Beta-receptors during aging in respiratory tissues.

Specific [125I] hydroxybenzylpindolol binding (33 fmol/mg protein) was detected in tracheal tissues from middle aged (417 g) and old (757 g) guinea pigs but binding was not measurable in tracheal tissues from young (118 g) animals. Similarly, receptor density increased in bronchial and parenchymal tissues during development but receptor affinity did not change. For any age, the receptor densities were parenchyma greater than bronchi greater than trachea (20/7/1); receptor affinities were identical. The potency of l-isoproterenol in relaxing bronchial muscle was reduced during development. In vivo, salbutamol reduced airway reactivity to histamine and was most potent in animals exhibiting airway hyperreactivity. In addition, l-propranolol sensitized airway muscle to the bronchoconstrictor effects of histamine in young guinea pigs; in old guinea pigs of the same airway reactivity, l-propranolol did not affect the induced bronchoconstriction. Our data suggest that there is a reduced sensitivity of airway muscle to catecholamines during development which may be due, in part, to increased density of beta-adrenoceptors which are not involved in eliciting the physiological response.