Expression of airway contractile properties and acetylcholinesterase activity in swine

Abstract

We studied the effect of maturation on contractile properties of tracheal smooth muscle from seventeen 2-wk-old swine (2ws) and fifteen 10-wk-old swine (10ws) in situ and in vitro. The response to parasympathetic stimulation was studied in situ in isometrically fixed segments. Contraction was elicited at lower frequencies [half-maximal response to electrical stimulation (ES50) = 6.7 +/- 0.05 Hz] in 2ws than in 10ws (ES50 = 9.1 +/- 0.4 Hz; P less than 0.01). Despite substantial differences in morphometrically normalized cross-sectional area in 2ws (0.012 +/- 0.003 cm2) and 10ws (0.028 +/- 0.001 cm2; P less than 0.01), maximal active tension elicited by parasympathetic stimulation was similar (12.4 +/- 3.2 g/cm in 2ws vs. 13.3 +/- 2.3 g/cm in 10ws; P = NS). In separate in vitro studies in 25 tracheal smooth muscle strips from 10 swine, concentration-response curves generated with potassium-substituted Krebs solution (KCl) were similar in 2ws and 10ws. In 58 other strips (10 swine), maximal active force elicited with acetylcholine (ACh) in 2ws was significantly greater than for 10ws (P less than 0.001). Removal of the epithelium had no effect. However, cholinesterase inhibition with 10(-7) M physostigmine augmented the response to ACh in 10ws (P less than 0.02) but not 2ws. We demonstrate increased force generation and sensitivity to vagal stimulation in 2ws vs. 10ws, which corresponds to increased reactivity to ACh in vitro. The relative hyperresponsiveness in 2ws is specific for cholinergic response and is attenuated at least in part by maturation of the activity of acetylcholinesterase enzyme.

Age differences in cholinergic airway responsiveness in relation with muscarinic receptor subtypes

“…2). This is consistent with the previous reports (Murphy et al, 1989;Panitch et al, 1989) and suggests that the degree of airway obstruction in young animals may be stimulus-specific. Age dependent airway responsiveness to agents other than ACh and the underlying mechanism remains to be studied.…”

“…The animal studies on this hypothesis have been controversial. The in vitro response to cholinergic stimulation, tachykinin or histamine was greater in immature airway smooth muscles from piglets (Murphy et al, 1989;Sparrow and Mitchell, 1990), guinea pigs (Brink et al, 1980;Preuss and Goldie, 1999;Wills-Karp, 1991), or rabbits (Shen et al, 1996;Tepper et al, 1995), whereas the opposite has also been found in sheep (Panitch et al, 1989) or piglets (Haxhiu-Poskurica et al, 1993;Rodriguez et al, 1994). None of these previous studies however measured airway responses in vivo to confirm that the airways from immature animals are truly more sensitive to stimuli.…”

Maturation of porcine tracheal cholinergic and inhibitory non-adrenergic, non-cholinergic innervation

“…When forces are compared by normalizing for cross-sectional area, some investigators have shown increased response to vagal stimulation in the immature pig. 16 However, they have also demonstrated down-regulation of all receptor-and non-receptor-dependent contractile forces in airway smooth muscle with maturation. 17 Specific studies of vagal responses may therefore be obscured by the maturational changes in nonspecific smooth muscle cell contractility if this method of normalization is used.…”

“…[13][14][15] Likewise, receptor-and nonreceptor-mediated stimulation of fetal and suckling porcine airway smooth muscle elicits in vitro a greater contraction per unit of muscle tissue than in adult preparations. [16][17][18] When such age-related differences in smooth muscle contractility are excluded, fetal porcine cholinergic nerves appear less sensitive to EFS than the adult cholinergic nerves. 2 This is surprising, given the relative lack of neural acetylcholinesterase until late in the neonatal period.…”

“…2 This is surprising, given the relative lack of neural acetylcholinesterase until late in the neonatal period. 16 However, iNANC nerves are also activated by EFS in addition to cholinergic nerves. The terminals of iNANC nerves release NO as the primary neurotransmitter in the human airway.…”