Abstract

The antagonist-sensitive binding of [³H]mepyramine to a washed homogenate of the longitudinal muscle from guinea pig small intestine was largely that expected for binding to histamine H₁-receptors. However, at pH 8.1 comparison of the extent of the inhibition of 10 nM [³H]mepyramine binding by mepyramine and promethazine revealed the presence of secondary nonreceptor binding sites. These sites were not apparent on making comparison at pH 7.5. Analysis of antagonist inhibition curves showed that for certain antagonists, most notably mepyramine, the Hill coefficient was significantly less than unity. However, the Hill coefficients for some antagonists approximated to unity, and there was in no case any significant difference between the level of antagonist-sensitive binding and that sensitive to inhibition by 2 µM promethazine. Histamine, 2-pyridylethylamine, and 2-thiazolylethylamine gave inhibition curves with Hill coefficients much less than unity. These agonists also inhibited [³H]mepyramine binding to a greater extent than did 2 µM promethazine. There were only small differences in the position of antagonist inhibition curves when measured in 50 mM phosphate buffer and in Krebs-phosphate solution. However, substitution of Krebs-phosphate for phosphate buffer produced an apparent increase in the affinity of the agonists. For histamine and 2-pyridylethylamine this increase in affinity occurred mostly in the high-affinity component of the curves. Omission of Ca²⁺ and Mg²⁺ from the Krebs-phosphate had no significant effect on the position of the inhibition curve for histamine.