Calcium-mediated enhancement of the cyclic AMP response in cultured bone cells

Extracellular calcium induces COX-2 in osteoblasts via a PKA pathway

We have shown that extracellular calcium [Ca(+2)](e) induces cyclooxygenase-2 (COX-2) expression and prostaglandin E(2) (PGE(2)) production via an ERK signaling pathway in osteoblasts. In this study, we examined the roles of protein kinase C (PKC) and A (PKA) signaling pathways in the [Ca(+2)](e) induction of COX-2 in primary calvarial osteoblasts from mice transgenic for -371 bp of the COX-2 promoter fused to a luciferase reporter. Neither PKC specific inhibitors nor downregulation of the PKC pathway by phorbol myristate acetate (PMA) affected the [Ca(+2)](e) stimulation of COX-2 mRNA or promoter activity. In contrast, PKA inhibitors, used at doses that inhibited forskolin-stimulated luciferase activity by 90%, reduced [Ca(+2)](e)-stimulated COX-2 mRNA expression and promoter activity by 80-90%. [Ca(+2)](e) also stimulated a 2- to 3-fold increase in cAMP production. Hence, the [Ca(+2)](e) induction of COX-2 mRNA expression and promoter activity was independent of the PKC pathway and dependent on the PKA signaling pathway.

Gap junctional intercellular communication contributes to hormonal responsiveness in osteoblastic networks

To evaluate whether intercellular coupling via connexin43 gap junction channels modulates hormonal responsiveness of cells in contact, we have created osteoblastic cell lines deficient in connexin43. Osteoblastic ROS 17/2.8 cells were transfected with a plasmid containing an antisense cDNA construct to rat connexin43. Control transfection did not alter cell-to-cell coupling nor connexin43 mRNA or protein expression relative to nontransfected ROS 17/2.8 cells. In contrast, stable transfection with an antisense connexin43 cDNA resulted in two clones, RCx4 and RCx16, which displayed significant decreases in connexin43 mRNA and protein expression and were dramatically deficient in cell-to-cell coupling. Phenotypically, all transfectants retained osteoblastic characteristics. However, cells rendered connexin43-deficient through antisense transfection displayed a dramatic attenuation in the cAMP response to parathyroid hormone. Alterations in hormonal responses were not due to changes in parathyroid hormone receptor number or binding kinetics nor to alterations in adenylyl cyclase activity. These results indicate that gap junctions may be required for mediating hormonal signals. Furthermore, these experiments support a regulatory role for connexin43-mediated intercellular communication in the modulation of hormonal responses within elaborately networked bone cells.

Effects of extracellular calcium on insulin-like growth factor II in human bone cells

Extracellular calcium concentration is critically important for normal function of the body. Recently, reports have shown that cells derived from parathyroid glands contain an extracellular calcium receptor that is responsive to changes in extracellular calcium. Bone is intimately involved in calcium homeostasis; therefore, we sought to test the hypothesis that extracellular calcium has direct effects on bone cells. Extracellular calcium was increased by the addition of varying concentrations of CaCl2 (0.4-2.0 mM) to the control medium. An increase in extracellular calcium increased cell proliferation, as assessed by 3H-thymidine incorporation, in a number of cell types including normal human bone cells derived from vertebrae (HBV155) and a number of human osteosarcoma cell lines. The increase in cell proliferation by elevated CaCl2 was dose dependent, whereas MgCl2 was not effective at the doses tested (up to 2 mM added MgCl2). To test the hypothesis that the mitogenic activity of elevated extracellular calcium involved a growth factor, levels of insulin-like growth factor II (IGF-II) were measured in the conditioned medium of HBV155 cells by radioimmunoassay after removal of binding proteins by size exclusion chromatography. The effects of an increase in extracellular calcium by 1 mM were: 1) increased culture media levels of IGF-II within 1 h of treatment, 2) the increase in IGF-II levels reached a maximum after 8 h of treatment, and 3) IGF-II levels were still elevated after 24 h of treatment. Furthermore, a blocking monoclonal antibody against IGF-II abolished the increased cell proliferation in HBV155 cells following elevation of extracellular calcium. Taken together, these findings suggest that an increase in extracellular calcium results in an increase in IGF-II which is required for the subsequent increase in cell proliferation.

