1,4-Dithiothreitol-induced changes in histamine H1-agonist efficacy and affinity in the longitudinal smooth muscle of guinea-pig ileum.

The effect of 1,4-dithiothreitol (DTT) on histamine H1-receptor agonist affinity and efficacy has been investigated in longitudinal muscle strips of guinea-pig ileum. Exposure of ileal smooth muscle to DTT significantly increased the maximal responses to the partial agonists SKF71473 and DE-2PEA, indicative of an increase in agonist efficacy. This effect was paralleled by a small decrease in EC50 values. In contrast, DTT produced a parallel displacement of the concentration-response curve to the full agonist histamine in the same muscle strips. Studies in which phenoxybenzamine and benzilylcholine mustard were used to reduce the maximum response to histamine suggested that DTT altered both agonist affinity and efficacy. The affinity constant for histamine, calculated by the method of Furchgott & Bursztyn (1967), increased by 2.7 fold in the presence of DTT. Furthermore, agonist efficacy also appeared to increase in the presence of DTT since the maximum response to histamine following phenoxybenzamine treatment increased on application of DTT. [3H]-mepyramine binding studies confirmed that DTT increased agonist affinity. DTT produced a significant parallel shift to the left of the displacement curves for histamine, 2-methylhistamine, 2-pyridylethylamine and 2-thiazolylethylamine. The results of this study therefore suggest that DTT potentiates H1-receptor-mediated contractile activity in guinea-pig ileum by increasing both agonist efficacy and affinity.