Prairie Dog Gallbladder Research Articles

Intracellular calcium modulates gallbladder ion transport

Although experimentally induced cholesterol gallstone formation has been associated with altered gallbladder (GB) absorption and increased biliary Ca 2+, the relationship between these events remains unclear. Recent studies suggest that extracellular Ca 2+ ([Ca 2+] ec) influences GB ion transport. Whether the effects of [Ca 2+] ec are mediated by changes in intracellular Ca 2+ ([Ca 2+] ic) has not been determined. This study was designed to define the effects of altered [Ca 2+] ic on GB ion transport. Prairie dog GBs were mounted in a Ussing chamber and short-circuit current ( I sc), potential difference ( V ms), and resistance ( R t) were recorded. Mucosal surfaces were exposed to either Dantrolene (Dt) or nickel (Ni 2+). Dt “traps” [Ca 2+] ic within intracellular organelles, thereby lowering cytosolic Ca 2+; and Ni 2+ prevents influx of [Ca 2+] ec, presumably by binding Ca 2+ channels. Although Dt reduced both I sc and V ms ( P < 0.01), these effects were transient. Transport recovery was probably due to increased [Ca 2+] ec influx with restoration of [Ca 2+] ic. Ni 2+ resulted in sustained decreases in I sc and V ms ( P < 0.05) despite subsequent addition of 10 m M Ca 2+. These findings are consistent with the prevention of [Ca 2+] ec influx by Ni 2+. We conclude that: (1) [Ca 2+] ic may be a modulator of GB ion transport and (2) previously reported [Ca 2+] ec effects on ion transport may be mediated through [Ca 2+] ic concentration changes.

Uncoupled basal sodium absorption and chloride secretion in prairie dog ( Cynomys ludovicianus) gallbladder

1. 1. Prairie dog gallbladders mounted in a Ussing-type chamber and bathed with symmetrical Ringer's solutions exhibited a transepithelial resistance ( R t ) of 51 ± 51Ω cm 2 , a lumen negative potential difference (V ms) of 11.5 ± 0.7 mV and a short-circuit current (I sc) of 6.9 ± 0.3 μEq/hr/cm 2. 2. 2. Radioisotopic ion flux experiments revealed that the basal I sc of 6.9 ± 0.3 μEq/hr/cm 2 was mostly accounted for by net Na + absorption of 3.2 ± 0.5 μEq/hr/cm 2 and net Cl − secretion of 2.9 ± 0.3 μEq/hr/cm 2. 3. 3. In HCO 3 − free Ringer's, net Na + flux was virtually abolished, net Cl − flux decreased by 50% and I sc was reduced by 77%. 4. 4. 10 −3 M mucosal amiloride and DIDS reduced I sc by 28 and 24%, respectively. 5. 5. Mucosal NaCl diffusion potentials indicated that the paracellular pathway was cation selective. 6. 6. Thin section electron micrographs showed a single cell population in this epithelium suggesting that net Na + absorption and Cl − secretion may emerge from the same cells. 7. 7. We conclude that prairie dog gallbladder epithelium is an electrogenic tissue and, in contrast to gallbladders of most other species, simultaneously but independently absorbs Na + and secretes Cl −.

Ca 2+ channel blockade inhibits gallbladder ion transport

Recent studies suggest that cholesterol gallstone (GS) formation is characterized by altered gallbladder epithelial ion transport and increased gallbladder (GB) luminal Ca 2+. Moreover, intracellular Ca 2+ has been reported to be an important modulator of intestinal ion transport. The aim of the present study was to determine the effects of Ca 2+ channel inhibition on GB ion transport. Prairie dog GBs were mounted in a Ussing chamber and bathed in warm oxygenated Ringer's solution, and short-circuit current ( I sc), transepithelial potential difference ( V ms), and tissue resistance ( R t) were recorded. Following stabilization, the mucosal surfaces of the GBs were exposed to 1 or 0.1 m M verapamil (VER). Effects on I sc were apparent within 10 sec with nadir values reached in 5 ± 1 min. Profound (76%) inhibition of I sc was seen with 1 m M verapamil exposure (26 ± 6 μA · cm −2) as compared to baseline values (170 ± 6 μA · cm −2) ( P < 0.001). Verapamil exposure (1 m M) also led to a marked inhibition of V ms ( P < 0.001 vs baseline) and a significant increase in R t ( P < 0.05 vs baseline). Similar trends were seen using 0.1 m M verapamil ( I sc nadir 133 ± 13 μA · cm −2). Verapamil-induced effects on gallbladder electrophysiology were largely reversible (75–90% recovery of baseline I sc after tissue washing). These data suggest that (1) verapamil induces rapid but reversible inhibition of ion transport and (2) Ca 2+ channel blockade inhibits ion transport in a dose-dependent fashion. We would propose that intracellular Ca 2+ may be a regulator of GB ion transport.

