Osteoclast ATP receptor activation leads to a transient decrease in intracellular pH

Mass transfer analysis of growth and substance metabolism of NSCs cultured in collagen-based scaffold in vitro

The aim of this study is to analyze the growth and substance metabolism of neural stem cells (NSCs) cultured in biological collagen-based scaffolds. Mass transfer and metabolism model of glucose, lactic acid, and dissolved oxygen (DO) were established and solved on MATLAB platform to obtain the concentration distributions of DO, glucose, and lactic acid in culture system, respectively. Calculation results showed that the DO influenced their normal growth and metabolism of NSCs mostly in the in vitro culture within collagen-based scaffolds. This study also confirmed that 2-mm thickness of collagen scaffold was capable of in vitro cultivation and growth of NSCs with an inoculating density of 1 × 10(6) cells/mL.

Purinergic signalling in the musculoskeletal system

It is now widely recognised that extracellular nucleotides, signalling via purinergic receptors, participate in numerous biological processes in most tissues. It has become evident that extracellular nucleotides have significant regulatory effects in the musculoskeletal system. In early development, ATP released from motor nerves along with acetylcholine acts as a cotransmitter in neuromuscular transmission; in mature animals, ATP functions as a neuromodulator. Purinergic receptors expressed by skeletal muscle and satellite cells play important pathophysiological roles in their development or repair. In many cell types, expression of purinergic receptors is often dependent on differentiation. For example, sequential expression of P2X5, P2Y1 and P2X2 receptors occurs during muscle regeneration in the mdx model of muscular dystrophy. In bone and cartilage cells, the functional effects of purinergic signalling appear to be largely negative. ATP stimulates the formation and activation of osteoclasts, the bone-destroying cells. Another role appears to be as a potent local inhibitor of mineralisation. In osteoblasts, the bone-forming cells, ATP acts via P2 receptors to limit bone mineralisation by inhibiting alkaline phosphatase expression and activity. Extracellular ATP additionally exerts significant effects on mineralisation via its hydrolysis product, pyrophosphate. Evidence now suggests that purinergic signalling is potentially important in several bone and joint disorders including osteoporosis, rheumatoid arthritis and cancers. Strategies for future musculoskeletal therapies might involve modulation of purinergic receptor function or of the ecto-nucleotidases responsible for ATP breakdown or ATP transport inhibitors.

Receptor-independent effects of 2'(3')-O-(4-benzoylbenzoyl)ATP triethylammonium salt on cytosolic pH

The effect of the relatively potent P2X7 receptor agonist 2'(3')-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate triethylammonium salt (BzATP-TEA) on cytosolic pH (pHi) was studied using MC3T3-E1 osteoblast-like cells, which endogenously express P2X7 receptors. pHi was measured fluorimetrically using the pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein. BzATP-TEA (0.3-1.5 mM) elicited fast-onset alkalinization responses. In contrast, adenosine 5'-triphosphate disodium salt (5 mM) failed to reproduce the BzATP-TEA-induced responses, indicating a P2 receptor-independent mechanism. We speculated that triethylamine, which is present in solutions of BzATP-TEA, permeates the plasma membrane, and is protonated intracellularly, leading to an increase in pHi. Consistent with this hypothesis, triethylammonium (TEA) chloride mimicked the effects of BzATP-TEA on pHi. Moreover, measurements using a Cytosensor microphysiometer revealed that TEA chloride transiently suppressed proton efflux from cells, whereas washout of TEA transiently enhanced proton efflux. BzATP-TEA also elicited a sustained increase in proton efflux that was blocked specifically by the P2X7 antagonist A-438079. Taken together, we conclude that BzATP-TEA-induced alkalinization is unrelated to P2X7 activation, but is due to the presence of TEA. This effect may confound assessment of the outcomes of P2X7 activation by BzATP-TEA in other systems. Thus, control experiments using TEA chloride are recommended to distinguish between receptor-mediated and nonspecific effects of this widely used agonist. We performed such a control and confirmed that BzATP-TEA, but not TEA chloride, caused the elevation of cytosolic free Ca(2+) in MC3T3-E1 cells, ruling out the possibility that receptor-independent effects on pHi underlie BzATP-TEA-induced Ca(2+) signaling.

