Effects of Acute Passive Tilting on Arterial Pressure, Renal Hemodynamics and Urinary Electrolyte Excretion in the Dog

¹ From the Department of Surgical Physiology, Walter Reed Army Institute of Research, Walter Reed Army Medical Center, Washington, D. C.

Renal hemodynamics, the excretion of water and electrolytes and mean arterial pressures in the abdominal aorta and in the carotid artery were studied in both conscious and anesthetized dogs when they were tilted at various angles, head-up or head-down, from the horizontal supine posture. Aortic pressure increased with head-up, and decreased with head-down tilting. Carotid pressure changes were the opposite but of smaller magnitude. Autonomic ganglionic blockade (hexamethonium) abolished the lower aortic response to both head-up and head-down tilting, and adrenergic stimulation (l-norepinephrine) prevented further pressor response to head-up tilting. Thirty minutes in the 45° head-up or head-down position did not alter renal plasma flow, glomerular filtration or the rates of excretion of sodium, potassium or water, but in subsequent control periods renal plasma flow decreased, suggesting compensatory renal vasoconstriction. This study demonstrated renal circulatory autoregulation over a mean arterial pressure range of 40 mm Hg. Over this degree of pressure change no evidence was found for an arterial baroreceptor concerned with the regulation of renal sodium excretion.