Acetylcholine-induced desensitization of the contractile response to histamine in guinea pig ileum is prevented by either pertussis toxin treatment or by selective inactivation of muscarinic M-3 receptors

We have studied the role of M-2 and M-3 muscarinic receptors in acetylcholi ne-mediated desensitization of the contractile response to histamine in the guinea pig ileum. Treatment of the isolated ileum with acetylcholine (30 m uM) for 20 min caused a marked desensitization of the contractile response to histamine. When measured 5 min after washout of acetylcholine, the EC,, value of histamine increased 5.8-fold compared with that estimated before a cetylcholine treatment, whereas the maximal response was unaffected. This s hift in the EC50 value of histamine was maximal at the earliest time measur ed after acetylcholine treatment (5 min), and normal sensitivity recovered in approximately 20 min. Acetylcholine-induced desensitization was prevente d by uncoupling of M-2 receptors from G(i) with pertussis toxin or by selec tive inactivation of M-3 receptors with N-2-chloroethyl-4-piperidinyl diphe nylacetate (4-DAMP mustard). The shifts in the EC50 values of histamine mea sured 5 min after acetylcholine treatment were only 2.0- and 1.8-fold in pe rtussis toxin- and 4-DAMP mustard-treated ilea, respectively. Both pertussi s toxin- and 4-DAMP mustard-treatment had little or no effect on histamine- induced contractions in control ileum. Measurement of histamine-stimulated inositol phosphate accumulation in the longitudinal muscle of the ileum sho wed little or no inhibitory effect of prior exposure to acetylcholine, indi cating that the majority of the heterologous desensitization occurs downstr eam from phospholipase C beta activation. Collectively, our results suggest that activation of both M-2 and M-3 receptors is required for heterologous desensitization of histamine-mediated contractions in the guinea pig ileum .