ATP-dependent uptake of Ca2+ by a microsomal fraction from rat incisor odontoblasts

Synchrotron X-ray fluorescence mapping of Ca, Sr and Zn at the neonatal line in human deciduous teeth reflects changing perinatal physiology

OBJECTIVES

Our first objective was to review the evidence describing the appearance and microstructure of the neonatal line in human deciduous teeth and to link this with known changes in neonatal physiology occurring at and around birth. A second objective was to explore ways to improve identification of the neonatal line by mapping the pre- and postnatal distribution of Ca, Sr and Zn in deciduous cuspal enamel and superimposing these maps onto transmitted light micrographs that included a clear true section of the neonatal line.

MATERIALS AND METHODS

We used synchrotron X-ray fluorescence to map elemental distributions in pre- and postnatal enamel and dentine. Two deciduous canines and 5 deciduous molars were scanned with an X-ray beam monochromatised to 17.0 keV at either 10.0, 2.5 or 1.0 μm resolution and 10 ms integration time.

RESULTS

Calcium maps distinguished enamel and dentine but did not clearly demarcate tissues formed pre- or postnatally. Strontium maps reflected presumed pre- and postnatal maternal serum levels and what are likely to be diet-dependent regions of Sr enrichment or depletion. Prenatal Zn maps, particularly for dentine, mirror elevated levels in the fetus and in colostrum during the first few days of life.

CONCLUSIONS

The neonatal line, enamel dentine junction and surface enamel were all Zn-rich. Within the neonatal line Zn may be associated with increased crystallinity but also with caries resistance, both of which have been reported previously. Elemental mapping may improve the identification of ambiguous NNLs and so be useful in forensic and archaeological studies.

Extracellular Ca2+ increases cytosolic free Ca2+ in freshly isolated rat odontoblasts

Recent evidence suggests that extracellular Ca2+ may modulate cell function in mineralized tissue. To determine whether dentinogenic cells, in particular, are sensitive to extracellular Ca2+, fura-2 microfluorometry was used to monitor intracellular calcium levels in odontoblasts freshly isolated from rat incisor. In response to applications of 0.5-4.0 mM extracellular calcium (CaCl2), most odontoblasts (84%; 107/128) showed an increase in intracellular calcium. For the majority of these cells (70%; 75/107), the typical response was biphasic; there was an initial, transient increase in intracellular calcium which reached peak levels within 30-50 s and decayed rapidly, followed by a slower (> 300 s) recovery toward basal levels. In general, the response of these cells to calcium was repeatable and the mean calcium concentration for the half-maximal response was approximately 1.3 mM. This effect could be partially blocked by either 200 microM lanthanum, a nonspecific blocker of Ca2+ channels, or 20 microM dantrolene, a potent inhibitor of Ca2+ release from internal stores. Used in combination, lanthanum, and dantrolene nearly abolished the calcium response completely. In addition, this response was sensitive to the dihydropyridine-sensitive calcium channel blocking agent nicardipine (60 microM), indicating a role for voltage-gated calcium channels during these events. These results show that odontoblasts respond to external calcium through mechanisms involving both influx of external calcium as well as release of calcium from internal stores and suggest a role for extracellular calcium in regulating the function of these cells.

Dentin mineralization and the role of odontoblasts in calcium transport

Dentin is formed by two simultaneous processes, in which the odontoblasts are instrumental--the formation of the collagenous matrix, and mineral crystal formation in this matrix. This pattern of formation is similar to that of bone, another mineralized connective tissue. Dentin and bone also have chemical compositions which are similar but with distinct differences. It is of fundamental importance to understand how the ions constituting the inorganic phase are transported from the circulation to the site of mineral formation and how this transport is regulated. For dentinogenesis, calcium is essentially the only ion for which data are available. Recent evidence suggests that a major portion of the Ca2+ ions are transported by a transcellular route, thus being under cellular control. The cells maintain a delicate Ca2+ ion balance by the concerted action of transmembraneous transport mechanisms, including Ca-ATPase, Na+/Ca2+ exchangers and calcium channels, and of intracellular Ca(2+)-binding proteins. The net effect of this is a maintenance of a submicromolar intracellular Ca2+ activity, and an extracellular accumulation of Ca2+ ions in predentin, at the mineralization front. Predentin can be regarded as a zone of formation and maturation of the scaffolding collagen web of the dentin organic matrix. In addition to collagen, it contains little but proteoglycan. Simultaneous with mineral formation, additional non-collagenous macromolecules are added to the extracellular matrix of dentin, these presumably being transported within the odontoblast process. Among these are highly phosphorylated dentin phosphoprotein (phosphophoryn) and another pool of proteoglycan.(ABSTRACT TRUNCATED AT 250 WORDS)

