Effect of cimetidine on exogenous histamine inhibition of histamine release in vivo.

We previously reported that exogenous histamine inhibits in vivo histamine release and eosinophil accumulation in ragweed-challenged skin sites of sensitive human subjects. The mechanism(s) involved were unclear. In this study, we repeated similar approaches in four of the same subjects pretreated for 3 days with cimetidine, an H2 receptor antagonist. The pattern of exogenous histamine effects was now different in that local exogenous histamine (50 ng/ml) did not significantly alter ragweed-induced mast cell alteration, histamine release, or the degree of eosinophil accumulation in skin challenge sites. These findings suggest that the observed exogenous histamine inhibitory effects may be mediated through the H2 receptor.