Characterization of a Na(+)-Ca(2+) exchanger in podocytes

Knowledge about Ca(2+) extrusion mechanisms in podocytes is limited. The aim of the study was to test whether a Na(+)-Ca(2+) exchanger (NCX) is present in differentiated podocytes and if so to examine its regulatory properties.

Intracellular Ca(2+) concentration ([Ca(2+)](i)) and intracellular pH were measured microspectrofluorometrically in single podocytes. Expression of NCX mRNA was studied by reverse transcription-polymerase chain reaction. NCX protein expression was investigated by immunocytochemistry.

Substitution of extracellular Na(+) (from 145 to 0, 5, 10, 20, and 30 mM) with N-methyl-D-glucamine resulted in a Na(+) concentration-dependent, reversible increase of [Ca(2+)](i). Complete extracellular Na(+) substitution (0 Na(+)) increased [Ca(2+)](i) reversibly from 95+/-5 to 275+/-16 and back to 66+/-5 nM (n=205). Raising the intracellular Na(+) concentration by application of 50 micro M monensin increased [Ca(2+)](i) from 105+/-22 to 192+/-45 nM (n=12). The [Ca(2+)](i) response induced by a low Na(+) concentration required extracellular Ca(2+) and did not correlate with changes of intracellular pH. The effect was blocked by the NCX inhibitor benzamil (IC(50) approximately 100 nM). Neither flufenamate (100 micro M, n=6), a blocker of non-selective cation channels, nor Hoe 694 (1 micro M, n=6), an inhibitor of the Na(+)-H(+) exchanger, did significantly influence the [Ca(2+)](i) response induced by extracellular Na(+) depletion. Activation of protein kinase C (PKC) by short-term application (5 min) of phorbol 12-myristate-13-acetate (PMA; 10 nM, n=4; 100 nM, n=7) inhibited Na(+)-Ca(2+) exchange, whereas PKC inhibition by long-term incubation (24 h) with PMA (100 nM, n=9) or bisindolylmaleimide I (100 nM, n=11) both increased Na(+)-Ca(2+) exchange, respectively. Expression of NCX mRNA was detected both in cultured differentiated podocytes and in podocytes directly pulled off from glomeruli ex vivo. NCX protein expression was detected by immunocytochemistry. In a different series of experiments, we studied the potential involvement of the exchanger in podocyte injury induced by the aminonucleoside puromycin. Pre-treatment of podocytes with 0.3 mM puromycin for 24 h significantly reduced the [Ca(2+)](i) response induced by extracellular Na(+) depletion (n=56). Compared with mRNA expression of the housekeeping gene GAPDH, NCX mRNA expression was significantly reduced by puromycin.

Our results demonstrate the presence of a Na(+)-Ca(2+) exchanger in podocytes and its regulation by PKC. Inhibition of Na(+)-Ca(2+) exchange by puromycin may contribute to podocyte injury in PAN nephrosis.