Changes in the response of guinea-pig airways in vivo and in vitro to cimetidine and propranolol during development.

1 Airway responses were examined in isolated tissues and in whole animal preparation of female albino guinea-pigs of known age. 2 Tone induced with acetylcholine in tracheal and bronchial tissues from young and old female guinea-pigs was not reduced by dimaprit or 4-methyl histamine even in tissues pretreated with mepyramine maleate. 3 Antagonism of H2-receptors with cimetidine did not affect the potency or efficacy of histamine in tracheal tissues from animals of either age group. 4 After cimetidine treatment the potency of histamine was increased in bronchial tissues from old but not young animals. The sensitizing effect was still demonstrable in tissues incubated with indomethacin. 5 In vivo airway sensitivity to threshold concentrations of histamine in animals from either age group was unaffected by cimetidine treatment. 6 Propranolol enhanced airway responses to histamine aerosols in young but not old guinea-pigs. 7 Cimetidine was without effect on histamine sensitivity in young guinea-pigs after propranolol treatment but significantly reduced airway sensitivity to histamine in old guinea-pigs. 8 Our data show that (a) H2-receptors are of no physiological significance for airway responses to histamine in vitro or in vivo and (b) during development the modulating actions of catecholamines upon airway responses are significantly reduced.