Kinetics of the O-methylating system for isoprenaline in the trachea and aorta of rabbit

Summary

• 1. Segments of tracheal smooth muscle or aorta from rabbits pretreated with reserpine (1 mg/kg) were incubated in ³H-isoprenaline (0.5–60 μmol/l). Steady-state rates of O-methylation were determined by measuring the formation of 3-O-methylisoprenaline (OMI) after incubation of tracheal and aortic tissues for 30 min and 10 min, respectively.

• 2. The steady-state O-methylation of isoprenaline in rabbit trachea was saturable, at least up to 60 μmol/l isoprenaline. In rabbit aorta, the O-methylation appeared to be saturable up to 30 μmol/l isoprenaline, but the rate of O-methylation increased for higher concentrations. The K _m values for the saturable component of O-methylation were 11.8 μmol/l in trachea and 3.03 μmol/l in aorta. The V _max values were 0.51 nmol·g⁻¹·min⁻¹ in trachea and 0.56 nmol·g⁻¹·min⁻¹ in aorta.

• 3. In tissues incubated in 0.5 μmol/l isoprenaline, 100 μmol/l corticosterone caused 78% inhibition of OMI formation in trachea and 86% inhibition in aorta. There was no inhibition of OMI formation by 100 μmol/l corticosterone in tracheal or aortic tissues incubated in 60 μmol/l isoprenaline.

• 4. Model calculations showed that the experimental results in trachea and aorta (3. above) were consistent with (a) entry of isoprenaline into the cells in the tissues by extraneuronal uptake and diffusion, and (b) exposure of the isoprenaline to intracellular catechol-O-methyltransferase with V _{max enzyme} ≪ V _{max uptake}.