Effect of bile acids on calcium efflux from isolated rat hepatocytes and perfused rat livers

The changes in intracellular Ca2+ concentration [( Ca2+]i) of hepatocytes induced by certain bile acids are biphasic: an initial increase is followed by a more gradual decrease. This latter decline in [Ca2+]i may be due to an efflux of Ca2+ across the plasma membrane. This hypothesis was tested by studying the effect of different bile acids on the efflux of 45Ca from preloaded rat hepatocytes and isolated perfused rat livers. The following bile acids were studied: cholic (C), ursodeoxycholic (UDC), chenodeoxycholic (CDC), and deoxycholic (DC) acids; their taurine (T) conjugates (TC, TUDC, TCDC, and TDC); and the taurine, sulfate (S), and glucuronide (Glu) derivatives of lithocholic acid (TLC, LS, TLS, and LGlu, respectively). At 0.3 mM, all bile acids except C, TC, TCDC, UDC, and TUDC significantly increased 45Ca efflux from preloaded hepatocytes without affecting cell viability. Dose-response studies revealed that the minimum effective concentration needed to induce 45Ca efflux was 0.06 mM for LS, 0.8 mM for TCDC, and 10 mM for TC. Efflux of 86Rb from preloaded hepatocytes was not significantly altered by 0.1 mM LS, indicating relative specificity for calcium. TDC and DC, but not TC, increased 45Ca efflux from preloaded perfused rat livers. These results showed that bile acids known to increase [Ca2+]i (CDC, DC, TDC, and TLC) also increased 45Ca efflux from hepatocytes and perfused livers and that efflux was also stimulated by LS, TLS, and LGlu. The extent of this efflux was related to the hydrophobicity of the steroid nucleus of the bile acid. It is speculated that bile acid-induced increases in [Ca2+]i activate the plasma membrane Ca2+ pump resulting in increased Ca2+ efflux.

Basal and bile salt-stimulated bile flow and biliary lipid excretion in ponies

The role of bile salt in biliary lipid excretion was studied in 3 healthy ponies with chronic external biliary fistulas. After endogenous bile salt pool depletion, micelle-forming taurocholate or taurochenodeoxycholate was infused to replace excreted bile salt. Enterohepatic circulations were held open (total biliary diversion) throughout each study. Results indicated that biliary lipid excretion in ponies (113 +/- 21 nmol/min/kg of body weight) is approximately 10 times less than that reported in rodents. Although the lipid composition (4.4% cholesterol, 5.6% phospholipid, and 90% bile salt) was within the predicted range for a single phase of micellar (or vesicular) liquid in solution, it was supersaturated with cholesterol because of low absolute concentrations of bile salt and phospholipid. Ponies, like guinea pigs, were determined to have a high bile salt-independent secretion of biliary lipid with little (or no) coupling to endogenous bile salt output. However, bile salt excretion induced by higher taurocholate infusion rates (ie, those greater than the physiologic range of 61 to 125 nmol/min/kg) was positively correlated with an increase in biliary phospholipid excretion, but not cholesterol excretion, thus indicating that a threshold intracellular bile salt concentration may be associated with enhanced biliary phospholipid excretion in ponies. The apparent cholerectic effects of endogenous bile salts, taurocholate, and taurochenodeoxycholate (that is, the increment in bile flow per increment in bile salt recovered) were greater in ponies than reported for any other mammal.

