Hepatitis B in patients with HIV infection: relationship to AIDS and patient survival.
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AIDS-Related Opportunistic Infections↗
Biomedical subjects
B F Scharschmidt
Publications and source records attributed to B F Scharschmidt.
Transmembrane electrical potential difference regulates Na+/HCO3- cotransport and intracellular pH in hepatocytes.
We have examined the hypothesis that a regulatory interplay between pH-regulated plasma membrane K+ conductance (gK+) and electrogenic Na+/HCO3- cotransport contributes importantly to regulation of intracellular pH (pHi) in hepatocytes. In individual cells, membrane depolarization produced by transient exposure to 50 mM K+ caused a reversible increase in pHi in the presence, but not absence, of HCO3-, consistent with voltage-dependent HCO3- influx. In the absence of HCO3-, intracellular alkalinization and acidification produced by NH4Cl exposure and withdrawal produced membrane hyperpolarization and depolarization, respectively, as expected for pHi-induced changes in gK+. By contrast, in the presence of HCO3-, NH4Cl exposure and withdrawal produced a decrease in apparent buffering capacity and changes in membrane potential difference consistent with compensatory regulation of electrogenic Na+/HCO3- cotransport. Moreover, the rate of pHi and potential difference recovery was several-fold greater in the presence as compared with the absence of HCO3-. Finally, continuous exposure to 10% CO2 in the presence of HCO3- produced intracellular acidification, and the rate of pHi recovery from intracellular acidosis was inhibited by Ba2+, which blocks pHi-induced changes in gK+, and by 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid, which inhibits Na+/HCO3- cotransport. These findings suggest that in hepatocytes, changes in transmembrane electrical potential difference, mediated by pH-sensitive gK+, play a central role in regulation of pHi through effects on electrogenic Na+/HCO3- cotransport.
Acidosis↗
Intracranial pressure monitoring and liver transplantation for fulminant hepatic failure.
Cerebral edema and intracranial hypertension, commonly present in fulminant hepatic failure, may lead to brainstem herniation and limit the survival of comatose patients awaiting liver transplantation before a donor organ becomes available. Also, they are likely responsible for postoperative neurological morbidity and mortality. Although intracranial pressure monitoring has been proposed to aid clinical decision making in this setting, its use in the prevention of brainstem herniation preoperatively, in the selection of patients for liver transplantation who have the potential for neurological recovery and in the maintenance of cerebral perfusion during liver transplantation has not been examined in detail. To address these issues, we established a protocol for intracranial pressure monitoring in comatose patients with fulminant hepatic failure as part of their preoperative and intraoperative management. Twenty adults and three children underwent intracranial pressure monitoring. Ten patients required preoperative medical therapy with mannitol, barbiturates or both for a rise in intracranial pressure above 25 mm Hg. Four patients had a sustained lowering of intracranial pressure, three of whom survived hospitalization. Six patients had intracranial hypertension refractory to medical management, were removed from a waiting list for a donor organ and died with brainstem herniation. Of the remaining 17 patients, 3 died of other causes while awaiting a donor organ, 2 recovered spontaneously without neurological sequelae and 12 underwent liver transplantation. All but one patient undergoing liver transplantation had transient intraoperative intracranial hypertension develop, requiring medical treatment. The 12 patients who had transplants recovered neurologically and were discharged from the hospital.(ABSTRACT TRUNCATED AT 250 WORDS)
Hepatic Encephalopathy↗
HCO3(-)-coupled Na+ influx is a major determinant of Na+ turnover and Na+/K+ pump activity in rat hepatocytes.
