Evidence that a ryanodine receptor triggers signal transduction in the osteoclast

We have investigated the effect of the alkaloid ryanodine on the release of intracellularly stored Ca2+ in response to activation of the osteoclast Ca2+ receptor by the surrogate agonist, Ni2+, Ni2+ (6 mM) in the presence of ethylene-glycol bis-(aminoethyl ether) tetraacetic acid (EGTA) (1.2 mM) and valinomycin (5 microM) induced a transient elevation of cytosolic [Ca2+] in fura 2-loaded osteoclasts. This transient was superimposed upon a small steady elevation of cytosolic [Ca2+] induced by the initial application of valinomycin alone. Ryanodine (10 microM) completely abolished such responsiveness. However, cytosolic [Ca2+] transients were restored when osteoclasts were depolarized by the extracellular inclusion of 100 mM-[K+] in the same solution. Thus, we demonstrate a sensitivity of the osteoclast signal transduction system to ryanodine for the first time to our knowledge.

Voltage-sensitive elevation of cytosolic [Ca2+] in guinea-pig cardiac myocytes elicited by calcitonin gene-related peptide

We have studied the effect of the 37-amino acid cardioactive peptide, calcitonin gene-related peptide (CGRP) on cytosolic [Ca2+] in guinea-pig ventricular myocytes following depolarization. Cytosolic [Ca2+] was measured in single myocytes using fura-2. The application of 20 mM K+ led to a transient rise of cytosolic [Ca2+] followed by an exponential decline. The subsequent application of 2 nM CGRP resulted in a marked increase in cytosolic [Ca2+]. In contrast, no such response was obtained without prior depolarization. The results suggest a basis for the cardiotropic effects of CGRP through an influence on cytosolic [Ca2+].

A quantitative description of components of in vitro morphometric change in the rat osteoclast model: relationships with cellular function

We describe the in vitro morphometric changes shown by rat osteoclasts that accompany their functional responses to the application of a range of regulatory agents of known physiological importance. We introduce a cellular motility parameter, mu, which was defined through a quantification of retraction-protrusion behaviour. This was used in conjunction with a net cell retraction, rho, which is derived from the change in total cell area following the application of an agent. These terms were used together for the description of cellular motility changes in response to specific cellular regulatory agents. The definition of retraction-protrusion was normalised against control cell area, to give a dimensionless variable independent of the net cell retraction. Thus, mutual terms present in either descriptor cancelled when the complementary parameter was held constant. Furthermore, the descriptor, mu remained time-invariant for extended intervals (around 20 min) even when rho was varying following cell introduction into culture. Interventions also with substances known to modify osteoclast function, were capable of altering each descriptor, to different extents. Thus elevation of the extracellular Ca2+ concentration ([Ca2+]e) at the osteoclast calcium "receptor" altered rho without changes in mu. In contrast, the polypeptide amylin (250 nM), within 20 minutes of application, elicited a marked change in mu, but only a relatively small change in rho. Finally, human calcitonin treatment (300 pM) influenced both descriptors. When combined together, these morphometric findings accordingly offer complementary descriptions of visible cellular changes in response to added agents of physiological relevance.(ABSTRACT TRUNCATED AT 250 WORDS)

A hypothesis for the local control of osteoclast function by Ca2+, nitric oxide and free radicals

Several important conclusions have recently emerged from in vitro studies on the resorptive cell of bone, the osteoclast. First, it has been established that osteoclast function is modulated locally, by changes in the local concentration of Ca2+ caused by hydroxyapatite dissolution. It is thought that activation by Ca2+ of a surface membrane Ca2+ receptor mediates these effects, hence providing a feedback control. Second, a number of molecules produced locally by the endothelial cell, with which the osteoclast is in intimate contact, have been found to affect bone resorption profoundly. For instance, the autocoid nitric oxide strongly inhibits bone resorption. Finally, reactive oxygen species have been found to aid bone resorption and enhance osteoclastic activity directly. Here, we will attempt to integrate these control mechanisms into a unified hypothesis for the local control of bone resorption.

The osteoclast Ca2+ receptor is highly sensitive to activation by transition metal cations

We report changes in the cytosolic Ca2+ concentration ([Ca2+]i) of single rat osteoclasts in response to Ca2+ receptor activation by micromolar concentrations of the transition metal cations, Cd2+ and Ni2+. The extracellular application of Cd2+ or Ni2+ resulted in a concentration-dependent elevation of cytosolic [Ca2+]. Each monophasic [Ca2+]i response consisted of an initial rapid rise of [Ca2+]i to a peak value followed by an exponential decay. Prior application of Cd2+ or Ni2+ induced refractoriness to a second application of the same cation. The results confirm the existence of a divalent cation-sensitive site on the osteoclast showing features of concentration-dependent activation and use-dependent inactivation.

Activation and inactivation of the osteoclast Ca2+ receptor by the trivalent cation, La3+

We report changes in the cytosolic Ca2+ concentration ([Ca2+]i) of single rat osteoclasts in response to Ca2+ receptor activation by micromolar concentrations of the lanthanide metal cation, La3+. The extracellular application of La3+ induced a concentration-dependent elevation of cytosolic [Ca2+]. Prior conditioning of osteoclasts with La3+ resulted in a concentration-dependent reduction of the response to a subsequent application of a maximally effective concentration of Ni2+, a known agonist of the osteoclast Ca2+ receptor. The results establish that the osteoclast Ca2+ receptor is highly sensitive to activation and inactivation by the trivalent cation, La3+.

Endothelin inhibits osteoclastic bone resorption by a direct effect on cell motility: implications for the vascular control of bone resorption

The abundance of endothelin (ET)-producing endothelial cells in bone marrow and the proximity of these cells to bone-resorbing osteoclasts prompted us to evaluate the action of ET-1 on osteoclast function. Osteoclasts disaggregated from neonatal rat long bones were settled onto devitalized cortical bone substrate, and resorption was quantified by morphometry. The supernatant tartrate-resistant acid phosphatase activity was determined by a spectrophotometric method using paranitrophenol phosphate as substrate. Cell motility was quantified by time lapse video- and computer-assisted image processing using an empirical procedure for morphometric analysis. Cytosolic free calcium levels ([Ca2+]i) were measured in single cells by an indo 1-based microspectrofluorimetric method. Using the area of bone resorbed per slice as response, we found that ET-1 caused a significant (P = 0.011) concentration-dependent inhibition of osteoclastic bone resorption (EC50 = 2.5 nM) without inhibiting acid phosphatase secretion. Exposure of isolated osteoclasts to ET-1 also led to a marked concentration-dependent inhibition of osteoclast motility (EC50 = 7.9 nM; P = 0.013; t1/2 = 18 min) without significant effects on cell spread area. These effects of ET-1 were reversible after removing the peptide, and the cells remained viable during the experiments. In addition, ET-1 did not elevate [Ca2+]i at the concentrations tested. The results suggest that ET-1 specifically interacts with an osteoclast receptor to inhibit osteoclastic bone resorption and cell motility. As the concentration of ET-1 required for osteoclast inhibition was similar to that reported for smooth muscle contraction, it is possible that ET-1, produced locally from the bone marrow endothelial cell, might play a primary role in osteoclast regulation.