Regulation of parathyroid hormonelike protein production in cultured normal and malignant keratinocytes

We have recently demonstrated that parathyroid hormone-like protein (PLP) production by cultured human squamous carcinoma cells (SCC) can be modulated by co-culture with fibroblasts. The interaction of SCC with fibroblasts, possibly occurring during the invasive phase of SCC, may be the stimulus for enhanced PLP production, thus contributing to the genesis of humoral hypercalcemia of malignancy in this type of cancer (Cancer Res 50:3589-3594, 1990). In the present study we show that the fibroblast-induced increase in PLP level in the medium of SCC-4 cells is paralleled by an increase in PLP messenger ribonucleic acid (mRNA) expression in these cells. We also found that the inhibition of secretion of PLP by monensin for 2 h resulted in a marked increase in immunodetectable PLP intracellularly, suggesting that secretion of PLP was a fast process. The modulation of the production of PLP by calcium and hydrocortisone was further examined in SCC-4 cells and was compared to that in normal keratinocytes and in SCC-9 cells. PLP levels in conditioned media were highest in poorly differentiating SCC-4 cells, intermediate in moderately differentiating SCC-9 cells, and lowest in normal keratinocytes showing high differentiating capacity. Furthermore, in each of the cell types used, PLP production was highest in cultures grown under low calcium conditions; at both calcium concentrations used, the presence of hydrocortisone reduced the PLP release into the medium. This reduction was probably due to a direct effect of hydrocortisone on PLP synthesis because the expression of PLP mRNA was also reduced in the presence of hydrocortisone when tested in SCC-4 cells. In conclusion, our findings indicate that the induction of differentiation in both normal and malignant keratinocytes is associated with the inhibition of PLP production.

Toxicity of root canal sealers on rat bone cells in primary culture

The cytotoxic effects on cultured rat bone cells of newly-developed root canal sealers and commercially available sealers were compared. Various root canal sealers were applied to cultured bone cells obtained from rat calvaria by the enzyme digestion method. Measurement of [3H]-thymidine incorporation, alkaline phosphatase activity, and calcium release were performed after 24 and 48 h. No significant difference was found in cellular DNA synthesis and alkaline phosphatase activity between cells exposed to New B-1, New B-5 and controls after exposure for 24 and 48 h. Cells in contact with Tubliseal, Diaket and AH-26 demonstrated a significant difference from controls in DNA synthesis and alkaline phosphatase activity. Calcium release at 24 h was significantly different in the cells treated with New B-1 and New B-5 than in controls. No appreciable difference was found, however, between New B-6, Sealapex and controls. At 48 h, cells treated with New B-1, New B-5 and New B-6 showed differed significantly difference from controls, but the cells exposed to Sealapex did not. The newly-developed root canal sealers had lower toxicity in vitro than five types of commercially available root canal sealers.

Calcium and protein kinase C enhance parathyroid hormone- and forskolin-stimulated adenylate cyclase in ROS 17/2.8 cells

Both parathyroid hormone (PTH)- and forskolin-stimulated adenylate cyclase activities in ROS 17/2.8 cells are enhanced by increasing the medium concentrations of CaCl2 from 10(-5) M to 3 x 10(-3) M. The ED50 for CaCl2 for both PTH- and forskolin-stimulated activities are similar. The tumor-promoting phorbol ester phorbol 12-myristate 13-acetate (PMA), a known activator of protein kinase C, also enhanced both PTH- and forskolin-stimulated adenylate cyclase. This action of PMA is specific for protein kinase C as phorbol esters that are not activators of protein kinase C had no effect on the system. The combined effects of PMA and CaCl2 were more than additive. The separate and combined effects of PMA and CaCl2 changed the rate of activation of the enzyme (Vmax) but did not modify the ED50 for PTH or for forskolin. PMA and CaCl2 both enhanced the potentiating effect of submaximal dose of forskolin on PTH-stimulated adenylate cyclase. It is concluded that calcium and PMA enhance PTH-sensitive adenylate cyclase and increase the production of cAMP by a mechanism that appears to involve the catalytic subunit of the enzyme and probably its interaction with a guanine nucleotide regulatory protein.

