Relaxation kinetics of formoterol and salmeterol in the guinea pig trachea in vitro.

The mechanisms producing long duration of action for formoterol and salmeterol are not fully understood. The aim of the current study was to examine how the concentration of long and short acting beta 2-adrenoceptor agonists affects their relaxation kinetics in airway smooth muscle. Onset (time to peak relaxation) and offset of action (reassertion of reversible relaxation followed repeated beta-adrenoceptor blockade and washout) were measured in the guinea pig trachea precontracted postjunctionally by carbachol 0.3 microM in vitro. At 10-1,000% (C10-C1,000) of the maximally effective concentration (C100: 150 nM formoterol, 10 microM salbutamol, 30 microM salmeterol), salbutamol had a shorter time to peak relaxation than did salmeterol. Formoterol and salmeterol had a similar time to peak relaxation at C10, but, in contrast to salmeterol, formoterol's time to peak relaxation became markedly shorter and similar to that of salbutamol as the concentration was increased up to C1,000. Significant reversible reasserted relaxation was demonstrated for salmeterol alone at C10. At C30-C1,000, however, salmeterol produced irreversible relaxation only, in spite of repeated beta-adrenoceptor blockade by sotalol 10 microM followed by washout. In contrast, formoterol produced an increasing reversible reasserted relaxation at C30-C1,000. Salbutamol produced significant, reversible reasserted relaxation at C1,000 only. In conclusion, the concentration determines the onset and offset of action for formoterol and to a lesser extent for salbutamol, but not for salmeterol. To cause sustained action, a submaximally effective concentration is sufficient for salmeterol, whereas formoterol requires a maximally effective concentration. The rank order of concentration dependence for the relaxation kinetics is not paralleled by the rank order of lipophilicity for formoterol, salbutamol, and salmeterol. Therefore, factors other than lipophilicity may also play a role in determining the relationship between concentration and relaxation kinetics for the investigated beta 2-agonists.