Modeling Ca-Polyanion Crosslinking in Secretory Networks. Assessment of Charge Density and Bond Affinity in Polyanionic Secretory Networks

Materials released by secretory cells are stored inside intracellular membrane-bound vesicles. These moieties are not freely diffusible in the vesicle but remain immobilized in a Ca²⁺-crosslinked condensed-phase polyanionic polymer matrix. During exocytosis a Na⁺/Ca²⁺ ion exchange process triggers a volume phase transition resulting in massive swelling and release of the materials to the extracellular space. Here we formulate a simple model to assess Ca²⁺-ion binding from the swelling kinetics of polymer networks. We found the diffusivity of the networks (D) exhibits a power-law dependency on the Ca²⁺ concentration where D ∝ Ca²⁺]^?2/3. The model yields an estimate of charge density and ionic affinity of the polymer chains. Studies of post-exocytic swelling kinetics in airway mucin granules, mast cell granules and granules from the microalga (Phaeocystis globosa) were used to validate predictions from our model. These results suggest that independent of the cell type, from animal to plant cells, a single polyelectrolyte interaction mechanism appear to be responsible for product release in exocytosis.