Novel mechanisms of calcium handling by the osteoclast: A review-hypothesis

The osteoclast is a cell that is unique in its ability to resorb bone and, in doing so, becomes exposed to unusually high millimolar Ca2+ concentrations. It is generally accepted that, during resorption, osteoclasts can "sense" changes in their ambient Ca2+ concentration. This triggers a sharp cytosolic Ca2+ increase through both Ca2+ release and Ca2+ influx. The change in cytosolic Ca2+ is transduced finally into inhibition of bone resorption. It has been shown that a type 2 ryanodine receptor isoform, expressed uniquely in the plasma membrane, functions as a Ca2+ influx channel and possibly as a Ca2+ sensor. Ryanodine receptors are ordinarily Ca2+ release channels that have a microsomal membrane location in a wide variety of eukaryotic cells, including the osteoclasts. However, only recently has it become obvious that ryanodine receptors are also expressed in osteoclast nuclear membranes, at which site they probably gate nucleoplasmic Ca2+ influx. Nucleoplasmic Ca2+ in turn regulates key nuclear processes, including gene expression and apoptosis. Here, we review the potential mechanisms underlying the recognition, movement, and effects of Ca2+ in the osteoclast. We will also speculate on the general biological significance of the unique processes used by the osteoclast to handle high Ca2+ loads during bone resorption.

Direct microsensor measurement of nitric oxide production by the osteoclast

Nitric oxide (NO) triggers marked osteoclast retraction which closely resembles that due to Ca2+. The effect of Ca2+ has been attributed to a stimulated release of NO. Here, we show for the first time, by direct measurement with a microsensor, that osteoclasts do indeed produce NO and that this production is enhanced by a high Ca2+. We also show that the Ca2+ ionophore, A23187, mimics the latter. Furthermore, osteoclasts on dentine produce more NO than osteoclasts on glass and NO release from dentine-plated osteoclasts is much less sensitive to stimulation by Ca2+. Finally, the microsomal Ca2+ store-depleting agent, thapsigargin, attenuates NO release only from osteoclasts on glass, suggesting that stored Ca2+ has the dominant effect in modulating NO release from non-resorbing cells. NO is a powerful inhibitor of bone resorption: a direct demonstration of its production is therefore strong evidence for a role in modulating osteoclast function.

Emerging insights into the role of calcium ions in osteoclast regulation

Osteoclasts are exposed to unusually high, millimolar, Ca2+ concentrations and can "sense" changes in their ambient Ca2+ concentration during resorption. This results in a sharp cystolic Ca2+ increase through both Ca2+ release and Ca2+ influx. The rise in cystolic Ca2+ is transduced finally into an inhibition of bone resorption. We have shown that a type 2 ryanodine receptor isoform, expressed uniquely in the osteoblast plasma membrane, functions as a Ca2+ influx channel, and possibly as a Ca2+ sensor. Ryanodine receptors are ordinarily microsomal membrane Ca2+ release channels. They have only recently been shown to be expressed a other sites, including nuclear membranes. At the latter site, ryanodine receptors gate nucleoplasmic Ca2+ influx. Nucleoplasmic Ca2+, in turn, regulates key nuclear processes, including gene expression and apoptosis. Here, we review potential mechanisms underlying the recognition, movement, and actions of Ca2+ in the osteoclast.

An unusual case of orbital hydatid cyst: a surgical emergency

A rare case of a 19-year-old man with an orbital hydatid cyst is presented. The lesion caused rapid deterioration of vision and was not responsive to the systemic use of mebendazole. It was treated successfully with emergency surgery. Early detection, surgical excision, and the systemic use of albendazole are suggested for the treatment of orbital hydatid cysts.

Retrograde Ca2+ signaling in C2C12 skeletal myocytes in response to mitochondrial genetic and metabolic stress: a novel mode of inter-organelle crosstalk

We have investigated the mechanism of mitochondrial-nuclear crosstalk during cellular stress in mouse C2C12 myocytes. For this purpose, we used cells with reduced mitochondrial DNA (mtDNA) contents by ethidium bromide treatment or myocytes treated with known mitochondrial metabolic inhibitors, including carbonyl cyanide m-chlorophenylhydrazone (CCCP), antimycin, valinomycin and azide. Both genetic and metabolic stresses similarly affected mitochondrial membrane potential (Deltapsim) and electron transport-coupled ATP synthesis, which was also accompanied by an elevated steady-state cytosolic Ca2+ level ([Ca2+]i). The mitochondrial stress resulted in: (i) an enhanced expression of the sarcoplasmic reticular ryanodine receptor-1 (RyR-1), hence potentiating the Ca2+ release in response to its modulator, caffeine; (ii) enhanced levels of Ca2+-responsive factors calineurin, calcineurin-dependent NFATc (cytosolic counterpart of activated T-cell-specific nuclear factor) and c-Jun N-terminal kinase (JNK)-dependent ATF2 (activated transcription factor 2); (iii) reduced levels of transcription factor, NF-kappaB; and (iv) enhanced transcription of cytochrome oxidase Vb (COX Vb) subunit gene. These cellular changes, including the steady-state [Ca2+]i were normalized in genetically reverted cells which contain near-normal mtDNA levels. We propose that the mitochondria-to-nucleus stress signaling occurs through cytosolic [Ca2+]i changes, which are likely to be due to reduced ATP and Ca2+ efflux. Our results indicate that the mitochondrial stress signal affects a variety of cellular processes, in addition to mitochondrial membrane biogenesis.

Molecular and functional evidence for calcineurin-A alpha and beta isoforms in the osteoclast: novel insights into cyclosporin A action on bone resorption

We provide the first molecular evidence for the presence of a functional serine/threonine phosphatase, calcineurin-A (CN-A), in the osteoclast. Polymerase chain reaction (PCR) of an osteoclast cDNA library, together with restriction mapping, revealed two isoform sequences, alpha and beta. We then examined the functionality of the detected CN-A by assessing the effect of a classical antagonist, cyclosporin A (CsA), in the osteoclast resorption (pit) assay. CsA (0.1 and 1 microg ml-1) potently inhibited bone resorption. The presence of lymphocytes, with or without prior exposure to CsA in vivo, failed to reverse the CsA-induced resorption-inhibition. Expectedly, CsA had no direct effect on cytosolic Ca2+ levels in fura-2-loaded osteoclasts. These studies are a prelude to further investigations into the possible role of CN-A in osteoclast regulation. Finally, mechanistic studies on the bone effects of CsA, a widely used immunosupressant, should proceed from these observations.

The effect of extracellularly applied divalent cations on cytosolic Ca2+ in murine leydig cells: evidence for a Ca2+-sensing receptor

1. The effect of extracellularly applied divalent cations upon cytosolic Ca2+ levels ([Ca2+]) was investigated in fura-2-loaded mouse Leydig (TM3) cells. 2. The extracellular application of Ca2+ (2.5-15 mM) or Ni2+ (0.5-5 mM) elicited concentration-dependent elevations in cytosolic [Ca2+] that were followed by decays to baseline levels. Extracellular Mg2+ (0.8-15 mM) failed to influence cytosolic [Ca2+]. 3. Conditioning applications of Ca2+ (2.5-10 mM), Mg2+ (2.5-15 mM) or Ni2+ (0.5-5 mM) all attenuated the cytosolic Ca2+ response to a subsequent test application of 5 mM [Ni2+]. 4. The amplitude of Ni2+-induced cytosolic Ca2+ signals remained constant in low-Ca2+ solutions. Such findings suggest a participation of Ca2+ release from intracellular stores. In parallel, depletion of Ca2+ stores by either ionomycin (5 microM, in low-Ca2+ solutions) or thapsigargin (4 microM) abolished or attenuated Ni2+-induced Ca2+ transients. 5. Ionomycin (5 microM) elevated cytosolic [Ca2+] in Ca2+-free solutions even after prior Ni2+ application, indicating the presence of Ni2+-insensitive stores. 6. Caffeine (250 and 500 microM) elevated cytosolic [Ca2+] and attenuated Ni2+-induced Ca2+ release. Furthermore, TM3 cells stained intensely with a specific anti-ryanodine receptor antiserum, Ab34. These findings suggest that Ca2+ release is regulated by ryanodine receptors. 7. Both membrane depolarization and hyperpolarization, brought about by changes in extracellular [K+] ([K+]e) in the presence of valinomycin (5 microM), altered the waveform of the Ni2+-induced cytosolic Ca2+ signal. Hyperpolarization, in addition, diminished the response magnitude. Such voltage-induced response modulation localizes the regulatory events to the Leydig cell plasma membrane. 8. We propose the existence of a cell surface divalent cation (Ca2+) receptor in Leydig cells, the activation of which triggers Ca2+ fluxes through ryanodine receptors.

