Polycations induce calcium signaling in glomerular podocytes

Background. The neutralization of the polyanionic surface of the podocyte b y perfusion of kidneys with polycations, such as protamine sulfate, leads t o a retraction of podocyte foot processes and proteinuria. This study inves tigates the effects of protamine sulfate or anionic, neutral, or cationic d extrans on the cytosolic calcium activity ([Ca2+]) in podocytes. Methods. [Ca2+](i) was measured in single cultured differentiated mouse pod ocytes with the fluorescence dye fura-2/AM. Results. Protamine sulfate caused a concentration-dependent and partially r eversible increase of [Ca2+](i) (EC50 approximately 1.5 mu mol/liter). Pret reatment of the cells with heparin (100 U/liter) inhibited the protamine su lfate-mediated increase of [Ca2+](i). Like protamine sulfate, diethylaminoe thyl dextran (DEAE-dextran) concentration dependently increased [Ca2+](i) i n podocytes (EC50 approximately 20 nmol/liter), whereas dextran sulfate or uncharged dextran (both 10 mu mol/liter) did not influence [Ca2+](i). A red uction of the extracellular Ca2+ concentration (from 1 mmol/liter to 1 mu m ol/liter) partially inhibited the protamine sulfate and the DEAE-dextran-in duced [Ca2+](i) response. Flufenamate (100 mu mol/liter) or Gd3+ (10 mu mol /liter), which are known to inhibit nonselective ion channels, did not infl uence 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 pr otamine sulfate-mediated retraction of podocyte foot processes.