EVIDENCE FOR SATURATION OF CATECHOL-O-METHYLTRANSFERASE BY LOW CONCENTRATIONS OF NORADRENALINE IN PERFUSED LUNGS OF RATS

Previous studies on the pulmonary removal and metabolism of catecholam ines in rat lungs have shown that, when the lungs are perfused with a low concentration (1 nmol/l) of noradrenaline, the amine is metabolize d by catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO), but is predominantly O-methylated, and the activities of COMT and MAO are 0.357 min(-1) and 0.186 min(-1), respectively. The aim of the pres ent study was to examine the changes in the metabolic profile of norad renaline in rat lungs over a range of concentrations, and to examine t he kinetics of the pulmonary O-methylation of noradrenaline and adrena line. In isolated lungs perfused with H-3-noradrenaline, there was a p rogressive decrease in the proportion of O-methylated metabolites and a corresponding increase in the proportion of deaminated metabolites, as the noradrenaline concentration in the perfusion solution was incre ased from 1 to 10 to 100 to 1000 nmol/l. Experiments designed to deter mine the rate of uptake of noradrenaline in lungs perfused with 1 nmol /l H-3-noradrenaline, under conditions of MAO inhibited, COMT inhibite d and COMT and MAO inhibited, showed that the results were compatible with co-existence of COMT and MAO in the pulmonary endothelial cells. Hence, it appeared that the changing metabolic profile with amine conc entration in the previous series of experiments was not due to saturat ion of noradrenaline uptake into cells that contained COMT but not MAO . Further experiments to examine the kinetics of O-methylation of nora drenaline and adrenaline (MAO inhibited) showed that the O-methylation of these amines in the lungs was predominantly saturable, with half-s aturation occurring at concentrations (9.8 nmol/l and 19.4 nmol/l, res pectively) that were two orders of magnitude lower than those required to half-saturate uptake, of the amines. Saturation of O-methylation b y these low concentrations of noradrenaline (i) provides the explanati on for the change in the metabolic profile of noradrenaline described above and (ii) appears to occur because V-max uptake much greater than V-max COMT for the metabolizing system consisting of non-neuronal upt ake(1) + COMT in the lungs, as has been described previously for the s ystem consisting of uptake(2) + COMT in extraneuronal sites in rat hea rt. The results show that the metabolic profile of catecholamines in t he pulmonary circulation will reflect that occurring at physiological levels only if studies are carried out with very low amine concentrati ons.