Recent studies in hepatocytes indicate that Na(+)-coupled HCO3- transport contributes importantly to regulation of intracellular pH and membrane HCO3- transport. However, the direction of net coupled Na+ and HCO3- movement and the effect of HCO3- on Na+ turnover and Na+/K+ pump activity are not known. In these studies, the effect of HCO3- on Na+ influx and turnover were measured in primary rat hepatocyte cultures with 22Na+, and [Na+]i was measured in single hepatocytes using the Na(+)-sensitive fluorochrome SBFI. Na+/K+ pump activity was measured in intact perfused rat liver and hepatocyte monolayers as Na(+)-dependent or ouabain-suppressible 86Rb uptake, and was measured in single hepatocytes as the effect of transient pump inhibition by removal of extracellular K+ on membrane potential difference (PD) and [Na+]i. In hepatocyte monolayers, HCO3- increased 22Na+ entry and turnover rates by 50-65%, without measurably altering 22Na+ pool size or cell volume, and HCO3- also increased Na+/K+ pump activity by 70%. In single cells, exposure to HCO3- produced an abrupt and sustained rise in [Na+]i from approximately 8 to 12 mM. Na+/K+ pump activity assessed in single cells by PD excursions during transient K+ removal increased congruent to 2.5-fold in the presence of HCO3-, and the rise in [Na+]i produced by inhibition of the Na+/K+ pump was similarly increased congruent to 2.5-fold in the presence of HCO3-. In intact perfused rat liver, HCO3- increased both Na+/K+ pump activity and O2 consumption. These findings indicate that, in hepatocytes, net coupled Na+ and HCO3- movement is inward and represents a major determinant of Na+ influx and Na+/K+ pump activity. About half of hepatic Na+/K+ pump activity appears dedicated to recycling Na+ entering in conjunction with HCO3- to maintain [Na+]i within the physiologic range.
Animals↗
Plasma membrane H(+)-HCO3- transport in rat hepatocytes: a principal role for Na(+)-coupled HCO3- transport.
Na(+)-coupled HCO3- transport has been demonstrated in the basolateral membrane of hepatocytes, but there is uncertainty regarding its stoichiometry or capacity compared with other mechanisms of H(+)-HCO3- transport. After preincubation in medium free of Na+, either in the presence or absence of HCO3(-)-CO2, rat hepatocytes in primary culture were reexposed to Na+ or HCO3(-)-CO2 alone or in combination. Transporter electrogenicity was assessed by measuring membrane potential difference (PD), and the relative capacities of Na(+)-coupled HCO3- transport, Cl(-)-HCO3- exchange, and Na(+)-H+ exchange were assessed by measuring the magnitude and rate of change of intracellular pH (pHi) using BCECF. In the absence of Na+, exposure to HCO3- alone had no consistent effect on PD or pHi. In the absence of HCO3-, reexposure to Na+ depolarized cells by 3 +/- 1 mV and caused an amiloride-inhibitable increase in pHi of 0.031 +/- 0.02 units/min. In the presence of HCO3-, reexposure to Na+ hyperpolarized cells by -14 +/- 5 mV and increased pHi at a rate of 0.133 +/- 0.11 units/min; both the hyperpolarization and alkalinization were inhibited by SITS but unaffected by amiloride. These changes in PD indicate that Na(+)-coupled HCO3- transport is electrogenic, consistent with coupling of more than one HCO3- to each Na+. Furthermore, SITS-inhibitable Na(+)-dependent alkalinization exceeds amiloride-inhibitable Na(+)-dependent alkalinization by an order of magnitude, suggesting that the transport capacity of Na(+)-coupled HCO3- transport exceeds that of Na(+)-H+ exchange.(ABSTRACT TRUNCATED AT 250 WORDS)
Animals↗
Na(+)-Ca2+ exchange in cultured rat hepatocytes: evidence against a role in cytosolic Ca2+ regulation or signaling.
