Spironolactone- and canrenone-induced changes in hepatic (Na+,K+)ATPase activity, surface membrane cholesterol and phospholipid, and fluorescence polarization in the rat

Spironolactone attenuates experimental uremic cardiomyopathy by antagonizing marinobufagenin

Spironolactone has been noted to attenuate cardiac fibrosis. We have observed that the cardiotonic steroid marinobufagenin plays an important role in the diastolic dysfunction and cardiac fibrosis seen with experimental renal failure. We performed the following studies to determine whether and how spironolactone might ameliorate these changes. First, we studied rats subjected to partial nephrectomy or administration of exogenous marinobufagenin. We found that spironolactone (20 mg/kg per day) attenuated the diastolic dysfunction as assessed by ventricular pressure-volume loops and essentially eliminated cardiac fibrosis as assessed by trichrome staining and Western blot. Next, we examined the effects of spironolactone and its major metabolite, canrenone (both 100 nM), on marinobufagenin stimulation of rat cardiac fibroblasts. Both spironolactone and canrenone prevented the stimulation of collagen production by 1 nM marinobufagenin but not 100 nM marinobufagenin, as assessed by proline incorporation and procollagen 1 expression, as well as signaling through the sodium-potassium-ATPase, as evidenced by protein kinase C isoform delta translocation and extracellular signal regulated kinase 1/2 activation. Both spironolactone and canrenone also altered ouabain binding to cultured porcine cells in a manner consistent with competitive inhibition. Our data suggest that some of the antifibrotic effects of spironolactone may be attributed to antagonism of marinobufagenin signaling through the sodium-potassium-ATPase.

Aging and the liver: functional aspects

The drastic decline in the function of the hepatic microsomal cytochrome monooxygenase system, initially reported in male rat livers, was shown to be due to a feminization of male rat livers with aging. In female rat livers as well as in mouse livers, this system was found to stay unchanged with age. Phase II reactions which showed some decline with aging in male rat livers again stayed fairly stable with age in female rat and mouse livers. Glutathione S-transferase (GST) enzyme activities, which are very stable with age in female rat and mouse livers, demonstrated highly age-dependent changes when dietary conditions were manipulated, suggesting a potential age difference in the homeostatic regulation of this enzyme system. Using the fluorescence recovery after photobleaching (FRAP) technique, unique studies revealed an age-dependent decline in the lateral mobility of proteins in hepatocyte surface membranes. The protease inhibitor model of aging, initially proposed by Ivy for brain cells, has been validated in hepatocytes, demonstrating an accumulation of lipofuscin-like granules in young animals treated with i.p. infusion of leupeptin for only 2 weeks. Antioxidant enzyme activities such as superoxide dismutase (SOD) and catalase (CAT) in the liver were clearly demonstrated no to be reduced in general terms with aging. Rather, a clear increase in CAT enzyme activities with age was demonstrated in female rat livers, thus challenging the concept that intracellular enzyme activities generally decline with aging. In this paper, studies performed in Japan on aging and the liver over the past 30 years, with a focus on its functional aspects, are critically reviewed in terms of the clinical implications of these studies as well as on theories of aging in general.

Effect of spironolactone on the expression of rat hepatic UDP-glucuronosyltransferase

Spironolactone (SL) increases the glucuronidation rate of several compounds. We analyzed the molecular basis of changes occurring in major rat liver UDP-glucuronosyltransferase (UGT) family 1 isoforms and in UGT2B1, a relevant isoform of family 2, in response to SL. UGT activity toward bilirubin, ethynylestradiol and p-nitrophenol was assayed in native and activated microsomes. Protein and mRNA levels were determined by Western and Northern blotting. The lipid composition and physicochemical properties of the microsomal membrane were also analyzed. Glucuronidation rates of bilirubin and ethynylestradiol (at both 3-OH and 17 beta-OH positions), determined in UDP-N-acetylglucosamine-activated membranes, were increased in SL group. Western blot analysis revealed increased levels of UGT1A1 and 1A5 (bilirubin and 3-OH ethynylestradiol conjugation), and 2B1 (17 beta-OH ethynylestradiol conjugation). Northern blot studies suggested transcriptional regulation by the steroid. Analysis of UGT activity in native vs. alamethicin-activated microsomes indicated increased latency, which was not associated to changes in physicochemical properties of the microsomal membrane. p-Nitrophenol glucuronidation rate and mRNA and protein levels of UGT1A6, the main isoform conjugating planar phenols, were not affected by the inducer. The data suggest transcriptional regulation of specific isoforms of hepatic UGT by SL, thus explaining previously reported increases in UGT activity toward selective substrates.

