Beta-adrenoceptors in human tracheal smooth muscle: characteristics of binding and relaxation

Specific binding of [125I]-(-)-cyanopindolol to human tracheal smooth muscle membranes was saturable, stereo-selective and of high affinity (Kd = 5.3 +/- 0.9 pmol/l and RT = 78 +/- 7 fmol/g tissue). The beta 1-selective antagonists atenolol and LK 203-030 inhibited specific [125I]-(-)-cyanopindolol binding according to a one binding site model with low affinity in nearly all subjects, pointing to a homogeneous beta 2-adrenoceptor population. In one subject using LK 203-030 a small beta 1-adrenoceptor subpopulation could be demonstrated. The beta-mimetics isoprenaline, fenoterol, salbutamol and terbutaline recognized high and low affinity agonist binding sites. Isoprenaline's pKH- and pKL-values for the high and low affinity sites were 8.0 +/- 0.2 and 5.9 +/- 0.3 respectively. In functional experiments isoprenaline relaxed tracheal smooth muscle strips having intrinsic tone with a pD2-value of 6.63 +/- 0.19.