Dentinogenesis

The formation of dentin, dentinogenesis, comprises a sophisticated interplay between several factors in the tissue, cellular as well as extracellular. Dentin may be regarded as a calcified connective tissue. In this respect, as well as in its mode of formation, it is closely related to bone. Using dentinogenesis as an experimental model to study biomineralization provides several practical advantages, and the results may be extrapolated to understand similar processes in other tissues, primarily bone. After describing dentin structure and composition, this review discusses items such as the morphology of dentinogenesis; the dentinogenically active odontoblast, transport, and concentrations of mineral ions; the constituents of the dentin organic matrix; and the presumed mechanisms involved in mineral formation.

Calcium ion transport kinetics during dentinogenesis: effects of disrupting odontoblast cellular transport systems

Due to strongly discrepant results in the literature, controversy exists about the timing of the transport of Ca2+ ions to the mineralization front during dentinogenesis and the role of the odontoblasts in this transport. The present study gives evidence, by means of autoradiography as well as by a radiochemical technique, that the transport time for Ca2+ ions into the dentin mineral phase is about 10-15 min in the rat incisor. The results also show that technical factors, such as mode of tracer injection and the use of perfusion fixation, may influence the results more or less strongly. Finally, by disturbing odontoblast microtubules, involved in intracellular transport processes, and by blocking odontoblast calcium uptake channels by nifedipine and neomycin, the Ca2+ ion transport into dentin mineral was found to be strongly impaired. This may be taken as an indication that transcellular calcium transport mechanisms have a role during dentinogenesis.

Calcium ion activity and pH in the odontoblast-predentin region: ion-selective microelectrode measurements

Ca2+ ion activities and pH were measured in the odontoblast/predentin region of rat incisors by means of the microelectrode technique. In Ringer solution, the apparent resting membrane potential of odontoblasts was determined to be -24 +/- 4 mV (mean +/- SE), whereas the odontoblast intracellular pH was found to be 6.66 +/- 0.02. The values obtained are within the range of other cell types, as measured in similar incubating solutions. The pH in the extracellular predentin was higher than the intracellular pH, 7.00 +/- 0.02. The Ca2+ ion activity in predentin (pCa = 2.94 +/- 0.15) was found to be significantly (P less than 0.001) higher than that in the dental pulp extracellular fluid (pCa = 3.37 +/- 0.14). The 2-3 times higher calcium activity extracellularly in predentin, compared with the dental pulp, implies the existence of some ion-concentrating mechanism across the odontoblast layer in the direction of the mineralization front.

Calcium transport during mineralization

The translocation of calcium from the extracellular fluid compartment into the mineralizing matrix during hard tissue formation is not well understood. There are two general means by which such calcium movement may occur: 1) diffusion through the pericellular space, or 2) transcellular transport. Cementum and bone are difficult tissues in which to study the system and little is known about the mechanisms involved. Dentin offers certain advantages for study and it appears that calcium movement into the mineralizing matrix is by transcellular transport. Information concerning the transport mechanism is meager. Enamel is more easily explored. The apparent existence of intercellular junctions tight to calcium in the ameloblast layer at all stages of enamel formation indicates that calcium movement occurs by transcellular transport. Based on published findings, a hypothesis concerning mechanisms of transcellular transport may be advanced. It is proposed that the relatively low level of calcium transport through secretory ameloblasts occurs without direct involvement of a calcium binding protein. During the maturation stage, when calcium influx to the matrix is greatly increased, a calcium binding protein (9 kd) appears and facilitates transport while preventing unphysiologic increases in the cytosolic free calcium ion concentration. Differences in the calcium ion concentrations of extracellular fluid and enamel matrix fluid appear to be critical to the influx of calcium across the proximal cell membrane and the efflux of calcium across the distal cell membrane.