Calcium modulation of the parathyroid hormone-sensitive adenylate cyclase in ROS 17/2.8 cells: effects of N-(6-aminohexyl-5-Cl-naphthalene sulfonamide) (W-7) and trifluoperazine (TFP)

The calcium modulation of the cyclic 3',5'-adenosine monophosphate (cAMP) response to parathyroid hormone (PTH) was studied in a clonal osteosarcoma cell line ROS 17/2.8. CaCl2 was found to stimulate the PTH-sensitive cAMP response of intact cells. At the maximal concentration of 1 mM CaCl2, the maximum response to PTH was increased, but the ED50 for PTH and the time course of maximal cAMP production were not affected. Verapamil blunted, while the cation ionophore A23187 enhanced, the stimulatory effect of CaCl2. Trifluoperazine (TFP) and N-(6-aminohexyl-5-Cl-naphthalene sulfonamide) (W-7) inhibited the stimulatory effect of CaCl2. In membranes prepared in the presence of 0.1 mM CaCl2, a biphasic effect of CaCl2 was demonstrated: stimulation at concentrations of 60-100 microM, and an inhibition above 200 microM, when adenylate cyclase was assayed in the presence of 200 microM EGTA. Addition of exogenous calmodulin to membranes prepared in the presence of EGTA did not have any effect on the PTH-sensitive adenylate cyclase activity, suggesting that endogenous calmodulin was not effectively stripped from the membranes by EGTA treatment. It is concluded that Ca2+ has both a stimulatory and an inhibitory role in modulating PTH-sensitive adenylate cyclase in ROS 17/2.8 cells by as yet unknown mechanisms, and that the involvement of endogenous calmodulin is implicated.

Quantitative interactions between Pb2+ and Ca2+ homeostasis in cultured osteoclastic bone cells

Cellular calcium homeostasis and calcium-mediated cell functions are conceptually attractive processes to be involved in the manifestation(s) of lead toxicity including impaired skeletal growth and cardiovascular and neurological dysfunction. Knowledge of Ca:Pb and Pb:Ca ratios in different structural and functional compartments of cells is essential for identifying, characterizing, and understanding the significance of Pb2+-Ca2+ interactions. Experiments were conducted to characterize the steady-state kinetic distribution and behavior of 45Ca in primary cultures of murine osteoclastic bone cells. Bone cells, derived from mouse calvaria, were enriched for osteoclasts by a sequential collagenase digestion and maintained in primary culture for 1 week. Cultures were labeled with 45Ca for two or 24 hr and the kinetic parameters were obtained by analysis of 45Ca washout curves. Cellular metabolism was based upon a model with three kinetic pools of intracellular Ca2+ containing approximately 45, 25, and 30% of the total cell calcium. In addition, we describe quantitative measurements of Ca:Pb and Pb:Ca ratios at important functional cell sites of Ca2+ transport and storage in intact cells. The intracellular relationships of Ca2+ and Pb2+ were calculated concurrently in individual cultures, using kinetic analysis of dual-label 45Ca and 203Pb washout curves. The Ca:Pb ratios of the rate constants and half-times were approximately 1:1, supporting the concept of similar cellular metabolism of the two elements. The Ca:Pb ratios for the kinetic pools and fluxes were considerably higher than 1:1. These in situ Ca:Pb relationships should be useful for designing and evaluating Ca-Pb studies with calmodulin, isolated mitochondria, and other individual components of the calcium messenger system. Moreover, these data demonstrate both similarities and differences in the kinetic distribution and behavior of Ca2+ and Pb2+ in osteoclastic bone cells.

Abnormal cell calcium concentrations in cultured bone cells obtained from femurs of obese and noninsulin-dependent diabetic rats

Cytoplasmic free calcium concentration [Ca2+]i was quantified in cultured bone cells with osteoblastic characteristics. The cells were obtained from femurs of obese (fa/fa) Wistar-Kyoto rats, from nonobese, noninsulin-dependent diabetic (NIDD) Sprague Dawley rats, and from their appropriate controls. [Ca2+]i was also determined in bone cells obtained from in vivo insulin-treated NIDD rats. Obese (Wistar Kyoto) rats had increased body weight (313 +/- 13 vs. 249 +/- 4 g; P less than 0.01), decreased femur weights (0.68 +/- 0.05 vs. 0.89 +/- 0.05 g; P less than 0.05), similar glucose levels (148 +/- 5 vs. 139 +/- 3 mg/dl), and higher plasma insulin levels (6.0 +/- 0.5 vs. 0.7 +/- 0.1 ng/ml; P less than 0.01) when compared with their nonobese [(fa/+); (+/+)] littermates. Nonobese, NIDD rats, compared with their appropriate controls (nondiabetic Sprague Dawley rats) had higher plasma glucose levels (235 +/- 32 vs. 145 +/- 3 mg/dl; P less than 0.01) but their plasma insulins, body weights, and femur weights were similar to controls (0.7 +/- 0.1 vs. 0.6 +/- 0.1 ng/ml; 302 +/- 4 vs. 318 +/- 14 g; 0.97 +/- 0.4 vs. 0.98 +/- 0.04 g, respectively). Long-term (4 weeks) daily insulin treatment (2 u/100 g) of the NIDD rats increased their plasma insulin (1.9 ng/ml; P less than 0.05) and body weight (369 +/- 13 g; P less than 0.05) but did not change their plasma glucose levels (225 +/- 5 mg/dl), or femur weights (0.98 +/- 0.4 g).(ABSTRACT TRUNCATED AT 250 WORDS)

