Zinc-Induced Effects on Osteoclastogenesis Involves Activation of Hyperpolarization-Activated Cyclic Nucleotide Modulated Channels via Changes in Membrane Potential

Investigation of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels in vitro inflammation model at molecular level

In our study, we aimed to create an inflammation model in endothelial and macrophage cell lines and to examine the changes in the expression of hyperpolarization activated cyclic nucleotide gated (HCN) channels at the molecular level. HUVEC and RAW cell lines were used in our study. 1 µg/mL LPS was applied to the cells. Cell media were taken 6 h later. TNF-α, IL-1, IL-2, IL-4, IL-10 concentrations were measured by ELISA method. Cell media were cross-applied to cells for 24 h after LPS. HCN1/HCN2 protein levels were determined by Western-Blot method. HCN-1/HCN-2 gene expressions were determined by qRT-PCR method. In the inflammation model, a significant increase in TNF-α, IL-1, and IL-2 levels was observed in RAW cell media compared to the control. While no significant difference was observed in IL-4 level, a significant decrease was observed in IL-10 level. While a significant increase in TNF-α level was observed in HUVEC cell medium, no difference was observed in other cytokines. In our inflammation model, an 8.44-fold increase in HCN1 gene expression was observed in HUVEC cells compared to the control group. No significant change was observed in HCN2 gene expression. 6.71-fold increase in HCN1 gene expression was observed in RAW cells compared to the control. The change in HCN2 expression was not statistically significant. In the Western-Blot analysis, a statistically significant increase in HCN1 level was observed in the LPS group in HUVEC cells compared to the control; no significant increase in HCN2 level was observed. While a statistically significant increase in HCN1 level was observed in the LPS group in RAW cells compared to the control; no significant increase in HCN2 level was observed. In immunofluorescence examination, it was observed that the level of HCN1 and HCN2 proteins in the cell membrane of HUVEC and RAW cells increased in the LPS group compared to the control group. While HCN1 gene/protein levels were increased in RAW and HUVEC cells in the inflammation model, no significant change was observed in HCN2 gene/protein levels. Our data suggest that the HCN1 subtype is dominant in endothelium and macrophages and may play a critical role in inflammation.

Effects of Axonal Demyelination, Inflammatory Cytokines and Divalent Cation Chelators on Thalamic HCN Channels and Oscillatory Bursting

Multiple sclerosis (MS) is a demyelinating disease of the central nervous system that is characterized by the progressive loss of oligodendrocytes and myelin and is associated with thalamic dysfunction. Cuprizone (CPZ)-induced general demyelination in rodents is a valuable model for studying different aspects of MS pathology. CPZ feeding is associated with the altered distribution and expression of different ion channels along neuronal somata and axons. However, it is largely unknown whether the copper chelator CPZ directly influences ion channels. Therefore, we assessed the effects of different divalent cations (copper; zinc) and trace metal chelators (EDTA; Tricine; the water-soluble derivative of CPZ, BiMPi) on hyperpolarization-activated cyclic nucleotide-gated (HCN) channels that are major mediators of thalamic function and pathology. In addition, alterations of HCN channels induced by CPZ treatment and MS-related proinflammatory cytokines (IL-1β; IL-6; INF-α; INF-β) were characterized in C57Bl/6J mice. Thus, the hyperpolarization-activated inward current (I_h) was recorded in thalamocortical (TC) neurons and heterologous expression systems (mHCN2 expressing HEK cells; hHCN4 expressing oocytes). A number of electrophysiological characteristics of I_h (potential of half-maximal activation (V_0.5); current density; activation kinetics) were unchanged following the extracellular application of trace metals and divalent cation chelators to native neurons, cell cultures or oocytes. Mice were fed a diet containing 0.2% CPZ for 35 days, resulting in general demyelination in the brain. Withdrawal of CPZ from the diet resulted in rapid remyelination, the effects of which were assessed at three time points after stopping CPZ feeding (Day1, Day7, Day25). In TC neurons, I_h was decreased on Day1 and Day25 and revealed a transient increased availability on Day7. In addition, we challenged naive TC neurons with INF-α and IL-1β. It was found that I_h parameters were differentially altered by the application of the two cytokines to thalamic cells, while IL-1β increased the availability of HCN channels (depolarized V_0.5; increased current density) and the excitability of TC neurons (depolarized resting membrane potential (RMP); increased the number of action potentials (APs); produced a larger voltage sag; promoted higher input resistance; increased the number of burst spikes; hyperpolarized the AP threshold), INF-α mediated contrary effects. The effect of cytokine modulation on thalamic bursting was further assessed in horizontal slices and a computational model of slow thalamic oscillations. Here, IL-1β and INF-α increased and reduced oscillatory bursting, respectively. We conclude that HCN channels are not directly modulated by trace metals and divalent cation chelators but are subject to modulation by different MS-related cytokines.