Na+/Ca2+ antiports in membranes of rat incisor odontoblasts

When incubated in the presence of Na+ ions, dissected rat incisor odontoblasts extruded Ca2+ ions, as detected by tetracycline fluorimetry. Also odontoblast mitochondria, obtained by subcellular fractionation and monitored by ion-specific mini-electrode technique, were found to extrude Ca2+ upon addition of Na+ ions. These findings demonstrate the presence of Na+/Ca2+ antiports in the odontoblast plasma membrane as well as in the odontoblast mitochondrial inner membrane. In addition, evidence was found for an ATP-dependent Ca2+ ion extrusion across the plasma membrane in these cells.

Effects of B-aminopropionitrile on mineralization during endochondral ossification in chick tibia

Two-week-old white leghorn chicks were fed a diet containing BAPN (0.05%) for three weeks. Thirty-six hours before sacrifice, the controls and BAPN fed chicks were dosed with 35S. The zone of provisional calcification was isolated, and 35S incorporation was estimated by liquid scintillation counting. Alkaline phosphatase and Ca+2-ATPase were biochemically analyzed. Microdensitometry, to assess the level of mineralization, was done on epiphysis and the metaphysis. Morphometry was performed on the various zones of growth plate. 35S incorporation was significantly lower in the bones of BAPN treated chicks as compared to the controls. The enzymatic studies showed a significant inhibition of alkaline phosphatase and Ca+2-ATPase. The microdensitometric studies showed a smaller area of highly mineralized bone in the zones of provisional calcification of the BAPN treated chicks as compared to the controls. Morphometry showed a reduction in the width of the zone of calcification in BAPN treated chicks as compared to the controls. On the basis of the above data, it is suggested that BAPN induced inhibition of mineralization during endochondral ossification may be the result of a lower synthesis of sulfur containing GAG's, the inhibition of enzymes alkaline phosphatase and Ca+2-ATPase and the derangement of cellular zones of the growth plate. The implications of these results lie in the fact that mineralization is dependent on multifactorial control of the microenvironment of bone and cartilage.

Regulation of free Ca2+ by subcellular fractions of rat incisor odontoblasts

A miniaturized Ca2+ electrode system was developed to monitor small and fast fluctuations of Ca2+ activity in the micromolar range in 100 microliters volumes. This was used to study Ca2+ influx/efflux cycling in suspensions of rat-odontoblast and liver-cell mitochondria and microsomes, as well as in whole odontoblasts with plasma membranes made permeable by digitonin. The steady-state free-Ca2+ activity maintained by mitochondria was pCa 6.2-6.4, and that of microsomes pCa 6.4-6.6. These levels were held upon repeated additions of Ca2+ and EGTA. The odontoblast mitochondria and microsomes had an intracellular Ca2+ buffering capacity similar to that of liver cells. The steady-state pCa level maintained in suspensions of digitonin-permeabilized whole odontoblasts was 6.4-6.6. Thus, this study gave no evidence for any specialized intracellular handling of Ca2+ in cells involved in mineralization.

