A small-conductance Ca(2+)-activated K+ current and Cl- current in rat dental pulp cells

Potassium Currents Activated by Depolarization in Odontoblasts

Increased intracellular free Ca²⁺ concentrations elicit plasma membrane depolarization, which leads to the activation of K⁺ currents. However, the precise properties of K⁺ currents activated by depolarization in odontoblasts remain to be elucidated. The present study identified biophysical and pharmacological characteristics of time-dependent and voltage-activated K⁺ currents in freshly dissociated rat odontoblasts using patch-clamp recordings in a whole-cell configuration. Using a holding potential of −70 mV, outwardly rectifying time- and voltage-dependent currents were activated by depolarizing voltage. To record pure K⁺ conductance, we substituted Cl⁻ in both the extracellular and intracellular solutions with gluconate⁻. Under these conditions, observation of K⁺ concentration changes in the extracellular solution showed that reversal potentials of tail currents shifted according to the K⁺ equilibrium potential. The activation kinetics of outward K⁺ currents were relatively slow and depended on the membrane potential. Kinetics of steady-state inactivation were fitted by a Boltzmann function. The half-maximal inactivation potential was −38 mV. Tetraethylammonium chloride, 4-aminopyridine, and α-dendrotoxin inhibited outward currents in odontoblasts in a concentration-dependent manner, suggesting that rat odontoblasts express the α-subunit of the time- and voltage-dependent K⁺ channel (Kv) subtypes Kv1.1, 1.2, and/or 1.6. We further examined the effects of Kv activity on mineralization by alizarin red and von Kossa staining. Continuous application of tetraethylammonium chloride to human odontoblasts grown in a mineralization medium over a 21-day period exhibited a dose-dependent decrease in mineralization efficiency compared to cells without tetraethylammonium chloride. This suggests that odontoblasts functionally express voltage-dependent K⁺ channels that play important roles in dentin formation.

Intercellular signal communication among odontoblasts and trigeminal ganglion neurons via glutamate

Various stimuli to the exposed surface of dentin induce changes in the hydrodynamic force inside the dentinal tubules resulting in dentinal pain. Recent evidences indicate that mechano-sensor channels, such as the transient receptor potential channels, in odontoblasts receive these hydrodynamic forces and trigger the release of ATP to the pulpal neurons, to generate dentinal pain. A recent study, however, has shown that odontoblasts also express glutamate receptors (GluRs). This implies that cells in the dental pulp tissue have the ability to release glutamate, which acts as a functional intercellular mediator to establish inter-odontoblast and odontoblast-trigeminal ganglion (TG) neuron signal communication. To investigate the intercellular signal communication, we applied mechanical stimulation to odontoblasts and measured the intracellular free Ca²⁺ concentration ([Ca²⁺]_i). During mechanical stimulation in the presence of extracellular Ca²⁺, we observed a transient [Ca²⁺]_i increase not only in single stimulated odontoblasts, but also in adjacent odontoblasts. We could not observe these responses in the absence of extracellular Ca²⁺. [Ca²⁺]_i increases in the neighboring odontoblasts during mechanical stimulation of single odontoblasts were inhibited by antagonists of metabotropic glutamate receptors (mGluRs) as well as glutamate-permeable anion channels. In the odontoblast-TG neuron coculture, we observed an increase in [Ca²⁺]_i in the stimulated odontoblasts and TG neurons, in response to direct mechanical stimulation of single odontoblasts. These [Ca²⁺]_i increases in the neighboring TG neurons were inhibited by antagonists for mGluRs. The [Ca²⁺]_i increases in the stimulated odontoblasts were also inhibited by mGluRs antagonists. We further confirmed that the odontoblasts express group I, II, and III mGluRs. However, we could not record any currents evoked from odontoblasts near the mechanically stimulated odontoblast, with or without extracellular Mg²⁺, indicating that N-methyl-d-aspartic acid receptor does not contribute to inter-odontoblast signal communication. The results suggest that a mechanically stimulated odontoblast is capable of releasing glutamate into the extracellular space via glutamate-permeable anion channels. The released glutamate activates mGluRs on the odontoblasts in an autocrine/paracrine manner, forming an inter-odontoblasts communication, which drives dentin formation via odontoblast-odontoblast signal communication. Glutamate and mGluRs also mediate neurotransmission between the odontoblasts and neurons in the dental pulp to modulate sensory signal transmission for dentinal sensitivity.

Sodium-calcium exchangers in rat ameloblasts

Although the central role of ameloblasts in synthesis and resorption of enamel matrix proteins during amelogenesis is well documented, the Ca(2+)-transport/extrusion mechanism remains to be fully elucidated. To clarify Ca(2+)-transport in rat ameloblasts, we investigated expression and localization of Na(+)-Ca(2+) exchanger (NCX) isoforms and the functional characteristics of their ion transporting/pharmacological properties. RT-PCR and immunohistochemical analyses revealed expression of NCX1 and NCX3 in ameloblasts, localized in the apical membrane. In patch-clamp recordings, Ca(2+) efflux by Na(+)-Ca(2+) exchange showed dependence on external Na(+). Ca(2+) influx by Na(+)-Ca(2+) exchange, measured by fura-2 fluorescence, showed dependence on extracellular Ca(2+) concentration, and it was blocked by NCX inhibitors KB-R7943, SEA0400, and SN-6. These results showed significant expression of NCX1 and NCX3 in ameloblasts, indicating their involvement in the directional Ca(2+) extrusion pathway from cells to the enamel mineralizing front.

Rapid penetration of lucifer yellow into vital teeth and dye coupling between odontoblasts and neighbouring pulp cells in the cat

The location and long process of odontoblasts appear well suited to detection of external stimuli. The odontoblasts may transmit the information not individually but as a syncytium via gap junction, which functions as a mechanism for intercellular linking cells and as the route for dye coupling. The aims of the present study were to examine dye penetration through enamel and dentin, and to confirm dye coupling between odontoblasts (OBs) or between odontoblasts and other pulpal cells beneath the odontoblastic layer (PCs). Either lucifer yellow (LY) or borate buffer (control) was applied to etched enamel surface of feline canines for 30 min at atmospheric pressure. In the decalcified sections, lucifer yellow positive cells were found not only in but also beneath the odontoblastic layer (experiment 1). In the isolated pulp cells, all OBs (27/27) and some PCs (6/9) that were immunocytochemically differentiated using two monoclonal antibodies were labelled with LY (experiment 2). These results indicate the remarkably quick movement of LYE through enamel and dentin into the superficial pulp. In experiment 3, fresh OBs and PCs were isolated from feline canines to which LY had not been applied. LY was iontophoretically injected into an OB-like cell that had an oval cell body and a long monopolar process. Some PCs and OBs identified immunocytochemically were labelled with LY, with the exception of a few LY-negative cells. These findings indicate that dye coupling exists not only between OBs but also between OBs and PCs. Thus, the coupling provides evidence for a functional link via which information is transmitted between OBs and PCs.