Because isometric systems differ substantially from methods of measuring airway smooth muscle tone that depend on airway diameter, we determined the sensitivity and significance of data derived from these different methods in 20 anesthetized dogs. The airway contractile response was measured directly from an isometric tracheal segment in situ and simultaneously as pulmonary resistance (RL) and dynamic compliance (Cdyn). The contractile response to intravenous (iv) methacholine (MC) (2.6 X 10(-11) to 2.6 X 10(-6) mol/kg) and norepinephrine (NE) (1.2 X 10(-11) to 1.2 X 10(-6) mol/kg) was measured in dose-response studies of beta-blocked and ganglion-blocked animals. A statistically significant change in isometric tracheal tension was first observed at 2.6 X 10(-10) mol/kg iv MC and 1.2 X 10(-9) mol/kg iv NE. Statistically significant changes in Cdyn and RL did not occur at doses less than 10(-8) mol/kg for either agonist. Substantial increase in isometric tracheal tension (greater than 10 g force/cm) occurred before any change in RL or Cdyn. These finding indicate that change in isometric tension reflects parallel changes in RL and Cdyn. For NE and MC, tracheal tension is a more sensitive and selective measurement of airway contraction than RL or Cdyn.
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