Polycations induce calcium signaling in glomerular podocytes

Abstract

Background. The neutralization of the polyanionic surface of the podocyte by perfusion of kidneys with polycations, such as protamine sulfate, leads to a retraction of podocyte foot processes and proteinuria. This study investigates the effects of protamine sulfate or anionic, neutral, or cationic dextrans on the cytosolic calcium activity ([Ca2+](i)) in podocytes. Methods. [Ca2+](i) was measured in single cultured differentiated mouse podocytes with the fluorescence dye fura-2/AM. Results. Protamine sulfate caused a concentration-dependent and partially reversible increase of [Ca2+](i) (EC50 approximately 1.5 μmol/liter). Pretreatment of the cells with heparin (100 U/liter) inhibited the protamine sulfate- mediated increase of [Ca2+](i). Like protamine sulfate, diethylaminoethyl dextran (DEAE-dextran) concentration dependently increased [Ca2+](i) in podocytes (EC50 approximately 20 nmol/liter), whereas dextran sulfate or uncharged dextran (both 10 μmol/liter) did not influence [Ca2+](i). A reduction of the extracellular Ca2+ concentration (from 1 mmol/liter to 1 μmol/liter) partially inhibited the protamine sulfate and the DEAE-dextran- induced [Ca2+](i) response. Flufenamate (100 μmol/liter) or Gd3+ (10 μmol/liter), which are known to inhibit nonselective ion channels, did not influence the [Ca2+](i) increase induced by protamine sulfate. In the presence of thapsigargin (50 nmol/liter), an inhibitor of the endoplasmic reticulum Ca2+-ATPase, both protamine sulfate and DEAE-dextran increased [Ca2+](i). Conclusions. The data indicate that polycations increase podocyte [Ca2+](i). The increase of [Ca2+](i) may be an early event in the pathogenesis of protamine sulfate-mediated retraction of podocyte foot processes.