Skeletal contribution of cyclic adenosine monophosphate in response to parathyroid hormone and calcitonin in vivo in the rat

Cyclic adenosine monophosphate (cAMP) is thought to be a second messenger for the actions of both parathyroid hormone (PTH) and calcitonin (CT). We examined the release of cAMP from rat bone in vivo after administration of synthetic rat PTH-(1-34) (rPTH), synthetic human PTH-(1-34) (hPTH), or synthetic human CT (hCT). Blood from the venous effluent of the femoral bone of rats (bone blood) was drawn at 5 and 10 minutes after the administration of hormones. The cAMP content of the bone blood was then compared to the cAMP content of arterial blood. In both kidney-clamped and non-kidney-clamped rats, hCT led to a significantly greater concentration of cAMP in the bone blood than in the arterial blood. We interpret this to be due to bone production and release of cAMP. Neither hPTH nor rPTH produced a significantly greater amount of cAMP in the bone blood than in arterial blood. These data do not preclude the possibility that there was a production of cAMP within the bone tissue itself after PTH but suggest that there was no release of cAMP from the bone into the bone blood.

Early effects of parathyroid hormone on membrane potential of rat osteoblasts in culture: role of cAMP and Ca2+

Microelectrodes were used to investigate the possible involvement of cAMP and Ca2+ ions in the parathyroid hormone's, bPTH(1-34), effect on the membrane potential of rat osteoblasts in primary culture. Parathyroid hormone (10(-7) M) depolarized cell membrane by 25.0 +/- 6.1 mV (mean +/- standard deviation, SD; n = 17). Blocking Ca2+ influx with the Ca channel blocker cobalt revealed two phases in the hormone effect: a rapid and slight membrane hyperpolarization followed by sustained depolarization. In addition, cobalt significantly (p less than 0.01) decreased the magnitude of the PTH depolarizing action. The addition of dibutyryl-cAMP (10(-3) M) to the perfusion solution also resulted in a biphasic effect. At a lower concentration (10(-4) M), dibutyryl-cAMP produced only membrane hyperpolarization, suggesting a cAMP dose dependence of the opposite membrane potential changes. Forskolin (10(-5) M) and the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) (10(-4) M) mimicked the depolarizing effect of PTH. IBMX at a low concentration (5 x 10(-6) M) potentiated the depolarizing effect of PTH. Increases in [Ca2+]i using Ca2+ ionophore A23187 and intracellular injection of CaCl2 or inositol trisphosphate decreased the PTH depolarizing action, whereas intracellular injection of EGTA enhanced this effect. These results indicate that PTH evokes a biphasic change in rat osteoblast membrane potential that seems to be mediated by an increase in cAMP and modulated by intracellular calcium.

Evidence that pulsed electromagnetic fields inhibit coupling of adenylate cyclase by parathyroid hormone in bone cells

To investigate the biochemical effects of pulsed electromagnetic fields (PEMF) on bone in particular and on cell membrane-associated activity in general, we have studied the modification by PEMF of cAMP metabolism in primary calvarial bone cells. We report that PEMF inhibited cAMP accumulation stimulated by bovine PTH(1-34) peptide. After a 1-hr PEMF exposure, the cAMP response to PTH (2-7 min) was decreased in exposed cells to 48-70% (p less than 0.05) of the response of unexposed cells; furthermore, this inhibition disappeared after 10-20 min with PTH. This inhibition occurred at submaximal PTH doses (2.4-7.3 nM) and no effect was observed at maximal PTH doses (24 nM). Thus with PEMF, the dose response curve for PTH became 0.5 log unit less sensitive. PEMF did not affect the cAMP response to cholera toxin and forskolin. However, when submaximal doses of both forskolin (0.5-1.0 microM) and PTH (0.24-2.4 nM) were used, forskolin prevented inhibition of cAMP production by PEMF in the range of fields and stimulus epochs which normally inhibit cAMP production. It is proposed that PEMF inhibits PTH-stimulated coupling of the adenylate cyclase system and that this inhibition does not affect the intrinsic activity of the G-protein and the catalytic subunit.

