The kinetic constants for the extraneuronal uptake and metabolism of 3H-(-)-noradrenaline in the perfused rat heart.

Abstract

1. Rat hearts were perfused with 50–2000 μM 3H-(−)-noradrenaline in the presence of 14C-sorbitol (to measure the extracellular distribution of the amine) and of 100 μM cocaine (to inhibit neuronal uptake). Initial rates of removal of noradrenaline were determined. Extraneuronal uptake determined in this way consisted of two components: a saturable and a non-saturable one. 2. When monoamine oxidase (MAO) was inhibited, the kinetic constants for saturable extraneuronal uptake of noradrenaline were: Km 60 μM, Vmax 50 nmoles · g−1 · min−1. U-0521, an inhibitor of catechol-O-methyl transferase (COMT) acted like a competitive inhibitor of extraneuronal uptake: the Km increased, Vmax remained unaffected. 3. In separate experiments either MAO or COMT was intact. Rat hearts were perfused with various concentrations of 3H-(−)-noradrenaline until the metabolites appeared at a constant rate in the venous effluent. These steady-state rates of metabolite formation were used to determine the kinetic parameters of the metabolizing systems. 4. O-methylation of noradrenaline in the intact heart was a saturable process and obeyed Michaelis-Menten kinetics; it had very low Km (1.7 μM) and Vmax (1.2 nmoles · g−1 · min−1). 5. Also the deamination of noradrenaline by the perfused heart was a saturable process, but Km and Vmax were considerably higher than for O-methylation (140 μM and 25 nmoles · g−1 · min−1, respectively). 6. It is concluded that “extraneuronal uptake and subsequent metabolism” of noradrenaline represents a site of loss that is highly effective at very low substrate concentrations (i.e., below the Km of the two metabolizing systems).