Effects of Posture on Renal Excretion of Sodium and Chloride in Orthostatic Hypotension

Abstract

Large decreases in the rates of sodium excretion, chloride excretion, and glomerular filtration were observed in patients with orthostatic hypotension when they assumed the upright posture following a period of recumbency. When they returned to the recumbent position, sodium excretion typically was not restored immediately in spite of immediate restoration of filtered sodium load. The decreased sodium excretion found to occur in patients with orthostatic hypotension when they are in the upright posture is considered to be due in part to the decreased filtered sodium load and in part to changes in tubular reabsorption that are independent of filtered sodium load.WV THEN the normal human subject changes from the recumbent to the erect position, arterial blood pressure is maintained,' and there is either no change or only a moderate decrease in renal sodium excretion.2-6 When the patient with orthostatic hypotension changes from the recumbent to the upright posture a marked fall in arterial blood pressure occurs7-"; therefore, it might be anticipated that the decreased arterial blood pressure in the upright posture would be reflected by a decreased intraglomerular pressure, decreased glomerular filtration rate, and decreased sodium excretion.The effect of posture upon the renal excretion of sodium in human subjects with orthostatic hypotension has not been reported previously. Studies

Some effects of change of posture on water and electrolyte excretion by the human kidney*

“…Bachman & Youmans (1953) and Pearce & Simons (1954) found no change in ammonium excretion in the upright position, while Goodyer & Seldin (1953) claimed a slight decrease; absence of effect on titratable acid excretion, in spite of a consistent change in pH and bicarbonate excretion, has been reported (Pearce & Simmons, 1954). Changes in urine bicarbonate excretion and pH are evident in the data of White, Rosen, Fischer & Wood (1926) and Bazett, Thurlow, Crowell & Stewart (1924); a fall in the ratio Cl: (Na + K) with a rise in pH after lying down was noted by Mills & Stanbury (1952), and this suggests a rise in bicarbonate excretion.…”

Basis of nocturnal polyuria in patients with autonomic failure

“…Patients with autonomic failure have been observed to excrete excessive quantities of salt and water when recumbent at night (Bachman and Youmans, 1953;Shear, 1963). Low rates of aldosterone secretion have been found in some patients with orthostatic hypotension (Slaton and Biglieri, 1967;Aminoff and Wilcox, 1972) and may therefore contribute to the excessive sodium loss of recumbency.…”

Supine Hypertension, Blood Pressure Variability and Circadian Rhythm in Autonomic Failure: The Role of Ambulatory Intra‐arterial Monitoring

“…Postural response of renin is preserved in some subjects with this con-dition (Bannister, Sever & Gross 1977) and this may play some role. Plasma volume may also undergo considerable circadian variation; certainly salt and water excretion have a highly abnormal pattern with daytime oliguria and night-time polyuria (Bachman & Youmans 1953;Wilcox, Aminoff & Slater 1977). This may be the fundamental cause of the gradual daytime rise in pressure.…”