Protamine augments stretch induced calcium increase in vascular endothelium

Human umbilical vein endothelial cells cultured on a transparent silicone chamber were subjected to a short stretch pulse (ca. 1 s, 5 - 25% stretch) of their substrate and following increases in intracellular Ca²⁺ concentration ([Ca²⁺]_i) were measured by fluorescence intensity ratiometry using fura-2. In response to mechanical stretch, the cells in HEPES buffered saline exhibited a Ca²⁺ transient in a dose dependent way. The response was completely dependent on external Ca²⁺ and inhibited by gadolinium (Gd³⁺), suggesting that ir was mediated by the activation of a stretch activated cation channel (SACatC). Interestingly, the stretch induced Ca²⁺ transient was significantly augmented in the presence of basic polypeptide, protamine. This augmented Ca²⁺ response was inhibited neither by Gd³⁺ nor by the deprivation of external Ca²⁺, indicating that the SACatC is not responsible for this phenomenon. In contrast, this augmentation was inhibited by depletion of intracellular Ca²⁺ stores with thapsigargin or by the pretreatment with phospholipase inhibitors such as U73122 and manoalide. 5 These results suggest the presence of a metabotropic mechanoreceptor distinct from the SACatC in vascular endothelium. This augmented [Ca^2-]_i increase may contribute to the vasodilating response induced by protamine during heparin neutralization in cardiac surgery.