Mechanism of hepatic bicarbonate secretion and bile acid independent bile secretion

Spatial hepatocyte plasticity of gluconeogenesis during the metabolic transitions between fed, fasted and starvation states

The liver acts as a master regulator of metabolic homeostasis in part by performing gluconeogenesis. This process is dysregulated in type 2 diabetes, leading to elevated hepatic glucose output. The parenchymal cells of the liver (hepatocytes) are heterogeneous, existing on an axis between the portal triad and the central vein, and perform distinct functions depending on location in the lobule. Here, using single cell analysis of hepatocytes across the liver lobule, we demonstrate that gluconeogenic gene expression ( Pck1 and G6pc ) is relatively low in the fed state and gradually increases first in the periportal hepatocytes during the initial fasting period. As the time of fasting progresses, pericentral hepatocyte gluconeogenic gene expression increases, and following entry into the starvation state, the pericentral hepatocytes show similar gluconeogenic gene expression to the periportal hepatocytes. Similarly, pyruvate-dependent gluconeogenic activity is approximately 10-fold higher in the periportal hepatocytes during the initial fasting state but only 1.5-fold higher in the starvation state. In parallel, starvation suppresses canonical beta-catenin signaling and modulates expression of pericentral and periportal glutamine synthetase and glutaminase, resulting in an enhanced pericentral glutamine-dependent gluconeogenesis. These findings demonstrate that hepatocyte gluconeogenic gene expression and gluconeogenic activity are highly spatially and temporally plastic across the liver lobule, underscoring the critical importance of using well-defined feeding and fasting conditions to define the basis of hepatic insulin resistance and glucose production.

Acetazolamide inhibits stimulated feline liver and gallbladder bicarbonate secretion

Bile acidification is a key factor in preventing calcium carbonate precipitation and gallstone formation. Carbonic anhydrase II (CA II), that is inhibited by acetazolamide, plays a role in regulation of the acid-base balance in many tissues. This study examines the effect of acetazolamide on secretin- and vasoactive intestinal peptide (VIP)-stimulated gallbladder mucosal bicarbonate and acid secretion. Gallbladders in anaesthetized cats were perfused with a bicarbonate buffer bubbled with CO2 in air. In 20 experiments VIP (10 microg kg(-1) h(-1)) and in 10 experiments secretin (4 microg kg(-1) h(-1)) were infused continuously intravenous (i.v.). Hepatic bile and samples from the buffer before and after perfusion of the gallbladder were collected for calculation of ion and fluid transport. During basal conditions a continuous secretion of H+ by the gallbladder mucosa was seen. Intravenous infusion of vasoactive intestinal peptide (VIP) and secretin caused a secretion of bicarbonate from the gallbladder mucosa (P < 0.01). This secretion was reduced by intraluminal (i.l.) acetazolamide (P < 0.01). Bile flow was enhanced by infusion of VIP and secretin (P < 0.01) but this stimulated outflow was not affected by i.v. acetazolamide. The presence of CA II in the gallbladder was demonstrated by immunoblotting. Biliary CA activity has an important function in the regulation of VIP- and secretin-stimulated bicarbonate secretion across the gallbladder mucosa.

