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 mu scle agonists, the tensile and viscoelastic properties change. It is n ot 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 Spra gue Dawley rats were suspended in an organ bath filled with Krebs solu tion (37 degrees C, pH=7.4) bubbled with 95%O-2/5%CO2. Resting tension (T) was set at 1.1 g and sinusoidal oscillations of 2.5% resting leng th (L(o)) at a frequency of 1 Hz were applied. Following 1 h of stress adaptation, measurements of length (L) and T were recorded under base line 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 Delta L/Delta t to the equation of motion. Hystere sivity (eta, the ratio of the energy dissipated to that conserved) was obtained from the equation eta=(R/E)2 pi f. 3 In order to determine w hether small airways or small vessels accounted for the responses to t he different pharmacologic agents, further studies were carried out in lung explants. Excised lungs from Sprague Dawley rats were inflated w ith agarose. Transverse slices of lung (0.5-1.0 mm thick) were culture d overnight. By use of an inverted microscope and video camera, airway and vascular lumen area were measured with an image analysis system. 4 NA, ACh and AII constricted the parenchymal strips, Airways constric ted after all agonists, vessels constricted only after AII. Atropine ( Atr) pre-incubation decreased the explanted airway and vessel response to Ail, but no difference was found in the parenchymal strip response . 5 Preincubation with the arginine analogue N-omega-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 (Pr op). 6 These results suggest that during ACh and NA challenge, small v essels do not contribute substantially to the parenchymal strip respon se. The discrepancy between results in airways, vessels and strips whe n Atr was administered prior to AII implicates a direct contractile re sponse in the parenchymal strip.