The cite of action of nerves in the pulmonary vascular bed in the dog.

1. The effects of stimulation of the thoracic vagosympathetic nerve or upper thoracic sympathetic chain on the pulmonary vascular resistance have been studied in atropinized, isolated, ventilated lung lobes under various conditions of pulmonary circulation perfusion. Throughout the nerve-stimulation tests bronchial circulation perfusion was maintained or temporarily interrupted.2. The pulmonary vascular resistance increase evoked by nerve stimulation (a) occurred in the absence of tidal air changes; (b) did not consistently differ during predominantly ;sluice' and ;non-sluice' conditions of pulmonary circulation perfusion; (c) was approximately one and a half times greater during constant pressure than during constant volume inflow perfusion of the pulmonary circulation; and (d) was greater during reverse than during forward perfusion.3. In lung lobes perfused in either direction at constant volume inflow nerve stimulation produced an increase in inflow pressure and a diminution in total lung blood volume reflected by a temporary increase in blood outflow.4. In lung lobes in which neither the pulmonary nor the bronchial circulations were perfused and the capillaries were completely blocked by high intratracheal pressures, thus isolating the pulmonary arterial system from the venous system, nerve stimulation produced a diminution in the blood volume of both systems.5. Nerve stimulation produced a rise in bronchial arterial pressure in the absence of pulmonary circulation perfusion.6. Further evidence is adduced that pulmonary vasomotor nerve responses do not depend upon the transfer of transmitter substances from the bronchial to the pulmonary circulation.7. The possible significance of these observations in relation to the site of action of pulmonary vasomotor nerves is discussed.