Role of bicarbonate in biliary excretion of diisothiocyanostilbene disulfonate

Hepatic transport of 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) was studied in isolated perfused rat livers and in isolated rat hepatocytes to determine if DIDS-induced decrease in biliary HCO3- excretion is due to a DIDS-HCO3- exchange and/or due to inhibition of Cl(-)-HCO3- exchange. In isolated perfused rat livers, DIDS reversibly decreased biliary HCO3- concentration and excretion. The changes in biliary HCO3- concentration were inversely related to biliary DIDS concentration. DIDS was concentrated in bile, indicating active hepatic transport. Replacement of perfusate HCO3- with equimolar dimethyloxazolidinedione (DMO) or tricine decreased biliary excretion, but not hepatic uptake, of DIDS. Biliary excretion of DIDS was also associated with a decrease in bile pH, and this decrease in pH was greater in the presence of HCO3-. HCO3-, but not DMO or tricine, stimulated DIDS efflux from preloaded hepatocytes. DIDS efflux was also temperature dependent and increased with increasing extracellular pH. Collectively, these results are consistent with the presence of a DIDS-HCO3- (OH-) exchange mechanism at the canalicular membrane. HCO3(-)-dependent Cl- uptake in hepatocytes was competitively inhibited by DIDS (Ki = 0.24 mM), confirming the presence of DIDS-inhibitable Cl(-)-HCO3- exchange. However, the ability of DIDS to decrease biliary HCO3- excretion persisted when perfusate Cl- was replaced by isethionate. Moreover, biliary HCO3- concentration returned to base line despite the presence of 2-6 mM DIDS in bile. Thus it seems unlikely that the inhibition of Cl(-)-HCO3- exchange by DIDS is a major mechanism of inhibition of HCO3- excretion. We, therefore, conclude that a DIDS-HCO3- (OH-) exchange at the canalicular membrane is the most likely explanation for the observed decrease in biliary HCO3- excretion.

Hepatotoxic bile acids increase cytosolic Ca++ activity of isolated rat hepatocytes

Effects of bile acids on cystolic Ca++ activity and cell viability of isolated rat hepatocytes were studied to test the hypothesis that bile acids may produce hepatotoxicity by increasing cystolic Ca++ activity. Changes in cystolic Ca++ activity were calculated from time-dependent changes in fluorescence of quin-2 loaded hepatocytes. Release of lactate dehydrogenase and changes in propodium iodide fluorescence were used to assess cell viability. Bile acids studied were unconjugated and taurine-conjugated cholate, chenodeoxycholate (and taurochenodeoxycholate), deoxycholate (and taurodeoxycholate) and lithocholate (and taurolithocholate). With the exception of cholate and taurocholate, bile acids increased cystolic Ca++ activity within 10 to 30 sec in a concentration-dependent fashion (0.05 to 1.0 mM) and in the order lithocholate = taurolithocholate greater than chenodeoxycholate = taurochenodeoxycholate = deoxycholate = taurodeoxycholate. The initial increase in cystolic Ca++ activity by bile acids was not due to cell damage, since bile acid-induced decreases in cell viability were not significant until 2 to 3 min. At higher concentrations of unconjugated bile acid, there was a secondary increase in quin-2 fluorescence corresponding temporally to the increase in propodium iodide fluorescence, indicating cell damage after the initial increase in cystolic Ca++ activity. The ability of conjugated and unconjugated bile acids to increase cystolic Ca++ activity was abolished and decreased (60 to 90%), respectively, in the absence of extracellular Ca++, indicating that extracellular Ca++ is the major source of the bile acid-induced increase in cystolic Ca++ activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of H+ efflux pathways in rat hepatocytes

A pH-stat method was used to characterize H+ efflux pathways in hepatocytes in order to determine if Na+/H+ and Ca++/H+ exchange are involved in H+ efflux from hepatocytes under basal conditions and if cyclic AMP analogs affect Na+/H+ exchange. Total H+ efflux of freshly prepared hepatocytes ranged from 10 to 15 nmoles per min per mg protein. A part of total H+ efflux (35 to 50%) was dependent on extracellular Na+. This Na+-dependent H+ efflux was (i) inhibited by amiloride with a half-maximal effect at 0.3 mM, (ii) inhibited by ouabain, (iii) dependent on extracellular pH and (iv) characterized by a Km of 15 +/- 3 mM Na+ and a Vmax of 9 +/- 0.07 nmoles per min per mg protein. Amiloride, ouabain and replacement of Na+ by choline also decreased intracellular pH determined from equilibrium distribution of dimethyloxazolidinedione. Li+ could partially substitute for Na+ in Na+-dependent H+ efflux and in maintaining intracellular pH. Efflux of CO2 and lactic acid from hepatocytes represented 80% of Na+-independent H+ efflux. Efflux of H+ in the presence and absence of Na+ was not significantly altered by extracellular Ca++ (less than 10 microM and 1.0 mM). Thus, Ca++/H+ exchange is unlikely to contribute significantly to total H+ efflux from hepatocytes. Cyclic AMP analogs, dibutyryl cyclic AMP and 8-bromo cyclic AMP, inhibited amiloride-sensitive Na+-dependent H+ efflux, and dibutyryl cyclic AMP decreased intracellular pH.(ABSTRACT TRUNCATED AT 250 WORDS)

