Early Maturation of Force Production in Pig Tracheal Smooth Muscle during Fetal Development

Abstract

The contractility of airway smooth muscle is fully established at late term at birth but its responsiveness during fetal life has not been defined. In this study, the contractile force of airway smooth muscle to acetylcholine (ACh), K+ depolarizing solution, and electrical field stimulation (EFS) was measured in tracheas from small fetal pigs. Contraction to either agonist and to EFS was detectable in fetuses of as low as 9 g body weight, which corresponds to approximately 36 days of gestation. Isometric force increased progressively with age, reaching 4.1 +/- 0.4 mN for K+ and 5.8 +/- 0.5 mN for ACh (10(-4) M) at 600 g fetal weight (90 days). However, when normalized for cross sectional area of smooth muscle, the stress was essentially the same from 17- to 600-g fetuses. (K+: 17 g = 74.4 +/- 10.6 mN/mm2, 600 g = 89.3 +/- 13.0 mN/mm2; ACh [10(-4) M]: 17 g = 76.3 +/- 16.0 mN/mm2, 600 g = 127.0 +/- 13.0 mN/mm2). The sensitivities of the various groups to ACh were not significantly different (e.g., EC50: 30 g = 4.0 +/- 0.2 x 10(-6) M, 600 g = 3.7 +/- 1.1 x 10(-6) M). EFS produced frequency-dependent contractile responses in all groups. With increasing fetal size, there was a corresponding increase in force. When this force was normalized to a maximum ACh response (10(-4) M), there was no significant difference between groups of fetuses. Histologic examination showed the major tissue components of the trachea were present in fetuses above 7 g. Immunocytochemistry detected myosin, caldesmon, and filamin in the smooth muscle from fetuses of 7 g and above, showing that contractile and actin-binding proteins were present from a very early age. It is concluded that smooth muscle contractile function is well developed very early in fetal life in pigs.

Effects of simulated tidal and deep breathing on immature airway contraction to acetylcholine and nerve stimulation

“…A number of maturational changes in airway innervation could potentially influence airway contraction under dynamic conditions, including changes in ACh release evoked by EFS, 28 altered density of peptide containing neurons 28–30 and inhibitory innervation of ASM 31 . In the pig, the predominant effect of EFS across a wide range of animal ages is activation of the cholinergic pathway 32,33 . In the present study, there was no evidence to implicate cholinergic control of the airways in the response to volume oscillation, because the inhibition produced by oscillation was the same for EFS as for 10 −5 mol/L ACh.…”

Foetal airway motor tone in prenatal lung development of the pig

“…Immunocytochemical procedures were as described by BOOTH et al [2]. At the end of an experiment, the bronchial tree was placed in Cryo-M-Bed and frozen in isopentane precooled in liquid nitrogen.…”

“…The functional maturity of the airway smooth muscle in foetal pigs has been studied at various ages of foetal development, using the trachealis [2]. The maximum isometric force generated in response to different modes of stimulation, including electrical field stimulation (EFS), when normalized as stress (force/cross sectional area of smooth muscle cells), was similar in 9-16 g foetal pigs (~36-41 days gestation) to that in late term foetuses (full term 115 days, weight 1,100 g).…”

Transition of functional innervation in the developing porcine airway from nitrergic to catecholaminergic

“…Rhythmic, myogenic contraction/relaxation phases are present in both human and porcine foetal bronchus causing the movement of foetal lung fluid (McCray, 1993; Sparrow et al , 1995). Foetal ASM contracts to acetylcholine, histamine, substance P and K + depolarizing solution suggesting that the cellular and molecular apparatus required for airway control is functional from early in development (Booth et al , 1992; Sparrow et al , 1994). Electrical field stimulation (EFS) of the mural nerves to the tracheobronchial tree of pre‐ and post‐natal pigs produces a contraction followed by relaxation.…”