The birth and postnatal development of purinergic signalling

The purinergic signalling system is one of the most ancient and arguably the most widespread intercellular signalling system in living tissues. In this review we present a detailed account of the early developments and current status of purinergic signalling. We summarize the current knowledge on purinoceptors, their distribution and role in signal transduction in various tissues in physiological and pathophysiological conditions.

NHE10, an osteoclast-specific member of the Na+/H+ exchanger family, regulates osteoclast differentiation and survival [corrected]

Bone homeostasis is tightly regulated by the balanced actions of osteoblasts (OBs) and osteoclasts (OCs). We previously analyzed the gene expression profile of OC differentiation using a cDNA microarray, and identified a novel osteoclastogenic gene candidate, clone OCL-1-E7 [J. Rho, C.R. Altmann, N.D. Socci, L. Merkov, N. Kim, H. So, O. Lee, M. Takami, A.H. Brivanlou, Y. Choi, Gene expression profiling of osteoclast differentiation by combined suppression subtractive hybridization (SSH) and cDNA microarray analysis, DNA Cell Biol. 21 (2002) 541-549]. In this study, we have isolated full-length cDNAs corresponding to this clone from mice and humans to determine the functional roles of this gene in osteoclastogenesis. The full-length cDNA of OCL-1-E7 encodes 12 membrane-spanning domains that are typical of isoforms of the Na(+)/H(+) exchangers (NHEs), indicating that this clone is a novel member of the NHE family (hereafter referred to as NHE10). Here, we show that NHE10 is highly expressed in OCs in response to receptor activator of nuclear factor-kappaB ligand signaling and is required for OC differentiation and survival.

The role of intracellular pH in cell growth arrest induced by ATP

In this study, we investigated ionic mechanisms involved in growth arrest induced by extracellular ATP in androgen-independent prostate cancer cells. Extracellular ATP reversibly induced a rapid and sustained intracellular pH (pH(i)) decrease from 7.41 to 7.11. Inhibition of Ca(2+) influx, lowering extracellular Ca(2+), and buffering cytoplasmic Ca(2+) inhibited ATP-induced acidification, thereby demonstrating that acidification is a consequence of Ca(2+) entry. We show that ATP induced reuptake of Ca(2+) by the mitochondria and a transient depolarization of the inner mitochondrial membrane. ATP-induced acidification was reduced after the dissipation of the mitochondrial proton gradient by rotenone and carbonyl cyanide p-trifluoromethoxyphenylhydrazone, after inhibition of Ca(2+) uptake into the mitochondria by ruthenium red, and after inhibition of the F(0)F(1)-ATPase with oligomycin. ATP-induced acidification was not induced by either stimulation of the Cl(-)/HCO(3)(-) exchanger or inhibition of the Na(+)/H(+) exchanger. In addition, intracellular acidification, induced by an ammonium prepulse method, reduced the amount of releasable Ca(2+) from the endoplasmic reticulum, assessed by measuring change in cytosolic Ca(2+) induced by thapsigargin or ATP in a Ca(2+)-free medium. This latter finding reveals cross talk between pH(i) and Ca(2+) homeostasis in which the Ca(2+)-induced intracellular acidification can in turn regulate the amount of Ca(2+) that can be released from the endoplasmic reticulum. Furthermore, pH(i) decrease was capable of reducing cell growth. Taken together, our results suggest that ATP-induced acidification in DU-145 cells results from specific effect of mitochondrial function and is one of the major mechanisms leading to growth arrest induced by ATP.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Regulation of bone resorption and formation by purines and pyrimidines

Growing evidence suggests that extracellular nucleotides, signalling through P2 receptors, might play important roles in the regulation of bone and cartilage metabolism. ATP and other nucleotides can exert impressive stimulatory effects on the formation and activity of osteoclasts (bone-resorbing cells) in addition to inhibiting bone formation by osteoblasts. In this review, the current understanding of the actions of nucleotides on skeletal cells and the probable receptor subtypes involved are discussed.

