NHE8 attenuates Ca²⁺influx into NRK cells and the proximal tubule epithelium

This is the authors' abstract. We don't add key points for this paper.

To garner insights into the renal regulation of Ca 2+ homeostasis, we performed an mRNA microarray on kidneys from mice treated with the Ca 2+ -sensing receptor (CaSR) agonist cinacalcet. This revealed decreased gene expression of Na + /H + exchanger isoform 8 (NHE8) in response to CaSR activation. These results were confirmed by quantitative real-time PCR. Moreover, administration of vitamin D also decreased NHE8 mRNA expression. In contrast, renal NHE8 protein expression from the same samples was increased. To examine the role of NHE8 in transmembrane Ca 2+ fluxes, we used the normal rat kidney (NRK) cell line. Cell surface biotinylation and confocal immunofluorescence microscopy demonstrated NHE8 apical expression. Functional experiments found 5-( N-ethyl- N-isopropyl)amiloride (EIPA)-inhibitable NHE activity in NRK cells at concentrations minimally attenuating NHE1 activity in AP-1 cells. To determine how NHE8 might regulate Ca 2+ balance, we measured changes in intracellular Ca 2+ uptake by live cell Ca 2+ imaging with the fluorophore Fura-2 AM. Inhibition of NHE8 with EIPA or by removing extracellular Na + -enhanced Ca 2+ influx into NRK cells. Ca 2+ influx was mediated by a voltage-dependent Ca 2+ channel rather than directly via NHE8. NRK cells express Cav1.3 and display verapamil-sensitive Ca 2+ influx and NHE8 inhibition-augmented Ca 2+ influx via a voltage-dependent Ca 2+ channel. Finally, proximal tubules perused ex vivo demonstrated increased Ca 2+ influx in the presence of luminal EIPA at a concentration that would inhibit NHE8. The results of the present study are consistent with NHE8 regulating Ca 2+ uptake into the proximal tubule epithelium.