Short-term effects of PTH on cultured rat osteoblasts: changes in membrane potential

The introduction of parathyroid hormone [bPTH (1-34)], 10(-8) M, into the medium of cultured rat osteoblasts results in rapid (less than 1 min) depolarization of the osteoblast membranes. Conventional and pH-sensitive microelectrodes were used to assess the mechanism underlying this change. PTH depolarized cell membrane independently of steady-state membrane potential (Vm). Blocking K+ conductance (Ba2+) and Ca2+-dependent K+ conductance (quinine) depolarized Vm by +13.1 +/- 4.6 (n = 6) and +14.8 +/- 6.7 mV (n = 6), respectively, and both abolished the effect of PTH on Vm. The rate of depolarization was reduced in low-Ca2+ medium. PTH inhibited low Na+-induced cell hyperpolarization, but intracellular pH was not altered by hormone addition. PTH-induced depolarization occurred even when the Na+-K+ pump was blocked with ouabain. A second slower response was seen in cells having a Vm lower than -60 mV, with an increase in negativity 5-15 min after hormone application. The results indicate that PTH rapidly modifies Vm by changes of K+ conductance, which may be the first step in hormonal stimulus-response coupling, and induces delayed, long-term changes in cell status.

Increase of adenylate cyclase catalytic-unit activity by dexamethasone in rat osteoblast-like cells

Glucocorticoids are known to increase the cyclic AMP response to parathyroid hormone (PTH) in cultured bone organs or bone cells. Using the osteoblast-like cell line ROS 17/2.8, which possesses receptors for both PTH and glucocorticoids, we investigated which component of the complex hormone receptor-guanine nucleotide regulatory unit--adenylate cyclase was affected by dexamethasone treatment. In response to PTH, isoproterenol or forskolin, a compound that is supposed to act directly on the catalytic unit, cyclic AMP production by intact cells and adenylate cyclase activity in purified plasma membrane were markedly increased by dexamethasone. Whereas NaF, guanosine 5'-[beta gamma-imido]triphosphate and Mn/ stimulated adenylate cyclase activity were similarly enhanced in membranes isolated from glucocorticoid-treated cells, the activity of the stimulatory guanine nucleotide regulatory unit, as assessed by reconstitution into membranes from the CYC- clone, which is genetically devoid of this component, was not altered. Thus in osteoblast-like cells dexamethasone appears to increase cyclic AMP synthesis by influencing the catalytic unit. Moreover, since it has been reported that glucocorticoids may produce changes in cell calcium metabolism, we evaluated cytoplasmic free Ca2+ concentration ([Ca2+]i) and intracellular Ca2+ stores mobilizable by the bivalent-cationophore ionomycin, by using the intracellular fluorescent indicator Quin-2. The results indicated that dexamethasone treatment did not influence [Ca2+]i but markedly decreased ionomycin-releasable Ca2+ stores.

Transient and sustained effects of hormones and calcium on membrane potential in a bone cell clone

