influences gallbladder bile volume, sodium, and pH as well as multiple inflammatory cytokine genes and genes related to water, sodium, chloride, and bicarbonate transport.

Gallstone formation. Local factors

Bile supersaturation is necessary for cholesterol gallstones to form. Not all people with supersaturated bile form gallstones, however, and additional factors must be present. The role of pronucleating substances has been extensively studied. Of these, proteins, especially mucin, are best understood. Mucin is secreted by the gallbladder epithelium and may act as a nidus for crystal nucleation. Other proteins that may act as pronucleators include alpha 1-acid glycoprotein, alpha 1-antichymotrypsin, phospholipase C, and a small calcium binding protein. The role of antinucleating factors is less well understood. Certain drugs, including octreotide and ceftriaxone, may also predispose to stone formation. Another local factor is gallbladder stasis, a well-known risk factor for pigment stone formation. More recent research has focused on the role of bacterial infection, which has long been believed to be a factor in pigment gallstone formation. Newer data also support a role for infection in cholesterol gallstone pathogenesis. Additionally, genetic factors that may predispose a patient to cholesterol gallstones have been identified in mice and in humans.

Establishment and characterization of primary gallbladder epithelial cell cultures in the prairie dog

BACKGROUND

The prairie dog has become the established animal gallstone model. This species has a unique propensity to form cholesterol gallstones in response to dietary manipulations. The development of a reliable gallbladder cell culture technique is critical for understanding pathogenic mechanisms of gallstone formation.

MATERIALS AND METHODS

Prairie dogs underwent laparotomy and cholecystectomy, followed by initiation of cell cultures. [3H]Thymidine incorporation was used to assess cell growth, and cell lines were assessed using routine histochemical and immunohistochemical staining.

RESULTS

Cell yields from prairie dog gallbladders were 4-8 x 10(6) viable cells per animal with viability ranging from 80 to 95%. When plated at 5 x 10(5) cells/cm2, cell clusters, visible within 24 h, coalesced into confluent monolayers within 3-5 days. Cultures remained viable for 6-8 weeks and could be passed for three to four subcultures. Immunohistochemical staining demonstrated a high degree of epithelial purity with immunopositivity for AE1/AE3, and cytokeratin, with no vimentin positivity (mesenchymal antigen). Intracytoplasmic vacuoles demonstrated positive staining for Alcian blue, periodic acid-Schiff, and mucicarmine and an anti-gallbladder mucin antibody confirmed the presence of the glycoprotein mucin.

CONCLUSIONS

This study demonstrates a reliable method for initiation and maintenance of prairie dog gallbladder epithelial cell cultures with a high degree of purity. This technique should allow further studies into the pathogenesis of cholesterol gallstones in this model.

Converting gallbladder absorption to secretion: the role of intracellular calcium

BACKGROUND

Experimental cholelithiasis is associated with elevated biliary calcium concentration and altered gallbladder absorption. Recent studies showed that extracellular calcium ([Ca2+]ec) plays a role in regulating gallbladder ion transport. The extent to which intracellular calcium ([Ca2+]ic) mediates the changes in gallbladder ion transport is not clear. We hypothesize that [Ca2+]ic is an important regulator of gallbladder ion transport.

METHODS

Prairie dog gallbladders were mounted in Ussing chambers, standard electrophysiologic parameters were recorded, and unidirectional Na+, Cl- and H2O fluxes were measured before and after mucosal exposure of 10-5 mol/L calcium ionophore A23187 was performed.

RESULTS

A23187 caused an increase in transepithelial short-circuit current and potential difference and a decrease in transepithelial resistance. A23187 inhibited mucosa to serosa Cl- flux and stimulated serosa to mucosa Na+ flux, resulting in increased net Cl- secretion and decreased net Na+ absorption. A23187 converted H2O from absorption to secretion. Transepithelial short-circuit current effect of A23187 was delayed by indomethacin pretreatment and was completely blunted by low bathing Ca2+.

CONCLUSIONS

This is the first demonstration that increased [Ca2+]ic converts the gallbladder from its normal absorptive state to a secretory one. Furthermore [Ca2+]ic appears to regulate ion transport through mechanisms that are partially prostaglandin-dependent. Studies are necessitated to define possible links between gallbladder secretion of Cl- and H2O and mucus hypersecretion, a well-described phenomenon associated with cholesterol gallstone formation.

Prostaglandin E2 stimulates ion transport in prairie dog gallbladder

The effects of prostaglandins, and specifically prostaglandin E2, on gallbladder ion transport were examined in the prairie dog. Gallbladders were mounted in an Ussing chamber and baseline short-circuit current, potential difference, and tissue resistance were measured. Addition of arachidonic acid (10(-4) M, mucosal surface) produced sustained elevations in short-circuit current and potential difference (P < 0.05), with mild reductions in resistance. In a second set of tissues, indomethacin exposure (10(-6) M) resulted in a significant (P < 0.02) decrease in short-circuit current and potential difference, with an increase in resistance. Subsequent addition of prostaglandin E2 (10(-7) M, serosal surface) fully reversed these changes and led to a significant increase in short-circuit current and potential difference (P < 0.001) with a return of resistance to baseline values. These findings suggest that endogenous prostaglandins mediate gallbladder ion transport.