Bronchodilation induced by muscular contraction in spontaneously breathing rabbits: neural or mechanical?

The respective contribution of mechanical and neural mechanisms to the bronchodilation occurring during exercise is not fully identified in spontaneously breathing animals. The airway response to electrically induced muscular contractions (MC) was studied after vagal cold block in 9 spontaneously breathing rabbits. The forced oscillation respiratory system resistance (Rrs) was measured at vagal nerve temperatures 37°C, 8°C and 4°C. Rrs was found to decrease significantly during MC in all conditions. The occasional occurrence of a deep breath was responsible for a sudden decrease in Rrs. However, when the deep breath was absent - after vagal cooling and in some experiments at 37°C - the bronchodilation was frequently dissociated from the change in breathing pattern, most likely illustrating a neural mechanism. Altogether, while some bronchodilation may be ascribed to the mechanical stretching of the airways, Rrs decreasing with little change in breathing pattern is likely related to a reflex effect, possibly a sympathetic-borne mechanism.