Plasma membrane Na(+)-Ca2+ exchange contributes importantly to the regulation of cytosolic Ca2+ concentration ([Ca2+]i) in excitable cells. Despite extensive study in excitable tissues, the role of this transporter in the regulation of [Ca2+]i in hepatocytes is unknown, and conflicting information has been reported regarding the presence of Na(+)-Ca2+ exchange in hepatocyte plasma membrane vesicles. We have therefore assessed the role of Na(+)-dependent Ca2+ transport in the regulation of [Ca2+]i in rat hepatocytes in primary culture under basal conditions and after exposure to vasopressin, a hormone that elevates [Ca2+]i. Ca2+ efflux, measured using 45Ca, did not differ in the presence or absence of extracellular Na+, either under basal conditions or in response to vasopressin. [Ca2+]i, measured using the Ca2(+)-sensitive dye fura-2, was not altered by transient or prolonged exposure to Na(+)-free media or by exposure to ouabain in concentrations sufficient to produce a five-fold elevation in intracellular Na+ concentration. The [Ca2+]i response to vasopressin was also unaffected by Na+ removal or ouabain. By contrast, in cultured rat cardiac myocytes, cells that possess Na(+)-Ca2+ exchange, transient or prolonged Na+ removal as well as ouabain exposure produced greater than fivefold increases in [Ca2+]i compared with controls. We conclude that Na(+)-Ca2+ exchange does not contribute to the regulation of [Ca2+]i in hepatocytes.
Animals↗
Impaired sulfoxidation in patients with primary biliary cirrhosis.
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Humans↗
Hepatocellular bile acid transport and ursodeoxycholic acid hypercholeresis.
This review focuses on mechanisms of bile acid transport across the basolateral and canalicular hepatocyte plasma membranes and on ursodeoxycholic acid (UDCA) hypercholeresis and biotransformation. Conjugated trihydroxy bile acids enter hepatocytes via a sodium-coupled mechanism localized to the basolateral membrane, which is saturable, concentrative, inhibited by other bile acids as well as by furosemide and bumetanide, and exhibits developmental changes in rats and probably also in humans. The stoichiometry of sodium-coupled bile acid uptake has been controversial. Hydrophobic, unconjugated dihydroxy and monohydroxy bile acids, including UDCA, enter hepatocytes more rapidly than does taurocholate, and their uptake is largely nonsaturable and sodium independent. A hydroxyl-exchange mechanism that mediates the uptake of cholic acid has also been reported, but its existence is controversial. Current evidence suggests that a 49-kDa protein mediates Na+-dependent taurocholate uptake and that a 54-kDa protein is involved in Na+-independent bile acid uptake. Studies with canalicular membrane vesicles have demonstrated saturable, sodium-independent taurocholate transport, which is sensitive to electrical potential, exhibits trans-stimulation, and appears to be mediated by a 100-kDa canalicular membrane glycoprotein. Studies in mutant rats with conjugated hyperbilirubinemia suggest the presence of a separate canalicular transport mechanism utilized by sulfated bile acids and organic anions such as bilirubin and sulfobromophthalein. UDCA produces in some species a dramatic hypercholeresis that is greater than expected based on the osmotic effect of the secreted bile acid. The hypercholeresis appears attributable to stimulation of biliary bicarbonate output and is decreased or abolished in the perfused rat liver by amiloride or perfusate Na+ substitution. These same maneuvers dramatically alter UDCA biotransformation (unconjugated UDCA disappears from bile, and UDCA glucuronide becomes a major metabolite) and lower hepatocyte intracellular pH. These and other findings indicate that UDCA hypercholeresis is tightly linked to biliary excretion of the unconjugated species and suggest that UDCA biotransformation may be influenced by intracellular pH.
Animals↗
Na+-H+ exchange activity in rat hepatocytes: role in regulation of intracellular pH.
