Histamine H1 receptor binding sites in mouse uterine horns

1. The specific binding of both [3H]histamine and [3H]pyrilamine to crude membrane preparations obtained from mouse uterine horns was studied with the aid of radioligand binding techniques. 2. Saturation isotherms and binding of 300 nM of [3H]histamine in the presence of increasing concentrations of unlabelled histamine suggested the presence of two populations of [3H]histamine binding sites (IC50: 56 +/- 5.6 nM and 100 +/- 9.8 microM, respectively). 3. Pyrilamine displaced the high affinity binding site (IC50: 100 +/- 10 nM), while cimetidine displaced the low affinity one (IC50: 98 +/- 10 microM). 4. Saturation curve analysis by using [3H]pyrilamine as radioligand, revealed that H1 binding site varied according to the predominance of sexual hormones (Kd: 29 nM; 102 nM and 130 nM in uterine membranes from ovariectomized, diestrous and estrous, respectively). At the end of pregnancy Kd was found to be 71.1 nM.