The funny current: Even funnier than 40 years ago. Uncanonical expression and roles of HCN/f channels all over the body

Discovered some 40 years ago, the I_f current has since been known as the "pacemaker" current due to its role in the initiation and modulation of the heartbeat and of neuronal excitability. But this is not all, the funny current keeps entertaining the researchers; indeed, several data discovering novel and uncanonical roles of f/HCN channel are quickly accumulating. In the present review, we provide an overview of the expression and cellular functions of HCN/f channels in a variety of systems/organs, and particularly in sour taste transduction, hormones secretion, activation of astrocytes and microglia, inhibition of osteoclastogenesis, renal ammonium excretion, and peristalsis in the gastrointestinal and urine systems. We also analyzed the role of HCN channels in sustaining cellular respiration in mitochondria and their participation to mitophagy under specific conditions. The relevance of HCN currents in undifferentiated cells, and specifically in the control of stem cell cycle and in bioelectrical signals driving left/right asymmetry during zygote development, is also considered. Finally, we present novel data concerning the expression of HCN mRNA in human leukocytes. We can thus conclude that the emerging evidence presented in this review clearly points to an increasing interest and importance of the "funny" current that goes beyond its role in cardiac sinoatrial and neuronal excitability regulation.

Transcriptomics-based analysis of the mechanism by which Wang-Bi capsule alleviates joint destruction in rats with collagen-induced arthritis

BACKGROUND

Rheumatoid arthritis (RA) is a chronic autoimmune disease accompanied with joint destruction that often leads to disability. Wang-Bi capsule (WB), a traditional Chinese medicine-based herbs formula, has exhibited inhibition effect on joint destruction of collagen-induced arthritis (CIA) animal model in our previous study. But its molecular mechanisms are still obscure.

METHODS

CIA rats were treated intragastrical with WB for eight weeks, and the effect of joints protection were evaluated by hematoxylin and eosin (H&E) staining, safranin O fast green staining, tartrate-resistant acid phosphatase (TRAP) staining and micro‑CT scanning analysis. The transcriptomic of tarsal joints were used to investigate how WB alleviated joint destruction.

RESULTS

The histological examination of ankle joints showed WB alleviated both cartilage damage and bone destruction of CIA rats. This protective effect on joints were further evidenced by micro-CT analysis. The transcriptomic analysis showed that WB prominently changed 12 KEGG signaling pathways ("calcium signaling pathway", "cAMP signaling pathway", "cell adhesion molecules", "chemokine signaling pathway", "complement and coagulation cascades", "MAPK signaling pathway", "NF-kappa B signaling pathway", "osteoclast differentiation", "PI3K-Akt signaling pathway", "focal adhesion", "Gap junction" and "Rap1 signaling pathway") associated with bone or cartilage. Several genes (including Il6, Tnfsf11, Ffar2, Plg, Tnfrsf11b, Fgf4, Fpr1, Siglec1, Vegfd, Cldn1, Cxcl13, Chad, Arrb2, Fgf9, Egfr) regulating bone resorption, bone formation and cartilage development were identified by further analysis. Meanwhile, these differentially expressed genes were validated by real-time quantitative PCR.

CONCLUSIONS

Overall, the protective effect of WB treatment on joint were confirmed in CIA rats, and its basic molecular mechanisms may be associated with regulating some genes (including Il6, Tnfsf11, Ffar2 and Plg etc.) involved in bone resorption, bone formation and cartilage development.

The impact of hyperpolarization-activated cyclic nucleotide-gated (HCN) and voltage-gated potassium KCNQ/Kv7 channels on primary microglia function

Background

Microglia are essential to maintain cell homeostasis in the healthy brain and are activated after brain injury. Upon activation, microglia polarize towards different phenotypes. The course of microglia activation is complex and depends on signals in the surrounding milieu. Recently, it has been suggested that microglia respond to ion currents, as a way of regulating their activity and function.

