Abstract
Summary
Daily determinations or fluid exchanges, urinary Na+ excretion, and plasma volume, Na+, K+, protein, and os-molality were made using 13 adult male rabbits during periods of ad libitum feeding, food deprivation for 4 days (water available), and 1 day of refeeding. Significant increases in urinary Na+ excretion and volume occurred after 24 hr of deprivation, and a significant increase in drinking took place after 2 days; maximal values were obtained by Day 3 for all three parameters. The plasma volume was significantly reduced from 33.4 to 29.3 ml/kg, coincident with the changes in urinary volume and Na+ excretion after 1 day of deprivation, and remained at this reduced level. Plasma osmo-lality was decreased after 1 day of deprivation and continued to decline. Plasma concentrations of Na+, K+, and protein did not change. In a second group of rabbits deprived of food and water, a marked natri-uresis was induced despite the expected reduction in urine volume. In a third group of rabbits, plasma glucose concentration was decreased after 24 hr of fasting but returned to the control level by Day 2 of fasting.
The data substantiate the assumption that the fasting-induced natriuresis and increased urine flow in the rabbit are accompanied by a reduction in plasma volume. The diuresis and natriuresis persist in fasting male rabbits with no further change in plasma volume after the first day of fasting. The natriuresis is clearly independent of the increased urine flow and fluid intake. The polydipsia is secondary to the reduction in plasma volume and enhances the increased urine flow to produce a marked polyuria.
The authors thank Mr. John Hegmann for his excellent technical assistance.
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