Mode of action of interleukin-6 on mature osteoclasts. Novel interactions with extracellular Ca2+ sensing in the regulation of osteoclastic bone resorption

We describe a physiologically significant mechanism through which interleukin-6 (IL-6) and a rising ambient Ca2+ interact to regulate osteoclastic bone resorption. VOXEL-based confocal microscopy of nonpermeabilized osteoclasts incubated with anti- IL-6 receptor antibodies revealed intense, strictly peripheral plasma membrane fluorescence. IL-6 receptor expression in single osteoclasts was confirmed by in situ reverse transcriptase PCR histochemistry. IL-6 (5 ng/l to 10 microg/l), but not IL-11 (10 and 100 microg/l), reversed the inhibition of osteoclastic bone resorption induced by high extracellular Ca2+ (15 mM). The IL-6 effect was abrogated by excess soluble IL-6 receptor (500 microg/l). Additionally, IL-6 (5 pg/l to 10 microg/l) inhibited cytosolic Ca2+ signals triggered by high Ca2+ or Ni2+. In separate experiments, osteoclasts incubated in 10 mM Ca2+ or on bone released more IL-6 than those in 1.25 mM Ca2+. Furthermore, IL-6 mRNA histostaining was more intense in osteoclasts in 10 or 20 mM Ca2+ than cells in 1.25 mM Ca2+. Similarly, IL-6 receptor mRNA histostaining was increased in osteoclasts incubated in 5 or 10 mM Ca2+. Thus, while high Ca2+ enhances IL-6 secretion, the released IL-6 attenuates Ca2+ sensing and reverses inhibition of resorption by Ca2+. Such an autocrine-paracrine loop may sustain osteoclastic activity in the face of an inhibitory Ca2+ level generated locally during resorption.

Effects of electromagnetic stimulation on the functional responsiveness of isolated rat osteoclasts

We report the effects of pulsed electromagnetic fields (PEMFs) on the responsiveness of osteoclasts to cellular, hormonal, and ionic signals. Osteoclasts isolated from neonatal rat long bones were dispersed onto either slices of devitalised cortical bone (for the measurement of resorptive activity) or glass coverslips (for the determination of the cytosolic free Ca2+ concentration, [Ca2+]). Osteoclasts were also cocultured on bone with osteoblastlike, UMR-106 cells. Bone resorption was quantitated by scanning electron microscopy and computer-assisted morphometry. PEMF application to osteoblast-osteoclast cocultures for 18 hr resulted in a twofold stimulation of bone resorption. In contrast, resorption by isolated osteoclasts remained unchanged in the presence of PEMFs, suggesting that osteoblasts were necessary for the PEMF-induced resorption simulation seen in osteoblast-osteoclast cocultures. Furthermore, the potent inhibitory action of the hormone calcitonin on bone resorption was unaffected by PEMF application. However, PEMFs completely reversed another quite distinct action of calcitonin on the osteoclast: its potent inhibitory effect on the activation of the divalent cation-sensing (or Ca2+) receptor. For these experiments, we made fura 2-based measurements of cytosolic [Ca2+] in single osteoclasts in response to the application of a known Ca2+ receptor agonist, Ni2+. We first confirmed that activation of the osteoclast Ca2+ receptor by Ni2+ (5 mM) resulted in a characteristic monophasic elevation of cytosolic [Ca2+]. As shown previously, this response was attenuated strongly by calcitonin at concentrations between 0.03 and 3 nM but remained intact in response to PEMFs. PEMF application, however, prevented the inhibitory effect of calcitonin on Ni2+-induced cytosolic Ca2+ elevation. This suggested that the fields disrupted the interaction between the calcitonin and Ca2+ receptor systems. In conclusion, we have shown that electromagnetic fields stimulate bone resorption through an action on the osteoblast and, by abolishing the inhibitory effects of calcitonin, also restore the responsiveness of osteoclasts to divalent cations.

A possible new role for vitamin D-binding protein in osteoclast control: inhibition of extracellular Ca2+ sensing at low physiological concentrations

Upon removal of its sialic acid or galactose residue, vitamin D-binding protein (DBP) becomes a potent macrophage-activating factor, DBP-MAF. Here we document a new function of DBP-MAF and its parent molecule, DBP, in osteoclast control. We show that all DBPs potently inhibit extracellular Ca2+ (cation) sensing at low nanomolar concentrations with the following rank order of potency: native DBP = sialidase-treated DBP > beta-galactosidase-treated DBP. This attenuation remains unaffected despite co-incubation either with the native DBP ligand, 1,25-dihydroxyvitamin D3, or with an asialoglycoprotein receptor modulator, asialoorosomucoid. Taken together, the results suggest that circulating DBP may play a role in the systemic control of osteoclastic bone resorption, a hitherto unrecognized action of the protein.

Functional and molecular evidence for P2X receptors in LLC-PK1 cells

Extracellular ATP affects a wide variety of cells via purinergic membrane receptors. One class of purinergic receptors, P2X, consists of ATP-gated, calcium-permeable, cation-selective channels. We performed whole cell patch-clamp studies, intracellular calcium concentration ([Ca2+]i) measurements, and reverse transcription-polymerase chain reaction (RT-PCR) to determine whether P2X receptors are expressed in LLC-PK1 cells. First, in patch-clamp studies, 100 microM ATP depolarized the cell membrane and increased the whole cell conductance of LLC-PK1 cells. This response was dose dependent and inhibited by 100 microM suramin, a P2 receptor antagonist. The ATP-induced conductance was cation selective but did not discriminate between Na+ and K+. ADP, alpha,beta-methylene-ATP, and beta,gamma-methylene-ATP had no effect on the whole cell conductance. Next, 10 microM ATP caused a rapid rise in [Ca2+]i in LLC-PK1 cells. This effect of ATP was inhibited by the absence of extracellular calcium and by suramin but not by pretreatment with pertussis toxin. ADP and beta,gamma-methylene-ATP had little or no effect on [Ca2+]i. Finally, RT-PCR produced a 330-bp fragment from LLC-PK1 cell RNA, whose sequence was 80% identical to the rat P2X1 receptor. We conclude that LLC-PK1 cells express purinergic receptors of the P2X class, which mediate depolarization and calcium entry when activated.

Upregulation of functional ryanodine receptors during in vitro aging of human diploid fibroblasts

We demonstrate for the first time that cellular aging in vitro is accompanied by a dramatic elevation in the levels of ryanodine receptor-bearing Ca2+ channels. These channels normally reside within microsomal membranes and gate Ca2+ release from intracellular stores. We therefore measured cytosolic Ca2+ levels in 'young' (30 mean population doublings, MPDs) and 'senescent' (53 to 58 MPDs) human diploid fibroblasts (HDFs). Application of the known ryanodine receptor modulators, caffeine or cyclic adenosine diphosphate-ribose (cADPr), triggered cytosolic Ca2+ signals in both young and senescent cells. The signal magnitude however was significantly greater in senescent compared with young HDFs. In parallel, incubation with a highly specific anti-ryanodine receptor antiserum resulted in specific immunofluorescence only in senescent HDFs. We envisage that elevated levels of functional ryanodine receptors may underlie the defective Ca2+ handling and cellular degeneration that occurs with aging.

