ATYPICAL SCHILD PLOTS WITH HISTAMINE H-1 RECEPTOR AGONISTS AND ANTAGONISTS IN THE RABBIT AORTA

Competitive antagonists of histamine H-1 receptor were investigated for their effects on histamine-induced responses in the rabbit aorta. Antazoline-induced antagonism gave linear Schild plots with slope equal 1, while the other antagonists ((+)-brompheniramine, (+/-)-chlorpheniramine, diphenhydramine and mepyramine) produced 'atypical' plots with slopes generally less than unity in the thoracic aorta. Schild plots obtained with these antagonists were evaluated using a two independent component model. The high affinity parameters thus estimated were compatible with those that have been reported for these antagonists. No such heterogeneity was observed in the abdominal aorta when diphenhydramine was investigated with different H-1 agonists. The results suggest the presence of at least two components in H-1-mediated responses in the thoracic aorta; these components are equally antagonized by antazoline, but differentially antagonized by the other antagonists used.