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. 2022 Jul 2;23(13):7377. doi: 10.3390/ijms23137377

Table 1.

Ca2+-permeable ion channels regulation by acidic pHe.

Ion Channel Cell Type Methodology Acidic pH Value and Treatment Time Effect of Low pH on Channel’s Activity/Expression Effect of Low pH on Ca2+ Signals Cellular Function Ref.
Piezo1 Piezo1-transiently transfected HEK293 cells Patch clamp
Mn2+ quenching assay		 pHe 6.3–6.7, acute treatment Stabilization of inactivated state, both acidic pHi and pHe inhibit channel’s activity Decreased Ca2+ influx Not assessed [62]
Murine pancreatic stellate cells (mPSCs) Mn2+ quenching assay
mPSCs spheroids viability and apoptosis assay		 pHe 6.6 and pHi 6.77 (obtained by 30 mM propionate) in acute treatment for Mn2+ quenching assay, while 24 h long treatment for spheroid histology Acidic pHe do not modify Piezo1 activity, while intracellular acidification inhibits channel’s activity Acidic pHe do not modify Ca2+ influx, while intracellular acidification decreases Ca2+ influx Acidic pHe (6.6) impairs PSCs spheroid’s integrity and viability, inducing cell apoptosis [63]
TRPM2 Inducible TRPM2-overexpressing HEK293 Patch clamp External solution with pH 5–8 superfused for 200 s. Internal solution with pH 6 superfused for 100 s; External solution with pH 3.5–6.5 in acute treatment or more prolonged periods (≥2 min) Extracellular acidification inactivates the channel in a voltage-dependent manner and [H+]-dependent manner. Intracellular acidification induces channel closure Not assessed, but recovery from acidic pH-induced inactivation requires external Ca2+ ions Not assessed [64]
Human neutrophils Patch clamp External solution with pH 5 in acute treatment External acidification negatively affects open probability and single-channel conductance, inducing channel closure Not assessed Not assessed [64]
TRPM2-overexpressing HEK293 Patch clamp External solution with pH 3.5–6 in acute treatment External acidification (up to pH 4.5) reversely decreases mean current amplitude in a [H+]-dependent manner, decreasing single-channel conductance Not assessed Not assessed [65]
TRPM2-overexpressing HEK293 Patch clamp External solution with pH 4.0–6.5. Different time exposition based on protocol (from <10 s to ≥2 min) Acidic pHe inactivates open channels in an irreversible manner. Exposition to pHe 4–5 negatively affects channel activation. Not assessed Not assessed [66]
TRPM2-overexpressing HEK293 Patch clamp External solution with pH 5.5, different exposition times (0, 30, 60, 90, and 120 s) Irreversible inhibition after ≤60 s exposure Not assessed Not assessed [67]
TRPM6 Pig isolated ventricular myocytes Patch clamp External solution with pH 5.5 and pH 6.5, ~5–10 min exposition External acidification decreases channel’s current amplitude in a pHe-dependent and voltage-independent manner. The inhibitory effect of acidic pHe is prevented by increasing intracellular pH buffering capacity Not assessed Not assessed [68]
TRPM6-overexpressing HEK293 cells Patch clamp External solution with pH 3–6, ~10 s-long exposition External acidification increases channel’s current amplitude in a pHe-dependent manner Not assessed Not assessed [69]
TRPM7 RBL-2H3 cells Patch clamp Acidification of intracellular side of membrane with ~200 s long 4–40 mM acetate treatment Pre-incubation in 40 mM acetate solution inhibits TRPM7 current in a reversible manner Not assessed Not assessed [70]
TRPM7-overexpressing Chinese Hamster Ovary (CHO-K1) cells Patch clamp Internal and external solution with pH 5.6 and variable exposition (~200–500 s) Internal and external acidification abolish channels’ current Not assessed Not assessed [70]
TRPM7-overexpressing HEK293 cells Patch clamp Internal solution with pH 6.1 and ~10 min exposition Internal acidification decreases TRPM7 currents’ density Not assessed Not assessed [71]
Mouse hippocampal neurons Patch clamp External solution with pH 6.5, 2 min exposition Extracellular acidification slows down channel’s activation in a voltage-independent way Not assessed Not assessed [72]
TRPM7-overexpressing HEK293T cells Patch clamp External solution with pH 4 and pH 6, acute treatment External acidification increases channel’s current amplitude in a pHe-dependent manner Not assessed Not assessed [69]
TRPM7-overexpressing HEK293T cells Patch clamp External solution with pH 3–7, ~50 s-long exposition External acidification determines a significant increase in TRPM7 inward current in an [H+] in a concentration-dependent manner Not assessed Not assessed [73]