The CaATPase activity of rat-incisor odontoblast vesicles

An homogenate of rat incisor odontoblasts had Ca2+ and Mg2+ ATPase activity and suitable storage conditions kept it stable for several days. Over 90 per cent of the activity was retained in a vesicle-rich microsomal fraction that removed about 85 per cent of the total material from the homogenate. This fraction was further characterized: the resolved Ca2+-activated ATPase activity, above the basal MgATPase activity, was 0.30 mumol Pi/min-mg total protein, and 50 per cent activated at free [Ca2+] equal 0.8 microM. This calcium dependency is consistent with an intracellular Ca2+-regulated enzymatic activity. The calcium ionophore, A23187, had no measurable effect on the CaATPase activity, which suggests that the odontoblast vesicles do not concentrate Ca2+ in a lipid bilayer compartment. Direct measurement of the uptake of 45Ca2+ by the filtration method and parallel measurements of CaATPase activity on the same preparations under identical conditions indicated that the odontoblast-derived vesicles have a coupling ratio of 0.024 Ca2+/ATP. This low coupling ratio and the lack of detectable compartmentalization of calcium indicate that the CaATPase activity of the odontoblast microsomes is not associated with a calcium pump. The [Ca2+] dependence of the activity suggests the CaATPase is under intracellular Ca2+ control, but its function is unknown.

Alkaline phosphatase activity and tetracycline incorporation during initial orthodontic tooth movement in rats

The activity of alkaline phosphatase and the incorporation of tetracycline as signs of bone formation were studied after orthodontic tooth movement for 10 h to 6 days in rats. Defined low or high forces were used. A moderate activity of non-specific alkaline phosphatase was found in the periodontal membrane (PDM) in untreated rats and in rats treated with low forces. In addition, all bone surfaces were outlined with a narrow band of intense non-specific alkaline phosphatase activity that was vanadate- and levamisole-resistant. Likewise, tetracycline was incorporated on all bone surfaces. The bone formation rate was low and uniform within the alveolus, indicating that no intra-alveolar drift of the molar occurred in the untreated rats. Orthodontic forces gradually inhibited vanadate- and levamisole-resistant alkaline phosphatases and tetracycline incorporation on the bone surfaces in the pressure zones of the PDM, depending on the magnitude of the force. It was suggested that the disappearance of these isoenzymes, in a limited area, as seen in the pressure zones, was associated with inhibited bone formation and subsequent initiation of bone resorption. On the tension side a slight reduction and redistribution of vanadate- and levamisole-resistant alkaline phosphatase activity could be noted irrespective of the magnitude of the applied force.

Further evidence of an intravesicular Ca2+- pump in odontoblasts from rat incisors

Intracellular vesicles containing alkaline phosphatases were isolated from isolated odontoblasts using several centrifugation techniques, gradient media and filtering procedures. With a combined centrifugation technique using 0.32 M sucrose layered on 1.23 M sucrose, a fraction containing alkaline phosphatases with a 36-40-fold increased specific activity was obtained. This fraction also revealed a high Ca2+-accumulating ability. The vesicle fraction was totally free from mitochondria but to some extent contaminated by lysosomes. Characteristics of Ca2+-uptake were obtained. The Ca2+-uptake was maximal at 37-40 degrees C whereas no Ca2+-accumulated at 4 degrees C. Temperatures above 40 degrees C strongly inhibited Ca2+-uptake. ATP was the most potent stimulator of Ca2+-uptake whereas ITP, GTP, CTP, ADP, PPi and AMP also promoted Ca2+-uptake. Cysteine, EDTA and Triton X-100 were inhibitory to Ca2+-uptake. A correlation between alkaline phosphatases in intracellular vesicles as well as their relation to extracellular matrix vesicles and to the mineralization process is suggested.

Ca2+- and Mg2+-activated ATP hydrolysis in normal and carious human dentine

Ca2+- and Mg2+-activated ATP hydrolysis was measured in various layers of hard and soft carious dentine of freeze-dried teeth. Developing, intact, fully-formed, carious and gangrenous teeth were studied. Ten milligramme samples of dentine were analysed for enzyme activity by applying a bioluminescence method for measurement of ATP. The highest ATP-hydrolysis in hard dentine was observed in the coronal circumpulpal areas of developing teeth (14.2 mumol/min X mg protein X 10(-5]. In fully-formed teeth, the corresponding values were lower (6.62 mumol/min X mg protein X 10(-5]. Ca2+-activated ATP hydrolysis gave higher values than Mg2+-activated ATP hydrolysis. In coronal circumpulpal areas, enzyme activity was higher than in apical circumpulpal areas. The highest rate of hydrolysis in soft carious dentine was 4.96 mumol/min X mg protein X 10(-5).