Lateral mobility of proteins and lipids of cell surface membranes during aging: do the data support 'The Membrane Hypothesis of Aging'?

Many previous studies regarding the change with age in surface membrane fluidity of different cell types, including hepatocytes, as determined by the fluorescence anisotropy method, are in conflict, demonstrating decreased, unchanged or even increased fluidity with age. In contrast, the results of our series of works using the fluorescence recovery after photobleaching (FRAP) technique, which measures protein lateral diffusion coefficients of hepatocyte surface membranes (Dp), have demonstrated that Dp generally declines in a linear fashion with age in hepatocytes of all animal strains and species examined. The major coworker (I. Zs.-Nagy) of these studies insists that our observations support his original hypothesis, 'The Membrane Hypothesis of Aging' (MHA), the primary assumption of which is that changes in cell surface membranes with age cause a general decline in intracellular enzyme activities. However, while it seems clear that cell surface membrane changes do occur with age, a number of past observations including those from the laboratory of this author, provide strong evidence that intracellular enzyme activities do not generally decline with age. This paper presents the data in detail, along with the author's view that the results do not support the main assumption of the MHA, but are more likely related to alterations in membrane functions with age.

Sex hormones differentially regulate isoforms of UDP-glucuronosyltransferase

PURPOSE

To investigate the role of sex hormones in the regulation of UDP-glucuronosyltransferase (UGT).

METHODS

We examined liver from adult, prepubertal, gonadectomised and gonadectomised plus hormone replaced rats of both sexes. Immunohistochemistry and immunoblots were performed using a polyclonal UGT antibody to a number of family 1 and family 2 UGT isoforms. Northern blot analysis was performed utilising cDNA probes to family 1 and family 2 isoforms.

RESULTS

Immunohistochemistry demonstrated variations in intensity and distribution of staining in the hormonally manipulated rats. Immunoblots showed variations in individual band intensity between rat groups. Immunoblots using a more specific antibody (anti-17 beta-hydroxysteroid UGT, which recognises UGT2B3 and UGT2B2) demonstrated marked differences between male and female rats and significant alterations after gonadectomy and testosterone replacement in the male rats. In northern analysis, UGT2B3 and 2B1 mRNA were significantly higher in adult males than females, and in prepubertal males compared to prepubertal females. In male rats, gonadectomy resulted in a 45-53% reduction in UGT2B3 and 2B1 levels respectively, which increased significantly with testosterone treatment to greater than normal adult levels. No change in UGT2B3 or 2B1 occurred after gonadectomy in females. In contrast, UGT1*1 mRNA tended to be higher in adult female and prepubertal female rats than in their male counterparts. In females, gonadectomy resulted in significant up-regulation of UGT1*1, while gonadectomy plus oestradiol treatment resulted in markedly reduced levels. UGT1*1 mRNA was not significantly altered by gonadectomy in males.

CONCLUSIONS

This study demonstrates the differential effects of sex hormones on the expression of isoforms from the two phylogenetically distinct UGT families.