Intercellular calcium signaling occurs between human osteoblasts and osteoclasts and requires activation of osteoclast P2X7 receptors

Signaling between osteoblasts and osteoclasts is important in bone homeostasis. We previously showed that human osteoblasts propagate intercellular calcium signals via two mechanisms: autocrine activation of P2Y receptors, and gap junctional communication. In the current work we identified mechanically induced intercellular calcium signaling between osteoblasts and osteoclasts and among osteoclasts. Intercellular calcium responses in osteoclasts required P2 receptor activation but not gap junctional communication. Pharmacological studies and reverse transcriptase-PCR amplification demonstrated that human osteoclasts expressed functional P2Y1 receptors, but, unexpectedly, desensitization of P2Y1 did not block calcium signaling to osteoclasts. We also found that osteoclasts expressed functional P2X7 receptors and showed that pharmacological inhibition of these receptors blocked calcium signaling to osteoclasts. Thus these studies show that calcium signaling between osteoblasts and osteoclasts occurs via activation of P2 receptors, but that different families of P2 receptors are required for calcium signaling in these two cell types. Intercellular calcium signaling among bone cells is therefore amenable to pharmacological manipulation that will specifically affect only bone-forming or bone-resorbing cells. P2 receptors may be important drug targets for the modulation of bone turnover.

Chloride dependent intracellular pH effects of external ATP in cultured human non-pigmented ciliary body epithelium

PURPOSE

To examine the effects of extracellular adenosine 5-triphosphate (ATP) on intracellular pH ([pH](i)) in cultured human non-pigmented ciliary body epithelium (HNPE).

METHODS

Intracellular pH was measured using spectrofluorescence video microscopy in isolated HNPE cells loaded with the cell-permeable acetoxymethyl ester form of the fluorescent probe BCECF.

RESULTS

In 5%CO(2)/HCO(3)(-) buffered Ringer's the resting [pH](i) was 7.25 +/- 0.006 (mean +/- SEM). Application of 10 microM ATP significantly decreased [pH](i) to 7.00 +/- 0.007 (P < 10(-5), n = 14). In the presence of 1 mM suramin, a P(2) receptor inhibitor, this process was significantly blocked. This [pH](i) effect required the presence of Cl(-) and was significantly inhibited by 0.1 mM diisothiocyanatostilbene-2-2'-disulfonic acid or acetazolamide (500 microM), indicating the involvement of a Cl(-)/HCO(3)( +) exchange mechanism. This response exhibited little dependence on external Na(+) and remained unaffected by the addition of the Na(+)/H( +) exchanger inhibitor amiloride (1 mM). Clamping intracellular calcium levels by incubation in the cell permeable calcium chelator, the acetoxymethyl ester form of BAPTA (100 microM) in low extracellular calcium solution (pCa9) did not affect the ATP-induced [pH](i) signal. In addition, the vacuolar H(+)-ATPase (V-ATPase) inhibitor, bafilomycin A(1) (1 microM), failed to alter the [pH](i) transient.

CONCLUSION

We have demonstrated that extracellular ATP leads to a sustained increase in [H(+)](i) in HNPE cells via a purinergic receptor activated pathway which is independent of the intracellular calcium signaling system. This study demonstrates that the ATP induced [pH]( i) transient is mediated through an upregulation in Cl(-)/HCO( 3)(-) exchange across the plasmamembrane in HNPE cells.