Osteoclasts from medullary bone of egg-laying Japanese quail do not express the putative calcium 'receptor'

The present study reports the contrasting effects of extracellular calcium ([Ca2+]e) elevation on cytosolic free calcium levels ([Ca2+]i) of osteoclasts, freshly isolated either from medullary bone of the egg-laying Japanese quail or from rat cortical bone. [Ca2+]i was measured in single osteoclasts using the Ca(2+)-sensitive fluorochrome, Indo-1. We found that elevation of [Ca2+]e failed to induce a rise of [Ca2+]i in quail osteoclasts, whilst causing an elevation of [Ca2+]i in rat osteoclasts. The calcium ionophore, ionomycin, led to a sustained elevation of [Ca2+]i in both cell types. These findings suggest that osteoclasts isolated from egg-laying quail do not possess the calcium sensor or 'receptor' that appears to be vital for the survival and function of rat osteoclasts.

Stimulation of osteoclastic bone resorption by hydrogen peroxide

The molecular mechanisms underlying the pathophysiology of bone destruction still remain poorly understood. We have found that hydrogen peroxide (H2O2), a reactive oxygen species (ROS), is a potent stimulator of osteoclastic bone resorption and cell motility. A marked enhancement of bone resorption was noted when rat osteoclasts, cultured on devitalised bovine cortical bone, were exposed to 10 nM [H2O2]. Apart from exposing osteoclasts to a low extracellular pH, which is known to enhance osteoclastic bone resorption, we provide first evidence for a molecule that stimulates osteoclastic bone resorption in osteoclast cultures that do not respond to parathyroid hormone and 1, 25 dihydroxyvitamin D3. We envisage that both basic biological and practical clinical implications may eventually follow from these studies.

Is the osteoclast calcium "receptor" a receptor-operated calcium channel?

Elevated extracellular calcium levels ([Ca2+]e) inhibit osteoclast function by elevating cytosolic free calcium levels ([Ca2+]i), presumably via the activation of a surface Ca2+ "receptor". It is unclear whether or not Ca(2+)-induced [Ca2+]i elevation involves the direct gating, by the putative "receptor", of a divalent cation channel. The results show that [Ca2+]i elevation in response to elevated [Ca2+]e comprises a distinct component of Ca2+ influx, the magnitude of which can be decreased and increased, respectively, by depolarising (100 mM-[K+]) and hyperpolarising (1 microM-[valinomycin]) the osteoclast membrane. In addition, activation of the putative Ca2+ "receptor" by elevated [Ca2+]e causes influx of the related divalent cation, magnesium (Mg2+). We suggest that Ca2+ influx induced by Ca2+ "receptor" activation is a major component of the observed [Ca2+]i response.

Regulation of cytosolic free calcium in isolated rat osteoclasts by calcitonin

It is now established that calcium is a second messenger mediating the action of calcitonin on the osteoclast. We have demonstrated that an increase in the concentration of intracellular free calcium ([Ca2+]i) is associated with (and possibly mediates) the functional effects of calcitonin, including an acute reduction of cell spread area (the R effect) and, in the longer term, a reduction in enzyme release. The present study addresses questions relating to mechanisms of calcitonin action on osteoclast [Ca2+]i. We have used asusuberic(1-7) eel and human calcitonin as agonists, and an indo-1-based dual-emission microspectrofluorimetric method for the measurement of [Ca2+]i in single osteoclasts. Whilst asusuberic(1-7) eel calcitonin caused a biphasic increase in [Ca2+]i, human calcitonin produced only a monophasic [Ca2+]i response of a much lower magnitude. Each biphasic response consisted of a rapid initial transient increase, occurring within seconds of exposure, followed by a sustained increase in [Ca2+]i. The magnitude of the latter response was more variable, but was consistently below the peak value of [Ca2+]i. The sustained phase of the calcitonin effect was abolished in extracellular Ca(2+)-free medium. This phase is therefore dependent on extracellular [Ca2+] ([Ca2+]e) whilst the rapid transient increase appeared to be dependent on Ca2+i redistribution. The effects of calcitonin on [Ca2+]i were concentration-dependent, with neither latency nor oscillations. Repetitive 30-s exposures to calcitonin failed to produce subsequent responses. There was a marked concentration-dependent correlation between changes in osteoclast [Ca2+]i and the magnitude of the R effect. Thus the likely components of the biphasic [Ca2-]i response are a rapid redistribution followed by the transmembrane flux of Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Islet amyloid polypeptide: production by an osteoblast cell line and possible role as a paracrine regulator of osteoclast function in man

1. The recently discovered peptide islet amyloid polypeptide shows considerable sequence homology with calcitonin-gene-related peptide, itself an alternative product of the calcitonin gene. The possibility that islet amyloid polypeptide might affect calcium homoeostasis and bone cell function was investigated. 2. Islet amyloid polypeptide messenger RNA was found to be expressed by human HTb 96 osteoblast-like cells in culture, and islet amyloid polypeptide immunoreactivity was present in the cell culture medium. 3. Infusion of islet amyloid polypeptide (150 pmol min-1 kg-1) caused a fall in serum calcium and phosphate concentrations in five patients with Paget's disease of the bone. This was similar to that caused by infusion of calcitonin (50 pmol min-1 kg-1). 4. These findings raise the possibility that islet amyloid polypeptide may act as a local factor within bone, produced by osteoblasts and regulating osteoclast function. The possibility of an action of islet amyloid polypeptide on the renal handling of calcium seems unlikely but is not totally excluded.

