| Summary: | A preparation is described utilizing a constant flow, right-heart bypass for perfusion of the isolated main pulmonary arteries at controlled pressures. It is demonstrated that stepwise increments of pressure in the pulmonary arteries are accompanied by increases in systemic vascular resistance and in hind-limb vascular resistance. These changes were demonstrated over the whole range of 5-120 cm H₂O pressure in the pulmonary arteries. In contrast there were no significant changes in renal vascular resistance or heart rate. It is .also shown that changing the temperature of the perfusate in the pulmonary artery from 37°C to 30°C is associated with a decrease in systemic vascular resistance. The effects of raising the pulmonary arterial pressure and of cooling the pulmonary artery were abolished by cervical vagotomy. It is suggested that there is a tonic reflex vasoconstrictor tone generated by activity of receptors lying in or close to the walls of the pulmonary artery. It is further suggested that the differential effects on systemic vascular resistance and renal resistance may distribute cardiac output preferentially to the kidney providing one mechanism by which changes in blood volume may lead to appropriate changes in renal solute excretion. The later hypothesis was put to test by collecting urine from the intact kidneys of animals with isolated pulmonary pouch preparation. A step increase in the pouch pressure evoked a corresponding rise in the urinary volume, osmolar
clearance, sodium excretion rate, but not the free water clearance and the potassium excretion rate. The response may be caused by renal hemodynamic changes as a result of the reflex increase in systemic arterial pressure. The rise in sodium excretion rate continued even after the release of the pressure in the pulmonary artery pouch thus the role of a natriuretic hormone in the reflex was suspected. A series of animals with one kidney intact and one kidney isolated and perfused with constant pressure was used in attempt to demonstrate the natriuretic action. These results confirmed the hemodynamic effect on the urinary function of the intact kidney. In the isolated kidney there was no statistically significant increase in sodium excretion rate, therefore the role of a natriuretic agent in the reflex response to distension of the pulmonary artery is still uncertain. === Medicine, Faculty of === Cellular and Physiological Sciences, Department of === Graduate
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