Methods and results

Under the hypothesis that HCN and KCNQ/Kv7 channels impact on microglia, we studied primary rat microglia in the presence or absence of specific pharmacological blockade or RNA silencing. Primary microglia expressed the subunits HCN1-4, Kv7.2, Kv7.3, and Kv7.5. The expression of HCN2, as well as Kv7.2 and Kv7.3, varied among different microglia phenotypes. The pharmacological blockade of HCN channels by ZD7288 resulted in cell depolarization with slowly rising intracellular calcium levels, leading to enhanced survival and reduced proliferation rates of resting microglia. Furthermore, ZD7288 treatment, as well as knockdown of HCN2 RNA by small interfering RNA, resulted in an attenuation of later microglia activation—both towards the anti- and pro-inflammatory phenotype. However, HCN channel inhibition enhanced the phagocytic capacity of IL4-stimulated microglia. Blockade of Kv7/KCNQ channel by XE-991 exclusively inhibited the migratory capacity of resting microglia.

Conclusion

These observations suggest that the HCN current contributes to various microglia functions and impacts on the course of microglia activation, while the Kv7/KCNQ channels affect microglia migration. Characterizing the role of HCN channels in microglial functioning may offer new therapeutic approaches for targeted modulation of neuroinflammation as a hallmark of various neurological disorders.

The effects of local and intraperitoneal zinc treatments on maxillofacial fracture healing in rabbits

OBJECTIVE

This study aimed to determine whether administration of topical and intraperitoneal zinc for maxillofacial fractures has any impact on the bone healing process.

MATERIAL AND METHOD

Thirty-two New Zealand rabbits were randomly assigned to four groups of eight each. The first group was the control group; fracture lines were fixed using titanium microplates and no medication was administered. The second group received fixations using zinc-coated titanium microplates. A single dose of 3 mg/kg zinc was administered intraperitoneally to the third group following fixations with titanium microplates. A single dose of 3 mg/kg zinc was administered intraperitoneally to the fourth group following fixations with zinc-coated titanium microplates. Zinc coating on to the titanium microplates was achieved using the physical vapor deposition technique. A fracture line was created in the nasal bones of all subjects and fixed with five-hole flat microplates and three 5-mm micro screws. All work groups were sacrificed at the end of the sixth week.

RESULTS

Histological examination showed that the number of osteoblasts were significantly higher in zinc-coated group (Group 2) than zinc uncoated, control group (Group 1), (415.6 ± 46.7 vs 366.3 ± 11.8) (p < 0.001). It was observed that intraperitoneal zinc treatment alone (Group 3) did not significantly increase in the osteoblast count compared to zinc un-coated group (Group 1), (390.6 ± 83.2 vs 366.3 ± 11.8), (p = 0.341). The immunoreactivity scores for IGF-1 were significantly higher in the zinc-coated group compared to control group (Group 2 vs 1), (9.3 ± 2.8 vs 3.7 ± 1.9) (p < 0.05). It was observed that intraperitoneal zinc treatment did not cause a significant difference in the aspect of IGF-1 for zinc-coated groups (Group 2 vs 4) (9.3 ± 2.8 vs 9.6 ± 2.2) (p = 0.791). The difference in the immunoreactivity score among whole groups for TGF-β was not statistically significant (Group 1 vs 2, 3.2 ± 1.7 vs 4.4 ± 2.3, p = 0.256; Group 1 vs 3, 3.2 ± 1.7 vs 3.8 ± 2.8, p = 0.524; Group 1 vs 4, 3.2 ± 1.7 vs 2.8 ± 1.3, p = 0.717; Group 2 vs 3, 4.4 ± 2.3, vs 3.8 ± 2.8, p = 0.610; Group 2 vs 4, 4.4 ± 2.3, vs 2.8 ± 1.3, p = 0.124; Group 3 vs 4, 3.8 ± 2.8, vs 2.8 ± 1.3, p = 0.311).

CONCLUSION

The local use of titanium microplates coated with zinc by PVD technique was found effective for fracture healing. Zinc coating of titanium microplates used in fracture treatment can accelerate fracture healing. It may be concluded that clinical studies should be performed now in order to explore if comparable results can be achieved in humans.

