Maturation of acetylcholinesterase expression in tracheal smooth muscle contraction.
We examined postganglionic development of acetylcholinesterase (AChase) activity and tracheal smooth muscle (TSM) contraction elicited by cholinomimetic activation and electrical field depolarization in vitro. Epithelium-intact tracheal strips excised from 21 2-wk-old swine (2ws) and 19 10-wk-old swine (10ws) were tethered isometrically at optimal resting length, and responses were expressed as percent of the maximum to 63 mM potassium-chloride (%KCl). Cumulative concentration-response curves to KCl were equivalent for TSM from 2 and 10ws. However, maximal contraction to ACh in 2ws (168 +/- 8.4 %KCl) was greater than for 10ws (142 +/- 2.3 %KCl; P less than 0.02). Stimulus-response curves (field electrodes; AC source) demonstrated greater sensitivity for TSM in 10ws (stimulus causing 50% of the maximal response = 3.32 +/- 0.13 V in 2ws vs. 2.25 +/- 0.12 V in 10ws; P less than 0.001), indicating that the greater cholinomimetic responsiveness of 2ws did not result from augmented presynaptic nerve conduction. The AChase inhibitor, physostigmine, caused 1) greater sensitivity of responses elicited by electrical field stimulation in 2ws (P less than 0.05) but not in 10ws (P = NS), 2) augmentation of maximal responses to exogenous ACh in 10ws (27% increase; P less than 0.01) but not 2ws (2% increase; P = NS), and 3) a greater increase in sensitivity to cholinomimetic activation in 2ws compared with 10ws (P less than 0.02). These data demonstrate increased cholinergic contraction of TSM in 2 vs. 10ws that results at least in part from reduced AChase activity in the trachea of immature animals.
Mitchell, RW; Murphy, TM; Kelly, E; Leff, AR
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