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Table of Contents
ORIGINAL ARTICLE
Year : 2021  |  Volume : 64  |  Issue : 2  |  Page : 80-87

Characterization of Ca2+-Sensing Receptor-Mediated Ca2+ Influx in Microvascular bEND.3 Endothelial Cells


1 Department of Internal Medicine, Division of Cardiology, Kiang Wu Hospital, Macau, China
2 Cardiovascular Division, Fu Jen Catholic University Hospital, Fu Jen Catholic University, New Taipei City; Department of Cardiology, Lotung Poh-Ai Hospital, Yilan County, Taiwan
3 Department of Physiology, China Medical University, Taichung, Taiwan
4 VIP Department, East Hospital, Tongji University School of Medicine, Shanghai, China
5 Department of Anesthesiology, China Medical University Hospital; Department of Anesthesiology, Kuang Tien General Hospital, Taichung, Taiwan
6 Division of Cardiology, Department of Medicine, Wan Fang Hospital, Taipei Medical University, Taipei, Taiwan

Date of Submission16-Nov-2020
Date of Decision06-Mar-2021
Date of Acceptance09-Mar-2021
Date of Web Publication19-Apr-2021

Correspondence Address:
Prof. Yuk-Man Leung
Department of Physiology, China Medical University, Taichung
Taiwan
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/cjp.cjp_93_20

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  Abstract 


Ca2+-sensing receptors (CaSR), activated by elevated concentrations of extracellular Ca2+, have been known to regulate functions of thyroid cells, neurons, and endothelial cells (EC). In this report, we studied CaSR-mediated Ca2+ influx in mouse cerebral microvascular EC (bEND.3 cells). Cytosolic free Ca2+ concentration and Mn2+ influx were measured by fura-2 microfluorometry. High (3 mM) Ca2+ (CaSR agonist), 3 mM spermine (CaSR agonist), and 10 μM cinacalcet (positive allosteric modulator of CaSR) all triggered Ca2+ influx; however, spermine, unlike high Ca2+ and cinacalcet, did not promote Mn2+ influx and its response was poorly sensitive to SKF 96365, a TRP channel blocker. Consistently, 2-aminoethoxydiphenyl borate and ruthenium red (two other general TRP channel blockers) suppressed Ca2+ influx triggered by cinacalcet and high Ca2+ but not by spermine. Ca2+ influx triggered by high Ca2+, spermine, and cinacalcet was similarly suppressed by A784168, a potent and selective TRPV1 antagonist. Our results suggest that CaSR activation triggered Ca2+ influx via TRPV1 channels; intriguingly, pharmacological, and permeability properties of such Ca2+ influx depended on the stimulating ligands.

Keywords: Ca2+, Ca2+-sensing receptors, endothelium, spermine, TRP channels


How to cite this article:
Leong IL, Tsai TY, Shiao LR, Zhang YM, Wong KL, Chan P, Leung YM. Characterization of Ca2+-Sensing Receptor-Mediated Ca2+ Influx in Microvascular bEND.3 Endothelial Cells. Chin J Physiol 2021;64:80-7

How to cite this URL:
Leong IL, Tsai TY, Shiao LR, Zhang YM, Wong KL, Chan P, Leung YM. Characterization of Ca2+-Sensing Receptor-Mediated Ca2+ Influx in Microvascular bEND.3 Endothelial Cells. Chin J Physiol [serial online] 2021 [cited 2021 Jul 28];64:80-7. Available from: https://www.cjphysiology.org/text.asp?2021/64/2/80/314084

The authors Iat-Lon Leong, Tien-Yao Tsai, Paul Chan and Yuk-Man Leung contribute to the study equally





  Introduction Top


Ca2+-sensing receptors (CaSRs) are G-protein-coupled receptors activated by elevated concentrations of extracellular Ca2+. One of the biological functions of CaSR is the modulation of parathyroid hormone (PTH) release.[1] Serum Ca2+ elevation stimulates thyroid cell CaSR, the latter then activates Gq/11 and phospholipase C (PLC). As a result, inositol 1, 4, 5-trisphosphate (IP3) is generated and Ca2+ is mobilized from internal stores. PTH release is consequently inhibited by cytosolic elevation of Ca2+ concentration.