Bile acids increase cellular free calcium in cultured kidney cells (LLC-PK)

Suspensions of LLC-PK1 cells were used to determine the effect of bile acids on the cellular homeostasis of inorganic ions. It is determined that bile acids alter cellular free calcium (Cai) levels in LLC-PK1 cells. A series of bile acids were compared and found to produce increases in Cai in the order: lithocholate sulfate (LCS) greater than deoxycholate greater than chenodeoxycholate greater than lithocholate glucuronide greater than cholate. LCS (300 microM) produces changes in Cai (measured using Fura-2) qualitatively similar to those produced by 1 microM ionomycin, except that only ionomycin is able to release calcium from intracellular stores. The effect on Cai is roughly proportional to LCS concentration between 50 and 300 microM. The presence of 40 mM Na in the extracellular medium reduces the LCS-induced rise in Cai to 20% of that observed in the absence of Na. This effect is specific for Na versus 150 mM extracellular K, Li, or TMA. The effect is not dependent on the Na gradient across the membrane. At concentrations of LCS which induce changes in Cai, no significant effect of LCS is observed on either cellular Na or K levels, or intracellular pH.

Pharmacokinetics of dantrolene sodium in horses

The pharmacokinetics of dantrolene sodium were investigated in horses following both intravenous (2 mg/kg) and intragastric (4 mg/kg) administration. Two ponies also received dantrolene sodium intravenously (2 mg/kg) in a pilot study to obtain preliminary kinetic data and to determine urinary and biliary excretion of the intact drug. Distribution and elimination of dantrolene was rapid, resulting in an elimination half-life of 129 +/- 8 (SEM) min and a whole body clearance of 4.16 +/- 0.52 ml/min/kg. Following intragastric administration, dantrolene rapidly acheived peak concentrations within 1.5 h, but was incompletely absorbed, with a bioavailability of 39 +/- 10%. Small amounts of intact drug were recovered in urine and bile. Based upon disposition kinetics of dantrolene in these studies, intravenous and intragastric dosage regimens were determined which would maintain blood dantrolene concentrations within the predicted clinically effective range.

Effects of ion substitution on transport and choleretic effect of ouabain

The role of inorganic ions in hepatic transport and choleretic effect of ouabain was studied in isolated perfused rat liver to verify whether Na+-coupled ouabain uptake into hepatocytes is responsible for the choleretic effect. Hepatic uptake and clearance of ouabain were not significantly affected when perfusate Na+ was replaced by Li+ or choline+, chloride by nitrate or isethionate, or bicarbonate by tricine. However, these ion substitutions, with the exception of Li+, significantly reduced ouabain-induced choleresis and biliary electrolyte excretion. When ouabain was infused at different rates followed by perfusion without ouabain, changes in bile flow paralleled biliary excretion of ouabain rather than hepatic uptake. These results indicate that hepatic uptake of ouabain is not Na+ dependent and that the osmotic effect of biliary excreted ouabain is responsible for its choleretic effect. A part of the choleretic effect (30%) must also involve other mechanisms, since a permeable anion-like nitrate failed to substitute for perfusate chloride. Results of infusion studies also showed that ouabain was concentrated in liver (liver/perfusate = 30) and in bile (bile/liver = 15), indicating that ouabain is transported against its concentration gradient across both sinusoidal and canalicular membranes.

