Renal oxygen consumption and sodium reabsorption during isotonic volume expansion in the developing rat.

Abstract

Summary: The aim of the present study was to obtain more information on oxygen consumption in relation to Na reabsorption in the immature kidney during hydropenia (HP) and isotonic volume expansion (VE) corresponding to 5% of the body weight. The study was performed in 24 male Sprague Dawley rats, which were divided into four groups containing six rats each: (1) 22− to 24-day-old rats during HP; (2) 22− to 24-day-old rats during VE; (3) 40− to 42-day-old rats during HP; and (4) 40− to 42-day-old rats during VE. Blood from the renal vein and from the artery was sampled for estimation of oxygen consumption. Glomerular filtration rate (GFR) was estimated by inulin clearance and renal plasma flow by renal extraction of inulin. Hemoglobin conentration and hematocrit are lower in younger rats than in older ones. VE induces a significant and equivalent fall in both age groups. During HP, renal blood flow (RBF) and GFR in relation to body weight are almost 50% lower in younger than in older rats (P < 0.001). During VE, RBF and GFR increase by almost 100% in younger rats (P < 0.001) and reach the same values as in older rats (P > 0.05). The RBF and GFR increased by about 15% in the older rats during the transition from HP and VE (P > 0.05). The amount of Na reabsorbed in 24-day-old HP rats was only 56% of the amount of Na reabsorbed in 40-day-old rats (P < 0.001). After VE, Na reabsorption increased more in 24-than in 40-day-old rats (by 52% and 12%, respectively). The fractional sodium excretion (CNa/Cin) was lower in the 24− than in the 40-day-old VE rats. Renal oxygen consumption (ROC) in the 24-day-old HP rats (including basal oxygen consumption of 1 μmole/g kidney weight) was 53% of the ROC in 40-day-old HP rats. During VE, ROC increased more in the 24-day-old rats (34%) (P < 0.01) than in the 40-day-old rats (4%) (P > 0.05). The renal Na/O2 ratio was the same in the 24-day-old HP, the 40-day-old HP and the 40-day-old VE rats. The 24-day-old VE rats had a significantly higher Na/O2 ratio than the other groups studied (P < 0.01). It is concluded that the energy demands of the immature kidney are related to the GFR and the amount of reabsorbed Na. If the GFR and the amount of reabsorbed Na increase, the oxygen consumption also increases. The amount of the increase, however, is less in the younger rats during VE than it would be if the renal Na/O2 ratio was fixed. Speculation: Equivalent volume expansion induces a considerably larger increase in glomerular filtration rate and a concurrent increase in Na reabsorption in the neonatal period. Because the increased Na reabsorption during VE is partly due to increased O2demanding Na transport, VE may be hazardous, at least in the neonatal period when the oxygen and energy resources are limited.