Measurements were made of the electrophysiological and cAMP response to changes in extracellular [Ca2+] and to hormone application in a bone cell clone. Both transient and long-term electrophysiological responses were studied. An increase in extracellular [Ca2+] usually resulted in a transient hyperpolarization of about 60-sec duration. In addition, increases in extracellular [Ca2+] from 0.9 to 1.8 mM and from 1.8 to 3.6 mM resulted in long-term hyperpolarization and increased potential fluctuations. Increasing bathing [Ca2+] until the membrane potential reached the K+ equilibrium level resulted in a significant decrease in fluctuations. Addition to the bathing medium of quinine, a putative blocker of the Ca2+-dependent K+ channel, resulted in long-term depolarization of the mean membrane potential, and a long-term decrease in potential fluctuations. Addition of Mg2+, a mild antagonist of Ca2+ entry into the cell, produced transient depolarization and reduction of potential fluctuations. These effects suggest that the potential fluctuations reflect cytoplasmic [Ca2+] fluctuations via Ca2+-dependent K+ membrane channels. Under an extracellular [Ca2+] of 1.8 mM, the application of prostaglandin E2 (PGE2), isoproterenol, and parathyroid hormone produced no significant effect on mean membrane potential or on the sustained potential fluctuations, but PGE2 did significantly raise intracellular cAMP. Under an increased bathing [Ca2+], significant changes in mean potential and fluctuations did occur in response to PGE2, but not in response to the other hormones, while the PGE2 effect on cAMP was not greatly changed. Hyperpolarizing transients of about 30-sec duration occurred in response to all of the hormones, particularly at an extracellular [Ca2+] of 3.6 mM. Thus, there are both transient and long-term electrophysiological responses to hormone application, with only the long-term response correlated with the production of cAMP. These electrophysiological responses may represent separate transient and long-term calcium transport responses to hormone application.

Modulation of PTH-stimulated cyclic AMP in cultured rodent bone cells: the effects of 1,25(OH)2 vitamin D3 and its interaction with glucocorticoids

Parathyroid hormone (PTH)-stimulated cyclic adenosine monophosphate (cAMP) in rat osteoblastlike (OB) cells has been shown to be modulated by steroid hormones; glucocorticoids are known to increase the level, while the effects of 1,25(OH)2D3 are inhibitory. In the present study, we found that the PTH-stimulated cAMP responses are similar in neonatal mouse and fetal rat OB cells. Dexamethasone (0.13-13 nM) augmented PTH-stimulated cAMP in both species. Mouse cells showed a higher maximal response to dexamethasone (100% increment) than rat cells (60-70% increment) with similar sensitivity to dexamethasone (ED50 approximately 1.0 nm). On the other hand, 1,25(OH)2D3 decreased PTH-stimulated cAMP, but the effect required pharmacological levels of hormone; mouse cells responded at a lower dose (1.3 nM) and were more sensitive than rat cells (responded at 13 nM) to 1,25(OH)2D3 treatment. Introduction of physiological concentrations of 1,25(OH)2D3 (0.013-1.3 nm) in addition to dexamethasone (13 nM) resulted in a synergistic enhancement of PTH-stimulated cAMP in rat cells. In contrast, a dose-dependent antagonistic effect was observed in mouse cells. In summary, our findings demonstrate species and concentration-dependent differences in hormonal responses to 1,25(OH)2D3 and a complex interplay among PTH, dexamethasone, and 1,25(OH)2D3.

Benzo(B)thiophene-2-carboxylic acid: calcium uptake and cyclic AMP production in isolated bone cells

The purpose of the present study was to investigate the mechanism of action on bone of Benzo(B)Thiophene-2-Carboxylic Acid (BL-5583). BL-5583, at a dose range of 0.01-100 micrograms/ml, inhibited spontaneous as well as A23187 and PTH-induced bone resorption in tissue culture. This compound also decreased calcium uptake in both osteoclastic and osteoblastic enriched bone cell populations obtained by sequential collagenase digestion of 1-2 day newborn rat calvariae. The decrease occurred after a 5 min. incubation with 45Ca and BL-5583. The effective dose range was 0.01-100 micrograms/ml. No effect on leucine incorporation or lactic acid production by bone cells was observed. BL-5583 also induced a transient decrease in calcium uptake in skin cells isolated from fetal rats by collagenase digestion, suggesting a lack of tissue specificity for this compound. No effect on cyclic AMP in isolated bone cells was observed with the same dose range that produced a calcium effect.

Effects of forskolin on bone. Stimulation of cyclic AMP accumulation and calcium efflux from chick embryo tibiae in organ culture