Effect of S-adenosyl-L-methionine on ethanol cholestasis and hepatotoxicity in isolated perfused rat liver

We investigated whether S-adenosyl-L-methionine (SAMe) influences the inhibitory effect of ethanol on bile secretion and ethanol hepatotoxicity in the isolated perfused rat liver. SAMe (25 mg/kg intramuscularly three times a day) was administered for three days consecutively. Liver was then isolated and perfused with taurocholate to stabilize bile secretion and exposed to 1% ethanol for 70 min. The effect of ethanol on bile flow, bile salt biliary secretion, oxygen liver consumption, AST and LDH release in the perfusate, and hepatic concentration of glutathione, malondialdehyde, and diene conjugates was compared between SAMe-treated livers (N = 11) and paired controls (N = 11). Control experiments without ethanol were also performed (N = 6). Exposure to 1% ethanol induced a significantly (P < 0.03) higher inhibition of bile flow (-35% vs 17%) and bile salt secretion (-28% vs 16%) in untreated compared with SAMe-treated livers. During 1% ethanol exposure, the release of LDH and AST in the perfusate was significantly lower (P < 0.02) in SAMe-treated livers. Oxygen liver consumption was markedly inhibited by 1% ethanol administration (P < 0.02 vs controls without ethanol), an effect almost totally prevented by SAMe treatment (P < 0.02 vs ethanol controls). The hepatic concentration of total glutathione was significantly (P < 0.02) decreased by 1% ethanol exposure, but this effect was less pronounced in SAMe-treated than in untreated controls (P < 0.02). The hepatic levels of malondialdehyde and diene conjugates were not significantly changed by ethanol exposure in either SAMe-treated or control livers in comparison to ethanol-free controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Metal induced changes in the erythrocyte membrane of rats

Lead, zinc and copper were administered i.p singly or in combination as acetate salts to rats for 14 consecutive days. It was observed that lead induced drastic changes, copper induced moderate changes but zinc did not cause any significant change in the cholesterol and phospholipid content, hexose, hexosamine and sialic acid levels and activities of the erythrocyte membrane enzymes--acetylcholinesterase (AChE), NADH dehydrogenase and Na(+)-K+ ATPase. In the combined metal treatment the presence of zinc considerably reduced the changes induced by lead and copper.

Physical-chemical-activity relationship of organic solvents: effects on Na(+)-K(+)-ATPase activity and membrane fluidity in mouse synaptosomes

Physical-chemical-activity relationship of aromatic hydrocarbons (n = 10) and alkyl acetates (n = 16) with respect to their in vitro effects on synaptosomal membranes was studied. Na(+)-K(+)-adenosine triphosphatase (Na(+)-K(+)-ATPase) activity and membrane fluidity, which was determined using the fluorescence probe 1,6-diphenyl-1,3,5-hexatriene, were used as potential indicators of neuronal cell toxicity. The potency of inhibition for the enzyme (IC50), the potency of increasing membrane fluidity (IC12.5), and n-octanol/water partition coefficient (P) were all determined experimentally for 26 solvents. Correlation analyses were made on aromatic hydrocarbons and on alkyl acetates. There were linear relationships between log P and pIC50 (log1/IC50) values, and between log P and pIC12.5 (log1/IC12.5) values, indicating that the hydrophobicity of the solvents determines their toxic ability to affect membrane environment; the more hydrophobic the solvents are, the more toxic they are. A direct linear relationship between Na(+)-K(+)-ATPase activity pIC50 and membrane fluidity pIC12.5 values was also shown. This predictive correlation suggests a similar mechanism of membrane surface interaction govering both processes that are common to the test solvents. The present results confirm the importance of the lipid environment of neuronal membranes in maintaining the normal function of membrane-bound protein.

S-adenosylmethionine prevents total parenteral nutrition-induced cholestasis in the rat

