Regional Variation in the Sensitivity of Longitudinal Smooth Muscle to Histamine at H1-receptors in Guinea-pig Ileum and Colon

The sensitivity of the distal ileum, proximal colon, medial colon, and distal colon of the guinea-pig to histamine has been evaluated. The rank order of sensitivity was ileum greater than medial colon greater than proximal colon approximately equal to distal colon. The mean -logEC50 values at receptors in the ileum, medial, proximal, and distal colon were 6.74, 6.18, 5.79, and 5.72, respectively. The apparent dissociation constant for the interaction of histamine with receptors in the various regions was determined. The -log Kd values at receptors in the ileum, proximal colon, medial colon, and distal colon were 4.68, 4.65, 4.62, and 4.44, respectively. The mean apparent -log Kd values for the antagonism of histamine by mepyramine were 9.0, 9.0, 9.1, and 8.9 for receptors on the ileum, proximal, medial, and distal colon, respectively. The results of these experiments provide no evidence that histamine receptors in the colon are distinguishable from H1-receptors as characterized on the ileum. The differences in sensitivity to histamine in the various regions of the intestine may be due to differences in the density of H1-receptors.