Search

Salmeterol and formoterol, two new long-acting beta 2-agonists were equipotent (values of negative log molar concentration eliciting half-maximal effect [pD2] 9.2 +/- 0.03 and 8.9 +/- 0.03, respectively) in relaxing maximally contracted guinea pig tracheal spirals (histamine, 100 microM). Both agonists were 10 times more potent than L-isoproterenol and fenoterol and 100 times more potent than albuterol. L-Isoproterenol and fenoterol induced > 90% relaxation (percentage of maximal aminophylline relaxation). Formoterol and albuterol were equally efficacious. Formoterol was more efficacious (86 +/- 5%) than salmeterol (62 +/- 3%) or soterenol (59 +/- 3%). In minimally contracted tissues (10 microM histamine), agonist potencies increased 10-fold and relaxation was complete. In [125I]iodocyanopindolol-labeled bronchial membranes, formoterol and salmeterol induced high-affinity states of the beta 2-receptor (pKh 9.6 +/- 0.4 and 10.4 +/- 0.7, respectively), the former inducing a higher percentage (57 +/- 6 versus 28 +/- 4, p < 0.05). Only low-affinity binding (pKI) was observed when guanine nucleotide was present. pD2 values were similar to pKh values and relative efficacies significantly correlated with percentage of pKI sites. Formoterol and salmeterol were highly selective for the beta 2 versus beta 1-subtype (pKI values were 8.2 +/- 0.09 and 6.25 +/- 0.06 and 8.3 +/- 0.04 and 5.7 +/- 0.04, respectively). Albuterol (5.83 +/- 0.06 and 4.71 +/- 0.16) and fenoterol (6.33 +/- 0.07 and 5.67 +/- 0.05) were less selective. These results can explain the potencies and efficacies of salmeterol and formoterol in humans.

Agonist binding to the beta 2-adrenoceptors and its mapping were studied using the newly developed radioligand [3H]formoterol. The results of [3H]formoterol saturation binding and formoterol inhibition of [3H]formoterol binding were consistent with binding to a single class of receptors (Kd = 1.34 +/- 0.15 nM, Bmax = 154.9 +/- 8.0 fmol/mg protein in guinea pig lung membranes, n = 8; Kd = 1.05 +/- 0.17 nM, Bmax = 67.8 +/- 8.1 fmol/mg protein in human lung membranes, n = 5) and competition assays with other agonists and antagonists disclosed only a single class of site. The nonhydrolyzable GTP analogue GTP gamma S caused a reduction in both Kd and Bmax, indicating that the receptors labelled by [3H]formoterol are coupled to a guanine nucleotide binding regulatory protein. Receptor mapping of [3H]formoterol binding sites shows that beta 2-adrenoceptors were widely distributed in both guinea pig and human lung, with dense labelling over airway epithelium and uniformly over alveolar walls, and sparse labelling of airway and vascular smooth muscle. In addition, submucosal glands were also sparsely labelled in human bronchus. The distribution of beta 2-adrenoceptors was similar to the pattern previously described with non-selective radiolabelled antagonists in the presence of selective beta 1-adrenoceptor antagonists.

Platelet-activating factor (PAF) administered i.v. or by aerosol to guinea pigs elicited an increase in airway responsiveness to both acetylcholine and histamine when compared with guinea pigs exposed to the vehicle bovine serum albumin (BSA). After i.v. PAF, the ED100 for acetylcholine and histamine was 5.4 and 3.4 micrograms/kg, respectively, in comparison with 17 and 6 micrograms/kg, respectively, before PAF, representing approximately a 3- and 2-fold increase in responsiveness, respectively. After aerosol PAF (500 micrograms), the ED100 for acetylcholine and histamine was 13 and 7 micrograms/kg, respectively, whereas after aerosolized BSA, the ED100 for acetylcholine and histamine was 23 and 12 micrograms/kg, respectively, representing approximately a 2-fold increase in responsiveness. However, airway smooth muscle obtained from these PAF- or BSA-treated animals did not exhibit any differences in contractile response to histamine or acetylcholine in vitro. Likewise, there were not significant differences in the binding affinity or receptor density between PAF- and BSA-treated tissues with [3H]quinuclidinylbenzilate or [3H]pyrilamine binding, which were used to identify muscarinic and H1-histamine receptors, respectively. Furthermore, histamine and carbachol-induced phosphoinositide hydrolysis was similar in PAF- and BSA-treated tracheal smooth muscle preparations. Thus, PAF induces airway hyperresponsiveness in the guinea pig that is not related to changes in airway smooth muscle or to changes in muscarinic and histamine (H1) receptor density or function.