Both an excess and an imbalance of amino acids have been associated with total parenteral nutrition-induced cholestasis. The present study was undertaken to further our understanding of this condition in light of observations that methyl donor amino acids may be protective. Rats were maintained on Travasol (3.4 g amino acids/24 h) and dextrose (10.2 g/24 h) with and without the "active methyl" S-adenosylmethionine at a dose of 75 mg/kg/24 h for 5 days, and compared to control rats on dextrose alone (10.2 g/24 h) with free access to rat chow. Bile flow (microliters/min) was lower (p < 0.025) in the Travasol (8.65 +/- 0.78) than in the control group (12.30 +/- 0.52) and was restored in the Travasol+S-adenosylmethionine animals (11.42 +/- 10). Furthermore, the bile acid secretory rate (mumol/h) was higher (p < 0.05) with S-adenosylmethionine (23.34 +/- 3.71) than without S-adenosylmethionine (14.16 +/- 2.19). As expected, the molar ratio of biliary cholesterol was lower (p < 0.005) in both total parenteral nutrition groups. However, in the total parenteral nutrition group without S-adenosylmethionine, there was also a decrease in the molar ratio of phospholipids which correlated well with the bile acid secretory rate. Analysis of liver plasma membranes showed that a lower activity of Na+K(+)-ATPase (mumol Pi/mg protein/h) (p < 0.005) in the Travasol animals (6.26 +/- 0.53) was restored to control values (15.20 +/- 1.43) by the addition of S-adenosylmethionine (17.07 +/- 2.87). In the three groups, a close correlation was observed between Na+K(+)-ATPase activity and bile flow.(ABSTRACT TRUNCATED AT 250 WORDS)

Organic solvents increase membrane fluidity and affect bile flow and K+ transport in rat liver

Following the earlier observation that inhalation of volatile lipid solvents and of narcotic gases causes cholestasis, we studied the effects of various organic solvents on bile flow, plasma membrane fluidity and potassium movement in rat liver. Both in vivo and in the isolated perfused liver, applications of CCl4, CHCl3, dichloromethane, trichloroethylene, halothane, benzene and cyclohexane elicited rapid and sustained but reversible cholestasis. A transient phase of choleresis was observed prior to and after cholestasis, during the increase and fall in liver tissue solvent concentrations, respectively. Tissue concentrations required to produce cholestasis were lower the higher the lipophilicity of the solvent. Membrane fluidity was measured in isolated basolateral liver cell membranes by fluorescence polarization. Fluidity increased with increasing solvent concentration, the increase being associated with either biphasic stimulation and inhibition of membrane enzymes (Na+,K(+)-ATPase, 5'nucleotidase) or with inhibition alone (Mg(2+)-ATPase). In the isolated perfused liver, application of organic solvents caused hepatic uptake of K+ that was followed by K+ release upon withdrawal of the solvent. The magnitude of K+ uptake elicited by the solvent was comparable with the effect of blocking K+ channels with 2 mM Ba2+, but Ba2+ was ineffective in the presence of the solvent. In contrast, application of ouabain caused K+ release in equal amounts in the absence and presence of the solvent, indicating that K+ uptake elicited by the solvent results from inhibition of K+ efflux through K+ channels rather than stimulation of the Na+,K+ pump. The data show that cholestasis elicited by lipid solvents is associated with an increase in membrane fluidity and with disturbance of liver K+ homeostasis. The significance of these observations is discussed with respect to other models of experimental cholestasis.

Effect of epomediol on taurocholate uptake in hepatocytes isolated from normal rats or rats with ethinyloestradiol-induced cholestasis

In this study the effect of epomediol (1,3,3-trimethyl-2-oxabicyclo 2.2.2 ocatan-6,7-endo, endo-diol) (Clesidren) on rat hepatocyte bile acid transport was evaluated using [3H]-taurocholate as a probe, in order to clarify the mechanism of action of this drug. To this purpose, primary cultures were prepared with hepatocytes obtained from normal rats (Group I), and rats receiving a 4-day treatment with either epomediol (100 mg/kg) (Group II), or ethinyloestradiol (5 mg/kg) (Group III), or ethinyloestradiol plus epomediol (5 mg and 100 mg/kg respectively) (Group IV) or vehicle DMSO 50 microliters/kg) (Group V). All hepatocytes were isolated 10 days after the end of treatment. Hepatocyte [3H]-taurocholate uptake was evaluated in vitro after 48 hr of incubation in the presence or absence of epomediol. In both cases no difference was found when evaluating the uptake of hepatocytes from Group I, II and V. In the absence of epomediol [3H]-taurocholate uptake in hepatocytes from rats of Group IV was significantly higher than that observed in hepatocytes from rats of Group III. On the other hand, the presence of epomediol did not influence [3H]-taurocholate uptake in hepatocytes from rats of Group III, which remained significantly lower compared to that of control hepatocytes (Group V). The protective effect obtained when administering epomediol simultaneously with ethinyloestradiol (Group IV) was not due to its ability to compete with ethynyloestradiol for the binding to oestrogen receptors. Our results indicate that epomediol is able to restore a normal hepatocyte bile acid uptake when given in vivo simultaneously with ethinyloestradiol but does not influence bile acid transport in cultured hepatocytes. Further studies are required to better define the choleretic activity of this drug.

