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 da ys, 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 tensi on at <7 days when compared with 2-3 and 10 wk. In vivo studies employ ing vagal stimulation before and after topical application of pirenzep ine (an M1 muscarinic receptor blocker) and in vitro pharmacological s tudies evaluating the inhibition of [H-3]quinuclidinyl benzilate (QNB) binding by pirenzepine demonstrated that immature M1-receptor functio n could not account for the weak tracheal smooth muscle responses in t he first week. Topical application of the cholinergic agonist methacho line to the tracheal segment also induced a significantly weaker contr actile response at <7 days when compared with 2-3 and 10 wk. Total den sity of muscarinic receptors, as well as the M1 and M3 muscarinic subt ypes, was not statistically different among <7-day-old, 1 - to 3-wk-ol d, and adult animals. Receptor binding studies in 1-3 wk and adult ani mals demonstrated biphasic dose-dependent inhibition of [H-3]QNB bindi ng in tracheal smooth muscle membranes by methacholine, with a high-af finity component dependent on the availability of G protein. These hig h-affinity muscarinic receptors coupled to G protein were absent in <7 -day-old piglets. We speculate that the weak tracheal smooth muscle co ntraction observed during the first week of life is in part secondary to immature G protein function.