CaSRs are also expressed in the nervous system and blood vessels. CaSRs, sensing extracellular Ca2+, regulate the excitability of nerve cells via opening nonselective cation channels.[2] CaSRs also promote proliferation and even migration of nerve cells.[3] Interestingly, isolated blood vessels are dilated by an increase in extracellular Ca2+: it was shown that CaSR stimulation in the porcine coronary artery endothelial cell (EC) caused the opening of intermediate conductance Ca2+-activated K+ channels.[4] As a consequence, EC hyperpolarized, followed by hyperpolarization of neighboring smooth muscle cells. In a recent work, it was shown that CaSR activation leads to a rise in free cytosolic Ca2+ concentration and NO release in human umbilical vein EC.[5]

In our previous work, we demonstrated protein expression and functions of CaSR in murine brain microvascular EC (bEND.3); the CaSR response (high Ca2+-elicited cytosolic [Ca2+] elevation) was refractory to suppression of PLC and involved Ca2+ influx.[6] In this work, we further studied the pharmacological properties of CaSR-mediated Ca2+ influx. We provided evidence that stimulation of CaSR in bEND.3 cells by three different means, namely high Ca2+, spermine, and cinacalcet, triggered Ca2+ influx via TRPV1 channels. It was found that, surprisingly, pharmacological and permeability properties of CaSR-triggered Ca2+ influx varied with the stimulating ligands.


  Materials and Methods Top


Materials and cell culture

Spermine, cyclopiazonic acid (CPA), and adenosine triphosphate (ATP) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Fura-2 AM was purchased from Calbiochem-Millipore (Darmstadt, Germany). Cinacalcet, 2-aminoethoxydiphenyl borate (2-APB), ruthenium red, CBA, SAR 7334, M084, GSK 2193874, tranilast, Pyr 3, A784168, and SKF 96365 were purchased from Tocris (Bristol, UK). Brain microvascular bEND.3 cells were cultured in Dulbecco's Modified Eagle Medium supplemented with 1% penicillin/streptomycin and 10% fetal bovine serum (Invitrogen, Carlsbad, CA, USA). Lung epithelial A549 cells were cultured in RPMI medium supplemented with 1% penicillin/streptomycin and 10% fetal bovine serum (Invitrogen, Carlsbad, CA, USA).

Microfluorometric measurement of cytosolic Ca2+ concentration and Mn2+ influx

Microfluorometric measurement of cytosolic concentration of Ca2+ was performed using fura-2 as Ca2+-sensitive dye.[7] Briefly, the cells were grown on small glass coverslips and incubated with 5 μM fura-2 AM (Invitrogen, Carlsbad, CA, USA) for 1 h at 37°C. The cells were then washed in bath solution, containing (mM): 140 NaCl, 4 KCl, 1 MgCl2, 2 CaCl2, and 10 HEPES (pH 7.4 adjusted with NaOH). Ca2+-free solution was the same as the bath solution described above except that Ca2+ was absent and 100 μM EGTA was supplemented. Cells were alternately excited with 340 nm and 380 nm (1 Hz switching frequency) with an optical filter changer (Lambda 10-2, Sutter Instruments, Novato, CA, USA). The emission wavelength was 500 nm and images were obtained with a CCD camera (CoolSnap HQ2, Photometrics, Tucson, AZ, USA) connected to an inverted Nikon TE2000-U microscope (Tokyo, Japan). When an influx of Mn2+ was measured, the excitation wavelength was 360 nm and emission wavelength was 500 nm. Data were analyzed by an MAG Biosystems Software (Sante Fe, MN, USA). Experiments were all conducted at room temperature (25°C). Fluorescence changes were measured and analyzed at a region of interest of single cells within a microscopic view (counted as an individual experiment). Experiments were repeated a few more times to obtain the means of all single cells.

