Functional Expression of Adenosine Receptors in Mesenchymal Stromal Cells

Extracellular adenosine is a signaling molecule involved in the regulation of physiological processes in a variety of cells, including mesenchymal stromal cells (MSCs). We have shown using microphotometry and Ca2+ probes that the population of MSCs isolated from human adipose tissue contained a small (5–10%) subpopulation of adenosinergic cells. In these cells, A1, A2A, and A2B adenosine receptors functioned; they were coupled with a phosphoinositide signaling cascade. Stimulation of adenosinergic MSCs by adenosine initiated the mobilization of intracellular Ca2+; Ca2+ responses to adenosine were generated according to the “all or nothing” principle: in small doses adenosine did not change the level of intracellular Ca2+, but it evoked Ca2+ responses of almost identical shape and amplitude at any concentration exceeding a threshold. It is noteworthy that the duration of the response delay from the moment of application of adenosine decreased as its concentration increased. Key stages of the generation of the Ca2+ response to adenosine were studied using the inhibitor analysis. The obtained data indicated that the adenosine receptor activated phospholipase C, which stimulated the production of IP3; the latter activated IP3 receptors and initiated the release of stored Ca2+, which most likely was dependent on the concentration of the agonist. This initial Ca2+ signal stimulated a large-scale release of Ca2+ from intracellular stores by the mechanism of Ca2+-induced Ca2+ release, which formed a secondary Ca2+ response of universal shape and amplitude.