Characterization of the anatomical structures involved in the contractile response of the rat lung periphery

1. When lung parenchymal strips are challenged with different smooth muscle agonists, the tensile and viscoelastic properties change. It is not clear, however, which of the different anatomical elements present in the parenchymal strip, i.e., small vessel, small airway or alveolar wall, contribute to the response. 2. Parenchymal lung strips from Sprague Dawley rats were suspended in an organ bath filled with Krebs solution (37°C, pH = 7.4) bubbled with 95%O₂/5%CO₂. Resting tension (T) was set at 1.1 g and sinusoidal oscillations of 2.5% resting length (L₀) at a frequency of 1 Hz were applied. Following 1 h of stress adaptation, measurements of length (L) and T were recorded under baseline conditions and after challenge with a variety of pharmacological agents, i.e., acetylcholine (ACh), noradrenaline (NB) and angiotensin II (AII). Elastance (E) and resistance (R) were calculated by fitting changes in T, L and ΔL/Δt to the equation of motion. Hysteresivity (η, the ratio of the energy dissipated to that conserved) was obtained from the equation η = (R/E)2πf. 3. In order to determine whether small airways or small vessels accounted for the responses to the different pharmacologic agents, further studies were carried out in lung explants, Excised lungs from Sprague Dawley rats were inflated with agarose. Transverse slices of lung (0.5-1.0 mm thick) were cultured overnight. By use of an inverted microscope and video camera, airway anti vascular lumen area were measured with an image analysis system. 4. NA, ACh and AII constricted the parenchymal strips. Airways constricted after all agonists, vessels constricted only after AII. Atropine (Atr) pre-incubation decreased the explanted airway and vessel response to AII, but no difference was found in the parenchymal strip response. 5. Preincubation with the arginine analogue N(ω)-nitro-L-arginine (L-NOARG) did not modify the response to ACh but mildly increased the oscillatory response to NA after co-preincubation with propranolol (Prop). 6. These results suggest that during ACh and NA challenge, small vessels do not contribute substantially to the parenchymal strip response. The discrepancy between results in airways, vessels and strips when Atr was administered prior to AII implicates a direct contractile response in the parenchymal strip.