Effects of plasma sample storage on blood ammonia, bilirubin, and urea nitrogen concentrations: cats and horses

Ten horses, a pony, and 13 cats were used to evaluate base-line blood ammonia, bilirubin, and urea nitrogen concentrations and to determine The effects of prolonged cold storage (-20 degrees C) before assay. Base-line plasma ammonia concentrations in cats (0.992 +/- 0.083 [SE] micrograms/ml) did not change significantly after 48 hours of storage (0.871 +/- 0.073 micrograms/ml); however, they were increased 4.2- and 13-fold after 168 and 216 hours of storage, respectively. In contrast to base-line plasma-ammonia values in cats, those of horses were significantly (0.265 +/- 0.044 micrograms/ml) lower, and significantly increased from base-line values after 48 hours of storage (0.861 +/- 0.094 micrograms/ml) and continued to increase 25.6-fold at 168 hours and 18.4-fold at 216 hours. Plasma urea nitrogen concentrations in cats (25.8 +/- 1.06 mg/dl) and horses (11.2 +/- 0.749 mg/dl) did not change significantly during 168 hours of storage. Total plasma bilirubin values from both cats (0.19 +/- 0.049 mg/dl) and horses (0.75 +/- 0.064 mg/dl) also did not change significantly during storage. These results indicate that feline plasma samples for ammonia determinations may be stored at -20 degrees C for up to 48 hours, whereas equine plasma ammonia values tend to increase during that time. The reason for the increase remains unexplained. Both feline and equine plasma urea nitrogen and total bilirubin are stable for at least 168 hours of storage at -20 degrees C.

Role of extracellular Ca2+ in hepatic bile formation and taurocholate transport

The role of extracellular Ca2+ in hepatic bile formation, biliary membrane permeability, and taurocholate (TC) transport was studied in isolated perfused rat livers and in isolated rat hepatocytes to determine the functional importance of paracellular permeability in biliary bile acid excretion. Each liver was perfused for 1 h with perfusate containing 1.3 mM Ca2+ (control period) followed by another hour with 1.3, 0.5, 0.1, 0.05, 0.03, or 0.01 mM Ca2+ (experimental period). Basal bile flow and biliary excretion of added TC declined significantly only at and below 0.05 mM perfusate Ca2+ and was associated with an increase in bile-to-perfusate concentration ratio of [3H]inulin (B/P inulin ratio). A twofold increase in the diffusional permeability coefficient at 0.05 mM and a sixfold increase at 0.03 and 0.01 mM perfusate Ca2+ could explain the increased in B/P inulin ratios. Time-dependent increases in cell-to-medium concentration ratios of inulin were less in the absence than in the presence of Ca2+. Hepatic uptake rates of TC determined in isolated hepatocytes and from perfusate disappearance of added TC and efflux rates of TC from preloaded hepatocytes were not significantly affected by Ca2+ removal. It is possible that the observed decline in biliary TC excretion at low perfusate Ca2+ is due to regurgitation of secreted TC back into the perfusate followed by reuptake. This was supported by an accumulation of perfusate radioactivity when TC uptake inhibitors (furosemide and bumetanide) were added to the perfusate (0.03 mM Ca2+) 60 min after the addition of [14C]TC.(ABSTRACT TRUNCATED AT 250 WORDS)

Furosemide choleresis in isolated perfused rat liver: partial dependency on perfusate sodium and chloride

The effect of furosemide on hepatic bile formation was studied in isolated perfused rat liver to determine if 1) the observed cholestatic effect at lower dose of furosemide in vivo is a primary effect or a secondary effect due to decreased hepatic blood flow caused by the furosemide-induced volume contraction and if 2) the observed choleretic effect at higher doses can be explained by the osmotic effect of furosemide and its metabolites in bile. A single dose of furosemide (initial perfusate concentration 0.01, 0.1 or 1 mM) produced choleresis, whereas 0.001 mM furosemide did not affect bile flow significantly. Because furosemide failed to produce cholestasis at tested doses, the observed cholestasis in vivo at similar blood concentrations must be a secondary effect. Furosemide choleresis was associated with biliary secretion of furosemide and its metabolites. However, the choleretic effect expressed as microliters per micromole of drug secreted declined with increasing dose and biliary secretion. Furosemide choleresis was also associated with an increase in the net biliary secretion of Na+ and Cl-. The effect of Na+ and Cl- replacement on furosemide choleresis was studied to determine if the choleresis was a result of direct effect of furosemide on hepatic electrolyte transport. Replacement of perfusate Na+ completely by Li+ or partially by choline+ resulted in a 30 to 50% reduction in choleretic effect and furosemide-induced biliary Cl- secretion. A similar decline in choleretic effect and net furosemide-induced biliary Na+ secretion was also observed when perfusate Cl- was replaced by nitrate, acetate or isethionate.(ABSTRACT TRUNCATED AT 250 WORDS)