Age-dependent decrease in the hepatic uptake of taurocholic acid resembles that for ouabain. A possible role of surface membrane protein mobility

Isolated hepatocytes were prepared from Wistar-derived male rats of different ages (4, 12 and 27-29 months) by the collagenase perfusion method. The hepatic uptake rate of taurocholate (TC) for the saturable fraction was calculated by subtracting the non-saturable fraction from the total hepatic uptake. The Vmax and the apparent affinity constant Km were computed for the saturable fraction by means of non-linear regression. The Vmax (nmol/mg protein/min, mean +/- SE) for young rats (N = 6) was 2.15 +/- 0.11, whereas in old rats (N = 4) the value was 50% lower (1.16 +/- 0.11, P less than 0.005). In contrast, Km (microM) values were not significantly different between young (25.88 +/- 1.90) and old (30.34 +/- 4.96) rats. There was a significant inverse linear relationship (r = 0.79; P less than 0.01) between the age of rats and the uptake velocity (nmol/mg/mg protein/min) at 1 microM of TC, suggesting a steady and almost linear decrease of TC uptake velocity with age. The rate of decrease per month (2.1%) was quite close to the value for ouabain uptake (2.8%) previously found by the authors. Furthermore, a marked linearity was observed between the average values for TC uptake rates for three age groups and corresponding lateral diffusion constants of hepatocyte plasma membrane proteins previously obtained by the authors using the fluorescence recovery after photobleaching method. The results support our previous proposal that protein mobility within the hepatocyte surface membrane may play at least a partial role in regulation of carrier-mediated hepatocyte uptake functions for various materials.

The effect of m-xylene on rat lung benzo[a]pyrene metabolism and microsomal membrane lipids: comparison with p-xylene

m-Xylene (1 g/kg, i.p., 1 h) was shown to decrease aryl hydrocarbon hydroxylase (AHH) activity, a detoxification pathway for benzo[a]pyrene (BaP), in the rat lung. Inhibition was maximal at 1 g/kg, 1 h after treatment and was sustained for at least 24 h. Reduction in cytochrome P-450 activity in rat lung was also observed, while liver activity was unchanged. p-Xylene has been previously shown to produce a similar pattern of MFO changes in rat lung. The lipid composition of the microsomal membrane is important to mixed function oxidase (MFO) regulation and function. Since the xylenes are lipophilic, these compounds were studied to determine whether they alter pulmonary microsomal lipids. p-Xylene produced an organ specific increase in lipid peroxidation in the rat lung. This was accompanied by decreases in lung microsomal total phospholipid (PL) and phosphatidylcholine (PC) content. Pulmonary microsomal membrane fluidity was also reduced by p-xylene administration. In comparison, m-xylene administration did not change any of the lipid membrane parameters tested. These divergent results leave unresolved the role of altered PL metabolism in solvent-induced inhibition of MFO activity.

