Pertussis toxin increases isoproterenol induced relaxation in field-stimulated ileum

We investigated the effects of pertussis toxin on contraction in field-stimulated guinea pig ileum in the absence and presence of isoproterenol. Field-stimulation elicited pertussis toxin-insensitive contractions. Cumulative addition of isoproterenol produced a maximal 52% reduction in the contractile response. Following pertussis toxin-treatment, the maximal inhibitory effect of isoproterenol increased to 83%. Pertussis toxin had no effect on the ability isoproterenol to inhibit contractions elicited by histamine agonists. Our results suggest that the increased effectiveness of isoproterenol in pertussis toxin-treated ileum is due to an uncoupling of the muscarinic M2 receptor contractile mechanism.