Statistical analysis

Data were means ± standard error of the mean. Unpaired or paired Student's t-test was employed where appropriate to make comparisons of two groups. ANOVA was used to compare multiple groups, followed by the Tukey's HSD post hoc test. P < 0.05 was considered statistically significant.


  Results Top


Spermine-stimulated Ca2+ influx was largely unrelated to store-operated Ca2+ entry in bEND.3 cells

In our previous report, we have shown that in bEND.3 cells, high Ca2+-elicited Ca2+ signal was insensitive to PLC inhibition, suggesting that Gq/11 may not be involved.[6] These data suggest that CaSR-stimulated Ca2+ signaling in bEND.3 cells does not depend on Ca2+ release but may involve Ca2+ influx. To confirm that CaSR response is mainly composed of Ca2+ influx but not Ca2+ release, we here employed the standard “Ca2+-free solution—Ca2+ replenishment” protocol [Figure 1]. Ca2+ as a CaSR agonist could not be used in this protocol since the addition of this agonist rendered the solution no longer Ca2+ free. Cinacalcet [Supplement 1], a positive allosteric modulator of CaSR,[8] is not suitable in this protocol since its action requires the presence of Ca2+ (as agonist). We therefore used spermine, a CaSR agonist, for this purpose. As shown in [Figure 1]a, the addition of spermine caused a substantial rise of Ca2+ in Ca2+-containing solution. However, in the absence of extracellular Ca2+, spermine triggered only a minimal (yet significant, P < 0.05) amount of Ca2+ release; subsequent Ca2+ replenishment resulted in Ca2+ influx [Figure 1]b comparable to the level reached at Ca2+-containing solution [Figure 1]a. Such a Ca2+ influx was retarded but not suppressed at a steady state in the presence of TRP blocker SKF 96365. Ni2+, a nonspecific Ca2+ channel blocker, abrogated spermine-stimulated Ca2+ influx [Supplement 2].
Figure 1: Ca2+ influx stimulated by spermine. (a) [Ca2+]i was measured in bEND.3 cells bathed in Ca2+-containing solution and cells were exposed to 3 mM spermine. (b) [Ca2+]i was measured in bEND.3 cells bathed in Ca2+-free solution. Cells were pretreated with or without 10 μM SKF 96365 for 8 min (red and black lines, respectively); after cells were stimulated by 3 mM spermine, 2 mM Ca2+ was replenished. The green line represents the results obtained when cells received no SKF 96365 pre-treatment and spermine stimulation before Ca2+ replenishment. There are significant (*P < 0.05) differences between the spermine-SKF group and spermine-no SKF group between 403 and 583 s. (c) [Ca2+]i was measured in bEND.3 cells bathed in Ca2+-containing solution and cells were treated with 30 μM cyclopiazonic acid. Spermine results obtained in Figure 1a are overlapped here for comparison. Significant (*P < 0.05) differences between the spermine and cyclopiazonic acid group began after 278 s. (d) [Ca2+]i was measured in bEND.3 cells bathed in Ca2+-free solution. After cells were treated with 30 μM cyclopiazonic acid, 2 mM Ca2+ was replenished. Spermine results obtained in Figure 1b are overlapped here for comparison. Significant (*P < 0.05) differences between the spermine and cyclopiazonic acid group began after 500 s. Results are mean ± SEM, with each group having 23–29 cells obtained from three to four separate experiments.