Amiloride-sensitive Na+-H+ exchange has been identified in basolateral membrane vesicles from rat liver, but little is currently known about its regulation or its role in maintenance of resting intracellular pH (pHi) in intact hepatocytes. We have assessed Na+-H+ exchange activity in isolated or cultured rat hepatocytes in nominally HCO3- free solution under basal conditions and after intracellular acidification by an NH4Cl pulse by measuring 1) pHi, using the pH-sensitive dye 2',7'-bis(carboxyethyl)-5(6)-carboxy fluorescein, 2) net H+ efflux by pH-stat titration, and 3) amiloride-inhibitable 22Na uptake. Under resting conditions, Na+-H+ exchange did not contribute measurably to Na+ uptake and accounted for less than 20% of net H+ efflux. Hepatocyte pHi averaged 7.07 +/- 0.03, significantly above H+ electrochemical equilibrium (6.92 +/- 0.08) determined using an electrogenic proton ionophore. Transient removal of extracellular Na+ or exposure to amiloride reversibly lowered pHi by 0.09 +/- 0.01 and 0.12 +/- 0.03 pH units, respectively, within 5-10 min. After intracellular acidification by an NH4Cl pulse, Na+ uptake rate increased about twofold, the increase being entirely amiloride inhibitable. Net H+ efflux increased about threefold, and 70% of the increase was amiloride inhibitable. Recovery of pHi after an NH4Cl pulse was reversibly blocked by exposure to amiloride or removal of Na+. Na+-H+ exchange activity (calculated from the rate of change in pHi and intracellular buffering capacity) was inversely related to pHi and was estimated to approach zero at pHi 7.25-7.50.(ABSTRACT TRUNCATED AT 250 WORDS)
Amiloride↗
Electrophysiological evidence for Na+-coupled bicarbonate transport in cultured rat hepatocytes.
Recent observations suggest that hepatocytes exhibit basolateral electrogenic Na+-coupled HCO3- transport. In these studies, we have further investigated this transport mechanism in primary culture of rat hepatocytes using intracellular microelectrodes to measure membrane potential difference (PD) and the pH-sensitive fluorochrome 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein to measure intracellular pH (pHi). In balanced media containing 25 mM HCO3-, PD averaged -32.1 +/- 0.6 (SE) mV and pHi averaged 7.22 +/- 0.03. PD became more negative (hyperpolarized) when extracellular [HCO3-] was increased and less negative (depolarized) when extracellular HCO3- was decreased. Acute replacement of extracellular Na+ by choline also resulted in membrane depolarization of 18.0 +/- 1.6 mV, suggesting net transfer of negative charge. This decrease in PD upon Na+ removal was HCO3- -dependent, amiloride insensitive, and inhibited by the disulfonic stilbene 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS). PD also decreased upon acute exposure to SITS. The degree of depolarization seen with removal of Na+ or HCO3- correlated directly with resting PD (r = 0.81 and 0.95, respectively), suggesting a voltage-dependent mechanism. Removal of extracellular Na+ also decreased pHi to 7.06 +/- 0.02, and this acidification was decreased in the absence of HCO3- or in the presence of SITS or amiloride. These studies provide direct evidence for electrogenic Na+-coupled HCO3- transport in rat hepatocytes. Further, they suggest that it represents a major pathway for conductive movement of Na+ across the membrane and that it contributes, along with Na+-H+ exchange, to the intracellular acidification observed upon removal of extracellular Na+.(ABSTRACT TRUNCATED AT 250 WORDS)
4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfo↗
Effect of pH on membrane potential and K+ conductance in cultured rat hepatocytes.
We have investigated the effects of extracellular (pHo) and intracellular (pHi) pH on membrane potential difference (PD) and cell conductance (gcell) in rat hepatocytes in primary culture. PD and pHi were measured continuously by using intracellular microelectrodes and the pH-sensitive fluorochrome 2',7'-bis(2-carboxyethyl)-5(6)-carboxy fluorescein (BCECF), respectively, during abrupt changes in the pHo or ionic composition of extracellular perfusate. In the presence of 25 mM HCO3-, PD, gcell, and pHi averaged (+/- SE) -32 +/- 1 mV, 16.4 +/- 1.0 nS, and 7.32 +/- 0.01, respectively. The transference number for K+ (tk+), which reflects the fractional contribution of K+ conductance to gcell, averaged 0.36 +/- 0.03. Exposure to 1 mM Ba2+ produced membrane depolarization and decreased tK+ by approximately 90%. Lowering pHo by a variety of maneuvers in the presence and absence of HCO3- consistently decreased pHi, decreased gcell (approximately 30 nS per unit change in pHi), and depolarized PD. Increasing pHo had opposite effects, but the changes in gcell were generally greater with intracellular acidification than alkalinization. The decrease in PD produced by lowering pHo was associated with a decrease in tK+ of 73 +/- 2% and was inhibited by Ba2+. Exposure to butyrate or withdrawal of NH+4, which lowered pHi without changing pHo, also caused depolarization of PD and a decrease in gcell that was inhibited by Ba2+. These observations indicate that the PD of hepatocytes is strongly influenced by pHi, with or without changes in pHo, and they further suggest that the effects of pH on PD are mediated through changes in plasma membrane K+ conductance.