Hepatobiliary transport of indocyanine green and sulfobromophthalein in fed and fasted horses

Fasting is associated with unconjugated hyperbilirubinemia in several species, including the horse. Studies in ponies showed that a 3-day fast decreased plasma clearance of bilirubin, cholic acid, and sulfobromophthalein (BSP). Since these organic anions are conjugated with different substrates, it is possible that observed differences in plasma clearance result from a general decrease in hepatic conjugating capacity during the animals' fasting. To test this hypothesis, the effects of a 3-day fast on plasma clearance of IV injected BSP (4.4 to 5.1 mg/kg), which is conjugated to glutathione, and indocyanine green (ICG; 0.8 to 1.1 mg/kg), which is not conjugated, were studied in 10 healthy horses and 2 ponies with diverted enterohepatic circulations (indwelling T tubes). Blood samples were obtained for 30 minutes after injection, and bile samples from ponies were obtained for 3 hours. Fasting increased plasma bilirubin concentration in all animals studied (from 1.03 +/- 0.337 mg/dl in control animals to 3.49 +/- 1.01 mg/dl in fasted animals). Kinetic values of ICG disappearance were determined from single exponential functions, and those for BSP were determined from both single and curvilinear (2-exponential) functions. Plasma clearance of BSP in fed horses (8.65 +/- 1.02 ml X min-1 X kg-1) was greater than clearance of ICG (3.54 +/- 0.67 ml X min-1 X kg-1), results similar to those reported in dogs, cats, rats, and persons. Fasting significantly decreased fractional plasma disappearance rate of both BSP (-36%) and ICG (-58%) and similarly reduced plasma clearance (BSP,-48%; ICG,-55%).(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of side-chain charge on hepatic transport of bile acids and bile acid analogues

The importance of side-chain charge on hepatic uptake and biliary secretion of bile acids and analogues was studied using the isolated, perfused rat liver and the anesthetized rat with a bile fistula. Derivatives of cholic acid with negative, neutral, zwitterionic, or positive charges on the side chain were synthesized and studied. Hepatic uptake by the isolated perfused liver, determined by measuring the rate of disappearance of a single 20-mumol bolus added to the perfusate, was strongly influenced by side-chain charge. A fully positively charged bile acid derivative (cholylcholamine) and two fully zwitterionic bile acid derivatives (CHAPS and cholyllysine) showed no appreciable uptake (less than 1% of the uptake rate of cholyltaurine). Bile acid derivatives existing mostly in cationic form (cholylamine) at pH 7.4, in neutral form (cholylglycylhistamine), or in divalent anion form (cholylaspartate and cholylcysteate) had an uptake rate that was greater but only 7-19% that of cholyltaurine. Side-chain charge also appeared to influence the rate of secretion into bile. Bile acids existing in mono- or dianionic form were well secreted (greater than 95% of dose in 2 h) into the bile, but all other derivatives had much lower secretion rates (less than 20% of dose in 2 h). When the biliary secretion of each bile acid derivative was expressed in relation to the amount that had entered the liver, relative secretion rates (presumably from liver cell) into bile decreased in the following order: cholyltaurine greater than cholylaspartate and cholylcysteate greater than CHAPS greater than cholyllysine greater than cholylglycylhistamine approximately equal to cholylamine. In bile fistula rats, cholylaspartate was quantitatively secreted into bile when infused at rates below its secretory maximum, whereas only very low biliary secretion rates of CHAPS were observed even during relatively high infusion rates; cholylamine was cholestatic. The above data show that, although uncharged and anionic derivatives of cholic acid may be taken up by the liver at a moderate rate, only anionic derivatives (both monovalent and divalent) are well secreted from within the liver cell into bile. A single negative charge on the side chain appears to be required for optimal transport of a bile acid from sinusoidal blood to bile.