Dispersion of ventricular repolarisation: a marker of ventricular arrhythmias in patients with previous myocardial infarction

OBJECTIVE

To examine whether, in coronary patients after myocardial infarction, the dispersion of ventricular repolarisation measured through QT and JT intervals from a surface electrocardiogram could allow separation of those with ventricular tachyarrhythmias (VT) complicating their myocardial infarct from those without.

DESIGN

A retrospective comparative study.

SETTING

University hospital.

PATIENTS

39 patients with myocardial infarction complicated by VT, 300 patients after myocardial infarction without arrhythmic events, and 1000 normal subjects. The myocardial infarction groups were divided into anterior, inferior, and mixed locations.

INTERVENTIONS

A computer algorithm examined an averaged cycle from a 10 second record of 15 simultaneous leads (12 lead ECG + Frank XYZ leads). After interactive editing, four intervals were computed: QTapex, JTapex, QTend, and JTend. For each interval, the dispersion was defined as the difference between the maximum and minimum values across the 15 leads.

RESULTS

The mean values of all four dispersion indices were higher in patients with myocardial infarction than in normal subjects (p < 0.01). In the infarct groups, patients with VT had significantly greater mean and centile dispersion values than those without VT. For instance, the 97.5th centile value of QTend was 65 ms in normal individuals, 90 ms in infarct patients without arrhythmia, and 128 ms in those with VT; 70% of the infarct patients who developed serious ventricular arrhythmias had values exceeding the 97.5th centile of the normal group, while only 18% of the infarct patients without arrhythmia had dispersion values above this normal upper limit. Among the infarct patients, nearly half of those (18 of 39) with tachyarrhythmias had dispersion values that exceeded the 97.5th centile of those without arrhythmia.

CONCLUSIONS

Dispersion of ventricular repolarisation may be a good non-invasive tool for discriminating coronary patients susceptible to VT from those who are at low risk.

Dispersion of ventricular repolarization in dilated cardiomyopathy

OBJECTIVE

Increased dispersion of ventricular repolarization has been shown to be a marker for increased risk of ventricular tachyarrhythmias in various cardiac disorders. The present study is aimed at comparing the values of four dispersion indices in four clinical groups: normal subjects (n = 23), patients with intraventricular conduction defects (QRS > 0.12 s) without underlying cardiac disease (n = 30), patients with dilated cardiomyopathy (n = 36), and patients with both dilated cardiomyopathy and ventricular conduction defects (n = 18).

METHODS

On an averaged cycle from a 10 s record of 15 simultaneous leads (12-lead ECG and XYZ leads), and after interactive editing, four intervals were computed: JTapex, JTend, QTapex and QTend. For each interval, the dispersion is defined as the difference between the maximal and minimal values across the 15 leads.

RESULTS

The mean values of all four dispersion indices were significantly smaller in the normal group than in the three other groups (P < 0.001). Among patients with dilated cardiomyopathy, those with intraventricular conduction defects had significantly higher dispersion values than those without, even disregarding the QRS duration (P < 0.01). Thus, patients with both dilated cardiomyopathy and ventricular conduction defects have larger dispersion values than patients with ventricular conduction defects alone (P < 0.01) and than those with dilated cardiomyopathy without intraventricular conduction defects.

CONCLUSION

Dispersion of ventricular repolarization is increased in patients with dilated cardiomyopathy, especially in those with ventricular conduction defects, suggesting that they are at higher risk of arrhythmic events.

Regulation of extracellular calcium sensing in rat osteoclasts by femtomolar calcitonin concentrations

Certain eukaryotic cells can sense changes in their extracellular Ca2+ concentration through molecular structures termed Ca(2+)-sensing receptors (CaRs). We have shown recently that in the bone-resorbing osteoclast, a unique cell surface-expressed ryanodine receptor (RyR), functions as the CaR. The present study demonstrates that the sensitivity of this receptor is modulated by physiological femtomolar concentrations of the bone-conserving hormone, calcitonin. Calcitonin was found to inhibit cytosolic Ca2+ responses to both Ca2+ and Ni2+. The latter inhibition was mimicked by amylin (10(-12) M), calcitonin gene-related peptide (10(-12) M), cholera toxin (5 micrograms/l) and dibutyryl adenosine 3',5'-cyclic monophosphate (cAMP) (2.5 x 10(-4) or 5 x 10(-4) M) and was reversed by the protein kinase A phosphorylation inhibitor, IP-20. Finally, using a quench flow module, we showed that cellular cAMP levels rise to a peak within 25 ms of calcitonin application; this is consistent with the peptide's rapid effect on CaR activation. We conclude, therefore, that cAMP plays a critical role in the control of CaR function by calcitonin.

Computer-assisted study of ECG indices of the dispersion of ventricular repolarization

A new computer-assisted method for the quantitative assessment of the dispersion of ventricular repolarization (DVR) has been developed. Through interactive editing of an averaged QRS-T cycle from a 15-lead electrocardiographic (ECG) record (12-lead ECG + XYZ leads), five ECG indices of DVR are automatically computed: they represent the maximal interlead difference of QT and the intervals from the J point to the T wave end, from the J point to the T wave apex, and from the T wave apex to the T wave end. The standard limits of these indices were then established in six clinical groups, including normal subjects and patients with left ventricular hypertrophy, with myocardial infarction, and with intraventricular conduction defect, all subjects being without ventricular arrhythmias and without interacting drugs. The mean values and percentile ranges of all DVR indices were lower in the normal group than in all pathologic groups. The 97.5th percentiles of the QT end dispersion and the JT end dispersion were, respectively, 65 and 76 ms in normal subjects, 84 and 86 ms in patients with inferior MI; 89 and 100 ms in those with anterior MI; 90 and 98 ms in those with left ventricular hypertrophy; and 94 and 99 ms in those with intraventricular conduction defects. This suggests that increased DVR is associated with the varieties of heart disease represented in this study, even in the absence of ventricular arrhythmias, and also that individual measurements of DVR used as predictors of future arrhythmic events should be referred to the standard range of their own clinical group.

Extracellularly applied ruthenium red and cADP ribose elevate cytosolic Ca2+ in isolated rat osteoclasts

We demonstrated recently that the divalent cation-sensing receptor on the osteoclast, the Ca2+ receptor (CaR), is a functional component of a cell surface-expressed ryanodine receptor-like molecule (RyR). The objective of the present study was to further characterize this putative RyR by use of the two well-known cell-impermeant RyR modulators, ruthenium red and adenosine 3',5'-cyclic diphosphate ribose (cADPr). We found that, when applied extracellularly, ruthenium red (5 x 10(-8)-10(-4) M) and cADPr (5 x 10(-6) M) triggered an elevation of cytosolic [Ca2+]. Depolarization of the cell membrane by the application of 0.1 M K+ in the presence of 5 x 10(-6) M. valinomycin resulted in a concentration-dependent increase in the magnitude of the cytosolic Ca2+ response to extracellular ruthenium red (5 x 10(-9) and 5 x 10(-5) M), a phenomenon that was not seen when osteoclasts were hyperpolarized using 5 x 10(-3) M K+ with 5 x 10(-6) M valinomycin. In the presence of an intact nonleaky cell membrane, these results would favor a plasma membrane locus of action for the two modulators. Furthermore, pretreatment of osteoclasts with either modulator resulted in a markedly attenuated cytosolic Ca2+ transient elicited in response to the CaR agonist Ni2+, thus confirming an interaction between the cADPr- and ruthenium red-sensitive sites and the osteoclast CaR. The inhibition of the cytosolic Ca2+ response to Ni2+ induced by ruthenium red remained unchanged in the face of membrane potential changes. Finally, the cytosolic Ca2+ response to caffeine (5 x 10(-4) M), another RyR modulator, was also strongly attenuated by pretreatment with 5 x 10(-9) M ruthenium red. We conclude that ruthenium red and cADPr act on plasma membrane-resident sites and that both these sites interact with the process of divalent cation sensing.