Forskolin (FSK), an activator of adenylate cyclase in many tissues, was investigated to determine its potential for studying cyclic AMP-mediated, physiological events in bone. Tibiae from 14-day chick embryos were cultured in modified Eagle's medium for up to 8 hr, and the cyclic AMP and Ca2+ efflux responses to parathyroid hormone (PTH) and FSK were observed. FSK caused a concentration-related increase in tissue cyclic AMP, with 10(-6) M FSK producing a cyclic AMP response of similar magnitude to that elicited by 10(-6) M PTH (1-34). However, while 10(-6) M PTH was the maximally-activating PTH concentration, a further elevation was produced by 10(-5) M FSK. Chick embryo tibiae cultured for 8 hr in the presence of PTH exhibited a concentration-related stimulation of Ca2+ efflux up to 10(-6) M PTH. When this physiological effect was examined in bones cultured in the presence of FSK, a biphasic response to increasing FSK concentrations was found; Ca2+ efflux was stimulated by 10(-6) M and 3 X 10(-6) M FSK, but not by 10(-7) M or 10(-5) M FSK. In summary, concentrations of FSK, which produced tissue cyclic AMP accumulation similar to that produced by physiologically effective PTH concentrations, also resulted in net Ca2+ efflux from bone. These results indicate that FSK may be a useful agent for studying the role of cyclic AMP in the response of bone to hormones.

The metabolism of lead in isolated bone cell populations: interactions between lead and calcium

Previous studies of lead metabolism in bone organ culture have defined, in part, an exchangeable bone lead compartment regulated by the same ions and hormones that normally control bone cell metabolism. This study was undertaken to further characterize this subcompartment of exchangeable lead and to examine possible interactions between lead and calcium in isolated bone cell populations. Bone cells, derived from mouse calvaria, were enriched for osteoclasts (OC) and osteoblasts (OB) by a sequential collagenase digestion. We found that (1) the uptake of 210Pb by OC cells was rapid, and OC cells had greater avidity for lead, compared to OB cells, at concurrent time points of incubation, (2) OB cells showed very little increase in lead uptake as medium lead concentrations were increased from 6.5 to 65 microM, in contrast, the uptake of lead by OC cells was almost linear, (3) after loading OC cells with 210Pb, significant release of label (approximately 15 to 30%) occurred within short time periods (less than or equal to 2 hr) during incubations in chase medium, (4) parathyroid hormone (PTH) at physiological concentrations effected a marked increase in 210Pb and 45Ca uptake in OC cells, after 5 min of incubation, Pb accumulation into OC cells continued as calcium uptake markedly decreased, (5) this PTH effect on 210Pb uptake was linear over PTH concentrations of 50 to 250 ng/ml, and (6) rising medium concentrations of lead (greater than or equal to 26 microM) markedly enhanced/exaggerated calcium uptake by OC cells, far above that produced by physiological concentrations of PTH. These data indicate that (1) quantitatively, OC cells are the predominant cell type in the metabolism of lead in this in vitro system of OC and OB cell monolayers, (2) mediated incorporation of lead into OC cells occurs and likely involves changes in membrane permeability effected by hormonal stimuli, such as PTH, and (3) modulations in cellular calcium metabolism induced by lead at low concentration may have the potential of disturbing multiple cell functions of different tissues that depend upon calcium as a second messenger.

A high affinity, calmodulin-responsive (Ca2+ + Mg2+)-ATPase in isolated bone cells

Although acute alterations in Ca2+ fluxes may mediate the skeletal responses to certain humoral agents, the processes subserving those fluxes are not well understood. We have sought evidence for Ca2+-dependent ATPase activity in isolated osteoblast-like cells maintained in primary culture. Two Ca2+-dependent ATPase components were found in a plasma membrane fraction: a high affinity component (half-saturation constant for Ca2+ of 280 nM, Vmax of 13.5 nmol/mg per min) and a low affinity component, which was in reality a divalent cation ATPase, since Mg2+ could replace Ca2+ without loss of activity. The high affinity component exhibited a pH optimum of 7.2 and required Mg2+ for full activity. It was unaffected by potassium or sodium chloride, ouabain or sodium azide, but was inhibited by lanthanum and by the calmodulin antagonist trifluoperazine. This component was prevalent in a subcellular fraction which was also enriched in 5'-nucleotidase and adenylate cyclase activities, suggesting the plasma membrane as its principal location. Osteosarcoma cells, known to resemble osteoblasts in their biological characteristics and responses to bone-seeking hormones, contained similar ATPase activities. Inclusion of purified calmodulin in the assay system caused small non-reproducible increases in the Ca2+-dependent ATPase activity of EGTA-washed membranes. Marked, consistent calmodulin stimulation was demonstrated in membranes exposed previously to trifluoperazine and then washed in trifluoperazine-free buffer. These results indicate the presence of a high affinity, calmodulin-sensitive Ca2+-dependent ATPase in osteoblast-like bone cells. As one determinant of Ca2+ fluxes in bone cells, this enzyme may participate in the hormonal regulation of bone cell function.