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To investigate renal proximal tubular sodium-hydrogen exchanger 3 (NHE-3) and H+-ATPase activities in young (5-week-old) spontaneously hypertensive rats (SHR) and normotensive Donryu (DRY) rats, in the period during which high blood pressure is developing.Five-week-old SHR and DRY rats were weighed and systolic blood pressure recorded. Proximal tubule cells were isolated, loaded with the intracellular pH dye, 2′-7′-bis-carboxyethyl-5(6)-carboxyfluoresceinacetoxymethyl-ester and acidified with a NH4+/NH3 prepulse. Na+-independent intracellular pH recovery rate (H+-ATPase activity) and initial Na+-dependent intracellular pH recovery rate (NHE-3 activity) were assessed. NHE-3 activity was assessed during inhibition of H+-ATPase with Bafilomycin A1 and during inhibition of any possible NHE-1 activity with Hoe 694.Mean body weight and systolic blood pressures of 5-week-old SHR and DRY rats were not significantly different. NHE-3 activity was higher in SHR, 1.08 ± 0.1 pH units/min compared with DRY rats, 0.73 ± 0.1 pH units/min (P <0.05) H+-ATPase activity was also higher in SHR, 0.119 ± 0.02 pH units/min, compared with DRY rats, 0.051 ± 0.02 pH units/min (P <0.05).Proximal tubule cells of 5-week-old SHR have higher NHE-3 and H+-ATPase activities compared with age-matched DRY rats. Enhanced proximal tubular fluid reabsorption is likely to contribute to development of high blood pressure in young SHR.