H1 and H2 histamine receptors in the in vitro nasal mucosa

The effect of histamine was studied on canine nasal blood vessels with an in vitro muscle tension measuring technique. Observations suggest that histamine has at least four different effects: 1) inhibits the contraction induced by nerve terminal stimulation, 2) relaxes the smooth muscle under sustained contraction, 3) contracts the smooth muscle, 4) enhances (nor)epinephrine-induced contraction. Both H1 and H2 histamine receptors were identified in the mucosal blood vessels of the canine nasal septum. H1 receptors were present at the surface of vascular smooth muscle and mediated the contractile response (3) due to opening calcium influx channels. H2 receptors existed at both sympathetic nerve terminals and the muscle cell membrane. Stimulation of either H2 receptor caused muscle relaxation. The presynaptic H2 receptors mediated vasodilation through the inhibition of norepinephrine release from the nerve endings. Postsynaptic H2 receptors caused direct muscle relaxation possibly through an increase in cyclic AMP. Both H1 and H2 antagonists worked well, whereas agonists did not properly produce theoretically expected results in these specimens.