The effects of exogenous norepinephrine on ventilatory mechanics in the rat

The effects of intravenous administration of norepinephrine on ventilatory mechanics were assessed in the anesthetized, spontaneously breathing rat. Pulmonary resistance, dynamic compliance and functional residual capacity remained unchanged during a 10-min infusion period. After paralysis, repetition of the norepinephrine infusion caused an increase in static deflation recoil pressure at lung volumes above the tidal range but maximum expiratory flow rates at corresponding lung volumes were not increased indicating an elevation in the resistance of upstream airways. Flow rate responses to low density gas breathing were not enhanced by norepinephrine suggesting peripheral airways narrowing as the cause of the increased upstream resistance. Isoproterenol protected animals against serotonin-induced bronchoconstriction whereas norepinephrine augmented the response to serotonin. It is concluded that in intact rats norepinephrine increases the flow-resistive properties of peripheral airways, does not measurably alter the caliber of central airways and possesses no discernible bronchodilator activity. The effects of norepinephrine on small airways appear to be secondary to changes in pulmonary hemodynamics rather than the result of direct stimulation of alpha adrenoceptors in airway smooth muscle.