Dissociative effects of volume expansion on renal tubular ionic transport

Recollection micropuncture and clearance studies were performed in chronically thyroparathyroidectomized (TPTX) dogs to evaluate the relative contributions of proximal and distal nephron segments to the composition of the urine during saline volume expansion (VE). Ionic and fluid reabsorption in late proximal tubule and final urine were compared during mild VE (2.5-3% body weight) and moderate VE (5-6% body weight). With mild VE, proximal fractional reabsorption (PFR) of tubular fluid, sodium, phosphate and bicarbonate declined but proximal calcium and potassium transport were unaltered. Both a mild natriuresis and bicarbonaturia occurred but there was no increase in urinary phosphate, calcium or potassium. With moderate VE, PFR of sodium and bicarbonate were inhibited to approximately the same degree as with mild VE but sodium and bicarbonate excretion were greater. A greater decline in phosphate PFR resulted in a phosphaturia. As in mild VE, calcium PFR was unchanged, but a mild calciuria occurred. A decline in potassium PFR was accompanied by a similar increase in percentage excretion. These data clearly demonstrate (1) and 'overwhelming' of distal reabsorptive capacity for phosphate due to greater proximal inhibition with 5% as compared to 2.5% VE in TPTX dogs, and (2) a dissociation of both proximal and whole kidney transport of calcium and potassium from that of sodium during mild VE. Also, distal inhibition of sodium and bicarbonate transport with moderate VE, but not with mild VE, is suggested by the data.