Dispersion of ventricular repolarization in hypertrophic cardiomyopathy

On an averaged QRS-T cycle from a 15-lead record (12-lead electrocardiogram + XYZ leads) and through interactive editing, four electrocardiographic indices of the dispersion of ventricular repolarization (DVR) are automatically computed and represent the maximal interlead difference of QT and JTend and QT and JTapex. The values of these indices were then examined in three clinical groups matched for age and sex: normal subjects (control), patients with left ventricular hypertrophy (LVH group), and patients with hypertrophic cardiomyopathy (HCM group) without ventricular arrhythmias and without interacting drugs. The mean values of all four DVR indices were significantly increased in the HCM group compared with the control group and the LVH group of another origin (ie, for the QTe dispersion index, the mean values and the 97.5th percentiles were, respectively, 65 +/- 18 ms and 97 ms in the HCM group, 41 +/- 25 ms and 79 ms in the LVH group, and 31 +/- 15 ms and 58 ms in the control group). The maximal QT interval was also significantly longer in the HCM group (464 +/- 30 ms) than in the LVH group (436 +/- 32 ms) and the control group (428 +/- 25 ms).

Disinfection and sterilization practices in Mexico

We evaluated antisepsis, disinfection, and sterilization procedures at 22 hospitals in the state of Yucatan, Mexico, which provide services for approximately 80% of the population. The percentage of hospitals that followed standard recommendations for diverse antisepsis procedures were as follows: surgical scrub, 41%; surgical site preparation, 68%; central intravenous (iv) catheters, 61%; peripheral iv catheters, 86%; urinary catheters, 41%, and umbilical cord care, 5%. Inappropriate procedures typically involved the use of benzalkonium chloride and mercury compounds. Adequate sterilization procedures were observed for sharp surgical instruments in 9%, for blunt surgical instruments in 81%; for linen in 90%; for surgical brushes in 9%; for metal prostheses in 100%; for plastic prostheses in 57%; and for disinfection of endoscopes in 30%. The most common errors were the use of too short an exposure time in steam sterilizers or dry heat sterilizers, and the use of benzalkonium chloride as a sterilizing agent. Only 14% of hospitals used commercial spore preparations for autoclave monitoring. For the reuse of disposables, 50-94% of hospitals employed inappropriate sterilization or low-level disinfection procedures. Similar conditions are likely to exist in hospitals throughout the country. The establishment of strict regulations and hospital training programmes for disinfection and sterilization procedures, and the reuse of disposable devices is urgently needed in Mexico.

A ryanodine receptor-like molecule expressed in the osteoclast plasma membrane functions in extracellular Ca2+ sensing

Ryanodine receptors (RyRs) reside in microsomal membranes where they gate Ca2+ release in response to changes in the cytosolic Ca2+ concentration. In the osteoclast, a divalent cation sensor, the Ca2+ receptor (CaR), located within the cell's plasma membrane, monitors changes in the extracellular Ca2+ concentration. Here we show that a RyR-like molecule is a functional component of this receptor. We have demonstrated that [3H] ryanodine specifically binds to freshly isolated rat osteoclasts. The binding was displaced by ryanodine itself, the CaR agonist Ni2+ and the RyR antagonist ruthenium red. The latter also inhibited cytosolic Ca2+ elevations induced by Ni2+. In contrast, the responses to Ni2+ were strongly potentiated by an antiserum Ab129 raised to an epitope located within the channel-forming domain of the type II RyR. The antiserum also stained the surface of intact, unfixed, trypan blue-negative osteoclasts. Serial confocal sections and immunogold scanning electron microscopy confirmed a plasma membrane localization of this staining. Antiserum Ab34 directed to a putatively intracellular RyR epitope expectedly did not stain live osteoclasts nor did it potentiate CaR activation. It did, however, stain fixed, permeabilized cells in a distinctive cytoplasmic pattern. We conclude that an RyR-like molecule resides within the osteoclast plasma membrane and plays in important role in extracellular Ca2+ sensing.

Extracellular cation sensing by the enterocyte: prediction of a novel divalent cation "receptor"

We report that the divalent cation Ni2+ elicits elevations in the cytosolic free Ca2+ concentration ([Ca2+]) in cultured enterocytes. These elevations were monophasic, each response consisting of a rapid initial transient rise of cytosolic [Ca2+] to a peak value followed by an exponential decline. The magnitude of the cytosolic [Ca2+] elevation varied with the concentration of applied Ni2+. In some cells, a single application of Ni2+ induced oscillatory changes in cytosolic [Ca2+]. There was also evidence for use-dependent inactivation: a conditioning application of Ni2+ substantially attenuated the response resulting from its subsequent application. Our findings thus demonstrate the existence of a divalent cation-sensing "receptor" on the enterocyte. This putative receptor may play a role in regulating mineral absorption across the enterocyte membrane.

Caffeine modulates Ca2+ receptor activation in isolated rat osteoclasts and induces intracellular Ca2+ release

A ryanodine-sensitive pathway is involved in intracellular Ca2+ release in response to activation of the osteoclast cell surface Ca2+ receptor. We now report that the ryanodine-receptor modulator, caffeine itself released intracellularly stored Ca2+ and, strongly inhibited Ca2+ release triggered in response to Ca(2+)-receptor activation by Ni2+, a surrogate cation agonist. Caffeine yielded a bell-shaped concentration-response curve (0.005-2 mM) and displayed use-dependent inactivation. Furthermore, responses to caffeine were abolished on prior application of Ni2+ (5 mM). Subthreshold (0.005 mM) caffeine concentrations abolished Ni(2+)-induced elevations in the cytosolic Ca2+ concentration ([Ca2+]). However, in a Ca(2+)-free, ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid-containing solution (extracellular [Ca2+] < 10 nM), caffeine (0.5 mM) neither elevated [Ca2+] nor inhibited the response to Ni2+. Finally, when caffeine was applied to intercept the plateau phase of the cytosolic Ca2+ signal triggered by extracellular Ca2+ elevation (10 mM), a rapid but reversible inactivation followed. These studies strongly indicate the existence of a caffeine-sensitive mechanism for the release of intracellularly stored Ca2+ in the osteoclast.

Effect of membrane potential on surface Ca2+ receptor activation in rat osteoclasts

Osteoclasts are known to possess a divalent cation-sensitive receptor, the Ca2+ receptor (CaR). The latter monitors changes in the local Ca2+ concentration generated as a result of hydroxyapatite dissolution. CaR activation elevates cytosolic [Ca2+] and thereby inhibits osteoclastic bone resorption. Recent studies have used Ni2+ as a surrogate CaR agonist to elicit changes in cytosolic [Ca2+]. This article examines the effects of membrane potential changes on the kinetics of the cytosolic [Ca2+] signal resulting from such Ni(2+)-induced CaR activation. Membrane potential was altered through variations in the extracellular [K] in combination with applications of the K+ ionophore, valinomycin. Membrane potential changes were confirmed by independent electrophysiological patch clamp studies of whole osteoclasts. The application of valinomycin produced a distinct, sustained elevation of cytosolic [Ca2+] in single fura 2-loaded cells, a "primary" response. This response was independent of valinomycin concentration (between 5 nM to 5 microM) and persisted in Ca(2+)-free, EGTA-containing solutions. It also persisted both in high (105 mM) and low (5 mM) extracellular [K+]. A gradual "secondary" elevation of cytosolic [Ca2+] then followed with the continued application of valinomycin, but this was eliminated by sequestering the extracellular [Ca2+] or by increasing extracellular [K+] from 5 to 105 mM. In a separate set of experiments, the presence of 5 microM [valinomycin]-([K+] = 5 mM) prolonged the cytosolic [Ca2+] signal elicited by 50 microM-[Ni2+] application. These prolonged kinetics persisted in low extracellular [Ca2+] (zero-added Ca2+), but reverted to a rapid time-course in the presence of 105 mM-[K+] or at higher [Ni2+] (500 microM and 5 mM). The experiments thus indicate that membrane voltage modifies the kinetics of CaR activation by Ni2+ and therefore suggests that the CaR is an integral protein in the osteoclast surface membrane.

