Pancuronium masks the prejunctional muscarinic autoreceptor in guinea pig tracheal smooth muscle.

The effect of pancuronium pretreatment on the function of the prejunctional muscarinic receptor in guinea-pig trachea was studied by using electrical field stimulation (EFS). The effects of cumulative doses of the muscarinic M2 receptor antagonist gallamine were investigated in tracheal smooth muscle strips from guinea-pigs after addition of pancuronium in vitro and in strips from guinea-pigs which had been pretreated with doses of pancuronium that caused 100% neuromuscular blockade. The results of both types of experiments were compared to those of control groups of the same size. In all strips a dose response curve with cumulative doses of methacholine was made before EFS was switched on. No differences were found between the mean pD2 value and slope of the concentration-response curves of untreated guinea-pigs and animals treated with anaesthetics and pancuronium. The animals showed variable responses to pancuronium. The bath concentration of pancuronium which decreased the EFS-induced contraction to half the original value varied between 14-61 microM. The intravenous dose necessary to paralyze the muscles, varied among the different guinea-pigs from 0.017-0.085 mg.kg-1. The EFS-induced contraction for the concentration range of gallamine 0.32 microM-0.32 mM was found to differ significantly between the strips treated with pancuronium in the organ bath and their control group. For the guinea-pigs anaesthetized and pretreated with pancuronium a significant difference with control was observed at gallamine concentrations ranging from 0.032-0.32 mM. These results show that pancuronium, added to the organ bath as well as administered intravenously to the guinea-pig, masked the inhibitory muscarinic receptor.