Subcellular and molecular mechanisms of bile secretion

One of the liver's principal functions is the formation of bile, which is requisite for digestion of fat and elimination of detoxified drugs and metabolites. Bile is a complex fluid made up of water, electrolytes, bile acids, pigments, proteins, lipids, and a multitude of chemical breakdown products. In this review, we have summarized the source of various biliary components, the route by which they end up in bile, including the underlying subcellular and molecular mechanisms, and their contribution to bile formation. One of the reasons why bile formation is so complex is that there are many mechanisms with overlapping substrate specificities, i.e., many biochemically unrelated biliary constituents share common transport mechanisms. Additionally, biliary constituents may reach bile by more than one pathway. Some biliary components are critical for bile formation; others are of minor significance for bile formation but play a major physiological role. The major driving force for bile formation is the uptake and transcellular transport of bile salts by hepatocytes. The energy for bile formation comes from the sodium gradient created by the basolateral Na+/K(+)-ATPase, to which bile salt transport is coupled. The secretory pathway for bile salts involves uptake at the basolateral surface of the hepatocyte, vectorial transcellular movement, and transport across the canalicular membrane into the canalicular lumen. Hydrophilic bile salts are taken up via a sodium-dependent, saturable, carrier-mediated process coupled to the Na+/K(+)-ATPase. This uptake mechanism is also shared by other substrates, such as electroneutral lipids, cyclic oligopeptides, and a wide variety of drugs. Hydrophobic bile acids are taken up by a sodium-independent facilitated carrier-mediated mechanism in common with other organic ions, including sulfated bile acids, sulfobromophthalein, bilirubin, glutathione, and glucuronides, or by nonsaturable passive diffusion. Two major carrier proteins have been identified on the hepatocyte basolateral membrane: a 48-kDa protein that appears to be involved with Na(+)-dependent bile salt uptake, and a 54-kDa protein, thought to be associated with Na(+)-independent bile salt uptake. The intracellular transport of bile salts may involve cytosolic carrier proteins, of which several have been identified. Some evidence suggests a vesicular transport mechanism for bile salts. Since bile acids clearly do not enter the cell by endocytosis, formation of transport vesicles must be a more distal event in the transcellular translocation process. Some bile salts appear to be transported within the same unilamellar vesicles that are involved in the secretion of cholesterol and phospholipid.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of phlorizin on hepatic bile production before and during glucose infusion

During intravenous infusion of glucose, bile secretion is reduced (cholestasis), indicating that hepatocellular metabolism of glucose could have harmful effects on the liver. Phlorizin has been identified as a compound capable of impeding glucose uptake of liver cells. To examine whether phlorizin had any effect on glucose-associated cholestasis, three groups of experiments were performed on anaesthetized pigs. In group I phlorizin (100 mg/kg body wt) during normoglycaemia stimulated bicarbonate-dependent bile secretion by 56 +/- 4%. After phlorizin, hyperglycaemia decreased both bile acid- and bicarbonate-dependent bile secretion by 37 +/- 4%. But after the glucose load normalization of plasma glucose concentration increased the bicarbonate-dependent fraction by 38 +/- 4%. In group II phlorizin (100 mg/kg body wt, infused intravenously) during hyperglycaemia stimulated bicarbonate-dependent bile secretion by 35 +/- 5%. In group III bile secretion was continuously stimulated by infusion of Na-taurocholate. Hyperglycaemia reduced bicarbonate-dependent bile secretion by 33 +/- 4%, but after phlorizin both bile acid- and bicarbonate-dependent bile secretion increased on average by 121 +/- 8%. The osmotic effect of hyperglycaemia cannot be blocked by phlorizin, but judged by the effect on bile secretion, phlorizin may decrease the cholestatic effect induced by metabolism of glucose. Phlorizin could be an interesting compound for use in solutions for organ preservation.

Effect of insulin on hepatic bile secretion during normoglycaemia and hyperglycaemia

The roles of hyperosmolality, hyperglycaemia, and insulin in glucose-induced reduction of bile secretion were examined in anaesthetized pigs. Compared with normoglycaemia, intravenous infusion of isotonic glucose reduced bile acid-dependent bile secretion at a plasma glucose concentration of 18 mmol/l, with 34 +/- 4%. Lowering of plasma glucose concentration to normoglycaemia after administration of insulin (10 U/kg body wt. low dose) increased bicarbonate-dependent bile secretion by 23 +/- 3%. Induction of hyperglycaemia (plasma glucose concentration, 16 mmol/l) and the combined infusion of isotonic glucose and the low insulin dose decreased bile secretion by 22 +/- 3%. During hyperglycaemia (plasma glucose concentration, 16 mmol/l) the combined infusion of isotonic glucose and a high dose of insulin (60 U/kg body wt) increased bile acid-dependent bile secretion by 26 +/- 3%. Hyperglycaemia reduces bile secretion without altering plasma osmolality. Endogenous production (or too rapid degradation) of insulin may be too small during intravenous glucose infusion to cope with the metabolic demands of hepatocellular glucose conversion. This may be overcome by administration of insulin in a large dose, which stimulates bile acid secretion.