Extracellular ADP is a powerful osteolytic agent: evidence for signaling through the P2Y(1) receptor on bone cells

There is increasing evidence that extracellular nucleotides act on bone cells via P2 receptors. This study investigated the action of ADP and 2-methylthioADP, a potent ADP analog with selectivity for the P2Y(1) receptor, on osteoclasts, the bone-resorbing multinuclear cells. Using three different assays, we show that ADP and 2-methylthioADP at nanomolar to submicromolar levels caused up to fourfold to sixfold increases in osteoclastic bone resorption. On mature rat osteoclasts, cultured for 1 day on polished dentine disks, peak effects on resorption pit formation were observed between 20 nM and 2 microM of ADP. The same concentrations of ADP also stimulated osteoclast and resorption pit formation in 10-day mouse marrow cultures on dentine disks. In 3-day explant cultures of mouse calvarial bones, the stimulatory effect of ADP on osteoclast-mediated Ca(2+) release was greatest at 5-50 microM and equivalent to the maximal effects of prostaglandin E(2). The ADP effects were blocked in a nontoxic manner by MRS 2179, a P2Y(1) receptor antagonist. Using in situ hybridization and immunocytochemistry, we found evidence for P2Y(1) receptor expression on both osteoclasts and osteoblasts; thus, ADP could exert its actions both directly on osteoclasts and indirectly via P2Y(1) receptors on osteoblasts. As a major ATP degradation product, ADP is a novel stimulator of bone resorption that could help mediate inflammatory bone loss in vivo.

Identification and Characterization of a Sodium/Calcium Exchanger, NCX-1, in Osteoclasts and Its Role in Bone Resorption

We provide the first demonstration for a Na+/Ca2+ exchanger, NCX-1, in the osteoclast. We speculate that by using Na+ exchange, NCX-1 couples H+ extrusion with Ca2+ fluxes during bone resorption. Microspectrofluorimetry of fura-2-loaded osteoclasts revealed a rapid and sustained, but reversible, cytosolic Ca2+ elevation upon Na+ withdrawal. This elevation was abolished by the cytosolic introduction (by gentle permeabilization) of a highly specific Na+/Ca2+ exchange inhibitor peptide, XIP, but not its inactive analogue, sXIP. Confocal microscopy revealed intense plasma membrane immunofluorescence with an isoform-specific monoclonal anti-NCX-1 antibody applied to gently permeabilized osteoclasts. Electrophysiological studies using excised outside-in membrane patches showed a low-conductance, Na+-selective, dichlorobenzamil-sensitive, amiloride-insensitive channel that we tentatively assigned as being an NCX. Finally, to examine for physiological relevance, an osteoclast resorption (pit) assay was performed. There was a dramatic reduction of bone resorption following NCX-1 inhibition by dichlorobenzamil and XIP (but not with S-XIP). Together, the results suggest that a functional NCX, likely NCX-1, is involved in the regulation of osteoclast cytosolic Ca2+ and bone resorption.

Resorptive state and cell size influence intracellular pH regulation in rabbit osteoclasts cultured on collagen-hydroxyapatite films

Diseases exhibiting excessive bone loss are often characterized by an increase in the size and number of osteoclasts in affected areas, suggesting that osteoclast size is associated with increased resorptive activity or efficiency. Because osteoclastic bone resorption depends on proton extrusion via a bafilomycin A1-sensitive vacuolar type H+ ATPase (V-ATPase), we investigated the relationship between osteoclast size and state of activity on the one hand, and proton-extruding mechanisms (bafilomycin A1-sensitive V-ATPase and amiloride-sensitive Na+/H+ exchange) on the other. In determining resorptive activities of individual osteoclasts, osteoclast-containing cell suspensions obtained from newborn rabbit long bones were cultured on apatite-collagen complex (ACC)-coated coverslips. Large osteoclasts resorbed 2.5 times more per cell than small osteoclasts, but the amount resorbed per nucleus was the same for the two categories. However, a much larger percentage of large osteoclasts was resorbing compared with small osteoclasts. To study pH regulatory mechanisms in individual large and small osteoclasts, the cells were loaded with the pH-sensitive indicator BCECF and analyzed by single-cell fluorescence. Small and large resorbing osteoclasts had significantly higher basal pH(i) than their nonresorbing counterparts. Also, small nonresorbing osteoclasts were insensitive to bafilomycin A1 addition or Na+ removal from the medium, large nonresorbing osteoclasts responded slightly, and all resorbing osteoclasts (small and large) responded strongly. Differences were also seen in the recovery from an acid load: both small and large nonresorbing osteoclasts were more sensitive to amiloride inhibition, while large resorbing cells were more sensitive to bafilomycin A1 inhibition. Small resorbing cells were inhibited equally by bafilomycin A1 and amiloride. These results clearly show that a greater proportion of large osteoclasts are active in resorption and that pH(i) regulation is associated with enhanced proton pump activity in actively resorbing osteoclasts. Thus, large and small osteoclasts differ in the proportion of cells that are resorbing, while pH regulatory mechanisms differ mainly between resorbing and nonresorbing cells.

