Histamine receptors in the human ureter

The occurrence of histamine H 1- and H 2-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 H 1-agonist 2-aminoethylthiazole contracted the ureter in a dose-dependent fashion whereas the H 2-agonists dimaprit and impromidine were ineffective. The H 1-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 H 2-antagonists cimetidine, metiamide and tiotidine potentiated the effect of histamine by a factor of 3 though high concentrations had to be used (25 − 30 g/ml). Both H 1 and H 2-antagonists were not able to modify the basal tone and/or motility showed by ureteral strips. All the above data suggested that H 1-receptors are predominant in the human ureteral muscle and the contraction induced by their stimulation completely mask the effect of the H 2-receptors stimulation. Since H 2-agonists were in-effective in basal conditions and H 2-antagonists potentiated the effect of histamine which is spasmogenic, we may suggest that H 2-receptors are less numerous than H 1-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).