Hyperosmolarity but not histamine evokes secretion of nasal fluid in the rat

Histamine (dihydrochloride or free base), mannitol or sodium chloride applied topically to the nasal mucosa of anaesthetized rats stimulated the secretion of fluid in a dose-dependent manner. Compound 48/80 applied in the same way and histamine given s.c. failed to evoke secretion. The secretion evoked by topically applied histamine was resistant to pretreatment with H1 and H2 receptor antagonists i.p. Furthermore, the secretion evoked by histamine and sodium chloride was resistant to pretreatment with atropine i.p. or lidocaine topically. The secretion evoked by mannitol or by sodium chloride was not reduced by H1 receptor antagonists and was not significantly different from that obtained after challenge with histamine of equivalent osmolality. We conclude that the response to topically applied histamine reflects the hyperosmolarity of the compounds applied and not the effect of histamine on histamine receptors.