The influence of ageing on muscarinic receptors, beta-adrenoceptors and adenylate cyclase activity in the bovine lung.

Muscarinic and beta-adrenoceptors were identified in airway epithelium, smooth muscle and lung parenchyma from Holstein-Friesian calves and cows and were characterized with [3H]quinuclidinyl benzilate and [3H]dihydroalprenolol, respectively. The muscarinic receptor density in the smooth muscle of cows (Bmax = 4803 +/- 245 fmol/mg protein) was 33% greater (p < 0.01) than in calves. Low receptor numbers were detected in the epithelium and parenchyma. In both calves and cows, the density of epithelial beta-adrenoceptors was twice as high as in smooth muscle and parenchyma. The quantity of beta-adrenoceptors in the tracheal epithelium (Bmax = 994 +/- 83 fmol/mg protein) and smooth muscle (Bmax = 492 +/- 41 fmol/mg protein) in cows was respectively 37% (p < 0.001) and 35% (p < 0.01) lower than in calves. Adenylate cyclase (AC) assays indicated that the basal and the (-)-isopropylnoradrenaline- (ISO-) stimulated cAMP production were not significantly different between the calves and cows. After stimulation with NaF, significantly higher cAMP production was found in all tissues from cows. Significant correlations were found between absolute AC responses to NaF and beta-adrenoceptor density in epithelium (r = -0.75, p < 0.001) and smooth muscle (r = -0.63, p < 0.01). It seems that, in older animals, the production of cAMP is independent of the number of receptors, indicating the presence of fully active compensatory mechanisms.