Mechanism of ursodeoxycholic acid- and canrenoate-induced biliary bicarbonate secretion and the effect on glucose- and amino acid-induced cholestasis

The mechanism of ursodeoxycholic acid (UDCA)- and canrenoate-induced bicarbonate choleresis was studied before and during the administration of glucose or amino acids in anaesthetized pigs. Previous studies have shown that the canalicular secretion has, on a molar basis, a relationship among the secretion of chloride, bicarbonate, and bile acids of 0.9, 0.3, and 1, respectively. Ductular secretion is associated with the transport of 0.25 mol chloride per 1 mol bicarbonate. In control experiments UDCA was associated with a biliary secretion of about 1.3 mol chloride and 0.5 mol bicarbonate per 1 mol bile acid, and canrenoate caused the secretion of 1.2 mol chloride per 1 mol bicarbonate. Intravenous infusion of glucose or amino acids increased these relationships, and after administration of UDCA or canrenoate, these relationships were still increased by at least 70% on average when compared with the control experiments. A reduction in bile secretion after glucose or amino acid infusion is opposed by UDCA or canrenoate. The effect of UDCA or canrenoate on bile secretion is not disturbed by glucose or amino acids. Both substances stimulate canalicular bicarbonate secretion and could be of importance in improving cholestatic conditions.

Dose-dependent effects of glycine, alanine, and glucose on hepatic bile secretion, oxygen consumption, and hemodynamics

To assess the effects of a small (0.5%) and a large dose (5%) of glycine and alanine and of hypertonic glucose on hepatic bile secretion, oxygen consumption, and hemodynamics, experiments were performed on anesthetized pigs. Only the large dose of amino acids exerted significant changes. Glycine, alanine, and glucose reduced bile acid-dependent bile secretion gradually, which was nearly halved from a control value of 0.32 +/- 0.04 ml/min. Oxygen consumption was thereby continuously stimulated during amino acid and glucose infusion and increased from 448 +/- 132 mumol/min before to 995 +/- 226 mumol/min after the infusion of glycine, alanine, and glucose. Hepatic arterial blood flow increased from 214 +/- 14 ml/min to 238 +/- 14 ml/min after glycine infusion, whereas portal venous blood flow decreased from 542 +/- 50 ml/min to 481 +/- 47 ml/min. Total hepatic blood flow remained unchanged. Alanine and glucose provoked no further changes in hepatic blood flow. Bile secretion is a sensitive marker of hepatic metabolism, whereas hepatic blood flow is not a dominant regulator of bile secretion. Stimulation of hepatic metabolism is not followed by changes in total hepatic blood flow.

Colchicine blocks the effects of secretin on bile duct cell tubulovesicles and plasma membrane geometry and impairs ductular HCO3- secretion in the pig

Secretin causes the bile duct cells to secrete HCO3-. To examine whether the transformation of duct cell ultrastructure that follows secretin stimulation depends on microtubules and is important for ductular HCO3- secretion, we examined the effect of colchicine on ductular HCO3- secretion and on the morphology of cells lining bile ductules of anaesthetized pigs. Colchicine blocked secretin-dependent cytoplasmic clearance of tubulovesicles and prevented expansion of the basolateral plasma membrane in duct cells and reduced the ductular HCO3- secretory response from 132 +/- 25 mumol min-1 to 97 +/- 14 mumol min-1. In contrast, lumicolchicine did not affect secretin-dependent tubulovesicle clearance or plasma membrane geometry or ductular HCO3- secretion. Accordingly, secretin-dependent cytoplasmic clearance of tubulovesicles in bile duct cells appears to depend on microtubules and to be important for ductular HCO3- secretion.

Amino acid- and glucose-induced cholestasis before and during secretin stimulation

To identify the mechanisms of reduced bile flow after hypertonic amino acid and glucose infusion, acute experiments were performed on anesthetized pigs. When secretin was not administered, amino acids or glucose reduced bile acid-dependent bile secretion to 65 +/- 3% of control. During secretin stimulation amino acids or glucose diminished bile acid-independent bile secretion to 78 +/- 2% of control. No changes in serum bilirubin, alanine aminotransferase, and aspartate aminotransferase were observed. Amino acids and glucose attack different mechanisms responsible for bile formation, but the result is that when secretin is not administered, biliary secretion of bile acids is reduced, and, accordingly, bile acid-dependent bile flow diminished. During secretin stimulation biliary NaHCO3 secretion is depressed, accounting for a fall in bile acid-independent bile flow. Amino acids exert no effect on bile acid secretion or, as a result, on bile acid-dependent bile flow after secretin infusion.

