MATURATION OF THE CHOLINERGIC RESPONSE OF TRACHEAL SMOOTH-MUSCLE IN THE PIGLET

Maturational changes of the contractile properties of airway smooth mu scle have been suggested to occur during the late prenatal and early p ostnatal period in the piglet. However, the timing and mechanisms invo lved remain elusive. Therefore, we studied isometric contraction of tr acheal smooth muscle (TSM) to evaluate the effects of maturation on 1) the response of TSM to the muscarinic agonist acetylcholine (Ach); an d 2) the regulatory activity of acetylcholinesterase (AChE) during the early postnatal period in the piglet. Studies were performed in 1-5-d ay-old (n = 6) and 14-22-day-old (n = 8) anesthetized, mechanically ve ntilated piglets. Changes in tracheal tension were measured in vivo fr om an open tracheal segment and normalized for both longitudinal lengt h of the tracheal segment and cross-sectional area. Dose-response curv es to locally applied Ach were obtained before and after the intraveno us administration of the AChE inhibitor physostigmine (Phys). Ach-indu ced changes in tracheal tension significantly increased with maturatio n during the first 3 weeks of life (P < 0.01), regardless of normaliza tion method. The addition of Phys significantly increased the choliner gic responses of TSM in both age groups (P < 0.02). However, this incr ease was comparable between age groups. There was also no difference i n AChE activity measured in tracheal smooth muscle homogenates between the 1-5 and 14-22-day-old animals. These data indicate that maturatio n significantly enhances the cholinergic responses of tracheal smooth muscle in the early postnatal period in the piglet. As the regulatory activity of AChE is equally developed in the first and third postnatal weeks, this does not account for the observed differences in force ge neration at these ages. (C) 1994 Wiley-Liss, Inc.