Locally injected ivabradine inhibits carrageenan-induced pain and inflammatory responses via hyperpolarization-activated cyclic nucleotide-gated (HCN) channels

Background

Recently, attention has been focused on the role of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels in the mechanism of and as a treatment target for neuropathic and inflammatory pain. Ivabradine, a blocker of HCN channels, was demonstrated to have an effect on neuropathic pain in an animal model. Therefore, in the present study, we evaluated the effect of ivabradine on inflammatory pain, and under the hypothesis that ivabradine can directly influence inflammatory responses, we investigated its effect in in vivo and in vitro studies.

Methods

After approval from our institution, we studied male Sprague–Dawley rats aged 8 weeks. Peripheral inflammation was induced by the subcutaneous injection of carrageenan into the hindpaw of rats. The paw-withdrawal threshold (pain threshold) was evaluated by applying mechanical stimulation to the injected site with von Frey filaments. Ivabradine was subcutaneously injected, combined with carrageenan, and its effect on the pain threshold was evaluated. In addition, we evaluated the effects of ivabradine on the accumulation of leukocytes and TNF-alpha expression in the injected area of rats. Furthermore, we investigated the effects of ivabradine on LPS-stimulated production of TNF-alpha in incubated mouse macrophage-like cells.

Results

The addition of ivabradine to carrageenan increased the pain threshold lowered by carrageenan injection. Both lamotrigine and forskolin, activators of HCN channels, significantly reversed the inhibitory effect of ivabradine on the pain threshold. Ivabradine inhibited the carrageenan-induced accumulation of leukocytes and TNF-alpha expression in the injected area. Furthermore, ivabradine significantly inhibited LPS-stimulated production of TNF-alpha in the incubated cells.

Conclusion

The results of the present study demonstrated that locally injected ivabradine is effective against carrageenan-induced inflammatory pain via HCN channels. Its effect was considered to involve not only an action on peripheral nerves but also an anti-inflammatory effect.

Bioactive multi-elemental PEO-coatings on titanium for dental implant applications

Bioactive PEO (Plasma Electrolytic Oxidation) coatings were generated on Grade I commercially pure titanium for dentistry applications using a Ca/P-based electrolyte with added Si, Mg, Zn or F species. Surface characteristics, chemical composition and ion liberation of the coatings were characterized using SEM/EDS, X-ray diffraction, optical profilometry, contact angle and ICP-OES. Corrosion resistance (OCP and DC polarization) was evaluated in SBF. Osteoblastogenesis and osteoclastogenesis processes on PEO-coated Ti and non-coated Ti controls were assessed after 7 days and 5 days of cell culture, respectively. Monolayer formation and metabolic activity were evaluated for the MC3T3 preosteoblastic cell line. All PEO coatings favoured differentiation processes over proliferation and presented three times greater quantity of secreted collagen than non-coated Ti control. All coating enabled osteoclast differentiation, with differences in number and size of the osteoclasts between the materials.

Cooperative electrogenic proton transport pathways in the plasma membrane of the proton-secreting osteoclast

A proton is a ubiquitous signaling ion. Many transmembrane H⁺ transport pathways either maintain pH homeostasis or generate acidic compartments. The osteoclast is a bone-resorbing cell, which degrades bone tissues by secreting protons and lysosomal enzymes into the resorption pit. The plasma membrane facing bone tissue (ruffled border), generated partly by fusion of lysosomes, may mimic H⁺ flux mechanisms regulating acidic vesicles. We identified three electrogenic H⁺-fluxes in osteoclast plasma membranes-a vacuolar H⁺-ATPase (V-ATPase), a voltage-gated proton channel (Hv channel) and an acid-inducible H⁺-leak-whose electrophysiological profiles and regulation mechanisms differed. V-ATPase and Hv channel, both may have intracellular reservoirs, but the recruitment/internalization is regulated independently. V-ATPase mediates active H⁺ efflux, acidifying the resorption pit, while acid-inducible H⁺ leak, activated at an extracellular pH < 5.5, diminishes pit acidification, possibly to protect bone from excess degradation. The two-way H⁺ flux mechanisms in opposite directions may have advantages in fine regulation of pit pH. Hv channel mediates passive H⁺ efflux. Although its working ranges are limited, the amount of H⁺ extrusion is 100 times larger than those of the V-ATPase and may support reactive oxygen species production during osteoclastogenesis. Extracellular Ca²⁺, H⁺ and inorganic phosphate, which accumulate in the resorption pit, will either stimulate or inhibit these H⁺ fluxes. Skeletal integration is disrupted by too much or too less of bone resorption. Diversities in plasma membrane H⁺ flux pathways, which may co-operate or compete, are essential to adjust osteoclast functions in variable conditions.

