Activation of dopamine D₁-like receptors induces acute internalization of the renal Na⁺/phosphate cotransporter NaPi-IIa in mouse kidney and OK cells

The Na⁺/phosphate cotransporter NaPi-IIa (SLC34A1) is the major transporter mediating the reabsorption of P_i in the proximal tubule. Expression and activity of NaPi-IIa is regulated by several factors, including parathyroid hormone, dopamine, metabolic acidosis, and dietary P_i intake. Dopamine induces natriuresis and phosphaturia in vivo, and its actions on several Na⁺-transporting systems such as NHE3 and Na ⁺-K⁺-ATPaSe have been investigated in detail. Using freshly isolated mouse kidney slices, perfused proximal tubules, and cultured renal epithelial cells, we examined the acute effects of dopamine on NaPi-IIa expression and localization. Incubation of isolated kidney slices with the selective Di-like receptor agonists fenoldopam (10 μM) and SKF-38393 (10 μM) for 1 h induced NaPi-IIa internalization and reduced expression of NaPi-IIa in the brush border membrane (BBM). The D₂-like selective agonist quinpirole (1 μM) had no effect. The D₁ and D₂ agonists did not affect the renal Na⁺/sulfate cotransporter NaSi in the BBM of the proximal tubule. Studies with isolated perfused proximal tubules demonstrated that activation of luminal, but not basolateral, D₁-like receptors caused NaPi-IIa internalization. In kidney slices, inhibition of PKC (1 μM chelerythrine) or ERK1/2 (20 μM PD-098089) pathways did not prevent the fenoldopam-induced internalization. Inhibition with the PKA blocker H-89 (10 μM) abolished the effect of fenoldopam. Immunoblot demonstrated a reduction of NaPi-IIa protein in BBMs from kidney slices treated with fenoldopam. Incubation of opossum kidney cells transfected with NaPi-IIa-green fluorescent protein chimera shifted fluorescence from the apical membrane to an intracellular pool. In summary, dopamine induces internalization of NaPi-IIa by activation of luminal D₁-like receptors, an effect that is mediated by PKA.