Pharmacological characterization of the muscarinic receptor antagonist, glycopyrrolate, in human and guinea-pig airways

1. In this study we have evaluated the pharmacological profile of the muscarinic antagonist glycopyrrolate in guinea-pig and human airways in comparison with the commonly used antagonist ipratropium bromide. 2. Glycopyrrolate and ipratropium bromide inhibited EFS-induced contraction of guinea-pig trachea and human airways in a concentration-dependent manner. Glycopyrrolate was more potent than ipratropium bromide. 3. The onset of action (time to attainment of 50% of maximum response) of glycopyrrolate was similar to that obtained with ipratropium bromide in both preparations. In guinea-pig trachea, the offset of action (time taken for response to return to 50% recovery after wash out of the test antagonist) for glycopyrrolate (t1/2 [offset]=26.4+/-0.5 min) was less than that obtained with ipratropium bromide (81.2+/-3.7 min). In human airways, however, the duration of action of glycopyrrolate (t1/2 [offset]>96 min) was significantly more prolonged compared to ipratropium bromide (t1/2 [offset]= 59.2+/-17.8 min). 4. In competition studies, glycopyrrolate and ipratropium bromide bind human peripheral lung and human airway smooth muscle (HASM) muscarinic receptors with affinities in the nanomolar range (K1 values 0.5-3.6 nM). Similar to ipratropium bromide, glycopyrrolate showed no selectivity in its binding to the M1-M3 receptors. Kinetics studies, however, showed that glycopyrrolate dissociates slowly from HASM muscarinic receptors (60% protection against [3H]-NMS binding at 30 nM) compared to ipratropium bromide. 5. These results suggest that glycopyrrolate bind human and guinea-pig airway muscarinic receptors with high affinity. Furthermore, we suggest that the slow dissociation profile of glycopyrrolate might be the underlying mechanism by which this drug accomplishes its long duration of action.