Polycations induce calcium signaling in glomerular podocytes.

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 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.

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.