Divalent cations mimic the inhibitory effect of extracellular ionised calcium on bone resorption by isolated rat osteoclasts: further evidence for a "calcium receptor"

Osteoclast activity is thought to be regulated by calcitonin, as well as by the level of ionised calcium generated locally as a result of bone resorption. The exposure of isolated osteoclasts to elevated ambient calcium levels has been shown to lower resorptive activity and to reduce rates of enzyme release. We have attempted to determine whether these effects are mediated by a divalent cation-sensitive "calcium receptor," as has been reported for the parathyroid chief cells. Thus, we compared the effect of alkaline earth metal cations on osteoclast function using a morphometric measure of bone resorption and a spectrophotometric method for measuring the activity of the released enzyme, acid phosphatase. The exposure of resorbing osteoclasts to between 5 and 20 mM extracellular ionised calcium ([Ca2+]e) inhibited bone resorption and enzyme release to an extent similar to that seen with 0.1 to 10 microM ionomycin. The effect of combining submaximal concentrations of [Ca2+]e (15 mM) and ionomycin (0.1 microM) resulted in additivity, suggesting that the influence of [Ca2+]e on bone resorption was mediated by elevated intracellular calcium levels ([Ca2+]i). The other cations studied (Mg2+, Ba2+) were effective and elicited similar effects, although some required higher concentrations. Thus, whilst Ca2+ and Mg2+ were effective at 10 to 15 mM levels, Ba2+ was effective only at high (20 mM) concentrations. These findings are consistent with an influence of [Ca2+]e on osteoclast activity through an action on a surface membrane "calcium receptor" that can also bind other divalent cations, rather than by passive changes of [Ca2+]i with [Ca2+]e elevation.

Correlates of osteoclast function in the presence of perchlorate ions in the rat

The effects of perchlorate anion (ClO4-) on osteoclast properties were investigated through a number of independent in vivo and in vitro procedures. Intravenous infusion of ClO4- significantly reduced plasma [Ca] in young (50 g) Wistar rats, in the absence of changes in plasma [Mg] or [albumin]. This effect was maximal at 20 min after administration, and at a dose of 600 mumol/rat. Scanning electron-microscope images suggested that the presence of 10 mM-perchlorate reduced both the total area of cortical bone resorbed by freshly disaggregated rat osteoclasts, and the number of osteoclastic excavations in vitro. Similar effects were observed in the presence of 5 mM [Ca2+]. The effects of Ca2+ were potentiated by otherwise ineffective (1 mM) doses of perchlorate. Indo-1 dual-emission microspectrofluorimetry indicated a transient sixfold elevation of cytosolic free [Ca2+], in isolated cultured osteoclasts, with addition of 10 mM-perchlorate. Records of time-lapse video images indicated that this was followed by a marked and sustained cell retraction, by up to 70% of control cell area. Such effects were not observed at thiocyanate concentrations (10 mM) that would have produced comparable lyotropic effects as perchlorate. However, perchlorate did not alter morphometric measures for pseudopodial motility and cell migration. Nor did it influence supernatant concentrations of tartrate-resistant (osteoclastic) acid phosphatase in cultures of resorbing osteoclasts. These findings suggest that perchlorate is a potent inhibitor of osteoclast function, and acts through an influence on intracellular [Ca2+], and in turn upon the degree of cell retraction.

Selective antagonism of calcitonin-induced osteoclastic quiescence (Q effect) by human calcitonin gene-related peptide-(Val8Phe37)

Exposure of isolated rat osteoclasts to calcitonin (CT) leads to an abrupt cessation of cell motility (Q effect) followed by cell retraction (R effect). We have previously shown that these effects are mediated by two G proteins that appear to activate separate post-receptor pathways. The present study demonstrates that the Q but not the R effect of CT (0.006 microM) is abolished in the presence of human calcitonin gene-related peptide (CGRP)-(Val8Phe37) (0.5 microM), a fragment analogue of human CGRP. This selective antagonism suggests that the Q effect could result from an action of CT upon a site that is distinct from that producing the R effect. The former site ('amylin site') also appears to interact with related peptides, amylin and CGRP, whilst the latter site ('CT site') specifically interacts with CT.

Hospital-acquired diarrhea in adults: a prospective case-controlled study in Mexico

OBJECTIVE

To know the incidence, etiology, risk factors, morbidity, and mortality of nosocomial diarrhea in adults.

DESIGN

Nested case-control study, matched by service, length of stay, date of admission, and presence of leukopenia and/or the acquired immunodeficiency syndrome (AIDS). Cases were those who developed nosocomial diarrhea. Controls were those who did not develop nosocomial diarrhea during a comparative period nor during the next ten days. Stool samples were processed in search for parasites, yeasts, bacteria, and rotavirus.

SETTING

Third-level referral center, in Mexico City, Mexico, for general internal medicine and surgical problems.

PATIENTS

Eligible subjects were all new admissions to the hospital from November 1987 to September 1988. Reasons for exclusion were presence of chronic diarrheal disease or melena. There were 115 cases and 111 controls.

RESULTS

Overall risk of acquiring nosocomial diarrhea was 5.5%, or 1.8 episodes per 100 patient-weeks. A potential pathogen was found in 59%. Yeasts and Entamoeba histolytica were the most frequently isolated pathogens. Mortality in cases was 18%, as compared with 5% in controls (p less than .01). Multivariate analysis showed enteral feeding, recent enemas, presence of Candida species, use of antacids/H2-blockers, and presence of nasogastric tubes as significant risk factors for nosocomial diarrhea.

CONCLUSIONS

Diarrhea is a common complication in hospitalized patients. It occurs more often than previously suspected and is linked with a substantial mortality. The spectrum of etiologic agents is different from that reported in pediatric hospitals. Given that nosocomial diarrhea may constitute, at least, a marker of severity of illness, it should receive more attention in general hospitals.

Single pulses of cytoplasmic calcium associated with phagocytosis of individual zymosan particles by macrophages

We have measured cytosolic free calcium levels ([Ca++]i) in individual macrophages during the phagocytosis of single zymosan particles. We report here that the contact of a macrophage with zymosan results in a rapid transient elevation of [Ca++]i. Each [Ca++]i pulse is symmetrical lasting for up to 30 seconds. In contrast, macrophage spreading is associated with repetitive [Ca++]i spiking occurring in salvos of up to four smaller spikes, each lasting for between 8 and 18 seconds. These qualitative and kinetic differences might suggest that the role of [Ca++]i in phagocytosis is distinct from its role in spreading.