Expression of P2 receptors in bone and cultured bone cells

Extracellular nucleotides acting through P2 receptors elicit a wide range of responses in many cell types. There is increasing evidence that adenosine triphosphate (ATP) may function as an important local messenger in bone and cartilage. In this study, we used immunocytochemistry, employing novel polyclonal antibodies against P2X(1-7) receptors, and in situ hybridization, using oligonucleotide probes corresponding to P2X(2,4) and P2Y(2,4) messenger RNAs (mRNAs), to localize P2 receptors on undecalcified bone sections and on cultured osteoblasts and osteoclasts. We provide the first direct evidence that the P2X(2) receptor subtype is expressed on osteoclasts, osteoblasts, and chondrocytes. We also obtained evidence for the expression of P2X(5) and P2Y(2) receptors on osteoblasts and chondrocytes, and for P2X(4) and P2X(7) receptors on osteoclasts. Our results confirm earlier reports of P2Y(2) and P2X(4) expression in human osteoclastoma and rabbit osteoclasts, respectively, and are consistent with ATP responses observed on bone cells using electrophysiological techniques. Our novel finding that P2X(2) is expressed by osteoclasts is of particular interest. P2X(2) is the only P2 receptor subtype that requires extracellular acidification to show its full sensitivity to ATP, and our recent functional studies have shown that the stimulatory action of ATP on resorption pit formation by mature osteoclasts is amplified greatly at low pH. These findings point to fundamental new mechanisms for the local modulation of bone resorption.

Differences in regulation of pH(i) in large (>/=10 nuclei) and small (</=5 nuclei) osteoclasts

Osteoclasts are multinucleated cells that resorb bone by extrusion of protons and proteolytic enzymes. They display marked heterogeneity in cell size, shape, and resorptive activity. Because high resorptive activity in vivo is associated with an increase in the average size of osteoclasts in areas of greater resorption and because of the importance of proton extrusion in resorption, we investigated whether the activity of the bafilomycin A(1)-sensitive vacuolar-type H(+)-ATPase (V-ATPase) and amiloride-sensitive Na(+)/H(+) exchanger differed between large and small osteoclasts. Osteoclasts were obtained from newborn rabbit bones, cultured on glass coverslips, and loaded with the pH-sensitive indicator 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Intracellular pH (pH(i)) was recorded in single osteoclasts by monitoring fluorescence. Large (>/=10 nuclei) and small (</=5 nuclei) osteoclasts differed in that large osteoclasts had a higher basal pH(i), their pH(i) was decreased by bafilomycin A(1) addition or removal of extracellular Na(+), and the realkalinization upon readdition of Na(+) was bafilomycin A(1) sensitive. After acid loading, a subpopulation of large osteoclasts (40%) recovered by V-ATPase activity alone, whereas all small osteoclasts recovered by Na(+)/H(+) exchanger activity. Interestingly, in 60% of the large osteoclasts, pH(i) recovery was mediated by both the Na(+)/H(+) exchanger and V-ATPase activity. Our results show a striking difference between pH(i) regulatory mechanisms of large and small osteoclasts that we hypothesize may be associated with differences in the potential resorptive activity of these cells.

Purinergic signalling: pathophysiological roles

In this review, after a summary of the history and current status of the receptors involved in purinergic signalling, we focus on the distribution and physiological roles of purines and pyrimidines in both short-term events such as neurotransmission, exocrine and endocrine secretion and regulation of immune cell function, and long-term events such as cell growth, differentiation and proliferation in development and regeneration. Finally, the protective roles of nucleosides and nucleotides in events such as cancer, ischemia, wound healing, drug toxicity, inflammation and pain are explored and some suggestions made for future developments in this rapidly expanding field, with particular emphasis on the involvement of selective agonists and antagonists for purinergic receptor subtypes in therapeutic strategies.