HCN4 pacemaker channels attenuate the parasympathetic response and stabilize the spontaneous firing of the sinoatrial node

Key points

The contribution of HCN4 pacemaker channels in the autonomic regulation of the sino‐atrial node (SAN) has been a matter of debate.

The transgenic overexpression of HCN4 did not induce tachycardia, but reduced heart rate variability, while the conditional knockdown of HCN4 gave rise to sinus arrhythmia.

The response of the SAN to β‐adrenergic stimulation was not affected by overexpression or knockdown of HCN4 channels.

When HCN4 channels were knocked down, the parasympathetic response examined by cervical vagus nerve stimulation (CVNS) was enhanced; the CVNS induced complete sinus pause. The overexpression of HCN4 attenuated bradycardia induced by CVNS only during β‐adrenergic stimulation.

We concluded that HCN4 pacemaker channels stabilize the spontaneous firing by attenuating the parasympathetic response of the SAN.

Abstract

The heart rate is dynamically controlled by the sympathetic and parasympathetic nervous systems that regulate the sinoatrial node (SAN). HCN4 pacemaker channels are the well‐known causative molecule of congenital sick sinus syndrome. Although HCN4 channels are activated by cAMP, the sympathetic response of the SAN was preserved in patients carrying loss‐of‐function mutations of the HCN4 gene. In order to clarify the contribution of HCN4 channels in the autonomic regulation of the SAN, we developed novel gain‐of‐function mutant mice in which the expression level of HCN4 channels could be reversibly changed from zero to ∼3 times that in wild‐type mice, using tetracycline transactivator and the tetracycline responsive element. We recorded telemetric ECGs in freely moving conscious mice and analysed the heart rate variability. We also evaluated the response of the SAN to cervical vagus nerve stimulation (CVNS). The conditional overexpression of HCN4 did not induce tachycardia, but reduced heart rate variability. The HCN4 overexpression also attenuated bradycardia induced by the CVNS only during the β‐adrenergic stimulation. In contrast, the knockdown of HCN4 gave rise to sinus arrhythmia, and enhanced the parasympathetic response; complete sinus pause was induced by the CVNS. In vitro, we compared the effects of acetylcholine on the spontaneous action potentials of single pacemaker cells, and found that similar phenotypic changes were induced by genetic manipulation of HCN4 expression both in the presence and absence of β‐adrenergic stimulation. Our study suggests that HCN4 channels attenuate the vagal response of the SAN, and thereby stabilize the spontaneous firing of the SAN.

The contribution of HCN4 pacemaker channels in the autonomic regulation of the sino‐atrial node (SAN) has been a matter of debate.

The transgenic overexpression of HCN4 did not induce tachycardia, but reduced heart rate variability, while the conditional knockdown of HCN4 gave rise to sinus arrhythmia.

The response of the SAN to β‐adrenergic stimulation was not affected by overexpression or knockdown of HCN4 channels.

When HCN4 channels were knocked down, the parasympathetic response examined by cervical vagus nerve stimulation (CVNS) was enhanced; the CVNS induced complete sinus pause. The overexpression of HCN4 attenuated bradycardia induced by CVNS only during β‐adrenergic stimulation.

We concluded that HCN4 pacemaker channels stabilize the spontaneous firing by attenuating the parasympathetic response of the SAN.

Role of lysosomal channel protein TPC2 in osteoclast differentiation and bone remodeling under normal and low-magnesium conditions

