Histamine receptors in the human ureter.

The occurrence of histamine H1- and H2-receptors in the human ureter was studied by means of relatively selective agonists and antagonists of both kinds of receptors. Isolated preparations of small strips of human ureters removed during surgery were used. Histamine and the H1-agonist 2-aminoethylthiazole contracted the ureter in a dose-dependent fashion whereas the H2-agonists dimaprit and impromidine were ineffective. The H1-antagonist chlorpheniramine shifted to the right the dose-response curves to histamine and to 2-aminoethylthiazole with the kinetics of the competitive antagonism. Conversely the H2-antagonists cimetidine, metiamide and tiotidine potentiated the effect of histamine by a factor of 3 though high concentrations had to be used (25 - 30 micrograms/ml). Both H1 and H2-antagonists were not able to modify the basal tone and/or motility showed by ureteral strips. All the above data suggested that H1-receptors are predominant in the human ureteral muscle and the contraction induced by their stimulation completely mask the effect of the H2-receptors stimulation. Since H2-agonists were ineffective in basal conditions and H2-antagonists potentiated the effect of histamine which is spasmogenic, we may suggest that H2-receptors are less numerous than H1-receptors and their stimulation cause a slight relaxation of the ureteral muscle. This situation is not uncommon in other smooth muscle system (e.g. respiratory system and gastrointestinal tract).