Functional and binding characteristics of long-acting beta(2)-agonists in lung and heart

Salmeterol and formoterol, two new long-acting beta(2)-agonists were equipotent (values of negative log molar concentration eliciting half-maximal effect [pD(2)] 9.2 +/- 0.03 and 8.9 +/- 0.03, respectively) in relaxing maximally contracted guinea pig tracheal spirals (histamine, 100 mu M). 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 mu M histamine), agonist potencies increased 10-fold and relaxation was complete. In [I-125]iodocyanopindolol-labeled bronchial membranes, formoterol and salmeterol induced high-affinity states of the beta(2)-receptor (pK(h) 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 (pK(l)) was observed when guanine nucleotide was present. pD(2) values were similar to pK(h) values and relative efficacies significantly correlated with percentage of pK(h) sites. Formoterol and salmeterol were highly selective for the Pt versus beta(1)-subtype (pK(l) 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.