The effect of smooth muscle tone on hysteresis of collateral channels was examined and compared with that of airways, in freshly excised dog lobes. A double lumen catherer was sealed into a branch off the main bronchus, and air (Vs) flowed through the outer lumen while the pressure in the segment (Ps) was measured by the inner lumen. Collateral conductance (Gcoll) was calculated as Gcoll = Vs/Ps. In another series of experiments a pleural capsule was used to measure either segmental airway flow (Vsaw) or capsule pressure (Pcap). Segmental airway conductance (Gsaw) was calculated as Gsaw = Vsaw/(Ps-Pscap). Hysteresis for Gcoll vs lung volume (Vl) curves were almost absent in control and after isoproterenol inflation Gcoll was greater than deflation Gcoll at a given Vl (clockwise history). Conversely, after histamine Gcoll vs Vl curves showed a counterclockwise history. The behaviors of Gcoll and Gsaw against a bronchomotor agent were similar. We conclude that major collateral channels have muscular walls, and being that of an airway type.
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