Polycations induce calcium signaling in glomerular podocytes.

Abstract

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. [Ca2+]i was measured in single cultured differentiated mouse podocytes with the fluorescence dye fura-2/AM. Protamine sulfate caused a concentration-dependent and partially reversible increase of [Ca2+]i (EC50 approximately 1.5 micromol/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 micromol/liter) did not influence [Ca2+]i. A reduction of the extracellular Ca2+ concentration (from 1 mmol/liter to 1 micomol/liter) partially inhibited the protamine sulfate and the DEAE-dextran-induced [Ca2+]i response. Flufenamate (100 micromol/liter) or Gd3+ (10 micromol/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. 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.