Osteoclastic inhibition: an action of nitric oxide not mediated by cyclic GMP

The osteoclast is unique in its ability to resorb bone, and excessive osteoclastic activity has been implicated in osteoporosis, Paget disease of bone, rheumatoid arthritis, and the growth of metastases in bone. The activity of this cell is controlled by the main circulating inhibitor, calcitonin, in association with locally produced modulators. We show that nitric oxide (NO) may be an important member of the latter group. NO is produced by the vascular endothelium and nervous system and is involved in both neurotransmission and the regulation of blood pressure. However, our results show that the autocoid is also a potent inhibitor of osteoclast function. NO (30 microM) produced a decrease to approximately 50% of the original osteoclast spread area. Similar effects were also produced by 3-morpholinosydnonimine or sodium nitroprusside, reagents that spontaneously release NO. These shape changes were associated with a reduction of bone resorption after a 24-hr incubation of isolated osteoclasts on devitalized bone slices. NO is thought to act by stimulating guanylate cyclase, with a consequent increase in cyclic GMP, but a different mode of action is likely in the osteoclast since dibutyryl or 8-bromo cyclic GMP have no effect. It should be noted that calcitonin can produce similar changes in shape and activity but is associated with an increase in osteoclast intracellular calcium and cessation of membrane movement; neither of these is produced by NO, suggesting that its mode of action is different. The abundance of NO-producing endothelial cells in bone marrow and their proximity to osteoclasts suggests that marrow endothelial cells may play a physiological role in the regulation of osteoclastic activity.

Enhanced circulating levels of neurally derived calcitonin gene related peptide in spontaneously hypertensive rats

STUDY OBJECTIVE

Calcitonin gene related peptide (CGRP) is a potent endogenous vasodilator. The peptide is released from perivascular nerve endings and can normally be detected in the circulation. An attempt was made to determine the concentrations and partially characterise the immunoreactive forms of circulating CGRP in a genetically hypertensive rat strain, the "spontaneously hypertensive rat" (SHR) and its genetic control, Wistar-Kyoto (WKY).

DESIGN

Immunoreactive plasma CGRP levels were measured using a highly sensitive carboxyl terminal specific CGRP radioimmunoassay together with high performance liquid chromatography. Plasma immunoreactive CGRP (i-CGRP) levels were also measured 6 h after intraperitoneal colchicine administration (10 mg.kg-1 body weight) to both groups of rats.

EXPERIMENTAL MATERIAL

Eight SHR rats were compared with eight WKY rats, both groups aged 7 weeks.

MEASUREMENTS AND MAIN RESULTS

Mean i-CGRP levels were threefold higher in the SHR group compared to controls, while plasma calcitonin levels were not different between the two groups. There was a significant correlation (Kendall's correlation coefficient, r = 0.57; p = 0.024) between i-CGRP levels and the mean systolic blood pressure (measured by the indirect tail cuff method) in SHR but not WKY rats. Similar profiles of i-CGRP were observed in both SHR and WKY rats, when acid-methanol extracts of pooled plasma were chromatographed under similar conditions. It was also confirmed that circulating CGRP in both SHR and WKY strains was derived from perivascular nerve endings, by demonstrating a complete abolition from plasma of previously detectable i-CGRP following the administration of colchicine.

CONCLUSIONS

The study shows that CGRP is normally released from vascular nerve endings, and that high concentrations of the circulating peptide are found in hypertension. This might represent a mechanism to counteract the excessive vasoconstrictor influences that underly the development and maintenance of hypertension.

Modularity of osteoclast behaviour and "mode-specific" inhibition of osteoclast function

This study is part of an attempt to understand the role of specific cellular activities in the bone resorptive process. Experiments were performed whereby known pharmacological agents were used to inhibit individual modes of osteoclastic activity, such as motility and secretion. The effects of such treatments on bone resorption were assessed by quantitative scanning electron microscopy. The compounds included colchicine, which was used to inhibit osteoclast motility; molybdate ions which were used to selectively inhibit the catalytic activity of secreted acid phosphatase, and omeprazole which was employed to inhibit the secretion of hydrogen ions. All compounds inhibited osteoclastic bone resorption, but singularly affected defined modes of activity. These findings suggest that each mode of osteoclastic activity is essential for the bone resorptive process, and that "mode-specific" inhibition may provide a means whereby excessive activity of the osteoclast can be regulated in disease.

"Calcium receptors" on eukaryotic cells with special reference to the osteoclast

There is a growing list of cells that are capable of detecting and responding to changes in the concentration of extracellular calcium. The two classic examples of this behaviour are the calcitonin-secreting parafollicular cells of the thyroid and parathyroid hormone-secreting chief cells of the parathyroid gland. A more recent addition to this list is the renin-secreting juxtaglomerular cell of the kidney. Particularly intriguing has been independently the discovery by two laboratories, that the resorptive cell of bone, the osteoclast, is capable of detecting changes in ambient calcium. A common theme amongst all these so called "calcium-responsive" cells is that extracellular calcium increases elevate intracellular calcium levels, and this intracellular signal is either stimulatory or inhibitory to the functional response. But how these cells detect changes in the concentration of extracellular calcium, and how these recognition events are subsequently transformed into intracellular signals that regulate cell function are somewhat less clear. The commentary reveals some recent developments that seemingly provide insights into these mechanisms, with special reference to the osteoclast.

Intracellular regulation of enzyme secretion from rat osteoclasts and evidence for a functional role in bone resorption

1. Osteoclasts are known to secrete acid phosphatase, an iron-containing phosphohydrolase. We have investigated (a) the possibility that acid phosphatase has a functional role in bone resorption and (b) the factors controlling enzyme secretion from isolated rat osteoclasts. 2. Osteoclasts were freshly disaggregated from neonatal rat long bones and dispersed at low densities on devitalized cortical bone slices or on plastic substrate. The levels of acid phosphatase in culture medium were measured spectrophotometrically using 4-nitrophenyl phosphate as hydrolysable substrate. The total plan area of bone resorbed was quantified by scanning electron microscopy in combination with image processing and analysis. 3. Ninety-three per cent of the total enzyme activity detected in the supernatant exposed to bone-osteoclast preparations was resistant to inhibition by D-tartaric acid and was bound to an antibody known to be highly specific for the osteoclast-derived isoenzyme, showing that it originated from osteoclasts. 4. A diminution in the level of supernatant enzyme activity achieved by incubating bone-osteoclast preparations with an antiserum specifically binding the osteoclast isoenzyme, or with a non-competitive inhibitor, molybdate or with competitive inhibitors, disphosphonates, led to a marked reduction of osteoclastic bone resorption. 5. The rate of the enzyme released into the culture supernatant, whether from resorbing (cultured on bone) or non-resorbing (cultured on plastic) osteoclasts declined gradually over 22 h, but that from the former was significantly depressed within the first 30 min of incubation. The supernatant enzyme concentration increased linearly up to 3 h; the levels released from resorbing osteoclasts remained consistently lower than those from non-resorbing cells. 6. Exposure of osteoclasts for 18 h to elevated [Ca2+]o levels produced a concentration-dependent inhibition of supernatant acid phosphatase levels. In the presence of 20 mM [Ca2+]o enzyme secretion from resorbing osteoclasts was significantly lower than that from non-resorbing cells. 7. Exposure of bone-osteoclast preparations to pertussis toxin produced no significant change of acid phosphatase release, while cholera toxin, dibutyryl cyclic AMP and forskolin produced a marked elevation of enzyme secretion. Ionomycin was found to inhibit enzyme release and this was less marked when osteoclasts were incubated on plastic substrate.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence that the action of calcitonin on rat osteoclasts is mediated by two G proteins acting via separate post-receptor pathways

