PROXIMAL NA- REABSORPTION IN UNINEPHRECTOMIZED RATS AFTER A SALINE LOAD(, FLUID AND HCO3)

We studied the effect of a saline load on proximal tubular Na+, fluid and HCO3- handling of control and uninephrectomized (UNx) rats. Glomer ular filtration rate (GFR) was measured by inulin clearance, Na+ reabs orption in the proximal tubule by lithium clearance (CLi+), proximal t ubule fluid reabsorption (Jv) by microperfusion with C-14 inulin and p roximal tubule HCO3- reabsorption (JHCO(3)(-)) by estimating pH of per fusate and collected fluid in vitro under CO2 equilibrated oil by mean s of Sb microelectrodes. No significant differences were found between control and UNx animals in baseline values of urine flow, GFR, fracti onal water and sodium excretion and blood and urine pH and pCO(2), sho wing that renal function was compensated 4 weeks after UNx. On the oth er hand, UNx rats showed a significant increase in proximal tubule Jv and JHCO(3)(-). After extracellular fluid volume expansion (EFVE) by i nfusion of HCO3- Ringer solution (O.1 ml/min) during 90-120 min, in bo th groups of animals hematocrit decreased by 15%, GFR remained unchang ed and fractional Na+ and water excretion increased significantly. How ever, after EFVE Jv and JHCO(3)(-) decreased significantly in control rats but were not modified in UNx animals. Fractional proximal Na+ rea bsorption as measured by CLi+ decreased in control rats after EFVE and was not altered in UNx rats. After EFVE the fall in fractional distal Na+ reabsortion was significantly higher in control than in UNx rats. The present results provide evidence that after a saline load the imp aired natriuretic response in UNx rats could be attributed to an alter ation in proximal and distal tubular Na+ and HCO3- handling. The data also showed a close correlation between proximal tubular function eval uated by CLi+ and by Jv.