Quantitative studies on the effect of prostacyclin on freshly isolated rat osteoclasts in culture

Prostaglandins exert marked but transient inhibitory effects on bone resorption. The present study examines the effects of prostacyclin (0.15 to 25 microM) on the morphology of freshly disaggregated rat osteoclasts. An area descriptor, rho, represented changes in total cell spread area, and a motility descriptor, mu, represented overall changes in cell motility. The application of prostacyclin intercepted the trend of an increasing cell spread area with time and produced a transient reduction of rho, an R effect. Its magnitude depended upon concentration and was marked at 25 microM prostacyclin. The subsequent recovery (+0.8/min) of rho at this concentration resembled the persistent spreading seen in the absence of the agonist. There was also a sustained decrease in mu to approximately 60% of its pretreatment value (a Q effect) following the application of 25 microM prostacyclin. The extracellular application of 20 mM [Ca2+] produced a similarly transient cell retraction preceded by a rise of cytosolic [Ca2+], but without a corresponding decrease in mu. In contrast, prostacyclin did not elevate cytosolic [Ca2+], suggesting the triggering of an alternative transduction pathway. A fully reversible retraction together with incomplete quiescence may explain the transience characteristic of the antiresorptive action of prostacyclin.

Calcium influx and release in isolated rat osteoclasts

Intracellular and extracellular sources of cytosolic [Ca2+] elevation in isolated rat osteoclasts were explored by a comparison of fura-2 signals in response to application of the Ca2+ ionophore, ionomycin, in Ca(2+)-containing and in Ca(2+)-free bathing solutions. Cytosolic [Ca2+] transients persisted in osteoclasts bathed in Ca(2+)-free, EGTA-containing solutions. They consisted of a peak cytosolic [Ca2+bd elevation followed by a full decay to baseline and were refractory to manipulations of surface membrane potential through changes in extracellular [K+]. They disappeared upon intracellular Ca2+ store depletion through repeated ionophore applications. They were therefore attributable solely to intracellularly stored Ca2+. In contrast, the fura-2 peaks in osteoclasts exposed to Ca(2+)-containing solutions decayed to sustained levels. Cytosolic [Ca2+] responses then persisted with repeated ionomycin application. These latter phenomena are accordingly attributable to extracellular Ca2+ entry. Finally, restoration of extracellular [Ca2+] to 1.25 mM following the depletion of intracellular Ca2+ stores by treatment with ionomycin elicited a cytosolic [Ca2+] 'overshoot' consistent with capacitative Ca2+ entry via a cytosolic route. These results demonstrate a refillable intracellular source of cytosolic Ca2+ that could function in osteoclastic regulation.

Dimensional analysis of osteoclastic bone resorption and the measurement of biologically active calcitonin

Calcitonin inhibits bone resorption through a direct action on the osteoclast. We report a quantitative analysis of bone resorption by disaggregated rat osteoclasts. We then used our findings to develop a formal bioassay for calcitonin. Osteoclasts were mechanically disaggregated from neonatal rat long bones and dispersed at low densities on slices of devitalized bovine cortical bone. The resulting areas of bone excavation were quantified to micrometric precision by scanning electron microscopy together with computer-assisted image analysis. These findings were correlated with the volumes of bone resorption in the same slices measured by confocal scanning microscopy for the first time. The total planar areas of bone resorption per slice correlated linearly (r = 0.78) with the confocal microscopic measurements of total volume resorbed, provided that volume was expressed to its two-thirds power. The latter transformation resulted in representations of the determined areas ([length]2) and volumes ([length]3) which were dimensionally consistent. These findings thus demonstrate that osteoclastic bone excavations show a consistent relationship between area and volume and that assessments of the area of excavations accordingly provide an empirical representation of the volume of bone resorbed. Furthermore, in view of the skewed nature of the distributions of area measurements, we assessed the effect of transforming the response variable to derive a metameter, (planar area of resorption)1/2. Such transformed data points, which expressed the data in the dimensions of [length], were more normally distributed than the raw data points and had more stable variances over a wider concentration range. We accordingly determined relative potencies using parallel line analyses on the transformed data. The latter offered a consistent correlation to the volume measurements when these were also converted to dimensions of [length] (r = 0.805). It was confirmed that the inhibition of bone resorption by calcitonins from various species, namely, pig, salmon and eel, was quantitatively dependent upon concentration of the respective peptides. The resulting assay was also found to be sufficiently sensitive to measure picomolar peptide concentrations with a precision, lambda (standard deviation/slope), ranging between 0.3 and 0.8. Finally, we identified factors affecting assay precision and sensitivity.

Modulation of the osteoclast Ca2+ receptor by extracellular protons: possible linkage between Ca2+ sensing and extracellular acidification

We report a sensitivity of the osteoclast cell surface Ca2+ receptor to extracellular protons. Freshly isolated rat osteoclasts were exposed to the known agonists of the Ca2+ receptor, Ca2+ and Ni2+, in extracellular solutions set at different pH values. Decreasing the extracellular pH from 7.8 to 4.0 units markedly potentiated the cytosolic Ca2+ signals elicited in response to Ca2+ receptor activation by either Ni2+ (50 microM, 500 microM or 5 mM) or Ca2+ (5 mM). Each response consisted of a rapid and usually transient elevation of cytosolic [Ca2+]. Maximal cytosolic [Ca2+] responses were obtained at pH values of 6.6 (for 5 mM-[Ni2+]) and 4.0 units (for 5 mM-[Ca2+]). Finally, the effects of extracellular pH persisted in Ca(2+)-free, EGTA-containing solutions, suggesting a modulation of intracellular Ca2+ release.

Elevated cytosolic calcium levels in human lymphocytes during surface virus infections

Generalised metabolic and electrolyte disturbances are known to accompany both plasma and surface virus infections. We have investigated whether these infections could impair the transport of Ca2+ from cells under conditions of controlled concentrations of the energy substrate glucose. Thus, cytosolic calcium levels ([Ca2+]i) were measured in single isolated lymphocytes obtained from healthy volunteers or those suffering from coryza. Before making measurements using a Ca(2+)-sensitive fluorescent dye indo 1, we incubated lymphocytes in buffers containing 0 mM-, 5.6 mM- or 11.2 mM-[glucose]. We found that [Ca2+]i of lymphocytes obtained from the sick were significantly higher than those from healthy controls both at 0 mM and 5.6 mM-[glucose], and that [Ca2+]i was inversely related to the media glucose concentration for both groups. These results suggest a diminished capacity of cation pumping in viral infections, such as coryza, in relationship to the available glucose as energy substrate.

Effect of raised extracellular calcium on cell spread area in quail medullary bone osteoclasts

The present study reports on the effects of extracellular calcium ([Ca2+]o) elevation and ionomycin on cell spread area of medullary bone osteoclasts freshly isolated from egg-laying Japanese quail. The responses were compared with those demonstrated in osteoclasts cultured for periods of 5-8 days and also to those previously demonstrated in neonatal rat osteoclasts. Freshly isolated medullary bone osteoclasts, unlike rat osteoclasts, were refractory to 20 mM [Ca2+]o, in that they showed no change in cell spread area. They did, however, show a modest (15%) reduction in cell spread area to ionomycin (7-50 microM), applied for 15-30 min. When medullary bone osteoclasts were precultured for 5-8 days, they exhibited a well-developed response to 20 mM [Ca2+]o with a 46% reduction in cell spread area. They also showed a similar reduction in cell spread area in response to ionomycin (4 microM). It is concluded that, unlike freshly isolated neonatal rat osteoclasts, those obtained from quail medullary bone appear refractory to inhibitory factors such as [Ca2+]o. However, when the avian cells are cultured for a few days they appear to recover their ability to respond to [Ca2+]o.