Essential role of sodium and chloride for theophylline-induced choleresis in the isolated perfused rat liver

Active secretion of electrolytes by hepatocytes is believed to be responsible for bile acid-independent canalicular bile flow (BAICF). Theophylline, which enhances BAICF, has been shown to enhance electrogenic Cl- secretion in a number of other epithelia. Such transport is dependent on Na+ and Cl-. Thus, the mechanism of theophylline choleresis may also involve stimulation of electrogenic Cl- secretion of the liver. This hypothesis was tested by studying the effect of ion substitution on theophylline choleresis in isolated perfused rat livers. Addition of theophylline (0.1 mmol) and dibutyryl cAMP (0.05 mmol) to 100 ml perfusate, in a single dose, increased bile flow and biliary secretion of Na+ and Cl- reversibly. These effects of theophylline were virtually abolished when perfusate Na+ (146 mM) was replaced by Li+ (146 mM) or choline+ (120 mM), and when Cl- (127 mM) was replaced by 120 mM NO-3, acetate- or isethionate-. Since even the permeable ions like Li+ and NO-3 could not substitute for Na+ and Cl-, these results show that the effect of theophylline on BAICF is specifically dependent on the presence of Na+ and Cl- in the perfusate. We propose, by analogy to other epithelia, that an electrogenic Cl- secretion mechanism is present in the liver. Theophylline, acting via cAMP, stimulates this transport process, thereby enhancing BAICF.

Sodium and chloride dependency of dibucaine- and procaine-induced choleresis in isolated perfused rat livers

The effect of local anesthetics, dibucaine and procaine, on hepatic bile formation was studied in the isolated perfused rat liver. Perfusate Na+ and Cl- were replaced by other ions to define the possible mechanism of action. A single dose (50 mumol) of dibucaine produced an initial cholestasis followed by choleresis. Whereas dibucaine produced only choleresis at a lower dose (10 mumol), only the cholestatic effect was seen at a higher dose (100 mumol). Procaine, on the other hand, produced only choleresis at all doses (1, 10 and 100 mumol); this choleresis was associated with biliary secretion of procaine and its metabolites. Neither dibucaine nor procaine affected the low endogenous bile acid secretion in these studies. The diffusion permeability coefficient of [carboxy-14C]inulin was not altered significantly by dibucaine and procaine, suggesting no significant alteration of biliary permeability. Biliary secretion of Na+ or Cl- declined during cholestasis and increased during choleresis. The initial cholestatic effect of dibucaine was still present when perfusates Na+ and Cl- were replaced by permeable Li+ or NO3-, but declined when Cl- was replaced by relatively impermeable isethionate, suggesting a nonspecific effect. The choleretic effect of both dibucaine and procaine, however, declined significantly when Na+ or Cl- was replaced by Li+, NO3- or isethionate-. These ion-substitutions did not affect significantly the biliary secretion of procaine and its metabolites. The ability to induce biliary secretion of Na+ and Cl- also decreased when Cl- was replaced by NO3- or isethionate and when Na+ was replaced by Li+, respectively. These results suggest that a part of the choleretic effect of both dibucaine and procaine is specifically dependent on Na+ and Cl-. This fraction is thus unlikely to be due to the osmotic effect of the secreted drug. Further studies showed that dibucaine inhibited Na+-dependent hepatic uptake of taurocholate, suggesting possible interference with other Na+-dependent transport processes. It is proposed that although a part of the choleresis is due to the osmotic effect of the secreted drug, the specific dependency of a portion of the choleretic effect on Na+ and Cl- is due to inhibition of Na+-coupled Cl- reabsorption from the canaliculi.

