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

Abstract Specific binding of [ 125 I]-(−)-cyanopindolol to human tracheal smooth muscle membranes was saturable, stereo-selective and of high affinity (K d =5.3±0.9 pmol/l and R T =78±7fmol/g tissue). The β 1 -selective antagonists atenolol and LK 203-030 inhibited specific [ 125 I]-(−)-cyanopindolol binding according to a one binding site model with low affinity in nearly all subjects, pointing to a homogeneous β 2 -adrenoceptor population. In one subject using LK 203-030 a small β-adrenoceptor subpopulation could be demonstrated. The beta-mimetics isoprenaline, fenoterol, salbutamol and terbutaline recognized high and low affinity agonist binding sites. Isoprenaline's pK H - and pK L - 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 pD 2 -value of 6.63±0.19.