MEDULLARY BONE AND AVIAN CALCIUM REGULATION

Medullary bone forms in egg-laying birds in response to gonadal steroids and is the most overtly oestrogen-dependent of all bone types. It acts as a labile reservoir for the supply of eggshell calcium. Previous studies indicate that feeding calcium- and vitamin-D-deficient diets to chickens results in resorption of cortical rather than medullary bone. More recent studies in calcium-stressed quail hens question this hypothesis and suggest that during the first 2 weeks of dietary calcium depletion the medullary bone is resorbed while cortical bone volume remains intact. The role of the osteoclast in bone resorption is the focus of much research that has recently included studies of medullary bone osteoclasts. The functional morphology of the avian cells, i.e. changes from quiescent to active osteoclasts with ruffled borders, reflects the rapid changes in medullary bone turnover that occur during the egg-laying cycle. Unlike mammalian osteoclasts, those from avian sources generally appear refractory to inhibitory factors such as calcitonin or raised extracellular calcium concentration. However, medullary bone osteoclasts cultured in vitro for several days recover their ability to respond to the latter factor by increasing their levels of free cytosolic Ca2+, reducing tartrate-resistant acid phosphatase secretion and reducing their cell spread area. It is suggested that factors such as ambient calcium levels and prostaglandins may form part of a system of rapid local control for medullary bone osteoclast activity.

Cyclosporine and cremaphor modulate von Willebrand factor release from cultured human endothelial cells

Cyclosporine has been associated with microangiopathic hemolysis (MAHA) and other thrombotic complications of bone marrow and renal transplantation. MAHA is characterized by intravascular platelet aggregation, which, in some situations, is thought to be mediated by hyperactive high molecular weight von Willebrand factor (vWF). We have hypothesized that transplant-related MAHA may be caused by CsA-mediated release of von Willebrand factor from endothelial cells. This hypothesis was tested by studying vWF release from human umbilical vein endothelial cells primed with either CsA or cremophor EL. CsA and cremophor alone did not increase vWF release until toxic concentrations were reached (50-100 micrograms/ml). However, at therapeutic concentrations (0.1-5 micrograms/ml) vWF release by cells stimulated with thrombin, histamine, PMA, and the calcium ionophore A23187 was enhanced by both CsA and cremophor in a concentration-dependent manner. In single isolated endothelial cells, the thrombin-induced increase in cytosolic free calcium was enhanced by both CsA and cremophor. Preincubation for 24 hr with CsA but not cremophor suppressed vWF release after thrombin stimulation. These observations were mirrored by a concentration-dependent suppression of [3H]thymidine uptake by CsA. We conclude that CsA vehicle, cremophor, enhances stimulated vWF release in vitro, probably by processes dependent upon increased cytosolic free calcium. This suggests a possible mechanism for thrombotic transplant complications.

Osteoclast function and its control

Bone resorption appears to be dependent on a range of processes. It requires an adequate number of osteoclasts to access bone mineral. These osteoclasts must be activated by a mechanism which is dependent upon prior osteoblastic stimulation. A range of factors then contribute to the formation of a functionally effective resorptive hemivacuole. These entail osteoclast adhesion to the bone surface leading to the formation of a sealing zone. Only then can subsequent processes such as H+ ion transport, enzyme secretion and matrix digestion become effective. Thus, any one process is potentially limiting to resorption and is a potential target for regulation. Long-range regulation takes place through the action of hormones, of which the mode of action of calcitonin has been the subject of recent investigations in isolated osteoclasts. Such studies have shown a possible involvement of distinguishable receptor subtypes, the occupancy of which may activate at least two types of triggering mechanism. It is likely that an eventual influence on motility properties through G protein mediation accounts for the actions of this hormone and of related peptides such as amylin and CGRP at the cellular level. Similar pathways may contribute to shorter range modulation of osteoclast activity by increases in ambient Ca2+. Finally, there is recent evidence for a contribution of endothelial cell-derived product to osteoclast regulation.

Amylin in bone conservation current evidence and hypothetical Considerations

Amylin, a 37-amino-acid long single-chain polypeptide, is structurally homologous to calcitonin and calcitonin gene-related peptide (CGRP). The peptide is secreted from pancreatic beta cells and is thought to have an anti-insulin action. Here, we review the recently described effects of amylin on calcium homeostasis and discuss its possible role in bone conservation. Amylin is a potent hypocalcemic and antiresorptive peptide. Studies using isolated osteoclasts have revealed that amylin inhibits cell motility (Q effect), without affecting cell spread area or elevating cytosolic [Ca(2+)]. Thus, amylin action is similar to that of calcitonin, but lower in potency. Lower circulating concentrations of amylin in type-1 diabetes may cause the bone loss associated with this condition.

Linkage of extracellular and intracellular control of cytosolic Ca2+ in rat osteoclasts in the presence of thapsigargin

Cytosolic [Ca2+] was measured in single osteoclasts using fura-2 in experiments investigating the effects of Ca2+ "receptor" activation using thapsigargin as a means of depleting intracellular Ca2+ stores. Application of 4 microM thapsigargin to osteoclasts in Ca(2+)-free solutions resulted in an elevation of cytosolic [Ca2+]. Under similar conditions, activation of the osteoclast Ca2+ receptor by the substitute divalent cation agonist, Ni2+, resulted in a transient elevation of cytosolic [Ca2+]. In both instances, restoration of extracellular [Ca2+] to 1.25 mM resulted in an "overshoot" of cytosolic [Ca2+]. Prior depletion of intracellular Ca2+ stores by thapsigargin markedly reduced the magnitude of the cytosolic [Ca2+] response to a subsequent application of 5 mM Ni2+. The application of 2 microM thapsigargin to intercept the falling phase of the Ni(2+)-induced cytosolic Ca2+ signal resulted in a sustained elevation of cytosolic [Ca2+], which was terminated by a second application of the same Ni2+. Furthermore, the sustained elevation of cytosolic [Ca2+] induced by thapsigargin application alone was abolished by late application of Ni2+. We conclude that activation of the surface membrane Ca2+ receptor on the osteoclast results in the cytosolic release of Ca2+ from intracellular storage organelles; the refilling of such stores depends upon a thapsigargin-sensitive Ca(2+)-ATPase; store depletion induces capacitative Ca2+ influx; and the Ca2+ influx pathway is sensitive to blockade by Ni2+.

[7 years' experience with neonatal tetanus in Yucatan]

Forty-six patients with neonatal tetanus (NT) were studied to identify factors for poor prognosis which could be used to select a therapeutic regimen with intravenous diazepam or neuromuscular blockade (NB) with pancuronium. Nine patients with NT grades II and III were successfully treated with diazepam; none of them died. Of the patients with NT grades IV and V, 27 received diazepam and ten received pancuronium. The mortality rate in these groups was 55% and 100%, respectively. Factors predicting poor prognosis among patients treated with diazepam were apneas (P = 0.01), and an age on admission of seven days or less (P = 0.0002). Patients who received diazepam and died, generally presented a rapidly fatal course (mean of four days); in this group tetanus was the main cause of death (73%). Patients treated with pancuronium survived a comparably longer period (mean = 15.7 days, P = 0.05), but generally died from nosocomial infections (70%, P = 0.04). On the basis of our results we propose that NT grades IV and V with the aforementioned factors for poor prognosis be treated with NB. In hospitals with limited resources and high rates of nosocomial infection, we suggest that NT grades IV and V without such factors initially be managed with diazepam, reserving NB for therapeutic failures. Finally, NT grades I-III may be effectively treated with diazepam alone.