Therapeutic doses of erythromycin esteolate is not cholestatic in rats in vivo

The effect of erythromycin esteolate (EE) on bile flow and bile acid secretion was studied in male Wistar rats in vivo. Daily oral treatment with a dose of up to 100 mg/kg for 1 week increased the bile flow and the bile acid secretion. Increasing the days of treatment to 4 weeks with a dose of 20 mg/kg did not alter the measured parameters significantly. Acute intravenous injection of erythromycin lactobionate (50 mg/kg) also increased bile flow and biliary bile acid secretion temporarily. The increase in bile flow may partly be due to the osmotic effect of the drug and its metabolites in bile. Since EE failed to produce cholestasis in the range of therapeutic doses, rats do not seem to be a suitable experimental model for studying EE-cholestasis.

Role of inorganic electrolytes in bile acid-independent canalicular bile formation

Ion-replacement studies were carried out in the isolated perfused rat liver to obtain insight into the role played by inorganic electrolytes in bile acid-independent canalicular bile flow (BAICF). The BAICF decreased significantly when Na+ (146 mM) was replaced by 120 mM K+, Rb+, Cs+, or choline and when Cl- (127 mM) was replaced by 120 mM acetate or isethionate; there was no reduction in BAICF when Na+ was replaced by Li+ (146 mM) and Cl- by NO-3. K+, Rb+, and Cs+, however, also caused a simultaneous decline in the perfusion rate. The BAICF decreased by 50% when HCO-3 was replaced by equimolar tricine; under this condition replacement of Cl- by NO-3, but not Na+ by Li+, decreased BAICF by 45%. Thus the hepatic transport of Cl- cannot be explained by simple diffusion only, and a special mechanism, probably Na+-coupled Cl- transport, may contribute about 30% of the BAICF. With Li+ replacing Na+ in the medium, the intracellular concentration of Li+ in isolated rat hepatocytes was less than that calculated for electrochemical equilibrium and was increased by 2 mM KCN, indicating active extrusion of this ion. Li+ was unable to activate Mg2+-ATPase of isolated rat liver plasma membranes, and 1 mM ouabain did not affect the Li+ distribution. These results suggest the potential importance of ion pumps other than Na+-K+-ATPase in BAICF.

Importance of solvent drag and diffusion in bile acid-dependent bile formation: ion substitution studies in isolated perfused rat liver

Ion substitution studies were carried out in the isolated perfused rat liver to define the importance of solvent drag and diffusion in bile acid-dependent bile formation. Two different methods, namely single injection (20 mu moles) and continuous infusions at 0.4, 0.8, 1.2, and 1.6 mu moles per min taurocholate (TC), were used to determine the bile acid-dependent bile flow (BADF). Both methods gave essentially the same results. Replacement of Na+ (146mM) by 120 or 146 mM Li+ and Cl-(127mM) by 120 mM NO3- increased BADF significantly. On the other hand, replacement of Na+ by 120 mM choline and Cl- y 120 mM isethionate decreased the BADF. The osmolarity of TC solution was not different when Na+ was replaced by 120 mM Li+ or choline and TC did not affect the osmotic activity of NaCl, and choline-Cl differently. Thus, the observed effect of Na+ replacement on BADF is not due to any change in the osmotic activity of the secreted TC. Substitution of HCO3- by equimolar tricine also decreased BADF. Under this condition, BADF increased when NaCl was replaced by equimolar NaNO3. Thus, HCO3- does not seem to be essential for TC choleresis. Since Li+ and NO3- are more permeable, and choline and isethionate are less permeable than Na+ and Cl-, respectively, these results suggest that the BADF is dependent on the permeability of the substituting cations and anions and thus support the hypothesis that solvent drag and diffusion play an important role in BADF.

