Topical application of the H2-histamine receptor agonist,
dimaprit (S-[4-N,N-dimethylaminopropyl]isothiourea), produced eosinophil chemotaxis into the anterior segment of rabbit eyes only when an H2-antagonist was co-administered.
Nordimaprit (S-[4-N,N-dimethylaminoethyl]isothiourea), a structural homologue of
dimaprit that lacked activity at
histamine receptors, produced eosinophil chemotaxis whether or not an H2-antagonist was co-administered. Onset of eosinophil chemotaxis began after 2 or more days of treatment, and was accompanied by
corneal edema, opacification, and ocular
inflammation. There was no concurrent
eosinophilia in the peripheral blood or in the conjunctiva. The response occurred in pigmented and albino rabbit eyes, and was facilitated by prior co-administration of
proparacaine eye drops. Another
dimaprit homologue without activity at
histamine receptors, homodimaprit (S-[4-N,N-dimethylaminobutyl]isothiourea), did not produce eosinophil chemotaxis when applied topically, nor did the H2-agonists
impromidine,
histamine, or
4-methylhistamine, whether co-administered with an H2-antagonist or not. It was concluded that
dimaprit and
nordimaprit produced a selective eosinophil chemotaxis unrelated to H1- and H2-histamine receptor activity. However, the H2-agonist activity of
dimaprit appeared to inhibit this response unless neutralized by an H2-antagonist. Topical application of
dimaprit with an H2-antagonist or
nordimaprit alone may allow large numbers of non-degranulated eosinophils, free of other cell types, to be harvested from the aqueous humor.