Maturation of catecholamine response and extraneuronal uptake in rabbit trachea

The present study investigated whether maturational changes in extraneuronal catecholamine clearance accounted for the ontogenetic attenuation of isoproterenol responsiveness in rabbit tracheal segments. Following half-maximal contraction with acetylcholine (ACh) or KCl, tracheal ring segments (TS) isolated from newborn, 1-mo, and adult rabbits were relaxed with cumulative administration of isoproterenol. With postnatal maturation, there occurred a significant decrease in both maximal relaxation and sensitivity to isoproterenol in both ACh- and KCl-contracted TS. Inhibition of extraneuronal catecholamine uptake with methylprednisolone resulted in enhanced sensitivity to isoproterenol in 1-mo and adult, but not in newborn tracheal segments. In separate studies, extraneuronal catecholamine uptake was directly assayed in tracheal segments isolated from rabbits of similar age and pretreated with neuronal uptake and monoamine oxidase inhibitors. Maximal extraneuronal uptake of [3H]norepinephrine significantly decreased with age in tracheal tissues. In contrast, the sensitivity of the uptake process to norepinephrine increased with age. The relative catecholamine reserve markedly decreased with age in rabbit tracheal segments and was directly related to the maximal degree of relaxation elicited with the beta-adrenergic catecholamine, isoproterenol, in age-matched TS precontracted with either ACh or KCl. These findings suggest that, with maturation, extraneuronal uptake more effectively competes with beta-adrenoreceptor stimulation and, accordingly, may contribute to the ontogenetic attenuation of beta-adrenergic responsiveness in rabbit tracheal tissues.