Pig isolated ventricular myocytes Patch clamp External solution with pH 5.5 and pH 6.5, ~5–10 min exposition External acidification decreases channel’s current amplitude in a pHe-dependent and voltage-independent manner. The inhibitory effect of acidic pHe is prevented increasing intracellular pH buffering capacity Not assessed Not assessed [68]
Rat basophilic leukemia cells (RBL) Patch clamp External solution with pH 5.5, pH 6 and pH 6.5, ~1-min-long exposition External acidification decreases channel’s current amplitude in a pHe-dependent manner Not assessed Not assessed [68]
HeLa cells Patch clamp
Cell death assays (fluometric analysis of caspase 3/7 activation, electronic sizing of cell volume, and triple staining with Hoechst/acridine orange and propidium iodide assay.		 External solution with pH 4 and pH 6, acute treatment for patch clamp experiments, and 1 h-long treatment with acidic pHe (4 and 6) for cell death assays External acidification increases channel’s current amplitude in a pHe-dependent manner Not assessed Acidosis promotes HeLa necrotic cell death [74]
Human atrial cardiomyocytes Patch clamp External solution with pH 4–6, acute treatment External acidification increases channel’s current amplitude in presence of divalent cations in the extracellular milieu Not assessed Not assessed [75]
TRPV1 TRPV1-expressing HEK293 cells Patch clamp Acidic solution with pH 5.5 applied intracellularly for ~50 s Acid treatment does not activate the channel in inside-out patches but potentiates 2-APB-evoked currents from the cytoplasmic side Not assessed Not assessed [76]
hTRPV1-transfected HEK293t cells Calcium imaging External solution with pH 4.3 and pH 6.1, ~4 min-long exposition Acidic pHe activates TRPV1 channel pHe 6.1 determines larger Ca2+ transients with respect to pHe 4.3 in physiological extracellular Ca2+ concentration, while, in presence of low extracellular Ca2+ concentration, cells exposed to pHe 6.1 show reduced Ca2+ entry respect to pHe 4.3 exposition Not assessed [77]
Defolliculated Xenopus laevis oocytes,
TRPV1-expressing HEK293 cells		 Patch clamp Extracellular solution with pH 6.4, cells pre-treated with acid bath solution for 2 min Acidic pHe potentiates heat-evoked TRPV1 current in oocytes; potentiation of capsaicin and heat-evoked TRPV1 currents in HEK293 cells Not assessed Not assessed [78]
Primary human adult dermal lymphatic endothelial cell (HDLECs) Cell viability assay
Cell invasion assay
in vitro tube formation assay
Transwell cell migration assay		 24 h long exposition to pHe 6.4, and 6 h long exposition for in vitro tube formation assay Acidic pHe activates TRPV1 channel Not assessed Acidic pHe affects HDLECs morphology, increasing their migration and invasive abilities, proliferation and promoting lymphangiogenesis via acidosis-induced TRPV1 activation [79]
TRPV2 TRPV2-expressing HEK293 cells Patch clamp Acute administration of extracellular solution with pHe 5.5 and 6 Extracellular acidosis potentiates the response of TRPV2 to 2-APB (and analogues) from the cytosolic side, while intracellular acidification and low pHe alone are not able to elicit any detectable current Not assessed Not assessed [80]
TRPV3 TRPV3-expressing HEK293 cells Patch clamp, calcium imaging Acute administration of extracellular solution with pHe 5.5 and 6 Extracellular acidosis potentiates the response of TRPV3 to 2-APB (and analogues) from the cytosolic side. Intracellular acidification activates the channel, eliciting small but detectable currents Extracellular acidosis increases Ca2+ entry following 2-APB stimulation Not assessed [80]
TRPV3-expressing HEK293 cells Patch clamp
Cell death assay (PI staining assay)		 Intracellular administration of acidic solution with pHe 5.5 and glycolic acid. Extracellular solution with pH 5.5. Intracellular solution with pH 5.5–7. Glycolic acid-induced intracellular proton release in presence of acidic solution activates the channel in a reversible way. Extracellular acidification does not activate TRPV3, while intracellular acidification alone activates the channel in a pH-dependent manner Not assessed Glycolic acid-induced acidification induces cell toxicity and cell death [81]
Human keratinocytes cells (HaCaT) Patch clamp, cell death assay (PI staining assay) Intracellular administration of acidic solution with pHe 5.5 and glycolic acid Glycolic acid-induced intracellular proton release in presence of acidic solution potentiates the channel’s response to 2-APB in a reversible manner Not assessed Glycolic acid-induced acidification induces cell toxicity and cell death [81]
TRPV4 Chinese hamster ovary cells Patch clamp External solution with pHe 4, 5.5 and 6, acute treatment Extracellular acidosis activates the channel in a pHe-dependent manner Not assessed Not assessed [82]
