Chronic volume expansion in the rat: proximal tubular Na+ transport and Na+ pump inhibition.

1. The lesser natriuresis of chronic volume expansion (ChVE) compared with that of acute volume expansion (AcVE) implies different homeostatic mechanisms. Because little information is available in the literature on proximal tubular (PT) Na+ transport and intracellular electrolyte concentrations, these were investigated in a rat model of ChVE. 2. Haematocrit was significantly lower and urine volume and Na+ excretion were significantly higher in ChVE rats compared with control rats. 3. Proximal tubular Na+ transport with artificial PT fluid was normal (3.67 +/- 0.09 x 10(-4) mm3 mm-2 s-1; mean+/-S.E.M.), while with endogenously harvested tubular fluid it was reduced to 2.78 +/- 0.07 x 10(-4) mm3 mm-2 s-1 in ChVE rats (P < 0.0001). 4. Intracellular Na+ was significantly elevated from 18.0 +/- 0.7 mmol (kg wet wt)-1 in control rats to 20.2 +/- 0.8 mmol (kg wet wt)-1 in ChVE rats (P = 0.044). The cells showed residual swelling, with dry weight and phosphorus values decreasing significantly compared with controls (19.5 +/- 0.4 to 18.5 +/- 0.03% and 130.4 +/- 3.7 to 117.8 +/- 2.8 mmol (kg wet wt)-1, P = 0.04 and 0.006, respectively). 5. The results demonstrate that in ChVE a tubular factor inhibits PT Na+ transport associated with an inhibition of the Na+ pump and this resembles one mechanism defined in AcVE.