Bicarbonate dependence of ion current in damaged bone

Calcium fluxes at the bone/plasma interface: Acute effects of parathyroid hormone (PTH) and targeted deletion of PTH/PTH-related peptide (PTHrP) receptor in the osteocytes

Calcium ion concentration ([Ca2+]) in the systemic extracellular fluid, ECF-[Ca2+], is maintained around a genetically predetermined set-point, which combines the operational level of the kidney and bone/ECF interfaces. The ECF-[Ca2+] is maintained within a narrow oscillation range by the regulatory action of Parathyroid Hormone (PTH), Calcitonin, FGF-23, and 1,25(OH)2D3. This model implies two correction mechanisms, i.e. tubular Ca2+ reabsorption and osteoclast Ca2+ resorption. Although their alterations have an effect on the ECF-[Ca2+] maintenance, they cannot fully account for rapid correction of the continuing perturbations of plasma [Ca2+], which occur daily in life. The existence of Ca2+ fluxes at quiescent bone surfaces fulfills the role of a short-term error correction mechanism in Ca2+ homeostasis. To explore the hypothesis that PTH regulates the cell system responsible for the fast Ca2+ fluxes at the bone/ECF interface, we have performed direct real-time measurements of Ca2+ fluxes at the surface of ex-vivo metatarsal bones maintained in physiological conditions mimicking ECF, and exposed to PTH. To further characterize whether the PTH receptor on osteocytes is a critical component of the minute-to-minute ECF-[Ca2+] regulation, metatarsal bones from mice lacking the PTH receptor in these cells were tested ex vivo for rapid Ca2+ exchange. We performed direct real-time measurements of Ca2+ fluxes and concentration gradients by a scanning ion-selective electrode technique (SIET). To validate ex vivo measurements, we also evaluated acute calcemic response to PTH in vivo in mice lacking PTH receptors in osteocytes vs littermate controls. Our data demonstrated that Ca2+ fluxes at the bone-ECF interface in excised bones as well as acute calcemic response in the short-term were unaffected by PTH exposure and its signaling through its receptor in osteocytes. Rapid minute-to-minute regulation of the ECF-[Ca2+] was found to be independent of PTH actions on osteocytes. Similarly, mice lacking PTH receptor in osteocytes, responded to PTH challenge with similar calcemic increases.

Elucidating the Role of Injury-Induced Electric Fields (EFs) in Regulating the Astrocytic Response to Injury in the Mammalian Central Nervous System

Injury to the vertebrate central nervous system (CNS) induces astrocytes to change their morphology, to increase their rate of proliferation, and to display directional migration to the injury site, all to facilitate repair. These astrocytic responses to injury occur in a clear temporal sequence and, by their intensity and duration, can have both beneficial and detrimental effects on the repair of damaged CNS tissue. Studies on highly regenerative tissues in non-mammalian vertebrates have demonstrated that the intensity of direct-current extracellular electric fields (EFs) at the injury site, which are 50-100 fold greater than in uninjured tissue, represent a potent signal to drive tissue repair. In contrast, a 10-fold EF increase has been measured in many injured mammalian tissues where limited regeneration occurs. As the astrocytic response to CNS injury is crucial to the reparative outcome, we exposed purified rat cortical astrocytes to EF intensities associated with intact and injured mammalian tissues, as well as to those EF intensities measured in regenerating non-mammalian vertebrate tissues, to determine whether EFs may contribute to the astrocytic injury response. Astrocytes exposed to EF intensities associated with uninjured tissue showed little change in their cellular behavior. However, astrocytes exposed to EF intensities associated with injured tissue showed a dramatic increase in migration and proliferation. At EF intensities associated with regenerating non-mammalian vertebrate tissues, these cellular responses were even more robust and included morphological changes consistent with a regenerative phenotype. These findings suggest that endogenous EFs may be a crucial signal for regulating the astrocytic response to injury and that their manipulation may be a novel target for facilitating CNS repair.

Electrical stimulation therapies for spinal fusions: current concepts

Electrical stimulation therapies have been used for more than 30 years to enhance spinal fusions. Although their positive effects on spinal fusions have been widely reported, the mechanisms of action of the technologies were only recently identified. Three types of technologies are available clinically: direct current, capacitive coupling, and inductive coupling. The latter is the basis of pulsed electromagnetic fields and combined magnetic fields. This review summarizes the current concepts on the mechanisms of action, animal and clinical studies, and cost justification for the use of electrical stimulation for spinal fusions. Scientific studies support the validity of electrical stimulation treatments. The mechanisms of action of each of the three electrical stimulation therapies are different. New data demonstrates that the upregulation of several growth factors may be responsible for the clinical success seen with the use of such technologies.

Bone as an ion exchange organ: evidence for instantaneous cell-dependent calcium efflux from bone not due to resorption

The current study tests the hypothesis that basal level and minute-by-minute correction of plasma Ca2+ by outward and inward Ca2+ fluxes from and into an exchangeable ionic pool in bone is controlled by an active partition system without contributions from the bone remodeling system. Direct real-time measurements of Ca2+ fluxes were made using the scanning ion-selective electrode technique (SIET) on living bones maintained ex vivo in physiological conditions. SIET three-dimensional measurements of the local Ca2+ concentration gradient (10 microm spatial resolution) were performed on metatarsal bones of weanling mice after drilling a 100-mum hole through the cortex to expose the internal bone extracellular fluid (BECF) to the bathing solution, whose composition mimicked the extracellular fluid (ECF). Influxes of Ca2+ towards the center of the cortical hole (15.1+/-4.2 pmol cm-2 s-1) were found in the ECF and were reversed to effluxes (7.4+/-2.9 pmol cm-2 s-1) when calcium was depleted from the ECF, mimicking a plasma demand. The reversal from influx to efflux and vice versa was immediate and fluxes in both directions were steady throughout the experimental time (>or=2 h, n=14). Only the efflux was nullified within 10 min by the addition of 10 mM/L Na-Cyanide (n=7), demonstrating its cell dependence. The timeframes of the exchanges and the stability of the Ca2+ fluxes over time suggest the existence of an exchangeable calcium pool in bone. The calcium efflux dependency on viable cells suggests that an active partition system might play a central role in the short-term error correction of plasma calcium without the contribution of bone remodeling.

