Postnatal developmental changes in adrenergic receptor responses of the dog tracheal muscle

Age-related changes in responses of tracheal muscle strip to norepinephrine (NE), epinephrine (E), phenylephrine (PE), dopamine (DA) and isoproterenol (IP) were examined in dogs 1-55 days of age. NE, E, PE and DA contract tracheal muscle in dogs 1-12 days of age. NE- and E-induced contractions are potentiated by propranolol. Contractile responses are little affected by atropine, but are completely blocked by phenoxybenzamine or phentolamine. NE-, E-, PE-, and DA-induced contractions gradually decrease with increasing age. Acetylcholine (ACh) contracts and IP slightly relaxes the tracheal muscle at all ages studied. On ACh-contracted tracheal muscle, PE and DA at all ages, and E and NE in 1-18 day old animals, induce a superimposed contraction (SIC) while IP causes a superimposed relaxation (SIR) at all ages. After approximately day 18, E and NE induce SIR while PE and DA still elicit a SIC on ACh-contracted tracheal muscle. These results suggest that E, NE, PE and DA contract the tracheal muscle through alpha adrenergic stimulation; however, beta adrenoceptors seem to be present and functioning to relax the tracheal muscle. As age advances the alpha receptor responses progressively decline; however, alpha receptors still respond to adrenergic agonists at relatively lower level of sensitivity in older animals.