Ageing and epithelial integrity as modulators of airway smooth muscle responsiveness to endothelin-1

This study has examined the effects of animal age on the contractile responsiveness and inositol phosphate-generating capacities of guinea-pig (0-104 weeks) and rat (4-104 weeks) isolated tracheal smooth muscle in response to endothelin-1 (ET-1). The influence of animal age on the specific binding of [125I]ET-1 to guinea-pig and rat isolated tracheal tissue was also examined. The potency (pD2) of ET-1 was three to four times greater in tracheal tissue taken from 4-week-old rats than in similar tissue from 12- to 32-week-old animals, although maximum response (Emax) was not significantly altered. Neither pD2 nor Emax were influenced by ageing in epithelium-intact guinea-pig tracheal preparations. In contrast, removal of the airway epithelium significantly increased the contractile potency of ET-1 by two- to three-fold in tissue from animals of 6-20 weeks of age, but not in tissue from newborn animals. Significant falls in specific [125I]ET-1 grain density with ageing were demonstrated during the maturation phase in both species. In the rat, the decrease between 4 and 12 weeks was reflected in the fall in ET-1 potency at 12 weeks. However, the age-associated reduction in airway smooth muscle ET receptor number in the guinea-pig was not mirrored by significant changes in sensitivity to ET-1, suggesting the presence of a functional receptor reserve. ET-1 (1 nM) caused significant increases in intracellular inositol phosphates, with levels generally higher in rat than in guinea-pig trachea. ET-1-induced inositol phosphate accumulation decreased significantly with respect to animal age in both guinea-pig and rat isolated tracheal tissue. However, this was not correlated with changes in contractile pD2 or Emax. For example, in both rat and guinea-pig, the smallest ET-1-induced increases in intracellular inositol phosphates were measured in airway smooth muscle from the oldest animals tested, although tissue sensitivity to ET-1 was stable in both species after 12 weeks of age. These data suggest that relatively low levels of inositol phosphates were required to elicit Emax, consistent with the presence of more than one signal transduction process.