Vagal effects on histamine, carbachol, and prostaglandin F2 alpha responsiveness in the dog

To investigate whether an individual dog's responsiveness to histamine correlates with its responsiveness to other bronchoconstrictor agents and to investigate whether varying vagal effects account for the previously described range of histamine responsiveness, we compared dose-effect relationships of histamine to those of two pharmacological dissimilar agents, carbachol and prostaglandin F2 alpha before and after vagal blockade. There was a highly significant correlation between histamine and both carbachol (P less than 0.001) and prostaglandin F2 alpha (P less than 0.001) responsiveness. The range of responsiveness to prostaglandin F2 alpha was greater than that for histamine or carbachol. When histamine and carbachol were given simultaneously, a purely additive effect was found. Vagal blockade had no significant effect on histamine or carbachol responsiveness, but significantly diminished the responsiveness to prostaglandin F2 alpha; however, it neither narrowed the range nor changed the rank order of responsiveness. We conclude that the range of responsiveness is not specific for any one agent and that vagal mechanisms do not play a role in producing this range.