Arterial plasma histamine concentrations were measured after exercise in 10 subjects with extrinsic atopic asthma, 10 who were non-atopic and non-asthmatic and seven who were atopic but non-asthmatic, by a single isotope radioenzymatic assay. Significantly higher plasma histamine concentrations were found in the asthmatic subjects before exercise than in the non-atopic controls (p less than 0.05). The mean histamine concentration rose after exercise in all groups but the increased levels were not significantly different from pre-exercise values. Similarly, mean circulating basophil counts increased in all groups after exercise, and a highly significant correlation was found between basophil counts and whole blood histamine concentrations (p less than 0.001). In vitro studies showed that there was a significant correlation between the number of basophils added to plasma samples and the concentrations of histamine subsequently detected. Although the mean concentrations of plasma histamine and whole blood histamine and number of basophils in the atopic control group were intermediate between those found in the atopic asthmatic and non-atopic controls, none of the differences was significant. Venous plasma histamine concentrations after exercise were measured in a further five subjects with extrinsic atopic asthma and five non-atopic, non-asthmatic subjects before and after exercise with the more sensitive and specific double isotope radioenzymatic assay. Concentrations of plasma histamine measured by this assay were about one tenth of those measured by the single isotope radioenzymatic assay. Although a small rise in mean plasma histamine concentration occurred in both groups after exercise there was no significant difference in these levels either between or within the groups. We find no evidence from these studies on measurement of peripheral blood histamine to support the hypothesis that mast cell mediator release is implicated in the pathogenesis of exercise induced asthma.
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