Activity of liver monooxygenase system in rats with low and high resistance to hypoxia

Abstract

Rats with low and high resistance to hypoxia are shown to differ in terms of the baseline activity of liver monooxygenases both in vivo and in vitro. Low-resistance animals are characterized by a significantly higher rate of elimination of antipyrine, hexenal, and nifedipine, as evidenced by shorter half-elimination period and higher urinary concentration of metabolites. The concentration of microsomal cytochromes P-450 and b5 as well as the rates of N-demethylation of amidopyrine, p-hydroxylation of aniline, and hydroxylation of diazepam are considerably higher in rats with low resistance to hypoxia. Key Words: liver monooxygenases, resistance to hypoxia; antipyrine; nifedipine; diazepamRecent investigations show that the mitochondrial respiratory chain provides a molecular basis for considerable species-specific and genetically determined [15] differences in the resistance to hypoxia occurring both at the organism and cellular levels [4]. Previously, we demonstrated opposite changes in the fatty acid composition of the endoplasmic reticulum membranes in hepatocytes from animals with high and low resistance to hypoxia in response to acute hypoxia [9]. However, the activity of the liver oxygen-depended monooxygenase system located in the endoplasmic reticulum in animals with different resistance to oxygen deficiency remains poorly investigated.The aim of the present study was to evaluate in vivo and in vitro the activity of the liver monooxygenase enzyme system in rats differing in individual resistance to oxygen deficiency. MATERIALS AND METHODSExperiments were performed on male Wistar rats weighing 180-220 g. Their resistance to hypoxia was Novosibirsk Medical Institute; Institute of Clinical and Experimental Lymphology, Siberian Division of the Russian Academy of Medial Sciences, Novosibirsk evaluated from the time before the second agonal inspiration during elevation to an "altitude" of 11,500 m at a rate of 50 m/sec in a pressure chamber [8]. Experiments on high-resistance (HR) and low-resistance (LR) animals were carried out 2 weeks after preliminary tests.The in vivo functional activity of liver monooxygenases was assessed by the elimination rate for antipyrine, diazepam, nifedipine, and hexenal. Antipyrine (18 mg/kg) and diazepam (20 mg/kg) were injected intraperitoneally, and nifedipine (5 mg/kg) was administered intragastrally. Blood was drawn from the caudal vein 0.5, 1, 1.5, and 2 h after administration of antipyrine and nifedipine, and 2, 2.5, 3, and 3.5 h after administration of diazepam. The concentrations of the drugs were measured by reverse-phase high-performance liquid chromatography (HPLC) with UV-detection on a Millichrom-lA chromatograph [7]. Elimination of the preparations was calculated by half-elimination time (T,/2) , elimination constant (K,), and clearance (C1) using conventional formulas [7]. The rate of hexenal elimination was evaluated by the duration of hexenal narcosis (100 mg/kg, intraperitoneally) as described elsewhere [2]. The activity of cytochrome P-450 was determined by mea...