Ammonium Chloride↗
Hepatic oleate uptake. Electrochemical driving forces in intact rat liver.
Recent observations suggest that the hepatic uptake of oleate may be sodium coupled. To assess the electrochemical forces driving fatty acid uptake, we used microelectrodes to monitor continuously the electrical potential difference across the plasma membrane in the perfused rat liver while simultaneously monitoring the rate of tracer [3H]oleate uptake from 1% albumin solutions. Isosmotic cation or anion substitution was used to vary the potential difference over the physiologic range. Depolarization of cells from -29 to -19 mV by substituting gluconate for chloride reduced steady-state oleate uptake by 34%. Conversely, hyperpolarization of cells to -52 mV by substituting nitrate for chloride increased uptake by 41%. Replacement of perfusate sodium with choline depolarized the cells to -18 mV and reduced uptake by 58%, an amount greater than expected from the degree of depolarization alone. Oleate in higher concentrations (1.5 mM in 2% albumin) depolarized cells by 3 mV in the presence of sodium, but had no effect in sodium-free buffer. These results suggest that a portion of oleate uptake in the intact liver occurs by electrogenic sodium cotransport. Uptake appears to be driven by both the electrical and sodium chemical gradients across the plasma membrane.
Algorithms↗
Rat hepatocytes exhibit basolateral Na+/HCO3- cotransport.
Primary cultures and plasma membrane vesicles were used to characterize Na+ and HCO3- transport by rat hepatocytes. Na+ uptake into hepatocytes was stimulated approximately 10-fold by 25 mM extracellular HCO3-.HCO3--stimulated Na+ uptake was saturable, abolished by 4-acetamido-4'-isothiocyano-2,2'-disulfonic acid stilbene (SITS), and unaffected by amiloride or Cl- removal. Neither propionate nor acetate reproduced this effect of HCO3-. 22Na efflux from preloaded hepatocytes was similarly increased approximately 10-fold by an in greater than out HCO3- concentration gradient. 22Na efflux was also increased by valinomycin and an in greater than out K+ concentration gradient in the presence but not absence of HCO3-. Intracellular pH (pHi) measured with the pH-sensitive fluorochrome 2',7'-bis-(2-carboxyethyl)-5-(and 6-)carboxyfluorescein (BCECF) decreased at a rate of 0.227 (+/- 0.074 SEM) pH units/min when extracellular HCO3- concentration was lowered from 25 to 5 mM at constant PCO2. This intracellular acidification rate was decreased 50-60% in the absence of Na+ or presence of SITS, and was unaffected by amiloride or Cl- removal. Membrane hyperpolarization produced by valinomycin and an in greater than out K+ concentration gradient caused pHi to fall; the rate of fall was decreased 50-70% by Na+ removal or SITS, but not amiloride. An inside positive K+ diffusion potential and a simultaneous out greater than in HCO3- gradient produced a transient 4,4'-diisothiocyano-2,2' disulfonic acid stilbene (DIDS) sensitive, amiloride-insensitive 22Na accumulation in basolateral but not canalicular membrane vesicles. Rat hepatocytes thus exhibit electrogenic basolateral Na+/HCO3- cotransport.
Animals↗
Amiloride and amiloride analogs inhibit Na+/K+-transporting ATPase and Na+-coupled alanine transport in rat hepatocytes.