Functional consequences of the interaction of Ni2+ with the osteoclast Ca2+ 'receptor'

Ni2+ was used as an extracellular activator of the Ca2+ 'receptor' in order to study the regulation of osteoclast function in vitro. Application of different micromolar concentrations of Ni2+ to osteoclasts bathed in 1.25 mM [Ca2+] and 0.8 mM [Mg2+] caused a concentration-dependent elevation of cytosolic [Ca2+] measured in single cells using fura-2 fluorescence. Cytosolic [Ca2+] responses to 5 mM [Ni2+] showed a rapidly developing and use-dependent inactivation, unlike those induced by the application of 10 mM [Ca2+]. Pre-treatment with 5 mM [Ni2+] reduced the magnitude of responses to a subsequent extracellular application of 10 mM [Ca2+] and vice versa. Ni2+ treatment elicited a number of functional effects. It produced an inhibition of osteoclastic bone resorption which was sustained over hours. This was associated with a pronounced cell retraction or R effect over the 40 min period following Ni2+ exposure as observed by time-lapse video image analysis. Both these effects varied with concentration. In contrast, granule movement, cell migration, and quantitative indicators of margin ruffling were all unchanged. These findings are consistent with the initiation of a causally related set of specific functional and morphometric events following activation of a specific membrane receptor sensitive to divalent cations.

The effect of tetracyclines on quantitative measures of osteoclast morphology

We report the effects of the tetracycline analogues 4-dedimethylaminotetracycline (CMT-1) and minocycline on osteoclast spreading and motility. Both agents influenced the morphometric descriptor of cell spread area, rho, producing cellular retraction or an R effect (half-times: 30 and 44 minutes for CMT-1 and minocycline, respectively). At the concentrations employed, the tetracycline-induced R effects were significantly slower than, but were qualitatively similar to, those resulting from Ca2+ "receptor" activation through the application of 15 mM-[Ca2+] (slopes: -1.25, -0.18, and -4.40/minute for 10 mg/l-[CMT-1], 10 mg/l-[minocycline] and 15 mM-[Ca2+], respectively). In contrast, the same tetracycline concentrations did not influence osteoclast margin ruffling activity as described by mu, a motility descriptor known to be influenced by elevations of cellular cyclic AMP. Thus, the tetracyclines exert morphometric effects comparable to changes selectively activated by occupancy of the osteoclast Ca2+ "receptor" which may act through an increase in cytosolic [Ca2+].

Tetracyclines modulate cytosolic Ca2+ responses in the osteoclast associated with "Ca2+ receptor" activation

We report the effects of tetracycline analogues on cytosolic Ca2+ transients resulting from application of ionic nickel (Ni2+), a potent surrogate agonist of the osteoclast Ca2+ "receptor". Preincubation with minocycline (1 mg/l) or a chemically modified tetracycline, 4-dedimethyl-aminotetracycline (CMT-1) (1 or 10 mg/l), resulted in a significant attenuation of the magnitude of the cytosolic [Ca2+] response to an application of 5 mM-[Ni2+]. Preincubation with doxycycline (1 or 10 mg/l) failed to produce similar results. In addition, application of minocycline alone (0.1-100 mg/l) resulted in a 3.5-fold elevation of cytosolic [Ca2+]. The results suggest a novel action of tetracyclines on the osteoclast Ca2+ "receptor".

Voltage sensitivity of the osteoclast calcium receptor

We demonstrated previously that osteoclasts possess a divalent cation-sensitive "receptor", the Ca2+ receptor. Activation of the Ca2+ receptor by the surrogate cation Ni2+ was shown to elicit an increase in cytosolic [Ca2+] to a peak value followed by an exponential decline. In the present study we examined the influence of surface membrane voltage on the kinetics of Ca2+ receptor inactivation. The K+ ionophore, valinomycin was applied to intercept the declining phase of the cytosolic [Ca2+] transient elicited by application of between 50 microM- and 5 mM-[Ni2+]. This resulted in a sustained elevation of cytosolic [Ca2+] or even a 'hump' followed by a gradual decline. Such a kinetic alteration persisted in a Ca(2+)-free solution, but was abolished in high extracellular [K+] (105 mM). Thus, we demonstrate for the first time to our knowledge, a modulatory effect of membrane potential on the function of the osteoclast Ca2+ receptor.

Cellular biology of bone resorption

Past knowledge and the recent developments on the formation, activation and mode of action of osteoclasts, with particular reference to the regulation of each individual step, have been reviewed. The following conclusions of consensus have emerged. 1. The resorption of bone is the result of successive steps that can be regulated individually. 2. Osteoclast progenitors are formed in bone marrow. This is followed by their vascular dissemination and the generation of resting preosteoclasts and osteoclasts in bone. 3. The exact pathways of differentiation of the osteoclast progenators to mature osteoclasts are debatable, but there is clear evidence that stromal cells support osteoclast generation. 4. Osteoclasts are activated following contact with mineralized bone. This appears to be controlled by osteoblasts that expose mineral to osteoclasts and/or release a factor that activates these cells. 5. Activated osteoclasts dissolve the bone mineral and digest the organic matter of bone by the action of agents secreted in the segregated microcompartments underlying their ruffled borders. The mineral is solubilized by protons generated from CO2 by carbonic anhydrase and secreted by an ATP-driven vacuolar H(+)-K(+)-ATPase located at the ruffled border. The organic matrix of the bone is removed by acid proteinases, particularly cysteine-proteinases that are secreted together with other lysosomal enzymes in the acid environment of the resorption zone. 6. Osteoclastic bone resorption is directly regulated by a polypeptide hormone, calcitonin (CT), and locally, by ionized calcium (Ca2+) generated as a result of osteoclastic bone resorption. 7. There is new evidence that osteoclast activity may also be influenced by the endothelial cells via generation of products including PG, NO and endothelin.

Activation of the Ca2+ "receptor" on the osteoclast by Ni2+ elicits cytosolic Ca2+ signals: evidence for receptor activation and inactivation, intracellular Ca2+ redistribution, and divalent cation modulation

Earlier studies have demonstrated that a high (mM) extracellular Ca2+ concentration triggers intracellular [Ca2+] signals with a consequent inhibition of bone resorptive activity. We now report that micromolar concentrations of the divalent cation, Ni2+, elicited rapid and concentration-dependent elevations of cytosolic [Ca2+]. The peak change in cytosolic [Ca2+] increased monotonically with the application of [Ni2+] in the 50-5,000 microM range in solutions containing 1.25 mM-[Ca2+] and 0.8 mM-[Mg2+]. The resulting concentration-response function suggested Ni(2+)-induced activation of a single class of binding site (Hill coefficient = 1). The triggering process also exhibited a concentration-dependent inactivation in which conditioning Ni2+ applications in the range 5-1,500 microM-[Ni2+] inhibited subsequent responses to a maximally effective [Ni2+] of 5,000 microM. Ni(2+)-induced cytosolic [Ca2+] responses were not dependent on extracellular [Ca2+]. Thus, when 5,000 microM-[Ni2+] was applied to osteoclasts in Ca(2+)-free, ethylene glycol bis-(aminoethyl ether) tetraacetic acid (EGTA)-containing medium (< or = 5 nM-[Ca2+] and 0.8 mM-[Mg2+]), cytosolic [Ca2+] responses resembled those obtained in the presence of 1.25 mM-[Ca2+]. Prior depletion of intracellular Ca2+ stores by ionomycin prevented Ni(2+)-induced cytosolic [Ca2+] responses, suggesting a major role for intracellular Ca2+ redistribution in the response to Ni2+. The effects of Ni2+ were also modulated by the extracellular concentration of the divalent cations, Ca2+ and Mg2+. When these cations were not added to the culture medium (0 microM-[Ca2+] and [Mg2+]), even low [Ni2+] ranging between 5 pM and 50 microM elicited progressively larger cytosolic [Ca2+] transients. However, the response magnitude decreased at higher, 250-5,000 microM-[Ni2+], resulting in a "hooked" concentration-response curve. Furthermore, increasing extracellular [Mg2+] or [Ca2+] (0-1 mM) diminished the response to 50 microM-[Ni2+], a concentration on the rising phase of the "hook." Similar increases (0-10 mM) in extracellular [Mg2+] or [Ca2+] increased the response to 5,000 microM-[Ni2+], a concentration on the falling phase of the "hook". These findings are consistent with the existence of a membrane receptor strongly sensitive to Ni2+ as well as the divalent cations, Ca2+ and Mg2+. Receptor occupancy apparently activates intracellular Ca2+ release followed by inactivation. Furthermore, repriming is independent of intracellular Ca2+ stores, suggesting that such inactivation operates at a transduction step between receptor occupancy and intracellular Ca2+ release.