Effects of digitoxin and lithium, used as a marker of passive Na transport, on secretin-dependent bile flow in the pig

The present study was performed in anaesthetized pigs, and the first aim was to assess the role of Na,K-ATPase in secretin-dependent biliary HCO3 secretion (JbHCO3). Intra-arterial administration of the cardiac glycoside digitoxin (0.2 mg/kg-1) reduced hepatic Na K-ATPase activity, JbHCO3 and secretin-dependent bile flow by 24, 55 and 34% respectively. In the second part of this study lithium (Li) was used as a marker of passive Na transport to assess the electrochemical gradient for Na flux into bile duct lumen during secretin-stimulated bile flow and impeded biliary osmotic water flow by i.v. infusion of glucose. At plasma glucose 85 (73-96) mmol l-1, bile [Na] and [Li] exceeded their concentrations in plasma by 57 and 47% respectively. By using the Nernst equation, transepithelial potential difference (PD) during hyperglycaemia was estimated to be -6.2 (0 to -10.8) mV (ductal lumen negative), which corresponds to a [Li]bile/[Li]plasma ratio of 1.3 (1.0-1.5). The ratio was not significantly different from the observed [Li]bile/[Li]plasma ratio of 1.4 (1.3-1.5). It is concluded (1) that Na, K-ATPase is necessary for JbHCO3, probably by sustaining the cell membrane PD (cell interior negative) which is a driving force for apical electrogenic HCO3 secretion, and (2) transepithelial Li (and hence Na) flux is driven solely by the negative transcellular PD during secretin-stimulated bile flow in the pig.

Secretin empties bile duct cell cytoplasm of vesicles when it initiates ductular HCO3- secretion in the pig

To determine whether secretin has any effect on bile duct cell ultrastructure, bile duct cells from liver biopsy specimens of pigs were analyzed morphometrically. During secretory rest, bile duct cell cytoplasmic vesicles totaled 96 (84-103) arbitrary units per cell volume (U). Secretin increased bile HCO3- secretion from 9 mumol/min (range 6-15) to 131 mumol/min (range 118-200) and lowered the bile duct cell vesicles to 5 U (range 3-9). Acute elevation of arterial PCO2 to 10.9 kPa (range 10.2-11.1) doubled vesicle number in resting duct cells and augmented the secretory response to secretin. At high arterial PCO2, secretin cleared the duct cell cytoplasm of vesicles and more than doubled the basolateral plasma membrane surface area. Taurocholate-induced canalicular choleresis, in contrast, did not alter duct cell morphology. It is concluded that secretin clears the bile duct cell cytoplasm of vesicles as it initiates ductular HCO3- secretion, possibly through causing exocytotic insertion of vesicle material into the basolateral plasma membrane.

Importance of carbonic anhydrase for canalicular and ductular choleresis in the pig

To assess the importance of carbonic anhydrase (CA) for canalicular and ductular choleresis, the effect of acetazolamide on bile secretion was measured in three experimental groups of anaesthetized pigs. CA activity in liver homogenate was 46 (43-54) U g-1 wet weight, 150 mg kg-1 b.w. acetazolamide completely abolished the CA activity. Acetazolamide reduced bile HCO3- secretion in six secretin infused, bile-acid depleted pigs by 67 (58-71)% at arterial pH 7.41 (7.38-7.46). By contrast, acetazolamide did not affect HCO3- secretion in six Na-taurocholate (TCA) infused pigs in the absence of secretin stimulation. Acetazolamide reduced ursodeoxycholic-acid- (UDCA) dependent HCO3- secretion by 24 (11-38)% in six other pigs in the absence of secretin stimulation. Histochemical examination using modifications of Hansson's method showed strong reaction in bile ductules and weaker reaction in peripheral zones of liver lobules. Because acetazolamide impairs HCO3- secretion from cells sustaining high rates of H+/HCO3- transport, it is suggested that high rates of H+/HCO3- transport are confined to bile ductules under conditions of secretin- and UDCA-induced choleresis.

