Intestinal Absorption and Hepatic Extraction of Propranolol and Metoprolol in Rats with Bilateral Ureteral Ligation
Biological and Pharmaceutical Bulletin
The intestinal absorption of orally administered propranolol is essentially complete, and the metabolism of this drug does not occur in the gut. 1,2) After oral administration of propranolol, the liver is the principal site of extensive pre-systemic and systemic metabolism, and less than 1% of the intact drug is found in the urine. 1,3) However, Bianchetti et al. showed that the area under the concentration-time curve for orally administered propranolol in renal failure patients not on
... sis is 7-to 8-fold higher than that in healthy volunteers. 4) The pharmacokinetics of propranolol has been extensively investigated using uranyl nitrate-induced renal failure model rats. 5,6) These studies showed the occurrence of increased bioavailability and reduced hepatic first-pass extraction of propranolol in rats with renal failure, although the precise biochemical and/or physiological mechanisms for the decreased presystemic clearance is unclear. 7, 8) The injection of uranyl nitrate is most effective and the easiest method for producing renal dysfunction in laboratory animals; however, changes in government regulations regarding the production and use of radioactive substances have made uranyl nitrate less available. 9,10) We therefore investigated the mechanisms responsible for the increased bioavailability of propranolol in cisplatin-induced renal failure rats. 11) The hepatic intrinsic clearance of propranolol was not significantly altered in rats with renal failure as compared with control rats. However, hepatic first-pass extraction of propranolol was dose-dependent and saturable in both renal failure and control rats, and the initial absorption rate of the drug from the intestine in rats with renal failure was significantly greater than that in control rats. Accordingly, the increased bioavailability of propranolol in rats with cisplatininduced renal dysfunction is mainly a result of the increased initial absorption rate in the intestine followed by the partial saturation of hepatic first-pass metabolism. 11) On the other hand, Laganiére and Shen investigated the pharmacokinetics of intravenously and orally administered propranolol in bilateral ureter-ligated (BUL) rats, and showed that its bioavailability is increased in 36-h BUL rats. 2) They also reported that the gastrointestinal absorption of propranolol is not altered in BUL rats as compared with control rats. Therefore, by inference, the increased bioavailability of propranolol in 36-h BUL rats is attributed to diminished hepatic first-pass metabolism. 2) The starting hypothesis of the present study was that the mechanism responsible for the increased bioavailability of propranolol in the BUL rat is different from that in the cisplatin-induced renal failure rat. We first examined the initial absorption rate of propranolol and metoprolol, another bblocker with high hepatic intrinsic clearance but low plasma protein binding, in the BUL rat intestine. We then evaluated the hepatic first-pass extraction of these drugs in BUL rats using intra-portal administration. Finally, we examined the intrinsic cytochrome P450 activity in hepatic microsomes and the generation rate of NADPH, a co-factor of P450, in the hepatic cytosol fraction. MATERIALS AND METHODS Materials Propranolol hydrochrolide, metoprolol tartrate, NADP ϩ , and NADPH were obtained from Nacalai Tesque (Kyoto, Japan). All other chemicals were of the highest grade available. Animals The animal experiments were performed in accordance with the Guidelines for Animal Experiments of Toyama Medical and Pharmaceutical University. Male Wistar rats (240-260 g) were purchased from Japan SLC Inc. (Hamamatsu, Japan). Before the experiments, the rats were housed in a temperature-and humidity-controlled room with free access to water and standard rat chow. The abdominal cavity was opened under pentobarbital anesthesia, and BUL was performed by isolating the ureters, placing two ligatures tightly around each ureter, and cutting between the ligatures. 12) The experiments were performed 24, 28, or 32 h after the operation. Sham-operated animals served as controls. To investigate the mechanism responsible for the increased bioavailability of propranolol in bilateral ureterligated (BUL) rats, the intestinal absorption and hepatic extraction of propranolol and metoprolol were evaluated. The initial absorption rate of these drugs after intra-intestinal administration was only slightly increased in the BUL rats, whereas the blood drug concentration in these rats was higher than that in control rats. The blood propranolol and metoprolol concentrations during intra-portal infusion in the BUL rat were significantly higher than that in the control rat. In the presence of NADPH, the intrinsic metabolic activity of metoprolol in hepatic microsomes was not altered by BUL. On the other hand, the NADPH generation rate in the hepatic cytosol in the BUL group was lower than that in the control group. These results indicate that the absorption rate-dependent decrease in hepatic first-pass clearance of propranolol and metoprolol due to saturation kinetics is marginal, and that the hepatic metabolic activity and extraction of the drugs is significantly decreased in BUL rats probably due to the reduced NADPH generation rate in the liver.