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The data in [Figure 1]a and [Figure 1]b suggest that spermine stimulated Ca2+ influx only with a minimal Ca2+ release, casting doubt on the nature of such Ca2+ influx as store-operated Ca2+ entry. As shown in [Figure 1]c, depletion of endoplasmic reticulum Ca2+ store with maximal (CPA, inhibitor of sarcoplasmic/endoplasmic reticulum Ca2+ pump) concentration resulted in a sustained Ca2+ signal in the presence of bath Ca2+. Also shown in the same figure for comparison is spermine-stimulated Ca2+ signal [Figure 1]a, which was significantly stronger than the CPA-stimulated one. [Figure 1]d shows CPA-stimulated Ca2+ release (in Ca2+-free solution) and Ca2+ influx (upon Ca2+ replenishment); spermine-stimulated Ca2+ release and Ca2+ influx [Figure 1]b were also shown for comparison. It is obvious that spermine stimulated less Ca2+ release but elicited a much stronger Ca2+ influx. Taken together, the results suggest spermine-stimulated Ca2+ entry, a major fraction of which was unrelated to store-operated Ca2+ entry.

Different sensitivity of high Ca2+-, spermine-, and cinacalcet-triggered Ca2+ influx to SKF 96365 in bEND.3 cells

We compared SKF 96365 inhibitory effects on Ca2+ signals triggered by high Ca2+, spermine, and cinacalcet (positive allosteric modulator of CaSR) in Ca2+-containing bath solution. As shown in [Figure 2]a and [Figure 2]b, Ca2+ signals triggered by high Ca2+ and cinacalcet were substantially suppressed by SKF 96365. Ca2+ signal triggered by spermine was retarded but only marginally suppressed at steadystate by SKF 96365 [Figure 2]c, an observation consistent with the results shown in [Figure 1]b. The results in [Figure 1] and [Figure 2] therefore suggest the sensitivity of CaSR-stimulated Ca2+ influx to SKF 96365 depended on the stimulating ligands.
Figure 2: Suppressive effects of SKF 96365 on Ca2+ signals triggered by high Ca2+, cinacalcet, and spermine. [Ca2+]i was measured in bEND.3 cells bathed in Ca2+-containing solution; cells were pretreated with or without 10 μM SKF 96365 for 8 min and then treated with (a) 3 mM Ca2+, significant (*P < 0.05) differences between the control and SKF group began after 103 s; (b) 10 μM cinacalcet, significant (*P < 0.05) differences between the control and SKF group began after 100 s; or (c) 3 mM spermine, significant (*P < 0.05) differences between the control and SKF group began after 119 s. Results are mean ± SEM, with each group having 16–29 cells obtained from three separate experiments.

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Unlike high Ca2+ and cinacalcet, spermine did not promote Mn2+ influx in bEND.3 cells

We then examined whether CaSR stimulation triggered Mn2+ influx. The results in [Figure 3]a and [Figure 3]b showed that high Ca2+ and cinacalcet stimulated Mn2+ influx. Unexpectedly, spermine did not stimulate Mn2+ influx [Figure 3]c.
Figure 3: Mn2+ influx was triggered by extracellular Ca2+ and cinacalcet but not spermine. (a) bEND.3 cells were bathed in Ca2+-free solution, treated with 1 mM Mn2+, followed by the addition of water or 3 mM Ca2+. Significant differences (*P < 0.05) between the control and CaCl2 group began after 462 s. (b) bEND.3 cells were bathed in 1 mM Ca2+-containing solution, treated with 1 mM Mn2+, followed by the addition of DMSO or 10 μM cinacalcet. Significant differences (*P < 0.05) between the control and cinacalcet group began after 200 s. (c) bEND.3 cells were bathed in Ca2+-free solution, treated with 1 mM Mn2+, followed by the addition of water or 3 mM spermine. (d) bEND.3 cells were bathed in Ca2+-free solution containing 10 μM SKF 96365, treated with 1 mM Mn2+, followed by the addition of water or 3 mM Ca2+. (e) bEND.3 cells were bathed in 1 mM Ca2+-containing solution containing 10 μM SKF 96365, treated with 1 mM Mn2+, followed by the addition of DMSO or 10 μM cinacalcet. Results are mean ± SEM, with each group having 26–50 cells obtained from three to five separate experiments.

