Phosphate handling by the rat nephron during saline diuresis

Phosphate handling by the rat nephron during saline diuresis. Inorganic phosphate handling by the nephron was investigated using renal micropuncture techniques in fasted rats undergoing sustained saline diuresis and in controls. The early distal and the late accessible proximal convolutions of the same nephron were micropunctured. Despite the presence of decreased fractional volume reabsorption at the end of the proximal tubule during saline diuresis, the tubular fluid to plasma phosphate ratio was unaffected. Fractional phosphate delivery at this site was not significantly higher. Fractional phosphate delivery at the early distal and late proximal segments of the same nephron were significantly correlated in both groups. The slopes of the regression lines were not different. The delivery of phosphate to the early distal tubule, for a given value of delivery from the late proximal, was lower during saline diuresis. Mean fractional phosphate reabsorption between the late proximal and the early distal tubules was twice as high during saline diuresis as in controls. This observation may account for the decreased fractional phosphate excretion in ureteral urine observed in fasting animals. Clearance studies indicated a different behavior between fed and fasted rats undergoing saline diuresis. Fractional phosphate excretion increased in the former and decreased in the latter