Parathyroid hormone and hydrochlorothiazide increase calcium transport by the luminal membrane of rabbit distal nephron segments through different pathways.

The present study was designed to investigate the effect of PTH on calcium (Ca2+) transport through the luminal membrane of proximal and distal rabbit tubule segments. Proximal tubule and distal tubule segment suspensions were incubated with the hormone, and the luminal membranes were subsequently purified. Incubation with 10(-8) M human PTH(1-34) strongly increased initial Ca2+ uptake by the distal membranes. The effect of PTH was dose dependent, with an apparent ED50 of 8.2 +/- 1.0 nM. We recently reported the presence of two kinetics of Ca2+ uptake by the distal luminal membranes. PTH affected exclusively the high affinity component, increasing the maximum velocity from 0.31 +/- 0.02 to 0.76 +/- 0.07 pmol/micrograms.10 sec (P < 0.001), and leaving the Michaelis-Menten constant Ca2+ unchanged. The addition of 500 microM hydrochlorothiazide (HCTZ) to the luminal membranes of distal tubules incubated with PTH further enhanced Ca2+ uptake. The effect of HCTZ was on the low affinity system. HCTZ (100 microM) enhanced the maximum velocity from 2.5 +/- 0.3 to 3.7 +/- 0.6 pmol/micrograms protein.10 sec (P < 0.01) without affecting the Michaelis-Menten constant. Whereas 1 microM nitrendipine alone did not affect Ca2+ transport by the distal tubule luminal membranes, the Ca2+ channel inhibitor completely abolished the effect of PTH. Conversely, 1 microM Bay K 8644 increased Ca2+ uptake by membranes from PTH-treated distal tubules but was ineffective in membranes from control tubules. Neither PTH nor nitrendipine nor Bay K 8644 had any effect on the luminal membranes from proximal tubules. These results suggest that: 1) the high affinity, low velocity Ca2+ transport system in the luminal membrane from distal cortical segments is sensitive to PTH; 2) the effects of nitrendipine and Bay K 8644 on Ca2+ uptake were observed only in membranes from tubules incubated with PTH; 3) this uptake is distinct from the thiazide-sensitive Ca2+ transport system; and 4) PTH does not influence Ca2+ transport by the luminal membrane of proximal tubules.

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