Calcitonin inhibits osteoclastic bone resorption and its action involves two separate acute effects on the osteoclast, both essential to the action of the hormone: abolition of cell motility (Q) and marked cellular retraction (R). The former was mimicked by dibutyryl cyclic AMP and cholera toxin and the latter by pertussis toxin, ionomycin and increases in ambient calcium. Aluminium fluoride ions produced both Q and R effects, while lithium prevented both. In addition, calcitonin elicited a biphasic elevation of cytosolic-free calcium in single isolated osteoclasts. We propose that the action of calcitonin is mediated by at least two G proteins, one responsible for the Q effect and the other for the R effect. In addition, two second messengers, cyclic AMP and calcium, are involved. These findings may help to explain the potency of calcitonin in inhibiting bone resorption, and may allow the rational design of new therapeutic agents designed to alter osteoclast behaviour.

Structure-activity relationship of human calcitonin-gene-related peptide

The calcitonin-calcitonin-gene-related peptide (CGRP) gene complex encodes a small family of peptides: calcitonin, CGRP and katacalcin. Calcitonin is a circulating hormone that prevents skeletal breakdown by inhibiting the resorption of bone by osteoclasts. CGRP, a potent vasodilator, is involved in normal regulation of blood flow. The calcitonins structurally resemble the CGRP peptides, and both are known to cross-react at each others' receptors. The present study was undertaken to examine the structural prerequisites for biological activity of the intact CGRP molecule. We therefore prepared eight chymotryptic and tryptic fragments of CGRP and synthesized its acetylated and S-carboxyamidomethylcysteinyl analogues. The analogues were purified by h.p.l.c. and their structures were confirmed by fast-atom bombardment mass spectrometry. We have examined the effects of structurally modified analogues and fragments of human CGRP in a calcitonin-receptor-mediated assay, the osteoclast bone resorption assay, and in one or two CGRP-receptor-mediated assays, the rabbit skin blood flow assay and the oedema formation assay. The results showed that (1) in the osteoclast bone resorption assay, both CGRP peptides, alpha and beta, were equipotent, and were both at least 1000-fold were both approx. 1000-fold more potent than salmon calcitonin; human calcitonin had no effect; (3) the bis- and N-acetylated CGRP analogues retained reduced levels of biological activity in all assays, whereas S-carboxyamidomethylcysteinyl-human CGRP was without activity; and (4) all tryptic and chymotryptic fragments of CGRP were without biological activity, with the exception of hCGRP-(Ala1-Lys35): this fragment had much reduced activity compared with the intact peptide in inhibiting osteoclastic bone resorption and increasing blood flow in the rabbit skin. The results suggest that: (1) calcitonin and CGRP act at distinct receptors to mediate different physiological effects; (2) minor amino acid substitutions, as between the alpha and beta forms of CGRP (these two forms have 94% structural similarity) do not result in differences in biological activity; (3) the intact peptide is required for full biological activity of the CGRP molecule, and even the loss of two amino acids at the C-terminus of the molecule results in a marked decrease in activity; (4) the disulphide bridge appears to play an important role in the interaction of the intact CGRP molecule with its receptor; and (5) the C-terminal region is probably necessary for the peptide to assume the right conformation in the interaction with the receptor.

Development and performance of a highly sensitive and specific two-site immunometric assay of calcitonin gene-related peptide

Calcitonin gene-related peptide (CGRP), a potent endogenous circulating vasodilator, is produced by the alternative splicing of the calcitonin/CGRP gene and is expressed mainly in neural and cardiovascular tissues. We recently reported a highly sensitive radioimmunoassay of CGRP, based on an antiserum recognizing the C-terminus of the molecule. We have also found that circulating immunoreactive CGRP is heterogeneous; thus we are unable to measure selectively the intact molecule with our one-site competitive approach. We therefore attempted to construct a two-site immunometric assay involving two antibodies, one that detects the C-terminus and another that recognizes the midregion of the molecule. To enhance assay sensitivity, we applied a colorimetric amplification system to this assay. This rapid, robust, and reproducible assay provides more nearly accurate estimates of circulating CGRP and offers a sensitive and more specific alternative to the radioimmunoassay, with advantages in speed, simplicity, and convenience.

Amylin-amide: a new bone-conserving peptide from the pancreas

Amylin-amide is a new member of the family of peptides encoded by the calcitonin multigene complex. In the present study, we have compared directly, the hypocalcaemic potency and duration of action of human amylin-amide and human calcitonin in an in vivo rat bioassay and an in vitro osteoclast bone resorption assay. Amylin-amide was found to have a potency approximately 40-fold lower than human calcitonin, whilst both peptides followed the same time course. This suggests that amylin-amide is the most potent non-calcitonin hypocalcaemic peptide so far reported. An important physiological implication follows. It would seem that amylin-amide can play a central role in the maintenance of the skeleton by virtue of its inhibitory influence on osteoclastic function.

Development and performance of a highly sensitive and specific two-site immunometric assay of calcitonin gene-related peptide

Calcitonin gene-related peptide (CGRP), a potent endogenous circulating vasodilator, is produced by the alternative splicing of the calcitonin/CGRP gene and is expressed mainly in neural and cardiovascular tissues. We recently reported a highly sensitive radioimmunoassay of CGRP, based on an antiserum recognizing the C-terminus of the molecule. We have also found that circulating immunoreactive CGRP is heterogeneous; thus we are unable to measure selectively the intact molecule with our one-site competitive approach. We therefore attempted to construct a two-site immunometric assay involving two antibodies, one that detects the C-terminus and another that recognizes the midregion of the molecule. To enhance assay sensitivity, we applied a colorimetric amplification system to this assay. This rapid, robust, and reproducible assay provides more nearly accurate estimates of circulating CGRP and offers a sensitive and more specific alternative to the radioimmunoassay, with advantages in speed, simplicity, and convenience.