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Since high Ca2+- and cinacalcet-stimulated Ca2+ signals were suppressed by SKF 96365, we next investigated whether high Ca2+- and cinacalcet-stimulated Mn2+ influx was sensitive to SKF 96365. The results in [Figure 3]d and [Figure 3]e show that high Ca2+ and cinacalcet did not stimulate Mn2+ influx in the presence of SKF 96365.

A784168-suppressed Ca2+ influx triggered by cinacalcet, high Ca2+, and spermine in bEND.3 cells

We also examined whether Ca2+ influx triggered by cinacalcet, high Ca2+, and spermine was inhibited by 2-APB and ruthenium red (general TRP channel blockers). As shown in [Figure 4]a and [Figure 4]b, Ca2+ signals triggered by high Ca2+ and cinacalcet were suppressed by 2-APB and ruthenium red. However, Ca2+ signal triggered by spermine was retarded but not suppressed at a steady state by these two agents [Figure 4]c. These results, being consistent with the effects of SKF 96365 shown in [Figure 2], suggest that high Ca2+ and cinacalcet differed from spermine in the manner they affected TRP channels.
Figure 4: Suppressive effects of 2-APB and ruthenium red on Ca2+ signals triggered by high Ca2+, cinacalcet , and spermine. [Ca2+]i was measured in bEND.3 cells bathed in Ca2+-containing solution; cells were pretreated with or without 50 μM 2-APB or 30 μM ruthenium red for 8 min and then treated with (a) 3 mM Ca2+; significant (P < 0.05) differences between the control and 2-APB group and ruthenium red group began after 183 (*) and 107 s (**), respectively; (b) 10 μM cinacalcet; significant (P < 0.05) differences between the control and 2-APB group and ruthenium red group began after 115 (*) and 100 s (**), respectively; or (c) 3 mM spermine; there were significant (P < 0.05) differences between the control and 2-APB group from 102 to 318 s (*), and between the control and ruthenium red group from 122 to 340 s (**). Results are mean ± SEM, with each group having 23–41 cells obtained from three to four separate experiments.

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Further experiments were performed to identify which TRP members were involved in CaSR-mediated Ca2+ influx. As shown in [Figure 5]a, [Figure 5]b, [Figure 5]c, Ca2+ influx triggered by high Ca2+, spermine, and cinacalcet was similarly suppressed by A784168 (1 μM), a potent and selective TRPV1 antagonist.[9] High Ca2+-triggered Ca2+ influx was not inhibited by CBA (TRPM4 inhibitor; 10 μM), SAR 7334 (TRPC6 inhibitor; 3 μM), Pyr 3 (TRPC3 inhibitor; 10 μM), M084 (TRPC4/5 inhibitor; 10 μM), GSK 2193874 (TRPV4 inhibitor; 300 nM), and tranilast (TRPV2 inhibitor; 10 μM) [Supplement 3].
Figure 5: Suppressive effects of A784168 on Ca2+ signals triggered by high Ca2+, cinacalcet, and spermine. [Ca2+]i was measured in bEND.3 cells bathed in Ca2+-containing solution; cells were pretreated with or without 1 μM A784168 for 8 min and then treated with (a) 3 mM Ca2+; significant (*P < 0.05) differences between the control and A784168 group began after 187 s; (b) 10 μM cinacalcet; significant (*P < 0.05) differences between the control and A784168 group began after 114 s; or (c) 3 mM spermine; significant (*P < 0.05) differences between the control and A784168 group began after 192 s. Results are mean ± SEM, with each group having 25–37 cells obtained from three to four separate experiments.

