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[Inquiry into the mechanism of long-acting bronchodilators]
1Institute of Lasermedicine, Heinrich-Heine University, Duesseldorf 40225, Germany.
Objective:
To compare the intrinsic activity and the affinity of long-(formoterol and salmeterol) and short-acting (fenoterol and salbutamol) bronchodilators with isoproterenol in membranes of calf tracheal smooth muscle.
Methods:
Membranes were prepared after isolation of intact cells using a mixture of collagenase and pronase. Affinities were measured with radioligand binding. Intrinsic activities (IA) of agonists and effective concentrations of agonists used for half maximum stimulation of adenylate cyclase (AC) (EC50) were determined as stimulants of beta 2-adrenoceptor coupled AC activity using alpha-22P-ATP as substrate.
Results:
The results showed that IA estimated as fraction of the maximum effect by isoproterenol (1.00) were 1.00, 0.76, 0.81 and 0.83 for formoterol, salmeterol, fenoterol and salbutamol respectively. Affinities measured as dissociation constants (pKD) exhibited higher affinities for long-acting (formoterol: 8.02, salmeterol: 8.43) than for short-acting agonists (fenoterol: 5.84, salbutamol: 5.84). Relative effects of agonists (pEC50) were much higher for formoterol (9.10) and salmeterol (8.70) than for fenoterol (6.69) and salbutamol (6.78).
Conclusions:
The results indicated that formoterol was characterized as a full agonist with a 20-fold higher affinity than isoproterenol whereas the other agonists acted as partial agonists. Significant amplifications of receptor signalling quantified as difference of values between pEC50 and pKD was found for the agonists in the tested receptor membranes. It is suggested that high IA, affinity and the signal amplification of formoterol might play a role in its long-acting effect.