Suppression by Muscarinic M2 Stimulation of the Contractile Mechanism via Histamine H, Receptors in the Longitudinal Muscle of Guinea Pig Ileum

The longitudinal muscle of the guinea pig ileum is well known to contract in response to stimulation of either muscarinic or hista-mine H1 receptors. However, we found that simuftaneous appli-cation of agonists to both receptors, methacholine (MeCh) and histamine (Hist), at their maximum concentrations (both at 1 0 M), induced a contraction of the muscle with a pattern very similar to that observed with MeCh alone. In the muscle con-tracted by MaCh, a muscannic antagonist, atropine, caused a transient relaxation, but a histamine H1 blocker, pynlamine, had no effect on the tension. Experiments using pirenzepine indicated that muscarinic-M1 receptors did not seem to be involved in the mechanism of this phenomenon. The atropine-induced relaxation was not affected byfhentolamine (1 0 M), proprancol (1 0 M) or tetrodotoxin (1 0 M). After pretreatment of the muscle wit