Effects of catechol-O-methyltransferase inhibition on the plasma clearance of noradrenaline and the formation of 3,4-dihydroxyphenylglycol in the rabbit.

The purpose of this study was to elucidate the finding of Friedgen et al. (1993 b) that catechol-O-methyltransferase (COMT) inhibition is much more effective in increasing the plasma concentration of endogenous dihydroxyphenylglycol (DOPEG) than in increasing the plasma concentration of infused DOPEG. To this end, reserpine-pretreated rabbits were anaesthetized and infused with noradrenaline and/or DOPEG, and the plasma clearances of infused noradrenaline (ClNA) and DOPEG (ClDOPEG) as well as the plasma DOPEG response to noradrenaline infusion [as reflected by the ratio of the steady-state increase in plasma DOPEG (delta DOPEG) to that in plasma noradrenaline (delta NA)] were determined before and after blockade of neuronal uptake by desipramine. Experiments were carried out either under control conditions or after COMT inhibition by i.v. administration of 3,4-dihydroxy-4'-methyl-5-nitrobenzophenone (Ro 40-7592). On the assumption that rates of neuronal noradrenaline uptake equal steady-state rates of neuronal DOPEG formation, the desipramine-sensitive components of ClNA and delta DOPEG/delta NA were used to estimate the apparent plasma clearance of DOPEG formed intraneuronally (Clf-DOPEG) in response to noradrenaline infusion. ClNA was 83.6 ml kg-1 min-1 in the absence and 48.1 ml kg-1 min-1 in the presence of desipramine. Neither the former nor the latter value was altered after COMT inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)