Direct effects of calcium on proximal tubule P i absorption have been examined only in a limited number of studies. Such an effect could arise indirectly by suppressing PTH release, or directly. Most studies report that elevating plasma calcium increases P i absorption ( 1, 39, 54). The first goal of the present studies was to determine the effect of luminal PTH, alone or in combination with basolateral PTH, on phosphate absorption by single mouse proximal straight tubules and to localize the PTH1R.Д®xtracellular calcium influences renal phosphate absorption. Although PTH sequestered the Na-P i cotransporter when applied to the luminal side of proximal tubules, the expression and functional role of apical PTH1Rs on phosphate absorption by intact segments of proximal tubules are not clear. PTH internalizes the apical NaP i-IIa cotransporter in mouse kidney when added either to mucosal or basolateral surfaces of proximal straight tubules ( 64). Functional type 1 PTH receptors (PTH1R) are located in both basolateral and, perhaps, on luminal membranes of proximal tubules ( 2). PTH inhibits inorganic phosphate (P i) absorption by internalizing the apical BBM type IIa Na-P i cotransporter ( 45). Type IIa Na-P i cotransporters account for в€ј70% of brush-border membrane (BBM) Na-P i cotransporter activity and are the target of parathyroid hormone (PTH) action ( 6). Renal phosphate reabsorption occurs mostly in proximal tubules through a cellular pathway. CaSR activation specifically regulates PTH-suppressible phosphate absorption. Stimulating apical or basolateral PTH1R inhibits phosphate absorption. In conclusion, PTH1Rs are expressed on apical and basolateral membranes of mouse proximal tubules. Experiments with proximal-like opossum kidney cells showed that elevated extracellular Ca 2+ or NPS R467, a type II calcimimetic, inhibited PTH action on P i uptake. Dopamine-inhibitable J Pi was not affected by Gd 3+. Gd 3+ did not affect basal, had no effect on J v when added to lumen or bath. Addition of Gd 3+ to the serosal bathing solution had no effect on PTH-sensitive J Pi. CaSR activation with luminal Gd 3+ abolished the inhibitory action of PTH on J Pi. CaSRs were localized primarily, but not exclusively, to brush-border membranes. Indirect immunofluorescence studies revealed abundant PTH receptor (PTH1R) expression on brush-border membranes, with lower amounts on basolateral membranes. Combined addition of PTH to perfusate and bathing solutions reduced J Pi to 1.5 pmol Inclusion of PTH in the luminal perfusate reduced J Pi to 2.1 pmol mm вЂ“1 under control conditions and decreased by 20% upon addition of serosal PTH.33P and FITC-inulin were added to the luminal perfusate to measure phosphate absorption ( J Pi) and fluid absorption ( J v), respectively. Tubules were bathed with DMEM culture media supplemented with 6% BSA and perfused with an ultrafiltrate prepared from the bathing solution. Single proximal S3 tubules were dissected from the kidneys of mice and studied by the Burg technique. In the present studies, we determined the effect of luminal and/or basolateral PTH on phosphate absorption and tested the hypothesis that CaSR activation blocks PTH-inhibitable phosphate absorption. The calcium-sensing receptor (CaSR) is expressed on apical membranes of proximal tubules. Inorganic phosphate (P i) is absorbed by proximal tubules through a cellular pathway that is inhibited by parathyroid hormone (PTH).

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