Bone as an ion exchange system: evidence for a link between mechanotransduction and metabolic needs

To detect whether the mutual interaction occurring between the osteocytes-bone lining cells system (OBLCS) and the bone extracellular fluid (BECF) is affected by load through a modification of the BECF-extracellular fluid (ECF; systemic extracellular fluid) gradient, mice metatarsal bones immersed in ECF were subjected ex vivo to a 2-min cyclic axial load of different amplitudes and frequencies. The electric (ionic) currents at the bone surface were measured by a vibrating probe after having exposed BECF to ECF through a transcortical hole. The application of different loads and different frequencies increased the ionic current in a dose-dependent manner. The postload current density subsequently decayed following an exponential pattern. Postload increment's amplitude and decay were dependent on bone viability. Dummy and static loads did not induce current density modifications. Because BECF is perturbed by loading, it is conceivable that OBLCS tends to restore BECF preload conditions by controlling ion fluxes at the bone-plasma interface to fulfill metabolic needs. Because the electric current reflects the integrated activity of OBLCS, its evaluation in transgenic mice engineered to possess genetic lesions in channels or matrix constituents could be helpful in the characterization of the mechanical and metabolic functions of bone.

Osteogenesis from cultured chick periostea has a specific requirement for chloride

Bone development, like embryonic development in general, depends on a particular internal electrical milieu. Ions are the carriers of currents that maintain this internal environment. In embryonic bone, chloride is a major carrier of such current. To explore the role chloride plays in embryonic bone development we performed several ion-removal experiments, using the chick periosteal osteogenesis (CPO) system as our model. We found that if chloride is reduced in the medium and replaced with a nontoxic anion, alkaline phosphatase (ALP) activity does not rise, nor does osteogenic development occur. However, acid phosphatase (AP) activity is not affected by level of chloride. Experiments using metabolic inhibitors showed that explants cultured in low chloride medium remain viable. Dose-response studies revealed that the response of ALP activity to chloride concentration is sigmoidal, with a [Cl-]0.5 of 45.9 mM. Reciprocal transfers of explants between complete and low chloride medium show that the rise in ALP activity depends on the length of time explants are cultured with chloride. In contrast, such transfer experiments show that osteogenesis requires chloride only during days 2-3 of culture.

Bone as an ion exchange system: evidence for a pump-leak mechanism devoted to the maintenance of high bone K(+)

To provide evidence of active accumulation of K(+) in bone extracellular fluid (BECF), electric currents driven by damaged living metatarsal bones of weanling mice, immersed in physiological media at different [K(+)], in the presence of blockers of the K(+) channels or of the Na(+)-K(+-)ATPase inhibitor, were measured by means of a voltage-sensitive two-dimensional vibrating probe. At 4 mM extracellular K(+) concentration ([K(+)](o)), an inward steady current density (7.85-38.53 microA/cm(2)) was recorded at the damage site, which was significantly dependent on [K(+)](o). At [K(+)](o) equal to that of BECF (25 mM), current density was reduced by 76%. At [K(+)](o) of 0 mM, the current density showed an increase, which was hindered by tetraethylammonium (TEA). Basal current density was reduced significantly after exposure to TEA or BaCl(2) and was unchanged after long- term exposure to ouabain. By changing control medium with a chloride-free medium, current density was reversed. The results support the view that K(+) excess in bone is maintained by a biologically active cellular system. Because the osteocyte-bone lining cell syncytium was at the origin of the current in bone, it is likely that this system controls the ionic composition of BECF.

Effect of an oral calcium load on urinary markers of collagen breakdown

Aim of this study was to investigate whether osteoclast activity changes as a consequence of even mild physiological perturbation of plasma calcium as such induced by an oral calcium load. Osteoclast activity was determined indirectly by measuring, in spot urines at two and four hours after oral calcium load, the urinary excretion of hydroxylysylpyridinoline (Pyr), deoxylysylpyridinoline (D-Pyr), hydroxyproline (Hyp) and galactosyl-hydroxylysine (GHyl). The occurrence of the metabolic perturbation of plasma calcium homeostasis was assessed by measuring three indexes: i.e. calcemic response, PTH reduction and calciuric response at times following oral calcium loading. A significant fall of urinary D-Pyr and Pyr followed the perturbation of calcium homeostasis induced by the oral calcium load in two groups of healthy young adult and postmenopausal women. The highest mean percent reduction was observed for D-Pyr and was quantitatively similar in the two groups. Since urinary D-Pyr is the most specific bone resorption marker, it may be inferred that the perturbation of plasma calcium homeostasis induced by an oral calcium load is able to acutely inhibit osteoclast activity. This supports the view that osteoclasts are involved in the short-term error correction of plasma calcium.