Amiloride, a commonly used inhibitor of Na+-H+ exchange, has been shown to exhibit a variety of nonspecific effects. Recently, the more potent amiloride analogs, 5-(N,N-dimethyl)amiloride hydrochloride (DMA) and 5-(N-ethyl-N-isopropyl)amiloride (EIA), have been used to control for the nonspecific effects of the parent compound. In the present study, we have explored the effects of these analogs on Na+/K+-transporting ATPase (Na+/K+-ATPase) and Na+-coupled alanine transport in primary rat hepatocyte cultures and rat liver plasma membranes, and we have compared the effects of these analogs with the effects of amiloride and ouabain. Amiloride, DMA, and EIA increased steady-state Na+ content and inhibited ouabain-sensitive 86Rb+ uptake in a reversible, concentration-dependent, ouabain-like manner, with estimated 50% inhibitory concentrations (IC50) of 3.0.10(-3) M, 5.2.10(-4) M, and 1.2.10(-4) M, respectively. Amiloride, DMA and EIA also inhibited ouabain-sensitive ATP hydrolysis in rat liver plasma membranes with similar potency (IC50 values of 2.2.10(-3) M, 2.2.10(-3) M, and 1.7.10(-4) M, respectively). In separate experiments, amiloride (5.10(-3) M), DMA (10(-3) M), and EIA (2.5.10(-4) M) decreased the uptake into hepatocytes of alanine by 20%, 61%, and 59%, respectively, and further studies with DMA (10(-3) M) demonstrated that this inhibition was largely due to a decrease in the Na+-dependent fraction of alanine uptake. These findings indicate that amiloride, DMA, and EIA inhibit hepatic Na+/K+-ATPase directly, reversibly, and with a relative rank order potency of EIA greater than DMA greater than amiloride. All three compounds also inhibit the hepatic uptake of alanine, and presumably could indirectly inhibit other Na+-coupled transport processes as well.
Adenosine Triphosphate↗
Effect of taurine on the isolated retinal pigment epithelium of the frog: electrophysiologic evidence for stimulation of an apical, electrogenic Na+-K+ pump.
The apical surface of the retinal pigment epithelium (RPE) faces the neural retina whereas its basal surface faces the choroid. Taurine, which is necessary for normal vision, is released from the retina following light exposure and is actively transported from retina to choroid by the RPE. In these experiments, we have studied the effects of taurine on the electrical properties of the isolated RPE of the bullfrog, with a particular focus on the effects of taurine on the apical Na+-K+ pump. Acute exposure of the apical, but not basal, membrane of the RPE to taurine decreased the normally apical positive transepithelial potential (TEP). This TEP decrease was generated by a depolarization of the RPE apical membrane and did not occur when the apical bath contained sodium-free medium. With continued taurine exposure, the initial TEP decrease was sometimes followed by a recovery of the TEP toward baseline. This recovery was abolished by strophanthidin or ouabain, indicating involvement of the apical Na+-K+ pump. To further explore the effects of taurine on the Na+-K+ pump, barium was used to block apical K+ conductance and unmask a stimulation of the pump that is produced by increasing apical [K+]o. Under these conditions, increasing [K+]o hyperpolarized the apical membrane and increased TEP. Taurine reversibly doubled these responses, but did not change total epithelial resistance or the ratio of apical-to-basal membrane resistance, and ouabain abolished these responses. Collectively, these findings indicate the presence of an electrogenic Na+/taurine cotransport mechanism in the apical membrane of the bullfrog RPE. They also provide direct evidence that taurine produces a sodium-dependent increase in electrogenic pumping by the apical Na+-K+ pump.
Animals↗
Inhibition of Na+/H+ exchange in the rat is associated with decreased ursodeoxycholate hypercholeresis, decreased secretion of unconjugated urodeoxycholate, and increased ursodeoxycholate glucuronidation.