A dual effect of calcitonin gene-related peptide on plasma calcium levels in the chick

Calcitonin gene-related peptide (CGRP) lowers plasma calcium in the rat and inhibits bone resorption by isolated rat osteoclasts. In our preliminary studies we found that rat CGRP elevates plasma calcium levels in the chick, a response that was somewhat similar to that of parathyroid hormone. Here, we report that human CGRP (alpha) produces a concentration-dependent elevation of plasma calcium levels. The two peptides did not follow precisely the same time course. Whereas at 15 minutes CGRP produced hypocalcaemia relative to the control plasma calcium levels, at 30 minutes both CGRP and PTH were found to be hypercalcaemic. These studies suggest that CGRP initially interacts with the calcitonin receptor to produce a calcitonin-like effect, which is followed by hypercalcaemia presumably by antagonising the action of endogenous circulating calcitonin.

Intracellular calcium in the control of osteoclast function. II. Paradoxical elevation of cytosolic free calcium by verapamil

We report here for the first time that verapamil elevates cytosolic calcium. We have found that in the isolated rat osteoclast, verapamil at low micromolar concentrations did not block the elevation of cytosolic free calcium ([Ca++]i) in response to elevated extracellular calcium concentrations ([Ca++]e). However, high micromolar concentrations of verapamil (300 microM or above) led to a rapid sustained elevation of [Ca++]i. These concentrations of verapamil had effects on osteoclast morphology and resorptive activity that were similar to those produced by elevated [Ca++]e: there was a marked dose-dependent fall in cell spread area and osteoclastic bone resorption. The sensitivity of osteoclasts to high micromolar concentrations of verapamil is unique and could not be mimicked in macrophages and lymphocytes.

Intracellular calcium in the control of osteoclast function. I. Voltage-insensitivity and lack of effects of nifedipine, BAYK8644 and diltiazem

We have recently demonstrated that in the rat osteoclast, a rise in the ambient calcium concentration induces a rapid elevation of cytosolic calcium, and that this phenomenon is accompanied by a complete inhibition of osteoclastic bone resorption. Here, we have attempted to characterise the electrophysiological nature of the putative 'calcium-activated' calcium channel. We have established that calcium influx into the osteoclast that occurs on exposure to elevated extracellular calcium is independent of membrane voltage and is insensitive to modulation by organic calcium channel modulators, namely nifedipine, BAYK8644 and diltiazem. The latter compounds were also unable to block the reduction of cell spread area and the inhibition of bone resorption produced in response to elevated extracellular calcium levels.

Amylin and amylin-amide lack an acute effect on blood glucose and insulin

Amylin-amide has been implicated in the pathogenesis of type II diabetes due to its proposed inhibitory effect on insulin release from beta cells of the pancreatic islets, and on glucose uptake by the skeletal muscle. In experiments with rats and rabbits we failed to demonstrate these anti-insulin actions of amylin and amylin-amide. A single bolus dose of the two peptides (500 pmol) administered i.v. failed to suppress plasma insulin levels or to elevate blood glucose levels. The continuous infusion of amylin-amide into rabbits also failed to suppress the release of insulin in response to hyperglycaemia produced by an i.v. bolus injection of glucose. These in vivo observations imply that the amylin peptides may not have a primary physiological role in carbohydrate metabolism, but in view of our previous findings, we speculate that the peptide has a more prominent role in calcium homeostasis.

A new approach for calcitonin determination based on target cell responsiveness

We report the development and validation of three microbioassays for calcitonin based on calcitonin-induced inhibition of the activity of isolated osteoclasts. Having precisely quantified osteoclast motility, spreading and bone resorptive activity, we have applied stringent analytical procedures to define assay characteristics. We have found that the appropriately transformed responses significantly regress on log dose of the peptides. Furthermore, potency estimates obtained using calcitonins from three species (human, salmon and a synthetic analogue of eel calcitonin) have been found to be consistent with those obtained using conventional calcitonin bioassays. In addition, the assays are remarkably sensitive (detection limit 10(-15) M), highly specific and precise. We have determined plasma levels of bioactive calcitonin on samples from patients with medullary thyroid carcinoma; these are several-fold lower than those obtained using our routine calcitonin radioimmunoassay. Our study thus, forms the basis of an entirely new approach for the determination of 'biologically active' calcitonin, and we envisage that such target cell-specific assays could become useful microanalytical methods.

Biological activity of calcitonin and its assessment

Radioimmunoassay has been the method of choice for the routine determination of calcitonin (CT) in biological fluids, mainly due to its precision, sensitivity, and ability to handle large numbers of samples. However, spuriously high results are often obtained due to undefined cross-reactivities of polyclonal antisera to precursors, polymers, metabolites, or fragments of CT, or to other physiological and pathological variants of the hormone. In routine practice, radioimmunoassays of CT are being replaced by immunometric two-site 'sandwich' assays utilizing monoclonal antibodies against defined antigenic epitopes. Hormone activity is, however, accurately quantifiable by biological assays based on the exquisite hormonal sensitivity of isolated osteoclasts. The assays are highly specific and remarkably sensitive. Their use in routine laboratories is nevertheless limited only to specific situations.

Kidney: a target organ for calcitonin gene-related peptide

In the present study we have evaluated the effect of calcitonin gene-related peptide (CGRP) on cyclic AMP levels in intact mouse kidneys. We have used an in vivo bioassay based on microwave irradiation as a means of rapid tissue fixation, followed by the determination of cyclic AMP concentration using a protein binding assay. CGRP was found to cause a dose-dependent elevation of renal cyclic AMP levels, and despite being slightly less potent than calcitonin (CT), the action of CGRP was significantly more prolonged. The extended time course of action of CGRP is in sharp contrast to its known effect on bone cyclic AMP levels and could be of physiological relevance. CGRP may act on a receptor different from that of CT to produce distinct functional effects.

