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 homeostat ic mechanisms. Because little information is available ill the literat ure on proximal tubular (PT) Na+ transport and intracellular electroly te 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) mm(3) 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) mm(3) mm(-2) s(-1) in ChVE rats (P < 0.0001). 4. Intracellula r 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 pho sphorus 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 w et wt)(-1), P = 0.04 and 0.006, respectively). 5. The results demonstr ate that in ChVE a tubular factor inhibits PT Na+ transport associated with an inhibition of the Na+ pump and this resembles one mechanism d efined in AcVE.