In the perfused rat liver, ursodeoxycholate in high dose produces an HCO3- -rich hypercholeresis which we have shown previously to be inhibited by replacement of perfusate Na+ with Li+ or by addition of amiloride (or amiloride analogues). In the present studies, we have determined whether such inhibition is associated with altered ursodeoxycholate biotransformation. Under control conditions, ursodeoxycholate infusion produced a 3.7-fold increase in bile flow and a 9.2-fold increase in biliary HCO3- output. By thin-layer chromatography, ursodeoxycholate radioactivity in bile was present in unconjugated form (15%) or as glycine or taurine amidates. Glucuronide conjugates of ursodeoxycholate accounted for less than 1% of biliary bile acids. Li+/Na+ substitution decreased ursodeoxycholate-stimulated bile flow and HCO3- secretion by greater than 90%, but decreased recovery of ursodeoxycholate and metabolites by only 25%. Amiloride or amiloride analogues decreased ursodeoxycholate-stimulated choleresis and HCO3- output by 38%-76%, yet did not cause decreased recovery of ursodeoxycholate and metabolites. Inhibition of the hypercholeresis was associated with a decrease in unconjugated ursodeoxycholate to less than 2% of total biliary bile acids, a striking increase in ursodeoxycholate glucuronides, and a reciprocal decrease in glycine and taurine amidates. With Li+/Na+ substitution, the predominant metabolites were a mixture of the 24-ester and the 3-aketal (ethereal) glucuronide (29%), and amidation with glycine appeared to be selectively inhibited; with amiloride or its analogues, only the 3-ethereal glucuronide was formed (20%-60% of biliary bile acids), and both taurine and glycine amidation were inhibited. Thus, maneuvers that decrease Na+/H+ exchange inhibit ursodeoxycholate hypercholeresis and cause replacement of unconjugated ursodeoxycholate in bile by its glucuronide. The secretion of unconjugated ursodeoxycholate, a lipophilic bile acid, appears to be necessary for hypercholeresis induced by high-dose ursodeoxycholate infusion.
Amiloride↗
Ursodeoxycholic acid choleresis: relationship to biliary HCO-3 and effects of Na+-H+ exchange inhibitors.
We have recently shown that substitution of Li+ for perfusate Na+ eliminates the HCO3(-)-rich choleresis produced by ursodeoxycholic acid (UDCA) in isolated perfused rat liver and that the increase in bile flow produced by both UDCA and taurocholic acid is partially inhibited by 1 mM amiloride. Although these findings are consistent with a role for Na+-H+ exchange in the choleresis produced by these bile acids, both Li+ substitution and amiloride affect other cellular processes, including Na+-K+-ATPase activity. We have now further explored both the relationship between UDCA-stimulated bile flow and biliary HCO3- secretion and the possible role of Na+-H+ exchange in this process by comparing the effects of amiloride with two of its more potent and presumably more specific analogues, 5-(N,N-dimethyl)amiloride hydrochloride (DMA) and 5-(N-ethyl-N-isopropyl)amiloride (EIA). In the absence of inhibitor, UDCA increased biliary HCO3- concentration ([HCO3-]) up to an apparent maximum of 60-70 mM, and bile flow and biliary HCO3- output appeared to be linearly related over a sixfold range of bile flow rates. Amiloride, DMA, and EIA each produced a concentration-dependent inhibition of UDCA-stimulated bile flow and biliary HCO3- output with an apparent rank order potency (EIA greater than DMA greater than amiloride) similar to that reported for inhibition of Na+-H+ exchange in other systems. None of the inhibitors significantly altered biliary UDCA output or the relationship between UDCA-induced bile flow and either biliary [HCO3-] or biliary HCO3- output. Effects of these inhibitors did not appear attributable either to nonspecific toxicity, as reflected by hepatic release of lactate dehydrogenase or K+, or to inhibition of hepatic Na+-K+-ATPase, measured as Na+-dependent uptake of 86Rb. In contrast to their effects on UDCA choleresis, these inhibitors had little or no effect on basal bile flow, biliary [HCO3-], and biliary HCO3- output. These findings indicate that UDCA-induced but not basal bile formation is closely coupled to biliary HCO3- concentration and output, and they provide additional evidence that UDCA choleresis requires an intact Na+-H+ exchange mechanism.
Amiloride↗
Proton transport by hepatocyte organelles and isolated membrane vesicles.
It is apparent that proton transport plays an important role in many essential hepatocyte functions. Important unanswered issues include the location of the H+-ATPase and its role in hepatic functions, the regulators of Na+-H+ exchange, the exact role of Na+-H+ exchange in bile formation and in hepatic regeneration, and the role of bile acids such as UDCA and nor-UDCA in mediating transepithelial proton transport.
Animals↗