**Acknowledgements**

generates an initial, local, and gradual Ca2+ signal. Otherwise, it is difficult to explain why in cells overloaded with NP-EGTA, agonist responses disappeared contrary to light responses

Note in conclusion that the specific features of agonist responses, including kinetics and magnitude, all-or-nothing behavior and gradual dose-response delay were correctly reproduced by Ca2+ signals elicited by Ca2+ uncaging (**Figure 5**). This supports the idea that agonist-evoked Ca2+ signaling in MSCs includes two different but coupled stages. Initially, agonists stimulate coupling of suitable GPCRs via appropriate G-proteins to PLC, thus trig-

receptors (IP<sup>3</sup>

ions from Ca2+ store. This machinery generates an initial, presumably local and gradual Ca2+ signal (**Figure 11**). When exceeding the threshold, this local Ca2+ signal stimulates CICR that

Ca2+ store. By involving the trigger-like mechanism CICR, a cell generates Ca2+ responses of virtually universal shape and magnitude at different agonist concentrations above the

**Figure 11.** Working model of agonist transduction in MSCs. The binding of agonists to GPCRs stimulates PLC-dependent

related Ca2+ store, producing an initial Ca2+ signal that gradually rises with agonist concentration (red curve). As soon as this signal reaches the threshold level (dotted line), the process determining agonist-dependent delay of a cellular

receptors (IP<sup>3</sup>

RCICR) in separated Ca2+ store and triggers CICR. This provides

Rgrad) mediates Ca2+ release from

. The consequent activation of IP<sup>3</sup>

a significant amplification mechanism that finalizes transduction with a large and global Ca2+ signal (blue curve).

receptors of another type (IP<sup>3</sup>

Rgrad) followed by the release of Ca2+

RCICR) presumably located in another, spatially separated

associated with Ca2+ uncaging (**Figure 10**).

production, activation of IP<sup>3</sup>

receptors (IP<sup>3</sup>

**5. Conclusion**

158 Calcium and Signal Transduction

gering IP<sup>3</sup>

is mediated by IP<sup>3</sup>

hydrolysis of PIP<sup>2</sup>

response, one stimulates IP<sup>3</sup>

to DAG and IP<sup>3</sup>

We thank Dr. V. Yu. Sysoeva for providing MSCs of the first passage. We are thankful to the Russian Science Foundation for support of studies of adrenergic and purinergic transduction (grant 18-14-0034) and P2Y receptors (grant 17-75-10127).
