Some Aspects of the Metabolism of Dimethylnitrosamine in the Rat

Nitrosamines are a potent and versatile group of carcinogens and by the generally accepted mechanisms of their carcinogenicity require metabolic transformation to alkylating agents or carbonium ions before they become biologically active (Hultin et al., 1960; Heath, 1962). Metabolic studies of dimethylnitrosamine in the rat have shown that it was readily metabolized to CO, (Magee, 1956) and that the formation of COz related to the decomposition of dimethylnitrosamine obeyed the Michaelis-Menten kinetic equation (Heath, 1962) These observations were based on investigations conducted on female rats intraperitoneally administered with 14C-labelled dimethylnitrosamine at dose levels ranging from 16 to 50mg/kg body wt. We have extended these metabolic studies to include dose levels of 14C-labelled dimethylnitrosamine ranging fromO.l to 50mg/kg body wt. administered by three parenteral routes and by oral intubation to male and female rats. In addition, the rates of disappearance of dimethylnitrosamine from the stomach and small intestine of the rat have been investigated. Some preliminary results are presented on the effects of pretreatment with dimethylnitrosamine, phenobarbitone and 20-methylcholanthrene on the metabolism of dimethylnitrosamine in the rat. Wistar albino rats of both sexes (200-2508 body wt.) were used in these studies. Metabolic studies were done on male and female rats at dose levels of 14C-labelled dimethylnitrosamine ranging from 0.1 to 50mg/kg body wt. administered by the intraperitoneal, intravenous, subcutaneous routes or by gastric intubation. The animals were placed in metabolic cages provided with facilities for the collection of respired air and urine and at intervals of time following the administration of '4C-labelled dimethylnitrosamine, radioactivity was measured in the respired CO, and in urine. Absorption studies of dimethylnitrosamine from the gastro-intestinal tract were done by using the operative techniques described by Matthews et al. (1968) and Casper et al. (1973). Following the partial laparotomy of rats anaesthetized with nembutal, the stomach was ligated at both ends, care being taken to avoid occluding blood vessels, and an appropriate dose of 14C-labelled dimethylnitrosamine dissolved in 0.9 % NaCl solution was injected into the stomach. After an absorptive period ranging from 6 to 30min, the stomach was excised, cut open, washed with 0.9% NaCl solution and the radioactivity measured. Similarly, absorption from the small intestines was determined following the injection of appropriate doses of 14C-labelled dimethylnitrosamine dissolved in 0.9 %NaCI solution into ligated small intestinal loops 5cm long, selected between 10 and 30cm from the pylorus. In the first series of experiments the rate and amount of 14C02 respired was determined following the intraperitoneal administration of 14C-labelled dimethylnitrosamine at doses of 0.1, 1 .O, 5 and 50mg/kg body wt. in male and femalerats. The total amount of 14C0, respired during 8 h at all doselevels investigatedand in both sexes was in the range of 57 to 59 % of the administered radioactivity. The 14C02 respiration rate in the intact animal was dose dependent and broadly obeyed the Michaelis-Menten kinetic equation, and the K,,, and V,,,. values obtained, 14.5mg/kg and 5.5nig/kg per h respectively, were comparable with those reported by Heath (1962). In the second series, the rate and amount of 14C0, respired was determined following the administration of 14C-labelled dimethylnitrosamine at a dose of 5nig/kg body wt., by the intraperitoneal, intravenous, subcutaneous and oral gavage rates to male and female rats. The amount of l4CO2 respired during 8 h by all four routes of administration in both sexes was about 57% of the administered dose and the 14C02 respiration rate of approximately 19 % of the dose/h was similar in all cases.