Pharmacological and histological examinations of regional differences of guinea‐pig lung: a role of pleural surface smooth muscle in lung strip contraction

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Abstract
Parenchymal lung strip preparations have been widely used as an in vitro model of peripheral airway smooth muscle. The present study examined functional responses of 4 consecutive guinea‐pig lung parenchymal strips isolated from the central region (segment 1) to the distal edge (segment 4) of the lower lung lobe. The middle two segments were designated as segments 2 and 3. Lung segments 1 and 4 exhibited significantly greater contraction than the other 2 segments to KCl when responses were expressed as mg force per mg tissue weight. Contractile responses to bronchospastic agents including histamine, carbachol, endothelin‐1, leukotrienes (LT) B4 and D4, and the thromboxane A2‐mimetic U46619 demonstrated no significant difference in EC50 values among the 4 lung segments. Contractile responses of segments 1 and 4 to antigen‐challenge (ovalbumin), ionophore A23187 and substance P were significantly greater than the other 2 segments with respect to either sensitivity or maximum responsiveness. U46619‐induced contractions of the 4 lung segments were relaxed in similar manner by papaverine and theophylline up to 100%, salbutamol up to 80%, and sodium nitroprusside by only 20%. In contrast, sodium nitroprusside markedly reversed U46619‐induced contraction of pulmonary arterial rings and bronchial rings. Histological studies identified 2–4 layers of smooth muscle cells underlying the lung pleural surface. Mast cells were prominent in this area. Moreover, morphometric studies showed that segment 4 possessed the least amount of smooth muscle structures from bronchial/bronchiolar wall and vasculatures as compared to the other 3 segments, and a significant difference in this respect was evident between segment 1 and segment 4. Since lung segments 1 and 4 are covered with larger surface area of lung pleura, the present results suggest that the significantly greater intrinsic contractile responses of segments 1 and 4 are associated with the presence of increased lung pleural surface possibly together with more mast cells. Thus, a primary contribution to the net contraction of the lung parenchymal strips may be smooth muscle from the lung pleura, alveolar ducts and interstitial contractile cells rather than from bronchi/bronchioles and micro‐vasculatures.