Abstract
The control and mechanisms of airway smooth muscle cell (SMC) contraction were investigated with a sequential series of lung slices from different generations of the same airway from the cardiac lobe of the mouse lung. Airway contraction was measured by monitoring the changes in airway lumen area with phase-contrast microscopy. Changes in intracellular calcium concentration of the SMCs were studied with a custom-built confocal or two-photon microscope. The distribution of the airway SMCs and the muscarinic M(3) or 5-HT(2A) receptors was determined with immunofluorescence. Methacholine and 5-HT induced a concentration-dependent airway contraction and Ca(2+) oscillations within the SMCs of each airway generation. The airway contraction in response to the same agonist concentration was greater in the middle generation compared with the distal or proximal generations of the same airway. Similarly, the Ca(2+) oscillations varied in different generations of the same airway, with a slower frequency in the SMCs of the distal zone as compared with the middle or proximal zones of airways. By contrast, high KCl induced minimal contraction and very slow Ca(2+) oscillations throughout the whole intrapulmonary airway. The slower agonist-induced Ca(2+) oscillations in the distal zone correlated with a reduced expression of agonist receptors. The layer of SMCs increased in thickness in the middle and proximal zones. These results indicate that the contractility of airway SMCs varies at different positions along the same airway and that this response partially results from different Ca(2+) signaling and the total amount of the SMCs.