P2 receptor-mediated signal transduction in Ehrlich ascites tumor cells

The mechanisms, by which the P2 receptor agonists adenosine 5'-triphosphate (ATP) and uridine 5'-triphosphate (UTP) evoke an increase in the free cytosolic calcium concentration ([Ca2+]i) and in intracellular pH (pHi), have been investigated in Ehrlich ascites tumor cells. The increase in [Ca2+]i evoked by ATP or UTP is abolished after depletion of intracellular Ca2+ stores with thapsigargin in Ca2+-free medium, and is inhibited by U73122, an inhibitor of phospholipase C (PLC), indicating that the increase in [Ca2+]i is primarily due to release from intracellular, Ins(1,4,5)P3-sensitive Ca2+ stores. ATP also activates a capacitative Ca2+-entry pathway. ATP as well as UTP evokes a biphasic change in pHi, consisting of an initial acidification followed by alkalinization. Suramin and 4,4'-diisothiocyano-2,2'-stilbene-disulfonic acid (DIDS) inhibit the biphasic change in pHi, apparently by acting as antagonists at P2 receptors. The alkalinization evoked by the P2 receptor agonists is found to be due to activation of a 5'-(N-ethyl-N-isopropyl)amiloride (EIPA)-sensitive Na+/H+ exchanger. ATP and UTP elicit rapid cell shrinkage, presumably due to activation of Ca2+ sensitive K+ and Cl- efflux pathways. Preventing cell shrinkage, either by incubating the cells at high extracellular K+ concentration, or by adding the K+-channel blocker, charybdotoxin, does not affect the increase in [Ca2+]i, but abolishes the activation of the Na+/H+ exchanger, indicating that activation of the Na+/H+ exchanger is secondary to the Ca2+-induced cell shrinkage.

ATP is a potent stimulator of the activation and formation of rodent osteoclasts

1. There is increasing evidence that extracellular ATP acts directly on bone cells via P2 receptors. In normal rat osteoclasts, ATP activates both non-selective cation channels and Ca2+-dependent K+ channels. In this study we investigated the action of ATP on the formation of osteoclasts and on the ultimate function of these cells, namely resorption pit formation. 2. We found that ATP stimulated resorption pit formation up to 5.6-fold when osteoclast-containing bone cell populations from neonatal rats were cultured for 26 h on ivory discs, with a maximum effect occurring at relatively low concentrations (0.2-2 microM). The stimulatory effect of ATP was amplified greatly when osteoclasts were activated by culture in acidified media (pH 6.9-7.0). Pit formation by acid-activated osteoclasts in the absence of ATP was inhibited by apyrase, an ecto-ATPase and by suramin, an antagonist of P2 receptors. 3. Over the same concentration range at which rat osteoclast activation occurred (0.2-2 microM), ATP also enhanced osteoclast formation in 10 day mouse marrow cultures, by up to 3.3-fold, with corresponding increases in resorption pit formation. Higher concentrations of ATP (20-200 microM) reduced or blocked osteoclast formation. Adenosine, a P1 receptor agonist, was without effect on either osteoclast activation or formation. 4. These results suggest that low levels of extracellular ATP may play a fundamental role in modulating both the resorptive function and formation of mammalian osteoclasts.