Effects of bumetanide on bile flow in the pig

This study was performed on 12 anaesthetized pigs in order to examine the effect of the 'loop' diuretic bumetanide (inhibitor of Na,K,Cl-co-transport) on ductular bile secretion. It has previously been shown that administration of furosemide (a less potent 'loop' diuretic) to dogs and rats increases bile flow due to inhibition of ductular reabsorption of electrolytes and water. In group I (n = 6) bumetanide (median biliary concentration: 8.4 x 10(-3) mol l-1) increased bile flow and biliary concentration of HCO3 by 200% (116-320) and 50% (26-96), respectively. Biliary concentration of Cl was significantly decreased by 6% (2-12) following administration of bumetanide. In group II (n = 6) bile secretion was measured during secretin infusion (3 CU kg b. wt h-1) in the arterial pH range of 7.40-7.00, both before and after bumetanide administration in each animal. Bumetanide (median biliary concentration: 2.7 x 10(-3) mol I-1) did not significantly alter biliary secretion of water, HCO3, Na, K or Cl. Bile acid secretion was reduced by 30% from 43 (28-55) to 30 (17-41) mumol min-1 (P less than 0.05) while hepatic venous concentration of bile acids was raised by 90% (54-126) from 184 (113-309) to 350 (229-502) mumol l-1 (P less than 0.05) at slightly increased hepatic blood flow. Hepatic venous serum concentration of bumetanide was 4.8 (2.1-7.4) x 10(-4) mol l-1 (unbound fraction).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of arterial pH and PCO2 on biliary HCO3- secretion in the pig

The purpose of the present study was to examine the effect of changes in arterial pH and PCO2 on biliary HCO3- secretion. This was done in order to further characterize the various ion transport mechanisms considered responsible for biliary HCO3- secretion in the pig. Experiments were performed on two groups of six pigs. In both groups arterial pH was varied in steps from pH 7.40 to 7.00, both at PCO2 5.5 kPa and PCO2 10 kPa. In group I (n = 6), data were obtained on the effect of arterial pH and PCO2 on ductular HCO3- secretion in bile acid depleted (cholestyramine pretreated), secretin-infused pigs. In group II (n = 6), the effect of pH and PCO2 on canalicular HCO3- secretion was studied in ursodeoxycholic acid (UDCA)-infused pigs (3 mumol min-1 kg-1 body wt). In group I, biliary HCO3- secretion exhibited PCO2-dependent, positive straight line relationships to arterial pH. An increment in biliary HCO3- secretion of 17 (11-24)% was seen during high PCO2 at pH 7.40. In group II, biliary HCO3- secretion exhibited PCO2-dependent, positive curvilinear relationships to arterial pH. A median increment in HCO3- secretion of 37 (20-62)% was seen during elevated PCO2 at arterial pH 7.40. The linear dependence of ductular HCO3- secretion on arterial pH and the effect of elevated PCO2 on HCO3- secretion fit well with findings in other epithelia, where proton transport is thought be driven by a proton pump. A computer simulation provided evidence suggesting that secretin-dependent HCO3- secretion does not involve the action of a Na+/H+ ion exchanger--in contrast to UDCA-dependent HCO3- secretion. It is concluded that ductular and canalicular HCO3- secretion could be mediated by a proton pump and a Na+/H+ ion exchanger in addition to canalicular HCO3- secretion due to solvent drag and diffusion, respectively.

The effect of amiloride on biliary HCO3- secretion in the anaesthetized pig

The present study was performed on 29 anaesthetized pigs and shows that the bile acid ursodeoxycholic acid (UDCA) produces a flow of bile rich in HCO3- compared with taurocholic acid (TCA). The slope relating biliary HCO3- secretion to bile acid secretion was 0.59 (0.44-0.82) and 0.33 (0.29-0.38) during venous infusion of UDCA and TCA, respectively. We next wanted to evaluate the importance of Na+/H+ ion exchange for biliary HCO3- secretion. High doses of amiloride were employed in order to impair the hepatic Na+/H+ ion exchanger. It was reasoned that any reduction in H+ efflux through the hepatic Na+/H+ ion exchanger involved in causing biliary HCO3- secretion would be translated into an equimolar fall in biliary HCO3- secretion. We found that amiloride (2.0 X 10(-4) mol l-1 plasma) reduced UDCA-dependent canalicular HCO3- secretion by 26 (14-35)% without concurrently reducing bile acid secretion. Amiloride (2.9 X 10(-4) mol l-1 plasma) did not significantly reduce secretin-dependent ductular HCO3- secretion. In this group of animals amiloride reduced bile acid secretion by 13 (5-22)%. It is concluded that Na+/H+ ion exchanger is essential for UDCA-dependent canalicular HCO3- secretion, but not for secretin-dependent ductular HCO3- secretion.