Correlation between the biliary excretion of ouabain and the lateral mobility of hepatocyte plasma membrane proteins in the rat--the effects of age and spironolactone pretreatment

The biliary excretion of intravenously injected ouabain and the diffusion constant of the lateral mobility of hepatocyte plasma membrane proteins were examined in control (saline-treated) and spironolactone-treated Wistar male rats of different ages (4, 14 to 15 and 24 months old). The biliary excretion of ouabain progressively decreased with age in control rats, the first 10-min biliary recovery in 24-month-old animals being one-third that of the youngest rats (4-month-old). The oral administration of spironolactone for 4 days (10 mg per 100 gm body weight on the first day and 20 mg per 100 gm body weight for the successive 3 days) caused a marked increase in the biliary recovery of ouabain in all age groups. Similarly, the average lateral diffusion constant of hepatocyte plasma membrane proteins as measured by fluorescence recovery after photobleaching showed a linear decrease with age, as was previously observed with F-344 rats of both sexes. Markedly and significantly (30 to 40%) higher diffusion constants were observed in rats pretreated with spironolactone for all three age groups, compared with the respective control values of corresponding ages. The parallelism between ouabain excretion and protein diffusion (i.e., a decrease with age and an increase with spironolactone pretreatment) suggests that the lateral mobility of proteins in the hepatocyte plasma membrane is a candidate mechanism for regulating ouabain excretion through the liver into the bile, most probably by regulating the hepatic uptake process for ouabain.

Biliary excretion of ouabain in aging male and female F-344 rats

The biliary excretion of ouabain was examined in Fischer-344 rats of both sexes and of different ages. The biliary recoveries for the first 10-min period after the intravenous injection of ouabain (0.1 mg/100 g body weight) were nearly 2-fold higher in female than in male rats for all age groups compared (4, 6, 12, 24, 28 months). In both sexes, the biliary recovery showed a gradual decrease with age, and the first 10-min value in 24-months old rats was about one third of those in the youngest rats. The 60-min total recovery values also significantly decreased with age. Some very old (males, 28 months; females, 31 months) rats, however, showed higher 60-min total recovery values than younger rats comparable to the youngest animal values. It was concluded that the biliary excretion of ouabain in rats decreases with age in both sexes which suggests that this is a general phenomenon in this animal species.

Drug-induced cholestasis

Intrahepatic cholestasis, defined as arrested bile flow, mimics extrahepatic obstruction in its biochemical, clinical and morphological features. It may be due to hepatocyte lesions of which there are three types, termed canalicular, hepatocanalicular and hepatocellular, respectively; or it may be due to ductal lesions at the level of the cholangiole or portal or septal ducts. Defective bile flow due to hepatic lesions reflects abnormal modification of the ductular bile. Defective formation of canalicular bile may involve bile acid-dependent or independent flow. It appears to result most importantly from defective secretion of bile acid-dependent flow secondary to defective uptake from sinusoidal blood, defective transcellular transport and defective secretion; or from regurgitation of secreted bile via leaky tight junctions. An independent defect in bile acid-independent flow is less clear. Defective flow of bile along the canaliculus may reflect increased viscosity and impaired canalicular contractility secondary to injury of the pericanalicular microfibrillar network. Impaired flow beyond the canaliculus may result from ductal injury. Sites of lesions that contribute to cholestasis include the sinusoidal and canalicular plasma membrane, the pericanalicular network and the tight junction and, less certainly, microtubules and microfilaments and Golgi apparatus. A number of drugs that lead to cholestasis have been found to lead to injury at one or more of these sites. Other agents (alpha-naphthylisothiocyanate, methylenedianiline, contaminated rapeseed oil, paraquat) lead to ductal injury resulting in cholestasis. Reports of inspissated casts in ductules (benoxaprofen jaundice) and injury to the major excretory tree (5-fluorouridine after hepatic artery infusion) have led to other forms of ductal cholestasis. Most instances of drug-induced cholestasis present as acute, transient illness, although important chronic forms also occur. The clinical features include the reflection of the cholestasis (pruritus, jaundice), systemic manifestations and extrahepatic organ involvement. While nearly all classes of medicinal agents include some that can lead to cholestasis, there are differences among the various categories. Phenothiazines and related antipsychotic and 'tranquillizer' drugs characteristically lead to cholestatic hepatic injury. The tricyclic antidepressants may lead to cholestatic or hepatocellular injury.(ABSTRACT TRUNCATED AT 400 WORDS)