The calcitonin gene peptides: biology and clinical relevance

The calcitonin/CGRP multigene complex encodes a family of peptides: calcitonin, its C-terminal flanking peptide, katacalcin, and a third novel peptide, calcitonin gene-related peptide (CGRP). The 32-amino acid peptide calcitonin inhibits the osteoclast, thereby conserving skeletal mass during periods of potential calcium lack, such as pregnancy, growth, and lactation. This hormonal role is emphasized by observations that lower circulating calcitonin levels are associated with bone loss and that calcitonin replacement prevents further bone loss. Structurally, CGRP resembles calcitonin and has been implicated in neuromodulation and in the physiological regulation of blood flow. Here we review the molecular genetics, structure, and function of the calcitonin-gene peptides as analyzed in the laboratory and focus on more recent clinical studies relating to disorders and therapeutics.

The effect of extracellular calcium elevation on morphology and function of isolated rat osteoclasts

Osteoclasts are large multinucleate cells unique in their capacity to resorb bone. These cells are exposed locally to high levels of ionised calcium during the process of resorption. We have therefore examined the effect of elevated extracellular calcium on the morphology and function of freshly disaggregated rat osteoclasts. Cell size and motility were quantitated by time-lapse video recording together with digitization and computer-centred image analysis. In order to assess the resorptive capacity of isolated osteoclasts, we measured the total area of resorption of devitalised cortical bone by means of scanning electron microscopy and computer-based morphometry. The results show that elevation of the extracellular calcium concentration causes a dramatic reduction of cell size, accompanied by a marked diminution of enzyme release and abolition of bone resorption. We propose that ionised calcium might play an important role in the local regulation of osteoclastic bone resorption.

Calcitonin gene-related peptide: endocrine distribution and characterization of circulating forms

This study reports the application of a highly sensitive and specific extraction-based radioimmunoassay for determining levels of calcitonin gene-related peptide (CGRP) in endocrine tissues and plasma of normal rats. In addition, we have characterized the immunoreactive material found in plasma using gel filtration and high performance liquid chromatography. The CGRP content of the thyroid was 100- to 1000-fold higher than that of the adrenals, pituitary or pancreas, while the gonads and kidneys contained appreciably smaller amounts of CGRP. There was no age- and sex-related difference in tissue content, with the exception of a subgroup of 450 g rats. These had a significantly raised thyroidal CGRP content compared to other rats of the same group. A significant correlation between thyroidal content and plasma levels was noted only in old (450 g) rats. CGRP-like immunoreactivity in plasma pooled from young adult (150 g) rats was found to be heterogeneous. Only 8% of the total immunoreactivity recovered from a Sephadex G50 column was found to coelute with synthetic rat CGRP when freeze-dried plasma was chromatographed and a significantly greater proportion (approximately 67%) eluted in the void volume. The void volume peak was markedly reduced when acid-methanol extracts of plasma were chromatographed under similar conditions. A major proportion of the immunoreactive material co-eluting with synthetic CGRP on gel filtration was also found to elute at a position corresponding to the monomer on HPLC: this was consistent with, though not proof of, structural identity.

Uncoupling of receptor-mediated cellular responses by ionic lithium

The propensity of ionic lithium to interfere with the coupling of receptors to guanine nucleotide binding proteins (G-proteins) has only recently been investigated using rat cortical membranes. In the present study we have used intact isolated osteoclasts to investigate lithium-induced uncoupling of the receptor-mediated actions of calcitonin. All actions of calcitonin on the osteoclast were abolished by ionic lithium. We believe that the cation prevents signal transduction by inhibiting G protein-receptor interaction, the first step in intracellular signalling.

Production of calcitonin gene-related peptide from human cancer cells

Calcitonin (CT) is produced ectopically from a variety of non-thyroidal cancers. The CT genes also encode another peptide, calcitonin gene-related peptide (CGRP) which is a potent vasodilator. In the present study we have used immunochemical and chromatographic methods to demonstrate the presence and characterize the molecular forms of CGRP in cultured human cancer cells. Using two highly sensitive and specific radioimmunoassays, we have detected immunoreactive CGRP (i-CGRP) in cell extracts and cell-exposed media of cultured promyelocytic leukaemia (HL60) and bronchogenic carcinoma (BEN) cells. The mean i-CGRP content of the HL60 and BEN cell extracts was 2 and 45 pmol/g wet weight respectively. On gel filtration and high performance liquid chromatography, the immunoreactive material was found to be heterogeneous, though a major proportion co-eluted with synthetic human CGRP(1-37), suggesting structural identity with the intact CGRP molecule. Finally, we have discussed some interesting features of CT-gene peptide expression in tumour cells.

'Calcium-activated' intracellular calcium elevation: a novel mechanism of osteoclast regulation

The osteoclast is unique in its capacity to resorb bone. An unbalanced increase in this activity causes osteoporosis, a crippling bone disease that poses a major public health problem. Despite this, our understanding of osteoclast regulation is very limited. Calcitonin is the only known physiological inhibitor of osteoclast function. We demonstrate here for the first time that the concentration of calcium ions at the resorptive site directly regulates osteoclast function by modulating the intracellular free calcium concentration. This represents an important feedback mechanism of osteoclast control.

In vivo and in vitro effects of amylin and amylin-amide on calcium metabolism in the rat and rabbit

Amylin is a new member of the calcitonin/CGRP family: it is a 37 amino acid polypeptide which was recently isolated from amyloid deposits in pancreatic islets obtained from type II diabetics. In the present study we investigated the effect of amylin and amylin-amide on calcium metabolism in the rat and rabbit. Two main methods were used: in vivo hypocalcaemic activity was assessed by measuring plasma calcium levels after injection of the peptide in 50 g rats; and in vitro resorption of cortical bone by disaggregated rat osteoclasts was quantified by scanning electron microscopy together with image analysis. We demonstrate that amylin and amylin-amide have calcitonin-like effects: both are powerful inhibitors of osteoclastic resorption and as a consequence lower plasma calcium in both rats and rabbits. We speculate that the peptide may exert systemic or local regulatory effects on bone cells.

Transforming growth factor-beta-induced mitogenesis of human bone cancer cells

We report for the first time the bifunctional effects of transforming growth factor-beta on the growth of cloned human osteosarcoma cells (Htb96). Cell growth was assessed by determining the cell number, replication index and [3H]-thymidine incorporation following 48 hours incubation of cultured Htb96 cells with the peptide. Exposure of cells to concentrations of TGF-beta upto 40 pM caused a mitogenic response, concentrations between 40 and 800 pM failed to stimulate cell growth and higher doses caused an inhibition of cell proliferation. The initial cell density was found to alter the responsiveness of Htb96 cells to TGF-beta; stimulation of proliferation was less profound at high and low cell densities. The observed cell density- and growth factor concentration-dependent effects of TGF-beta on the growth of tumour cells might suggest the existence of an autocrine regulatory mechanism. Furthermore, by demonstrating a sensitivity to inhibition by indomethacin, we conclude that the proliferative effect of TGF-beta is at least, in part, dependent on the de novo synthesis of prostaglandins.

