The affinity constant for doxepin obtained from inhibition of histamine‐induced contraction of guinea‐pig intestinal smooth muscle at 30°C was 2.6 ± 0.18 ± 1010m−1. The slope of a Schild plot was not significantly different from unity.
The affinity constant of doxepin did not vary markedly with temperature. At 37°C it was 3.75 ± 0.02 ± 10m−1 and at 25°C 2.1 × 10m−1.
Doxepin was a competitive inhibitor of [3H]‐mepyramine binding to guinea‐pig cerebellar homogenates. The affinity constant derived for doxepin at 30°C was 1.12 ± 0.45 ± 10m−1.
Hill coefficients for curves of doxepin or mepyramine inhibition of [3H]‐mepyramine binding in guinea‐pig cerebellum, cerebral cortex and hippocampus did not differ significantly from unity.
The mean affinity of mepyramine for histamine H1‐receptors in rat brain homogenates at 30°C was 3.5 × 108m−1. Hill coefficients for curves of doxepin or mepyramine inhibition of [3H]‐mepyramine binding to homogenates of rat cerebral cortex or rat whole brain were near unity.
These studies provide no evidence that doxepin binds preferentially to a sub‐class of histamine H1‐receptors in rat brain.
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