Characteristics of the cocaine-sensitive accumulation and O-methylation of ³H-(−)-noradrenaline by rabbit endometrium

Summary

1. The extraneuronal uptake and O-methylation of 2,5,6 ³H-(−)-noradrenaline was studied in segments of uterine endometrium from rabbits pretreated with 17β-oestradiol and progesterone. 2. The uptake of ³H-noradrenaline was measured in MAO- and COMT-inhibited tissues and obeyed Michaelis-Menten kinetics with an apparent K _m of 78 μmol/1 and a V _max of 5.4 nmol/g min. Uptake was inhibited by low Na⁺ and by potential substrates in the order dopamine > (−)adrenaline > (−)isoprenaline = 5-hydroxytryptamine. 3. Following uptake at 1.2 μmol/1, efflux of ³H-noradrenaline was slow and appeared to be from two compartments, of which the first (I) had a t1/2 of 53 min and a capacity of 1.8 nmol/g. The presence of the second compartment (II) was inferred from the tissue content of ³H after 60 min of efflux, which was 3–4 times greater than predicted if the ³H was present in compartment I only. Following incubation with ³H-noradrenaline in the presence of cocaine 30 μmol/1 the ³H efflux was rapid and the combined capacities of compartments I and II were greatly decreased. 4. ³H-NMN formation, measured in MAO-inhibited tissues, obeyed Michaelis-Menten kinetics with a half-saturating outside concentration of 12 μmol/1 and a V _max of 0.9 nmol/g · min. The formation was inhibited by the neuronal uptake inhibitors, desipramine 3 μmol/1 and metaraminol 100 μmol/1 (each by 80%), but was unaffected by the extraneuronal uptake inhibitor, NMN 100 μmol/1, and by oxytetracycline 100 μmol/1 and methoxamine 10 μmol/1. 5. ³H-NMN formation was inhibited to a small extent (by 30%) by hydrocortisone in a concentration which inhibited ³H-NMN formation in the myometrium by 84%. 6. It is concluded that the extraneuronal uptake of exogenous noradrenaline in the endometrium (a) resembles uptake in sympathetic nerves in its sensitivities to sodium ions and uptake inhibitors, and (b) resembles corticosteroid-sensitive extraneuronal uptake in that it has a low affinity for noradrenaline, but is linked with O-methylation in a fashion which renders uptake and O-methylation a relatively high-affinity, low-capacity system for removing noradrenaline.