Influence of bivalent cations, phosphate and complexing substances on inorganic pyrophosphate in the microsomal fraction of isolated rat odontoblasts

The influence of three diphosphonates, ethane-1-hydroxy 1.1. diphosphonate, methane-diphosphonate and dichloromethane diphosphonate, on inorganic pyrophosphatase (PPiase, E.C. 3.6.1.1.) was investigated. The inhibition by the diphosphonates appeared to be due to their complexation with a bivalent cation, probably Zn2+, which acts as a cofactor for the enzyme. The influence of calcium and inorganic phosphate (Pi) on the PPiase activity was studied in the presence and absence of several complexing substances, including the diphosphonates. Ca2+ alone inhibited the enzyme whereas, in the presence of EDTA, a Ca2+ stimulation of the PPiase was observed which was substrate-dependent and had an optimal activity at a Ca2+:PP4-i ratio of 1:1. There was a small inhibition of the PPiase activity by Pi which was not influenced by the substrate used.

A biochemical and morphological investigation of alkaline phosphatase and Ca+2-ATPase during initial mineralization in chick embryonic tibia

Biochemical analysis of the enzymes alkaline phosphatase and Ca+2-ATPase in the membrane fraction obtained from chick embryonic tibias showed a positive correlation between the elevated activity of these two enzymes and the onset of mineralization. Histochemistry further showed an increased intensity of alkaline phosphatase at the time of the onset of mineralization. It seems that both of these enzymes are involved in the process of mineralization.

Alkaline phosphatases in endochondral ossification of rats low in calcium and vitamin D deficient

Young rats fed a low calcium and vitamin D deficient diet for 2 weeks developed hypocalcemia and increased alkaline phosphatase activity in serum. The serum alkaline phosphatase activity (pNPPase) was found to be of skeletal origin. In accordance, the total non-specific alkaline phosphatase (pNNPase) activity in the microsomal fraction of tibial epiphyseal cartilage and metaphysis increased in the deficiently fed group when compared to the normal group. An increased activity in the microsomal fraction of tibial epiphyseal cartilage and metaphysis was shown both for inorganic pyrophosphatase and total ATP-degrading enzyme activity in the deficient group. This was also found in the presence of R 8231, indicating an increased activity of Ca2+-ATPase, shown to be present in both the epiphyseal plate and the metaphysis. These increased enzyme activities were consistent with the known effects of hypocalcemia and/or parathyroid hormone (PTH) on bone alkaline phosphatase activity. The increase in Ca2+-ATPase might, however, be a direct response to the hypocalcemia present in the deficient animals. Furthermore, the findings in the present study support the view that the same alkaline phosphatase iso-enzyme is present at different calcification loci.

Properties of alkaline phosphatases from cellular cementum of rat molars

The characteristics of nonspecific alkaline phosphatase, (APase, EC 3.1.3.1.) measured as beta-glycerophosphatase (GPase, EC 3.1.3.1.), inorganic pyrophosphatase (PPiase, EC 3.6.1.1.) and adenosine triphosphatase (ATPase, EC 3.6.1.3.) were studied in detail of butanol extracts prepared from rat molar cementum. Mg2+ was not absolutely essential to any of the activities, but at low levels was stimulatory in all cases. Higher concentrations were inhibitory. Ca2+ stimulated ATPase activity weakly at low levels, but was slightly inhibitory to the other enzyme activities. All enzyme activities showed nearly identical sensitivities to heat inactivation and to L-p-bromotetramisole and levamisole, which caused nearly complete inhibition. About 10-15% of the ATPase activity was insensitive to L-p-bromotetramisole and levamisole. The data are consistent with the concept that GPase, PPiase and ATPase activities of cementum to a major part stem from one enzyme, namely nonspecific alkaline phosphatase.