Src family tyrosine kinase regulates intracellular pH in cardiomyocytes

The Anion Cl-/HCO3- Exchangers AE1, AE2, and AE3 are membrane pH regulatory ion transporters ubiquitously expressed in vertebrate tissues. Besides relieving intracellular alkaline and CO2 loads, the AEs have an important function during development and cell death and play a central role in such cellular properties as cell shape, metabolism, and contractility. The activity of AE(s) are regulated by neurohormones. However, little is known as to the intracellular signal transduction pathways that underlie this modulation. We show here that, in cardiomyocytes that express both AE1 and AE3, the purinergic agonist, ATP, triggers activation of anion exchange. The AE activation is observed in cells in which AE3 expression was blocked but not in cells microinjected with neutralizing anti-AE1 antibodies. ATP induces tyrosine phosphorylation of AE1, activation of the tyrosine kinase Fyn, and association of both Fyn and FAK with AE1. Inhibition of Src family kinases in vivo by genistein, herbimycin A, or ST638 prevents purinergic activation of AE1. Microinjection of either anti-Cst.1 antibody or recombinant CSK, both of which prevent activation of Src family kinase, significantly decreases ATP-induced activation of AE. Microinjection of an anti-FAK antibody as well as expression in cardiomyocytes of Phe397 FAK dominant negative mutant, also prevents purinergic activation of AE. Therefore, tyrosine kinases play a key role in acute regulation of intracellular pH and thus in cell function including excitation-contraction coupling of the myocardium.

Purinergic transmitters inhibit bone formation by cultured osteoblasts

Adenosine triphosphate (ATP) and other purinoceptor agonists cause a transient rise in [Ca2+]i in cultured osteoblast-like cells and have a mitogenic effect, as does parathyroid hormone (PTH), and there is evidence that ATP and PTH can act synergistically on osteoblasts. The likelihood that nucleotides, acting through purinoceptors, are important local factors in bone remodeling is therefore considerable. However, their effect on bone formation is unknown. We recently developed a culture system in which appositional bone formation occurs only in narrow grooves cut in a substratum. We have used this as an assay to measure the effects of ATP (50 and 500 mumol/L), ATP gamma S (20 mumol/L), 2-MeSATP (2 and 20 mumol/L), uridine triphosphate (UTP) (0.2, 2, and 20 mumol/L), adenosine (20 mumol/L), bovine PTH (0.25 and 0.5 IU/mL), rat PTH1-34 (10(-8) and 10(-7) mol/L), and rat PTHrP1-40 (10(-9) and 10(-8) mol/L) on bone formation by rat calvarial osteoblasts. The culture medium was renewed 3 times/week (every 2 or 3 days), and the number of bone loci and length and area of Alizarin red-stained mineralized bone formed in the grooves of each specimen in 16-29 days were measured. Compared with controls, ATP gamma S, 2-MeSATP, and ATP reduced the amount of bone formed in a 2-3 week culture period. Adenosine had no effect, and UTP either had no effect or at 2 mumol/L stimulated bone formation. PTH and PTHrP completely abolished bone formation in 4 week cultures. Our findings are consistent with evidence for more than one P2 purinoceptor subtype in bone, and show for the first time that the effect of ATP on appositional bone formation by osteoblasts in vitro is, like PTH and PTHrP, inhibitory.

Nuclear versus perinuclear and cytoplasmic calcium in osteoclasts

We measured fluorescence from the calcium indicator Fluo-3 in multinucleated osteoclasts. In the initial state, each nucleus is surrounded by a ring of bright fluorescence. Following activation of purinergic receptors by 100 microM ATP there is a pulse of cellular fluorescence increase, and nuclear fluorescence intensity becomes greater than that of the cytoplasm. This is followed by a period during which the fluorescence of the cell decreases below that of the initial state. During the pulsed increase following purinergic receptor activation, the perinuclear fluorescence intensity does not increase as much as that in the nuclear centers and, following this pulse, the perinuclear fluorescence intensity decreases more than that in the nuclear centers, relative to the initial state. Measurements in which Mn2+ was introduced into the cell show that the number of Fluo-3 molecules per unit horizontal area in the nuclear centers is slightly greater than that in the perinuclear regions, and more than twice that in the surrounding cytoplasm. These results show that there is a much higher free calcium concentration in the perinuclear regions than in the nuclear centers in the initial state, with a release of free calcium from the perinuclear regions following activation of the purinergic receptors. These data also provide evidence that the free calcium concentration in the nuclear centers is lower than in the cytoplasm in the initial state.