EFFECTS OF THORACIC VOLUME EXPANSION ON CARDIORENAL FUNCTION IN THE CONSCIOUS RAT

Background: Exposure to microgravity results in the loss of fluid and electrolytes. Hypothesis: This study was designed to determine whether loss of fluid and electrolyte by the kidney occur by increased filtra tion or decreased tubular reabsorption and to investigate the mechanis ms involved. Methods: Vascular and bladder catheters were implanted an d the effects of preferential thoracic volume expansion were studied i n conscious rats using a new hindlimb supported head-down tilt model d esigned to simulate the effects of microgravity. Control rats maintain ed at 0 degrees tilt(NT) were compared to rats at 40 degrees head-down tilt (HDT). Results: HDT immediately increased central Venous pressur e from 1.4 +/- 0.3 to 2.7 +/- 0.3 mm Hg (p < 0.01); which peaked after 8 h. Compared to NT, cumulative sodium excretion significantly increa sed within 6 h of HDT and remained increased at 24 h (198.8 +/- 40.3 v s. 72.8 +/- 18.4 mu Eq; p < 0.01). HDT also significantly increased gl omerular filtration rate (GFR) at both 6 (p < 0.05) and 24 h (p < 0.01 ). In contrast, fractional proximal reabsorption (assessed by lithium clearance) was unchanged over the period of HDT, indicating an appropr iate proximal tubule response to increased filtered sodium. HDT had no significant effect on plasma catecholamine or atrial natriuretic pept ide concentration nor on plasma renin, while plasma aldosterone concen tration was increased after 24 h (72.8 +/- 24.0 vs. 32.4 +/- 8.7 ng/dl ; p < 0.05); presumably in response to sodium loss during HDT. Conclus ion: HDT-induced thoracic volume expansion significantly increases sod ium excretion, primarily as a result of an increase in GFR.