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Effects of TRP channel blockers on spermine-triggered Ca2+ influx in A549 cells

Since spermine-triggered Ca2+ influx differed pharmacologically from that triggered by high Ca2+ and cinacalcet, we further investigated spermine actions in another cell type, the human alveolar epithelial A549 cell. As shown in [Figure 6]a, the addition of spermine triggered Ca2+ signal in Ca2+-containing solution. In the absence of extracellular Ca2+, spermine did not trigger Ca2+ release; subsequent Ca2+ replenishment resulted in Ca2+ influx [Figure 6]b. SKF 96365 strongly suppressed [Figure 6]a, while A784168 did not inhibit spermine-triggered Ca2+ influx [Figure 6]c. These results with SKF 96365 and A784168 as probes in A549 cells appeared to be different from those obtained in bEND.3 cells.
Figure 6: Effects of SKF 96365 and A784168 on Ca2+ signals triggered by spermine in A549 cells. (a) [Ca2+]i was measured in A549 cells bathed in Ca2+-containing solution; cells were pretreated with or without 10 μM SKF 96365 for 8 min and then treated with 3 mM spermine; significant (*P < 0.05) differences between the control and SKF 96365 group began after 138 s; (b) [Ca2+]i was measured in A549 cells bathed in Ca2+-free solution. Cells were treated with 3 mM spermine and then 2 mM Ca2+ was replenished. (c) [Ca2+]i was measured in A549 cells bathed in Ca2+-containing solution; cells were pretreated with or without 1 μM A784168 for 8 min and then treated with 3 mM spermine. Results are mean ± SEM, with each group having 67–88 cells obtained from four separate experiments.

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  Discussion Top


In a previous report, we have shown protein (Western blot) and functional expression of CaSR in bEND.3 cells.[6] We have shown that CaSR response (high Ca2+-triggered Ca2+ signaling) in bEND.3 is unaffected by PLC inhibition caused by two chemically unrelated inhibitors, edelfosine and U73122,[6] therefore suggesting that the Ca2+ signaling is not caused by Ca2+ release by IP3 produced via Gq/11-PLC-mediated cleavage of phosphatidyl 4,5-bisphosphate (PIP2). The “Ca2+-free solution—Ca2+ replenishment” protocol allows one to dissect Ca2+ influx from Ca2+ release, but this protocol, as explained, precludes the use of high Ca2+ and cinacalcet to stimulate CaSR. We therefore used spermine as a CaSR agonist and found that it stimulated Ca2+ influx largely unrelated to store-operated Ca2+ entry; this finding is consistent with the insensitivity of high Ca2+-triggered Ca2+ signaling response to PLC inhibition. CaSR has been demonstrated to be coupled to a variety of G proteins, for instance, G12/13, Gq/11, Gi, and Gs.[10],[11],[12],[13] CaSR activation has also been shown to stimulate ERK1/2.[14],[15] None of the above is directly coupled to Ca2+ mobilization except Gq/11, but our data are not supportive of a role of Gq/11 in causing Ca2+ influx.

We have excluded the involvement of the reverse mode of Na+/Ca2+ exchanger in high Ca2+-triggered Ca2+ signaling.[6] The suppression of high Ca2+- and cinacalcet-triggered Ca2+ signals and Mn2+ influx by SKF 96365, 2-APB, and ruthenium red suggests that CaSR-mediated Ca2+ influx was via TRP channels [Figure 2] and [Figure 3]. As distinguished from voltage-gated Ca2+ channels such as L-, T-, and N-type voltage-gated Ca2+ channels, TRP channels are a large family of nonvoltage-gated Ca2+-permeable channels.[16] It is noted that bEND.3 cells express multiple TRP channels such as various members of TRPM, TRPC, and TRPV.[17] Indeed, CaSRs interact with various TRP channels in mesangial cells,[18] aortic smooth muscle cells,[19] and mesenteric artery ECs.[20] The results in [Figure 1], [Figure 2], [Figure 3], [Figure 4] suggest high Ca2+-, spermine-, and cinacalcet-triggered Ca2+ influx via TRP channels; the Ca2+ influx was similarly suppressed by A784168 (a potent and selective TRPV1 antagonist) [Figure 5], suggesting the involvement of TRPV1 channels. This finding is considered novel, as it is noted that, on the contrary, CaSR activation leads to inhibition of TRPV1 in rat vagal bronchopulmonary sensory neurons.[21] How CaSR is coupled to TRP channels is hitherto undefined.[22]

