Development of cholinergic innervation and muscarinic receptor subtypes in piglet trachea

We studied physiological and pharmacological maturation of cholinergic innervation to tracheal smooth muscle in piglets at three ages: < 7 days, 2-3 wk, and 10 wk. Change in tracheal tension was measured in vivo from a tracheal segment and normalized for its size. Electrical vagal stimulation induced a significantly weaker increase in tracheal tension at < 7 days when compared with 2-3 and 10 wk. In vivo studies employing vagal stimulation before and after topical application of pirenzepine (an M1 muscarinic receptor blocker) and in vitro pharmacological studies evaluating the inhibition of [3H]quinuclidinyl benzilate (QNB) binding by pirenzepine demonstrated that immature M1-receptor function could not account for the weak tracheal smooth muscle responses in the first week. Topical application of the cholinergic agonist methacholine to the tracheal segment also induced a significantly weaker contractile response at < 7 days when compared with 2-3 and 10 wk. Total density of muscarinic receptors, as well as the M1 and M3 muscarinic subtypes, was not statistically different among < 7-day-old, 1- to 3-wk-old, and adult animals. Receptor binding studies in 1-3 wk and adult animals demonstrated biphasic dose-dependent inhibition of [3H]QNB binding in tracheal smooth muscle membranes by methacholine, with a high-affinity component dependent on the availability of G protein. These high-affinity muscarinic receptors coupled to G protein were absent in < 7-day-old piglets. We speculate that the weak tracheal smooth muscle contraction observed during the first week of life is in part secondary to immature G protein function.