The bone is the main storage site for Ca²⁺ and Mg²⁺ ions in the mammalian body. Although investigations into Ca²⁺ signaling have progressed rapidly and led to better understanding of bone biology, the Mg²⁺ signaling pathway and associated molecules remain to be elucidated. Here, we investigated the role of a potential Mg²⁺ signaling-related lysosomal molecule, two-pore channel subtype 2 (TPC2), in osteoclast differentiation and bone remodeling. Previously, we found that under normal Mg²⁺ conditions, TPC2 promotes osteoclastogenesis. We observed that under low-Mg²⁺ conditions, TPC2 inhibited, rather than promoted, the osteoclast differentiation and that the phosphatidylinositol 3,5-bisphosphate (PI(3,5)P₂) signaling pathway played a role in the TPC2 activation under low-Mg²⁺ conditions. Furthermore, PI(3,5)P₂ depolarized the membrane potential by increasing the intracellular Na⁺ levels. To investigate how membrane depolarization affects osteoclast differentiation, we generated a light-sensitive cell line and developed a system for the light-stimulated depolarization of the membrane potential. The light-induced depolarization inhibited the osteoclast differentiation. We then tested the effect of myo-inositol supplementation, which increased the PI(3,5)P₂ levels in mice fed a low-Mg²⁺ diet. The myo-inositol supplementation rescued the low-Mg²⁺ diet-induced trabecular bone loss, which was accompanied by the inhibition of osteoclastogenesis. These results indicate that low-Mg²⁺-induced osteoclastogenesis involves changes in the role of TPC2, which are mediated through the PI(3,5)P₂ pathway. Our findings also suggest that myo-inositol consumption might provide beneficial effects in Mg²⁺ deficiency-induced skeletal diseases.

Does incorporating zinc in titanium implant surfaces influence osseointegration? A systematic review

STATEMENT OF PROBLEM

Titanium implant surfaces have been modified to improve osseointegration; however, the evidence for incorporating zinc into titanium implants to improve new bone formation and osseointegration is not clear.

PURPOSE

The purpose of this systematic review was to assess the efficacy of treating titanium surfaces with zinc on the osseointegration of implants.

MATERIAL AND METHODS

The focused question addressed was, "Does incorporating zinc in titanium implant surfaces influence osseointegration?" Indexed databases were searched up to January 2016 using the key words "Bone to implant contact"; "implant"; "zinc"; "osseointegration." Letters to the editor, case reports/case series, historic reviews, and commentaries were excluded. The pattern of the review was customized to summarize the pertinent data.

RESULTS

Ten experimental studies were included, all of which were performed in animals (5 in rabbits, 4 in rodents, and 1 in goats). The number of titanium implants placed ranged from 10 to 78. The results from all studies showed that incorporating zinc into titanium implants enhanced new bone formation and/or bone-to-implant contact around implants. One study reported that zinc enhanced the removal torque on implants.

CONCLUSIONS

The current available evidence on adding zinc to titanium implants surfaces to enhance osseointegration remains unclear. Further investigation is necessary to assess its effectiveness and safety in humans and to establish a standard methodology and ideal compound for incorporating zinc ion into titanium implant surfaces in a clinical setting.

Acid-inducible proton influx currents in the plasma membrane of murine osteoclast-like cells

Acidification of the resorption pits, which is essential for dissolving bone, is produced by secretion of protons through vacuolar H(+)-ATPases in the plasma membrane of bone-resorbing cells, osteoclasts. Consequently, osteoclasts face highly acidic extracellular environments, where the pH gradient across the plasma membrane could generate a force driving protons into the cells. Proton influx mechanisms during the acid exposure are largely unknown, however. In this study, we investigated extracellular-acid-inducible proton influx currents in osteoclast-like cells derived from a macrophage cell line (RAW264). Decreasing extracellular pH to <5.5 induced non-ohmic inward currents. The reversal potentials depended on the pH gradients across the membrane and were independent of concentrations of Na(+), Cl(-), and HCO3 (-), suggesting that they were carried largely by protons. The acid-inducible proton influx currents were not inhibited by amiloride, a widely used blocker for cation channels/transporters, or by 4,4'-diisothiocyanato-2,2'-stilbenesulfonate(DIDS) which blocks anion channels/transporters. Additionally, the currents were not significantly affected by V-ATPase inhibitors, bafilomycin A1 and N,N'-dicyclohexylcarbodiimide. Extracellular Ca(2+) (10 mM) did not affect the currents, but 1 mM ZnCl2 decreased the currents partially. The intracellular pH in the vicinity of the plasma membrane was dropped by the acid-inducible H(+) influx currents, which caused overshoot of the voltage-gated H(+) channels after removal of acids. The H(+) influx currents were smaller in undifferentiated, mononuclear RAW cells and were negligible in COS7 cells. These data suggest that the acid-inducible H(+) influx (H(+) leak) pathway may be an additional mechanism modifying the pH environments of osteoclasts upon exposure to strong acids.