CaSR stimulation by high Ca2+ and cinacalcet has been shown to activate intermediate conductance Ca2+-activated K+ channels and thus hyperpolarize porcine coronary artery ECs.[4] Such hyperpolarization increases the electrochemical driving force for Ca2+ influx. Therefore, in our case, it is possible that high extracellular Ca2+ stimulated CaSR, TRP channel opening, and Ca2+ influx; the latter was enhanced further by hyperpolarization. Spermine-stimulated Ca2+ influx via TRP was also expected to be modulated by membrane potential changes since spermine inhibits inward rectifier K+ (Kir) channels, whose main role is to set the resting membrane potential.[23] It has been suggested that a small efflux of K+ above equilibrium K+ potential is the usual function of Kir channels.[23] Therefore, spermine inhibition of Kir channels may indeed depolarize bEND.3 cells.

What is rather unexpected is that spermine-stimulated Ca2+ influx, in contrast to the responses to high Ca2+ and cinacalcet, was much less sensitive to SKF 96365. Consistently, high Ca2+- and cinacalcet-triggered Ca2+ influx was significantly inhibited by 2-APB and ruthenium red (general TRP channel blockers), but spermine-triggered Ca2+ influx could only be slowed down but not suppressed by them. These data suggest that high Ca2+ and cinacalcet differed from spermine in the manner they interacted with the CaSR-TRP channel complex. Furthermore, while high Ca2+ and cinacalcet stimulated Mn2+ influx, spermine failed to do so, although it markedly stimulated an influx of Ca2+. It therefore appears that upon activation by different ligands, CaSR-triggered Ca2+ influx exhibited different pharmacological and permeability properties. CaSR activation has been known to exhibit biased agonism, in which binding by distinct ligands selectively activates certain signaling pathways such as G12/13, Gq/11, Gi, Gs, or ERK1/2.[14],[22] Ca2+, spermine, and cinacalcet may bind to distinct sites of CaSR, causing the latter to adopt different active conformations, eventually leading to biased agonism.[22] Biased agonism may also explain why CaSR activation could be coupled to different TRP channel members.[18],[19],[20],[21] In line with this notion, we have shown in [Figure 6] that spermine-triggered Ca2+ influx may be via TRP channels other than TRPV1 in A549 cells, a result different from what was observed in spermine-stimulated bEND.3 cells. Our work here further reveals that distinct CaSR ligands could differentially affect the pharmacological and permeability properties of TRP channel opening.


  Conclusion Top


CaSR activation in bEND.3 cells led to Ca2+ influx largely unrelated to store-operated Ca2+ entry; such Ca2+ influx might be via TRPV1 channels. Interestingly, different CaSR ligands appeared to affect differently the pharmacological and permeability properties of the elicited Ca2+ influx.

Acknowledgements

Y.M.L. and K.L.W. would like to thank China Medical University and China Medical University Hospital, Taiwan, for providing fundings (DMR-108-083; DMR-109-093; CMU108-S-31). This work was also supported by a grant from the Natural Science Foundation of Shanghai (18ZR1430900).

Financial support and sponsorship

Y.M.L. and K.L.W. would like to thank China Medical University and China Medical University Hospital, Taiwan, for providing fundings (DMR-108-083; DMR-109-093; CMU108-S-31). This work was also supported by a grant from the Natural Science Foundation of Shanghai (18ZR1430900).

Conflicts of interest

There are no conflicts of interest.



 
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