Effects of 3-methylhistamine and Phenylethylamine on Histamine Action on Isolated Guinea-pig Trachea Rings

It is well known that histamine produces constriction via H1 receptors and decreases tracheal smooth muscle tone via H2 and H3 receptors. In addition, it has already been reported that 3-methylhistamine and phenylethylamine are competitive antagonists of histamine N-methyl-transferase (HMT), the enzyme responsible for rapid inactivation of histamine. Our results suggest the possibility that 3methyl-histamine and phenylethylamine as competitive antagonists of histamine N-methyl-transferase lead to potentiation of histamine induced constriction of isiolated guinea-pig trachea, probably due to the decrease of histamine methylation and consequent inactivation. In as much, 3-methyl-histamine and phenylethylamine had no effect on the basal tone of isolated trachea smooth muscle, as well as on other mechanisms leading to increased responsiveness of guinea-pig tracheal smooth muscle (acetylcholine, KCl, electro stimulation).