Stereospecific reduction of 3- and 7-oxo groups of oxocholanic acids in isolated perfused rat liver

The metabolism of 3- and 7-oxo groups of oxocholanic acids was studied in isolated perfused rat liver. The metabolites in bile were determined enzymatically using 3 alpha- and 7 alpha-hydroxysteroid dehydrogenases. The 3-oxo group of all the oxocholanic acids tested (dehydrocholate, glycodehydrocholate, taurodehydrocholate, 3,7-dioxocholanate, 3,12-dioxocholanate and tauro-7,12-dihydroxy-3-oxocholanate) was reduced stereospecifically to 3 alpha-hydroxy metabolites. On the other hand the 7-oxo group was excreted partially unchanged (30% of the dose) and partially as 7 alpha-hydroxy metabolites (6-10% of the dose). The remainder of the 7-oxo group was concluded to have been reduced to 7 beta-hydroxy metabolites. These results indicate that the 7-oxo group of oxocholanic acids is reduced predominantly to 7 beta-hydroxy metabolites in rats rather than to 7 alpha-hydroxy metabolites as found in man.

Increased biliary GSSG-secretion and loss of hepatic glutathione in isolated perfused rat liver after paraquat treatment

Perfusion of isolated rat livers with 1 mM paraquat for 3 hours led to a stimulated release of oxidized glutathione into the effluent caval perfusate and into the bile. Whereas the biliary stimulation was 245%, stimulation in to the perfusate was only 19.2%. In addition, the glutathione content in the paraquat-treated livers decreased from 2.74 +/- 0.23 to 0.80 +/- 0.07 mumol/g liver. The hepatic content of GSSG, however, was not changed by paraquat. This resulted in an elevated ratio of GSSG/GSH+2GSSG from 0.036 to 0.113. The total amount of GSSG released via bile and perfusate is less than the total loss of hepatic GSH. These findings are discussed in view of mechanisms by which paraquat decreases in GSH in the liver.

Altered hepatobiliary transport of taurocholic acid in aged rats

Hepatobiliary transport of taurocholic acid was studied in adult (3 months) and old (2 years) rats using an isolated perfused rat liver technique in order to determine the effect of age on hepatic uptake and secretion of bile acids simultaneously. The results were analyzed using a steady-state compartmental model to estimate the uptake and secretion of taurocholic acid. Hepatic secretion was decreased to a greater extent than the uptake in old rats. These changes in transport activities were associated with increases in perfusate and liver bile acid pool sizes. These results can explain the decrease in total pool size and synthesis rate of bile acids observed previously in old rats using in vivo studies. It has been suggested that the age-dependent decrease in bile acid transport capacity of the liver is secondary to the altered lipid composition of the liver plasma membranes of old rats.

Effect of dehydrocholic, chenodeoxycholic, and taurocholic acids on the excretion of bilirubin

The effects of IV bile acid infusion (at approx 20% of normal excretion rate) on the biliary excretion of 3-alpha-hydroxy bile acids and bilirubin were investigated in ponies prepared surgically with chronic external biliary fistulas. Endogenous bile acid excretion (approx 45 mumol/min) decreased to the hepatic synthesis rate (approx 1.5 mumol/min) during the initial 4 to 5 hours of bile drainage. In type 1 studies, both chenodeoxycholic and taurocholic acid infusion (8 to 9 mumol/min) increased bilirubin excretion by 58% to 82% following 5 hours of biliary diversion. During type 2 studies, 3-hour IV infusions (10.5 mumol/mon) of dehydrocholic acid, 4 hours following biliary diversion, increased bile flow by 45% to 62% and excretion of 3-alpha-hydroxy bile acid by 34% to 36% above preinfusion (hepatic synthesis) levels. Bilirubin excretion was not significantly changed during those increases in bile flow and bile acid excretion. Immediately after dehydrocholic acid infusion, taurocholic acid infusion (8.1 mumol/min) greatly increased bilirubin excretion for 1 hour (a reversal of hepatic storage identical to that found during type 1 studies), prolonged excretion (mg/2 hours) being two to three times that caused by dehydrocholic acid infusion. Bilirubin excretion appeared to correlate with the micelle-forming capacity of endogenous bile acids as opposed to the nonmicelle-forming characteristic of synthestic dehydrocholic acid.