Calcium-induced calcium release mediates all-or-nothing responses of mesenchymal stromal cells to noradrenaline

Abstract

By using Ca²⁺ imaging and Fluo-4 dye, we examined the capability of certain agonists of G-protein coupled receptors to stimulate Ca²⁺ signaling in cultured mesenchymal stromal cells (MSC) derived from the human adipose tissue. In particular, a small subpopulation (∼5%) MSC was found to respond to noradrenaline with Ca²⁺ transients. The all-or-nothing fashion was characteristic of adrenergic Ca²⁺ signaling in MSC, that is, while at low concentrations noradrenaline stimulated undetectable Ca²⁺ transients, virtually maximal responses were elicited by this agonist at any concentration above the threshold of 100–200 nM. In some experiments, MSC were loaded with the photosensitive Ca²⁺ chelator NP-EGTA to produce local or global jumps in cytosolic Ca²⁺ concentration by virtue of Ca²⁺ uncaging. Global uncaging eliciting a high enough Ca²⁺ jump triggered a Ca²⁺ transient in the MSC cytoplasm, which was similar to a noradrenaline response kinetically and by magnitude. When Ca²⁺ uncaging was produced locally, it initiated a Ca²⁺ signal that traveled along a cell with a speed that exceeded an expected one by two orders of magnitude, should Ca²⁺ signal transfer be mediated merely by passive Ca²⁺ diffusion in the presence of Ca²⁺ buffer. These findings implicated Ca²⁺-induced Ca²⁺ release (CICR) as a mechanism amplifying local Ca²⁺ signals in MSC. Of Ca²⁺ targets involved in CICR, the ryanodine receptor and IP₃ receptor are only known. The inhibitory analysis revealed IP₃ receptors to be principally responsible for CICR in MSC, whereas a contribution of ryanodine receptors was negligible. Altogether, our results suggest that an initial noradrenaline-dependent rise in cytosolic Ca²⁺ stimulates, should it reach the threshold level, IP₃ receptors, thereby triggering an avalanche-like Ca²⁺ release from Ca²⁺ stores and underlying the all-or-nothing dependence of cellular responses on the agonist concentration.