Effect of ageing on nicotine-induced contraction of guinea-pig bronchial preparation.

1. Nicotine-induced contraction of bronchial preparations isolated from 3, 6, 10, 50 and 100 week old guinea-pigs were studied. 2. Contractile responses to nicotine were inhibited by hexamethonium and a substance P antagonist but not by atropine or tetrodotoxin, suggesting that the nicotine-contraction was mediated through a release of substance P-like material(s). 3. The pD2 value (potency) of nicotine did not change with age from 3 to 10 weeks but decreased thereafter to 100 weeks, while no age-related change was found in the pD2 value of substance P. The pA2 value of hexamethonium did not change with age, suggesting that the affinity of drugs to nicotine receptors does not change with age. 4. These results suggest that possibility that age-related change in the pD2 value of nicotine is due to a change in the amount of nicotine receptors but not to a change in the affinity of nicotine to its receptors.