Extracellular Ca2+ sensing by the osteoclast

An increasing number of cell types appear to detect changes in the extracellular Ca2+ concentration and and accordingly modify their function. We review recent evidence for the existence and function of such a mechanism in the osteoclast. Elevated external [Ca2+] in the mM range reduces bone resorption and results in motile changes in the cells. These changes may partly result from elevations of cytosolic [Ca2+] triggered through activation of a surface Ca2+ receptor. Closer analyses of the increases in cytosolic [Ca2+] associated with receptor activation are hindered by the action of this ion both as extracellular agonist and intracellular second messenger. Variations in the peak cytosolic [Ca2+] response to external Ca2+ with changes in cell membrane potential by K+ and valinomycin establish a contribution from extracellular Ca2+. Use of CIO4-, Ni2+ and Cd2+ as surrogate activators in low extracellular [Ca2+] indicate a contribution from Ca2+ release from intracellular stores as well. Such agonists also modify Ca2+ redistribution in other systems, such as skeletal muscle. Thus, we may gain insights into osteoclast extracellular Ca2+ detection and transduction from known features of more well-characterised cell systems.

Role of the endothelial cell in osteoclast control: new perspectives

The osteoclast is of central importance in the process of bone remodeling. Its function is regulated by hormones and locally produced factors. Endothelial cells occur in close proximity to the osteoclast. Some endothelial cell-derived products, including endothelins, nitric oxide, and reactive oxygen species, have been recently implicated as modulators of osteoclast function. Endothelins inhibit bone resorption and osteoclast margin ruffling (quiescence or Q effect) at concentrations similar to those effective for their primary vasoconstrictive action. Contrary to expectations, however, it has been shown that endothelin action on the osteoclast is not mediated through an elevation of cytosolic Ca2+. Nitric oxide (NO) produces marked cell retraction (retraction or R effect), but its detailed mode of action is unknown. However, it is clear that the effects of this autocoid are not due to enhanced cyclic guanosine monophosphate (cGMP) production, a transduction system commonly used by NO. Finally, the reactive oxygen species H2O2 has been shown recently to enhance osteoclastic activity. Thus, the reported effects of the endothelial cell-derived products on the osteoclast are generally consistent with a regulatory role for endothelial cells in osteoclast control and suggest the existence of unique activation pathways, well worth exploring further. Unravelling the responsible mechanisms may also help understand the pathophysiology of a range of bone and joint diseases. For example, in rheumatoid arthritis, there is increased H2O2 production from activated neutrophils, and bone resorption is a major pathophysiological feature.

Amylin inhibits bone resorption by a direct effect on the motility of rat osteoclasts

We have performed a set of independent studies on the effects of the circulating pancreatic polypeptide, amylin, on rat osteoclast function, in vitro. Time-lapse video observations, measuring cell protrusions and retraction, showed that 250 nmol l-1 amylin or 250 nmol l-1 beta-calcitonin gene-related peptide (beta-CGRP) inhibited osteoclast motility (quiescence or Q effect). Both amylin and beta-CGRP produced inhibitory responses with a significant first-order regression over time (half-times, 19 and 28 min respectively). In contrast, 250 nmol l-1 amylin or 250 nmol l-1 beta-CGRP produced no change of osteoclast spread area, whilst 300 pmol l-1 calcitonin (CT) application resulted in cell retraction (R effect). Forskolin (10 mumol l-1) mimicked amylin and CGRP in inhibiting osteoclast motility (half-time, 8.6 min), and similarly lacked an effect on cell spread area. Neither amylin nor beta-CGRP (62.5-1250 nmol l-1) elevated cytosolic free calcium levels ([Ca2+]i) in single osteoclasts whilst 300 pmol l-1 salmon calcitonin (sCT) produced a rapid phasic elevation of [Ca2+]i, confirming previous results with asusuberic (1-7) eel calcitonin. The osteoclast-bone resorption assay revealed the following potency difference in direct comparison of the area of resorption per bone slice: beta-CGRP/amylin, 0.1; sCT/amylin, 800 and human CT/amylin, 12. The potency of deamidated amylin approached that of beta-CGRP. Assay precision ranged between 0.3 and 0.8. Amylin (250 nmol l-1) also significantly (P < 0.05) reduced supernatant (tartrate-resistant) acid phosphatase in the bone-osteoclast cultures. These measures independently indicate an effect of amylin on osteoclast motility through mechanisms distinct from those of calcitonin, possibly through different selectivities for receptor subtypes, the cyclic AMP-linked 'amylin subtype' and the [Ca2+]i-linked 'calcitonin subtype'.

Stimulation of a Gs-like G protein in the osteoclast inhibits bone resorption but enhances tartrate-resistant acid phosphatase secretion

Previous studies have demonstrated that G-protein agonists induce quiescence (Q effect) or retraction (R effect) in isolated osteoclasts. We now report the functional effects of such agonists on osteoclastic bone resorption and enzyme release. Exposure of osteoclasts to tetrafluoro-aluminate anions (AlF4-), a universal G protein stimulator, resulted in a marked concentration-dependent inhibition of bone resorption. This was associated with a dramatic increase in the secretion of the osteoclast-specific enzyme, tartrate-resistant acid phosphatase (TRAP). Cholera toxin, a Gs stimulator and a selective Q effect agonist, similarly abolished bone resorption and enhanced TRAP secretion. In contrast, pertussis toxin, a Gi inhibitor and a selective R effect agonist, inhibited bone resorption significantly, but slightly reduced enzyme release. The results suggest an involvement of a Gs-like G protein in TRAP secretion from the osteoclast, possibly through a cyclic AMP-dependent mechanism.

Further studies on the mode of action of calcitonin on isolated rat osteoclasts: pharmacological evidence for a second site mediating intracellular Ca2+ mobilization and cell retraction

Calcitonin is a circulating polypeptide that inhibits bone resorption by inducing both quiescence (Q effect) and retraction (R effect) in osteoclasts. Two structurally related members of the calcitonin gene peptide family, calcitonin gene-related peptide (CGRP) and amylin, inhibit osteoclastic bone resorption selectively via the Q effect. In the present study, we have made measurements of cell spread area in response to the application of amylin, CGRP and a peptide fragment of CGRP, CGRP-(Val8Phe37). We found that, over a wide concentration range (50 pmol/l to 2.5 mumol/l), the selective Q effect agonists did not produce an R effect. Furthermore, the peptides, when used at a 50-fold higher molar concentration than calcitonin, did not antagonize calcitonin-induced cell retraction. Additionally, experiments designed to measure changes in the intracellular free calcium concentration ([Ca2+]i) in single osteoclasts revealed that, unlike calcitonin, the non-calcitonin Q effect agonists did not produce a rise in [Ca2+]i. The peptides were also unable to attenuate the peak rise in [Ca2+]i induced by calcitonin. The results support our hypothesis that the inhibitory activity of calcitonin on osteoclastic bone resorption is mediated by two sites which may or may not be part of the same receptor complex. One of these is the classical Q effect site coupled to adenylate cyclase via a cholera toxin-sensitive Gs. This site can be activated by nanomolar concentrations of calcitonin, amylin, CGRP or CGRP-(Val8Phe37). A novel R effect site, possibly coupled via a pertussis toxin-sensitive G protein to a [Ca2+]i elevating mechanism is predicted from this study.(ABSTRACT TRUNCATED AT 250 WORDS)