mTRPV4-overexpressing primary cultured mouse esophageal epithelial cells Ca2+ imaging External solution with pHe 5, acute treatment Not assessed Extracellular acidic pH decreases Ca2+ entry, lowering cytosolic Ca2+ concentration Not assessed [83]
TRPV6 Jurkat cells Patch clamp External solution with pH 6, acute treatment Extracellular acidosis suppresses TRPV6-mediated currents Extracellular acidic pH reduces Ca2+ entry, lowering cytosolic Ca2+ concentration Not assessed [84]
TRPA1 HEK-293t cells expressing hTRPA1, mTRPA1, or rTRPA1 Patch clamp
Calcium imaging		 Acidic solutions with pH 7.0, 6.4, 6.0, and 5.4, 30 s-long treatment in calcium imaging experiments Extracellular acidosis activates inward currents via hTRPA1 and potentiates acrolein-evoked currents of hTRPA1 in a pHe-dependent and reversible manner, while failing to activate mouse and rodent TRPA1. Extracellular acidosis increases Ca2+ entry in hTRPA1, no effect on mTRPA1 and rTRPA1. Not assessed [85]
DRG neurons derived from TRPV1/TRPA1−/− mice and overexpression hTRPA1 Calcium imaging Acidic solutions with pH 5, 60 s-long treatment Not assessed Acidic pHe induces Ca2+ entry Not assessed [85]
Neuroblastoma ND7/23 cells expressing hTRPA1 Patch clamp Acidic solution with pH 5, acute treatment Acidic pHe activates hTRPA1 Not assessed Not assessed [85]
TRPC5 TRPC5-transiently transfected HEK293 cells Patch Clamp External acidic solution with pH 4.2, 5.5, 6.5, 7, ~100 s-long treatment G protein-activated and spontaneous currents are potentiated by extracellular acidic pH by increasing the channel open probability, with a maximum effect at ~pH 6.5, while more acidic values inhibit the channel Not assessed Not assessed [86]
TRPC4 TRPC4-transiently transfected HEK293 cells Patch Clamp External acidic solution with pH 4.2, 5.5, 6.5, 7, ~100 s-long treatment G protein-activated currents are potentiated by extracellular acidic pH, with a maximum effect at ~pH 6.5 and complete inhibition at pHe 5.5 Not assessed Not assessed [86]
mTRPC4-stably transfected HEK293 cells Patch Clamp External acidic solution with pH 6.8 Low pHi (6.75–6.25) accelerates Gi/o-mediated TRPC4 activation, and this requires elevations in intracellular calcium concentration. Intracellular protons inhibit Englerin A-mediated TRPC4 activation Not assessed Not assessed [87]
TRPC6 TRPC6-transiently transfected HEK293 cells Patch Clamp External acidic solution with pH 4.2, 5.5, 6.5, 7, ~100 s-long treatment Acidic pHe inhibits channel’s inward and outward currents starting from pHe 6.5 and the inhibition is potentiated by more acidic pHe values. Not assessed Not assessed [86]
ORAI1/STIM1 Human macrophages Patch clamp External acidic solution with pH 6 and 8, ~200 s-long treatment Extracellular acidosis inhibits ORAI1 channel in a pHe-dependent and reversible manner Not assessed Not assessed [88]
H4IIE rat liver cells overexpressing ORAI1 and STIM1 Patch clamp External acidic solutions with pH 5.1 and 5.9 ORAI1 and STIM1-mediated ICRAC are inhibited by acidic pHe, with maximal effect at pHe 5.5 Not assessed Not assessed [89]
RBL2H3 mast cell line, Jurkat T lymphocytes and heterologous ORAI1-2–3/STIM expressing HEK293 cells Patch clamp External and intracellular acidic solutions with pH 6 and 6.6 External and internal acidification inhibits IP3-induced ICRAC in RBL2H3 mast cell line, Jurkat T lymphocytes, and in heterologous ORAI/STIM-mediated ICRAC in HEK293 cells in a reversible manner Not assessed Not assessed [90]
ORAI1/STIM1-transiently transfected HEK293 cells Patch Clamp External acidic solution with pH 5.5 Acidic pHe inhibits ORAI1-2–3/STIM1 current amplitude in a reversible and pH-dependent manner, with a maximal effect at pHe 4.5 Not assessed Not assessed [91]
ORAI1/STIM1-transiently transfected HEK293 cells Patch Clamp Intracellular acidic solution with pH 6.3 Intracellular acidosis inhibits ORAI1/STIM1 current, regulating the amplitude of the current and the Ca2+-dependent gating of the CRAC channels Not assessed Not assessed [92]
SH-SY5Y human neuroblastoma cells Ca2+ signals quantification by Mn2+ quench technique External acidic solution with pH 6.8 and 7 and 7.2. Different treatment time, ranging from ~3–4 min to ~8 min for carbachol-mediated Ca2+ entry and ~7 min for thapsigargin-mediated Ca2+ entry Not assessed Tumour acidic pHe inhibits carbachol- and thapsigargin-mediated Ca2+ entry in a reversible manner, while intracellular acidification or alkalinization leads to no effects in carbachol-mediated Ca2+ entry Not assessed [93]