Effect of spironolactone on bilirubin conjugation by the rat liver

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.

Formation of bilirubin monoglucuronide and diglucuronide in isolated rat hepatocytes. Effect of spironolactone

The formation of bilirubin monoglucuronide (BMG) and diglucuronide (BDG) was studied in isolated rat hepatocytes with appropriate viability. Isolated cells were obtained from normal rats and from rats pretreated with spironolactone (SP). A fixed number of cells (4.8 X 10(6)) was incubated in a medium containing uridine diphosphoglucuronic acid (UDPGA, 3.4 mM) and bilirubin (11.3 microM, 29 microM, 50 microM and 81 microM) for different time intervals (from 0 to 25 min). The pellet of cells and the supernatant fraction were subjected to alkaline methanolysis, and the proportions of BMG and BDG were estimated by thin-layer chromatography. No conjugates were detected at time O or in the absence of UDPGA in the incubation system. BMG and BDG were detected after 2 min of incubation and then they increased up to 15 min of incubation. Both conjugates were mostly found in the supernatant fraction, and a predominance of BMG was apparent. Normal cells also synthesized increasing amounts of BMG and BDG with the increase of bilirubin substrate concentration up to 50 microM. When hepatocytes from SP-treated rats were used, a more rapid rate of glucuronidation, that was mainly produced at the expense of BMG found in the supernatant fraction, was clear. The results probably indicate that enzymic conversion of BMG to BDG may be rate limiting in isolated hepatocytes although other possible mechanisms were not excluded.

Effect of spironolactone on p-nitrophenol glucuronidation in isolated rat hepatocytes

The effect of spironolactone (SP) on p-nitrophenol (PNP) glucuronidation was studied in isolated rat hepatocytes with appropriate viability conditions. A significant increase of protein concentration and PNP glucuronidation was found in the hepatocytes from SP-treated rats. Increased enzyme activity apparently was related to the SP dose. The results favor the conclusion that SP may induce PNP glucuronidation in the hepatocyte.

Effect of spironolactone on bilirubin metabolism in rat liver and small intestinal mucosa

In vitro and in vivo experimental models were designed for the study of the effect of spironolactone (SP) on bilirubin metabolism in rat liver and small intestinal mucosa. In vitro studies included uptake of bilirubin by liver slices and intestinal sheets, determination of glucuronyltransferase activity in mucosal homogenates, and the handling of bilirubin by the isolated perfused liver after bilirubin overload. In vitro studies were carried out to measure the plasma disappearance rate of bilirubin and to determine the extent of bilirubin conjugation and biliary excretion of the pigment infused intravenously. The results obtained suggested that the mechanisms involved in the uptake of bilirubin by tissues were not influenced by SP pretreatment. Glucuronyltransferase activity in the small intestinal mucosa was significantly induced by SP, as previously observed in rat liver. Isolated perfused livers from SP-treated rats, as well as treated living rats, exhibited a greater than normal capacity for bilirubin excretion into bile at the expense of bilirubin diglucuronide. Conjugated bilirubin in the small intestinal mucosa of rats infused with unconjugated pigment was also increased after SP pretreatment. The results favoured the conclusion that SP is an inducer of bilirubin conjugation in the livers as well as in extrahepatic tissues, such as the small intestinal mucosa.

Biliary excretion of sulfobromophthalein in isolated perfused livers from normal and spironolactone-treated rats

Bile flow and biliary excretion of sulfobromophthalein (BSP) was examined in isolated perfused livers from normal and spironolactone(SP)-treated rats. BSP biliary excretion contributed to the bile production in both groups. Moreover SP increased BSP biliary excretion but transfer of dye from plasma into liver was not affected.