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 filtration or decreased tubular reabsorption and to investigate the mechanisms involved. Methods: Vascular and bladder catheters were implanted and the effects of preferential thoracic volume expansion were studied in conscious rats using a new hindlimb supported head-down tilt model designed to simulate the effects of microgravity. Control rats maintained at 0°tilt (NT) were compared to rats at 40° head-down tilt (HDT). Results: HDT immediately increased central venous pressure from 1.4 ± 0.3 to 2.7 ± 0.3 mm Hg (p < 0.01); which peaked after 8 h. Compared to NI, cumulative sodium excretion significantly increased within 6 h of HDT and remained increased at 24 h (198.8 ± 40.3 vs. 72.8 ± 18.4 μEq; p < 0.01). HDT also significantly increased glomerular filtration rate (GFR) at both 6 (p < 0.05) and 24 h (p < 0.01). In contrast, factional proximal reabsorption (assessed by lithium clearance) was unchanged over the period of HDT, indicating an appropriate proximal tubule response to increased filtered sodium. HDT had no significant effect on plasma catecholamine or atrial natriuretic peptide concentration nor on plasma renin, while aldosterone concentration 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. Conclusion: HDT-induced thoracic volume expansion significantly increases sodium excretion, primarily as a result of an increase in GTR.