Cholestasis in late metabolic acidosis of prematurely born infants

Serum concentrations of bile acids and tyrosine were determined in 14 premature infants with late metabolic acidosis and in 13 comparable controls without acidosis (protein intake 2 g/kg X d). At the same time the bile acids and the catalytic activity concentrations of lipase and trypsin were estimated in the duodenal juice. The daily faecal excretion and the percentage of fat eliminated were measured. In 8 patients with late metabolic acidosis the duodenal studies were repeated one week after late metabolic acidosis. Infants with late metabolic acidosis showed significantly higher concentrations of bile acids and tyrosine in the serum than the controls (p less than 0.0005). In the duodenal juice the activities of lipase and trypsin and the concentration of bile acids--especially of dihydroxy bile acids--were decreased (p less than 0.001). The faecal excretion during late metabolic acidosis was significantly increased, with high percentage of fat. Eight days after late metabolic acidosis all duodenal parameters equalled the range of the control group. The relations between acidosis, cholestasis, and amino acid transport to the liver are discussed.

DCCD (N,N'-dicyclohexylcarbodiimide) inhibits biliary secretion of HCO-3

To study whether a proton pump is an integral part of the mechanism responsible for secretin-dependent biliary secretion of HCO-3 ions, the proton pump inhibitor N,N'-dicyclohexylcarbodiimide (DCCD) was systemically administered to six anesthetized, secretin-infused pigs. Because biliary HCO-3 secretion varies with arterial pH, secretion rate was measured at several different arterial pH values, before and after DCCD (25 mumol/kg). At arterial pH 7.45, bile flow was 2.1 (1.6-2.9) ml/min, and HCO-3 secretion was 224 (157-311) mumol/min. DCCD reduced bile flow and HCO-3 secretion by 30% and 40%, respectively, independent of arterial pH. In contrast, bile acid secretion, 46 (41-59) mumol/min, was not changed by DCCD. The hepatic adenosine triphosphatase (ATP) level, 2.0 (1.8-2.1) mumol/g wet tissue, was not changed by DCCD. DCCD (10(-4) mol/l) affected neither Na,K-ATPase nor carbonic anhydrase activities in separate in vitro assay systems. The reduction in biliary HCO-3 secretion induced by the proton pump inhibitor DCCD may indicate that a proton pump is integrated into the mechanism responsible for secretin-dependent biliary secretion of HCO-3.

Role of osmosis in biliary NaCl secretion and bile formation

To challenge the osmotic hypothesis of biliary NaCl secretion and bile formation, experiments were performed in anaesthetized pigs. An increase in plasma osmolality of 7 +/- 1 mosm/kg H2O induced by intravenous sucrose infusion decreased NaCl secretion, NaHCO3 secretion, and bile flow by 36 +/- 3%, 34 +/- 2%, and 34 +/- 3%, respectively. There was no change in the biliary concentration of NaCl and NaHCO3. When bile acids were infused intravenously, the secretion of 1 mmol bile acids caused an osmotic flow of 12.0 ml bile containing 0.92 mmol NaCl and 0.30 mmol NaHCO3 in an isotonic solution. Bile acids are therefore much stronger choleretic substances than NaHCO3. When the plasma sodium concentration was increased to 200 mM, bile flow increased by 31 +/- 5% and the secretion of bile acids, NaHCO3, and NaCl was increased by 63 +/- 3%, 96 +/- 4%, and 93 +/- 4%, respectively. These data are consistent with osmotic transport as the main mode of bile formation, but diffusion could be responsible for a small fraction. A raised plasma sodium concentration stimulates osmotic formation of bile by increasing both the bile acid-dependent and -independent secretion through stimulation of biliary bile acid and NaHCO3 secretion.

