AJP - Gastrointestinal and Liver Physiology, Vol 241, Issue 2 93-G97, Copyright © 1981 by American Physiological Society
Histamine receptor in bullfrog gastric mucosa
S. J. Hersey
The histamine receptor of isolated bullfrog gastric mucosa was
characterized in terms of respiration and acid secretory responses to
histamine antagonists and agonists. Cimetidine, a selective H2-antagonist,
showed competitive antagonism of histamine responses with a pA2 value of
6.55. In contrast, the H1-antagonist, mepyramine, showed inhibition only at
very high concentrations. Based on these results, the histamine receptor
would be classified as the H2 type. Measurements of agonist potency
sequence revealed a marked difference between the amphibian and mammalian
gastric histamine receptors. The selective H1-agonists, 2-pyridylethylamine
and 2-aminoethylthiozol, were found to be more efficacious than the
selective H2-agonists, dimaprit and impromidine. The lack of efficacy for
dimaprit and impromidine does not appear to be due to a lack of binding to
the receptor because these agents inhibit responses to histamine. For
dimaprit, the inhibition was found to be competitive with an apparent pA2
value of 5.37. These results indicate that there is a molecular difference
between H2-receptors in mammals versus amphibians.
