Distribution and quantitative developmental changes in guinea pig pulmonary β-receptors

Studies using tissue homogenates have demonstrated an increase in pulmonary β-receptors during development. However, techniques using disrupted tissue have not permitted the precise anatomic localization of pulmonary β-receptors or identification of structures where increases occur. Using L-[³H]dihydroalprenolol, β-receptors were radioautographically localized and quantitated in sections of newborn (NB) and adult (A) guinea pig lung. Scatchard analysis showed a single class of binding sites with a maximum binding capacity of 189 ± 3 (NB) and 305 ± 37 (A) fmol·mg^-1 protein (P < 0.02). Binding was of high affinity with the dissociation constant (K(d)) = 1.46 ± 0.2 (NB) and 1.26 ± 0.3 (A) nM (NS). The majority of β-receptors were localized in alveolar wall and airway epithelia (alveolar ≥ bronchiolar > bronchial) (P < 0.0001). Airway and vascular smooth muscle had significantly fewer demonstrable β-receptors. The increased number of β-receptors in the adult appeared to be due primarily to a 2.0 ± 0.12-fold increase in alveolar wall and airway epithelia as opposed to only a 1.3 ± 0.18-fold increase in the already low number in airway and vascular smooth muscle (P < 0.05). While apparent receptor density may not necessarily correlate with physiological response or importance, radioautographic localization of pulmonary β-receptors may significantly enhance our understanding of their role in normal and pathologic states.