Effect of systemic pH, PCO2 and bicarbonate concentration on biliary bicarbonate secretion in the rat

The effect of acute metabolic or respiratory acid-base disturbances on biliary bicarbonate secretion was examined in bile fistula rats. Animals were infused with ursodeoxycholate at a rate that stimulates bicarbonate secretion (1 mumole . min-1 X 100 gm-1), in control conditions and during acute acid-base disturbances. Metabolic acidosis or alkalosis were induced by HCl or NaHCO3 infusions, and respiratory acidosis or alkalosis were created respectively by adding CO2 to the inspired gas or by hyperventilation in artificially ventilated animals. Biliary bicarbonate concentration was always higher than plasma bicarbonate concentration. During metabolic disturbances, changing the plasma bicarbonate concentration from 9.2 to 30.2 mM stimulated biliary bicarbonate secretion by 113%. During respiratory disturbances, changing the plasma PCO2 from 25.5 to 59.8 mm Hg also increased biliary bicarbonate secretion by 89%. Biliary bicarbonate output was thus independent of plasma pH. When all animals were considered, bile flow was positively correlated with biliary bicarbonate concentration (r = 0.71, p less than 0.001). Acetazolamide significantly decreased ursodeoxycholate-induced bile flow and bicarbonate secretion by 20 and 22%, respectively. These results support the hypothesis that there is a relationship between ursodeoxycholate-induced bicarbonate secretion and bile flow. They are also consistent with the view that ursodeoxycholate-stimulated biliary bicarbonate secretion in the rat is strongly affected by plasma bicarbonate and PCO2, but not by plasma pH, and involves carbonic anhydrase.

Techniques for studying biliary secretion: electrolytes in bile

A major limitation in understanding bile formation has been technical. The liver and ductular epithelium are relatively inaccessible, necessitating indirect techniques of uncertain validity. This is well seen in attempts to define the role of electrolyte secretion in bile. It is widely agreed that bile salts stimulate a component of canalicular flow and that inorganic electrolyte secretion is stimulated by bile salts. The choleretic efficiency of a bile salt is directly related to the magnitude of the electrolyte effect. But there is no consensus regarding how and where bile salts stimulate electrolyte secretion. Some evidence points to a paracellular route by processes of solvent drag and diffusion. Other studies suggest stimulation of specific transcellular electrolyte pathways. It has been believed that canalicular bile salt-independent bile flow is generated by active blood-to-bile electrolyte transport. Actually, available methods do not permit us to conclude with absolute certainty that there is canalicular bile salt-independent flow, although there is considerable evidence for it. New studies suggest that electrolyte transport in this type of flow is passive and that flow is due to transport of organic anions. Ductular flow does seem to be due to active transport of electrolytes, particularly bicarbonate. Better and more direct techniques are required to settle the controversies in this area.

Role of sodium, bicarbonate, and plasma osmolality in biliary secretion

To examine the effect of changes in biliary sodium and bicarbonate secretion on bile formation, experiments were performed on fasted, pentobarbital-anesthetized pigs. During continuous intravenous secretin infusion (2.7 CU X kg body wt-1 X h-1) sodium secretion was altered by increasing or reducing plasma sodium concentration. Bicarbonate secretion was altered by varying arterial plasma pH. At increased biliary sodium secretion, bile formation was depressed, but changes in bicarbonate secretion were accompanied by parallel alterations in bile formation. Bile acid secretion was increased during elevated plasma sodium concentration, whereas reduced plasma sodium concentration depressed bile acid secretion. To distinguish between the effect of changes in plasma osmolality and sodium concentration, bile formation was also studied during intravenous sucrose infusion at normal plasma sodium concentration. About 50% of the effect on bile formation of changing plasma sodium concentration is solely caused by the changes in plasma osmolality. During secretin stimulation bile formation is mainly determined by bicarbonate. Changes in plasma osmolality affect bile secretion through alterations in the net osmotic force across the hepatocellular membrane. Sodium has an impact on the bile-acid-dependent fraction, whereas bicarbonate is the mediator of the bile-acid-independent fraction of bile secretion.