Further evidence for the origin of circulating calcitonin gene-related peptide in the rat

1. We have attempted to determine the origin of a potent circulating vasodilator, calcitonin gene-related peptide (CGRP), by assessing the effects of administration of capsaicin and colchicine, and of thyroidectomy on plasma levels. Plasma CGRP, neurokinin A and calcitonin levels were measured using highly sensitive and specific radioimmunoassays, and the circulating forms of CGRP characterized by size-exclusion gel and high-performance liquid chromatography. 2. Seven minutes following the intraperitoneal injection of capsaicin (10 mg/kg body weight) plasma CGRP and neurokinin A levels were found to rise by 15- and 4-fold respectively, while there was no change in circulating calcitonin levels. Chromatography revealed that the immunochemical forms of CGRP released into the circulation by capsaicin were similar to those found in normal plasma, which included a peak co-eluting with the intact CGRP molecule. 3. Six hours after the intraperitoneal administration of colchicine (10 mg/kg body weight), CGRP was not detectable in the circulation of 40-day-old rats and plasma levels were significantly lowered in the group of greater than or equal to 200-day-old rats. Chromatography revealed that peaks of circulating immunoreactivity corresponding to monomeric CGRP and its fragments were substantially reduced following colchicine treatment. 4. The magnitude of capsaicin-evoked elevation of plasma CGRP was only marginally (significant at 0.1 greater than P greater than 0.05) reduced by pre-treatment with colchicine. 5. Thyroidectomy alone caused a significant reduction of plasma CGRP levels in greater than or equal to 200-day-old rats; a greater reduction was observed when thyroidectomy was combined with colchicine administration. However, when 40-day-old rats were thyroidectomized, there was an unexplained elevation of plasma CGRP levels. 6. The results suggest that while CGRP is normally released into the circulation from nerve terminals, both in young adult (40-day-old) and old (greater than or equal to 200-day-old) animals, a thyroidal origin is obvious only in old rats.

Inhibition of osteoclastic acid phosphatase abolishes bone resorption

Osteoclastic acid phosphatase is a member of a widely-distributed class of iron-containing proteins with acid phosphatase activity. Antibodies raised against one member of this class cross-react with other members from the same or different species, but not with acid phosphatase isoenzymes of different types. When antibodies to one such protein, porcine uteroferrin, are added to medium in which rat osteoclasts are incubated on devitalised cortical bone, both bone resorption and acid phosphatase activity are markedly inhibited. Furthermore, addition of molybdate (an inhibitor of this class of acid phosphatases) also inhibits both bone resorption and enzyme activity. These observations strongly suggest a functional role for osteoclastic acid phosphatase in bone resorption.

Production and characterisation of immunoreactive calcitonin gene-related peptide (CGRP) from a CGRP receptor-positive cloned osteosarcoma cell line (UMR 106.01)

A subclone of an osteoblast-like osteosarcoma cell line (UMR 106.01) has recently been shown to possess specific binding sites for calcitonin gene-related peptide (CGRP) linked to adenylate cyclase. The present study provides the first demonstration for the production of immunoreactive CGRP from CGRP-receptor positive osteosarcoma cells. Mean immunoreactive CGRP levels were 15 pmol/g and 1 pmol/l for acid extracts of cells and cell-exposed media respectively. On gel filtration and high performance liquid chromatography, a major proportion of immunoreactive CGRP was found to co-elute with synthetic rat CGRP(1-37). Only negligible quantities of calcitonin were detected in cell extracts or cell-exposed supernatant. The production of authentic CGRP from a CGRP-receptor positive tumour suggests that the peptide may have autocrine effects on its producer cell.

Epidemic of Serratia marcescens bacteremia and meningitis in a neonatal unit in Mexico City

A case-control study was conducted on an epidemic of bacteremia and meningitis caused by Serratia marcescens in the neonatal intensive care unit and special care nursery of a general hospital in Mexico City, Mexico. A 19.9% incidence of bacteremia and meningitis was recorded in contrast to 1.4% and 3.7% during preepidemic and post-epidemic periods; a 69% mortality rate was observed. Peripheral IV catheters and the use of mixed IV fluids prepared in the wards were the major risk factors (P less than 0.001). Rectal and nasopharyngeal cultures were positive in 68% of asymptomatic neonates and hand cultures were positive in 16.7% of personnel. Strains were resistant to all aminoglycosides and broad-spectrum penicillins, and belonged to the A5/8 biogroup. Containment of this outbreak was difficult because of failure to identify colonized infants early in the epidemic and because of persistent carriage of S marcescens by personnel. Comparisons between this hospital and tertiary care centers in Mexico suggest that in developing countries nosocomial infections could be of greater magnitude in secondary than in tertiary level centers.

Effects of peptides from the calcitonin genes on bone and bone cells

The calcitonin-calcitonin gene-related peptide (CGRP) gene complex encodes a family of novel peptides--calcitonin, CGRP and katacalcin. Whereas calcitonin is a circulating hormone involved in skeletal maintenance, the physiological function of CGRP still remains unclear. In the present study we have compared the biological activity of CGRP with that of calcitonin using three experimental systems. We have demonstrated that both peptides inhibit bone resorption by active rat osteoclasts and thus lower plasma calcium when injected into young rats. In both respects the CGRP homologues (rat, human alpha and human beta) were found to be 100- to 1000-fold less potent than human calcitonin. The effects of the CGRP peptides and calcitonin were only additive. Human CGRP (alpha) also caused a marked dose-dependent elevation of bone cyclic AMP levels in mice, somewhat like calcitonin. Though from our studies it would seem reasonably clear that CGRP is weakly agonistic for the calcitonin receptor on the osteoclast to produce effects on bone resorption and plasma calcium, it is still unclear whether the elevation of bone cyclic AMP simply represents an osteoclastic effect or an additional, more important, effect on osteoblasts. It is highly unlikely that CGRP may exert systemic effects on bone. Nevertheless, the peptide may be an important local regulator of bone cell function.