Effect of systemically administered H1- and H2-receptor antagonists on nasal blood flow as measured with laser Doppler flowmetry in a provoked allergic reaction.

Twelve subjects with seasonal allergic rhinitis were challenged topically with birch pollen extract in a double-blind, double-dummy, placebo-controlled, randomized cross-over study. Pre-treatment was performed with either a selective histamine-1 (H1-) antagonist (terfenadine), a selective H2-antagonist (cimetidine), a combination of these drugs or a placebo. Nasal mucosa microcirculatory blood flow was measured with the use of laser Doppler flowmetry. The allergen challenge induced a decrease in the microcirculatory blood flow of the nasal mucosa. Pre-treatment with the H1-antagonist inhibited this effect and allergic symptoms, while pre-treatment with the H2-antagonist did not. No signs of an additive effect were seen after combination of the antagonists. Thus, H1-receptors but not H2-receptors, seem to be of importance in the pathophysiology of the allergic rhinitis.