Morphology of the prairie dog gallbladder: normal characteristics and changes during early lithogenesis.

Studies were undertaken to describe the normal structure of the prairie dog gallbladder and adjacent cystic duct, and then to determine sequential changes that occurred as abnormalities in bile composition developed during high cholesterol feeding. Control animals were fed a diet with trace cholesterol, while experimental animals were fed a diet enriched with 1.2% cholesterol for 1, 2, 3, or 4 weeks. Light microscopy and scanning and transmission electron microscopy were used to characterize morphologic changes at each time interval. Biliary lipid composition was altered in all experimental groups, evidenced by significant decreases in bile-acid-to-cholesterol ratios. Cholesterol crystals appeared in experimental bile at 1 and 2 weeks, while stones formed at 3 and 4 weeks. The cystic duct and neck of the gallbladder occasionally displayed goblet cells. Little mucus was demonstrable in principal cells of the gallbladder, but much more in those lining the cystic duct. After 2 weeks of lithogenic diet, there was an increase in mucus content and secretion from all areas, as well as an influx of polymorphonuclear and mononuclear leukocytes. Accumulation of plasma cells in the lamina propria was an especially prominent feature of experimental tissues. These results suggest that 1) there is regional heterogeneity in the mucus content of the gallbladder and cystic duct of the prairie dog, and 2) both regions respond to lithogenesis with mucus hypersecretion and acute and chronic inflammatory changes prior to the appearance of cholesterol gallstones.

The effects of amiloride on biliary calcium and cholesterol gallstone formation.

Recent studies indicate that gallbladder absorption increases during the early stages of experimentally-induced cholesterol gallstone formation. The purpose of the present study was to ascertain whether pharmacologic inhibition of gallbladder ion transport and absorption reduces the incidence of experimentally-induced cholesterol gallstones. Prairie dogs were fed either a control chow or a 1.2% cholesterol-enriched chow for 15 days. One group of cholesterol-fed animals received saline via an orogastric tube; another group received amiloride, a drug known to inhibit in vitro ion transport in the prairie dog gallbladder. The incidence of gallstones in cholesterol-fed animals was reduced from 83% to 13% (p less than 0.025) when the animals were treated with amiloride; this occurred despite a cholesterol-saturation index comparable to that observed in gallstone animals. Additionally, although biliary calcium decreased in the gallbladder, hepatic bile did not in the amiloride-treated animals. These data provide further evidence that altered gallbladder absorption and increased biliary calcium are important factors in the pathogenesis of cholesterol gallstones.

Arachidonic acid stimulates mucin secretion in prairie dog gallbladder.

Mucin glycoprotein secretion from prairie dog gallbladder explants was studied in 24-h organ culture using [3H]glucosamine as a precursor. Indomethacin caused a reversible dose-dependent inhibition of both mucin release (50% inhibition between 10(-6) and 10(-5) M indomethacin) and 6-ketoprostaglandin F1 alpha (6-keto-PGF1 alpha) release (66% inhibition at 10(-7) M). Sepharose 4B chromatography of secreted [3H]-glucosamine-labeled glycoproteins revealed that indomethacin inhibited release of a high-molecular (greater than 10(6) daltons) mucin-type glycoprotein. Addition of sodium arachidonate (10(-4) M) to organ culture medium caused an approximate two- to fivefold increase (P less than 0.02) in mucin release compared with control and an increase in the secretory component of total glycoprotein synthesis from 37.5% in control explants to 75.2% (P less than 0.01). The stimulatory effect of arachidonate on mucin secretion was blocked by indomethacin. We incubated explants for 1 h with [14C]arachidonate and studied prostaglandin and thromboxane products in medium by high-performance liquid chromatography. The only conversion product identified was 6-keto-PGF1 alpha, the stable breakdown product of prostacyclin. Release of gallbladder mucin is significantly inhibited by indomethacin and increased by arachidonate. Mucin secretion may be regulated in part by endogenous levels of prostacyclin, the major cyclooxygenase product in prairie dog gallbladder epithelium.

Roles of lithogenic bile and cystic duct occlusion in the pathogenesis of acute cholecystitis

The hypothesis that the elements essential for the induction of acute cholecystitis are the presence of lithogenic bile and cystic duct occlusion was tested in the prairie dog gallstone model. Neither the presence of gallstones alone nor acute cystic duct occlusion alone resulted in acute inflammation of the gallbladder. Acute cholecystitis developed in prairie dog gallbladders containing cholesterol-saturated bile, with or without gallstones, shortly after cystic duct occlusion. These data suggest that the factors essential for the induction of acute cholecystitis are the presence of lithogenic bile and cystic duct occlusion and that gallstones, although frequently present, are not an essential prerequisite to acute inflammation.