Specialized Ca(2+)-dependent ion channels ubiquitously couple intracellular Ca(2+) signals to a change in cell polarization. The existing physiological evidence suggests that Ca(2+)-activated Cl(-) channels (CaCCs) are functional in taste cells. Because Ano1 and Ano2 encode channel proteins that form CaCCs in a variety of cells, we analyzed their expression in mouse taste cells. Transcripts for Ano1 and Ano2 were detected in circumvallate (CV) papillae, and their expression in taste cells was confirmed using immunohistochemistry. When dialyzed with CsCl, taste cells of the type III exhibited no ion currents dependent on cytosolic Ca(2+). Large Ca(2+)-gated currents mediated by TRPM5 were elicited in type II cells by Ca(2+) uncaging. When TRPM5 was inhibited by triphenylphosphine oxide (TPPO), ionomycin stimulated a small but resolvable inward current that was eliminated by anion channel blockers, including T16Ainh-A01 (T16), a specific Ano1 antagonist. This suggests that CaCCs, including Ano1-like channels, are functional in type II cells. In type I cells, CaCCs were prominently active, blockable with the CaCC antagonist CaCCinh-A01 but insensitive to T16. By profiling Ano1 and Ano2 expressions in individual taste cells, we revealed Ano1 transcripts in type II cells only, while Ano2 transcripts were detected in both type I and type II cells. P2Y agonists stimulated Ca(2+)-gated Cl(-) currents in type I cells. Thus, CaCCs, possibly formed by Ano2, serve as effectors downstream of P2Y receptors in type I cells. While the role for TRPM5 in taste transduction is well established, the physiological significance of expression of CaCCs in type II cells remains to be elucidated.

Electrogenesis in mesenchymal stromal cells (MSCs) remains poorly understood. Little is known about ion channels active in resting MSCs and activated upon MSC stimulation, particularly, by agonists mobilizing Ca in the MSC cytoplasm. A variety of Ca-gated ion channels may couple Ca signals to polarization of the plasma membrane. Here, we studied MSCs from the human adipose tissue and found that in cells responsive to ATP and adenosine with Ca transients or exhibiting spontaneous Ca oscillations, Ca bursts were associated with hyperpolarization mediated by Ca-gated K channels. The expression analysis revealed transcripts for KCNMA1 and KCNN4 genes encoding for Ca-activated K channels of large (K1.1) and intermediate (K3.1) conductance, respectively. Moreover, transcripts for the Ca-gated cation channel TRPM4 and anion channels Ano1, Ano2, and bestrophin-1, bestrophin-3, and bestrophin-4 were revealed. In all assayed MSCs, a rise in cytosolic Ca stimulated K currents that were inhibited with iberiotoxin. This suggested that K1.1 channels are invariably expressed in MSCs. In ATP- and adenosine-responsive cells, iberiotoxin and TRAM-34 diminished electrical responses, implicating both K1.1 and K3.1 channels in coupling agonist-dependent Ca signals to membrane voltage. Functional tests pointed at the existence of two separate MSC subpopulations exhibiting Ca-gated anion currents that were mediated by Ano2-like and bestrophin-like anion channels, respectively. Evidence for detectable activity of Ano1 and TRPM4 was not obtained. Thus, K1.1 channels are likely to represent the dominant type of Ca-activated K channels in MSCs, which can serve in concert with K3.1 channels as effectors downstream of G-protein-coupled receptor (GPCR)-mediated Ca signaling.

The current knowledge of electrogenesis in mesenchymal stromal cells (MSCs) remains scarce. Earlier, we demonstrated that in MSCs from the human adipose tissue, transduction of certain agonists involved the phosphoinositide cascade. Its pivotal effector PLC generates DAG that can regulate ion channels directly or via its derivatives, including arachidonic acid (AA). Here we showed that AA strongly hyperpolarized MSCs by stimulating instantly activating, outwardly rectifying TEA-insensitive K+ channels. Among AA-regulated K+ channels, K2P channels from the TREK subfamily appeared to be an appropriate target. The expression of K2P channels in MSCs was verified by RT-PCR, which revealed TWIK-1, TREK-1, and TASK-5 transcripts. The TREK-1 inhibitor spadin antagonized the electrogenic action of AA, which was simulated by the channel activator BL 1249. This functional evidence suggested that TREK-1 channels mediated AA-dependent hyperpolarization of MSCs. Being mostly silent at rest, TREK-1 negligibly contributed to the “background” K+ current. The dramatic stimulation of TREK-1 channels by AA indicates their involvement in AA-dependent signaling in MSCs.

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