Renal tubular sites of altered calcium transport in phosphate-depleted rats

characteristic of chronic phosphate (PO) depletion (PD). To study the changes in tubular transport and the site of the hypocalciuric effect of P04 administration, clearance and micropuncture experiments were per-formed in intact rats pair fed either a control diet (0.5% P04) or a P04-depleted (PD) diet (0.01 % P04) plus AI(OH3) and in parathyroidectomized (PTX) PD rats, infused either with saline or with neutral sodium P04. Intact PD rats, compared with intact rats on a control diet, exhibited a lower plasma ultrafiltrable (UF) P04 (5.8±0.5 vs. 7.8+0.3 mg/dl), higher fractional excretion (FE) of Ca (4.1±1.2 vs. 0.6±0.1%), and re-duced FE P04 (0.1+0.01 VS. 10.2+ 1.8%). Tubular fluid/ plasma inulin was lower in the late proximal tubule of PD rats, associated with increases in fractional delivery (FD) from the proximal tubule of Na and Ca. The %FD of Ca to the early distal tubule of PD rats was increased (20+3 vs. 11+2%), but this difference was abolished by the late distal tubule (5.1 + 1.2 vs. 3.3±0.9%). In PTX-PD rats, P04 infusion increased plasma UF P04 (13.8±0.7 vs. 7.8±0.7 mg/dl). FE of Ca was reduced (1.08±0.35 vs. 4.59±1.57%) without correcting the increased Ca delivery to the late distal tubule. These data indicate that PD impairs Ca reabsorp-tion in tubular segments before but not within the distal convoluted tubule, so that hypercalciuria is ul