Tetrodotoxin-sensitive fast Na+ current in embryonic chicken osteoclasts

Vasa recta voltage-gated Na+ channel Nav1.3 is regulated by calmodulin

Rat descending vasa recta (DVR) express a tetrodotoxin (TTX)-sensitive voltage-operated Na(+) (Na(V)) conductance. We examined expression of Na(V) isoforms in DVR and tested for regulation of Na(V) currents by calmodulin (CaM). RT-PCR in isolated permeabilized DVR using degenerate primers targeted to TTX-sensitive isoforms amplified a product whose sequence identified only Na(V)1.3. Immunoblot of outer medullary homogenate verified Na(V)1.3 expression, and fluorescent immunochemistry showed Na(V)1.3 expression in isolated vessels. Immunochemistry in outer medullary serial sections confirmed that Na(V)1.3 is confined to alpha-smooth muscle actin-positive vascular bundles. Na(V)1.3 possesses a COOH-terminal CaM binding motifs. Using pull-down assays and immunoprecipitation experiments, we verified that CaM binds to either full-length Na(V)1.3 or a GST-Na(V)1.3 COOH-terminal fusion protein. In patch-clamp experiments, Na(V) currents were suppressed by calmodulin inhibitory peptide (CIP; 100 nM) or the CaM inhibitor N-(6-aminohexyl)-5-chloro-1-naphthalene-sulphonamide hydrochloride (W7). Neither CIP nor W7 altered the voltage dependence of pericyte Na(V) currents; however, raising electrode free Ca(2+) from 20 to approximately 2,000 nM produced a depolarizing shift of activation. In vitro binding of CaM to GST-Na(V)1.3C was not affected by Ca(2+) concentration. We conclude that Na(V)1.3 is expressed by DVR, binds to CaM, and is regulated by CaM and Ca(2+). Inhibition of CaM binding suppresses pericyte Na(V) currents.

A slow outward current and a hypoosmolality induced anion conductance in embryonic chicken osteoclasts

In this paper we report on a hypoosmolality induced current, I(osmo), in embryonic chicken osteoclasts, which could only be studied when blocking a simultaneously active, unidentified slow outward current, I(slo). I(slo) was observed in all of the examined cells when both the intracellular and extracellular solutions contained sodium as the major cation and no potassium. The current was outwardly rectifying and activated at membrane potentials more positive than -44 +/- 12 mV (n = 31). The time to half activation of the current was also voltage dependent and was 350 ms at Vm = +80 mV, and 78 ms at Vm = +120 mV. The current did not inactivate during periods up to 5 s. Extracellular 4-AP (5 mM), TEA (5 mM) and Ba2+ (1 mM), blockers of K+ conductances in chicken osteoclasts, did not influence I(slo). However, I(slo) was inhibited by 50 microM extracellular verapamil, which allowed us to study I(osmo) in isolation. Exposure of the osteoclasts to hypotonic solution resulted in the development of a depolarization activated I(osmo). It developed after a 1-min delay and reached its maximum within 10 minutes. Half-maximal activation occurred after 4.4 +/- 0.9 min (n = 9). The current activated within a few ms upon depolarization and did not inactivate during at least 5 sec. I(osmo) reversed around the calculated Nernst potential for Cl- (E(Cl) = +7.3 mV and V(rev) = +5.4 +/- 3.6 mV, n = 9). The underlying conductance, G(osmo) exhibited moderate outward rectification around 0 mV in symmetrical Cl- solutions. Ion substitution experiments showed that G(osmo) is an anion conductance with P(Cl) approximately = P(F) > P(gluc) >> P(Na). I(osmo) was blocked by 0.5 mM SITS but 50 microM verapamil, 5 mM TEA, 5 mM 4-AP, 1 mM Ba2+, 50 microM cytochalasin D and 0.5 mM alendronate did not have any effect on the current. Cl- currents have been implicated in charge neutralization during osteoclastic acid secretion for bone resorption. The present results imply that osmolality may be a factor controlling this charge neutralization.

Electrophysiological characterization of ion channels in osteoclasts isolated from human deciduous teeth

Ion channels contribute to several important processes in osteoclasts, including proton transport and volume regulation. Although ion channels have been described in osteoclasts from several species, little is known about their properties in human osteoclasts. We devised a method for isolation of authentic human osteoclasts from deciduous teeth undergoing root resorption, and characterized currents in these cells using patch-clamp techniques. Three types of K(+) current were identified. Hyperpolarization elicited an inwardly rectifying K(+) current in most osteoclasts, which was inhibited by Ba(2+) in a voltage- and time-dependent manner. Depolarization elicited an outwardly rectifying and tetraethylammonium-sensitive current, consistent with a large-conductance Ca(2+)-dependent K(+) channel. In addition to these basal currents, extracellular adenosine 5'-triphosphate (ATP) elicited a linear current that was identified as a Ca(2+)-dependent K(+) current, based on its reversal potential close to that predicted for K(+), its blockade by quinine, and its activation by Ca(2+) ionophore. Last, an outwardly rectifying current was observed to activate spontaneously or in response to ATP, with properties of a swelling-activated Cl(-) current. This current reversed direction close to the Cl(-) equilibrium potential and was blocked by the anion channel blocker, niflumic acid, identifying it as a Cl(-) current. In summary, we have developed a novel method for isolation of authentic human osteoclasts and have characterized K(+) and Cl(-) currents. Cl(-) current mediates charge compensation during electrogenic H(+) transport, so activation of Cl(-) current may contribute to the stimulatory effects of extracellular ATP on bone resorption.

Voltage-gated sodium channels in cardiac microvascular endothelial cells

The goal of this study was to determine whether inward Na+ or Ca2+ currents could be measured in cardiac microvascular endothelial cells (CMEC). CMEC were isolated from rat ventricular muscle and studied during days 1-4 in culture. Differential uptake of fluorescently labeled acetylated low-density lipoproteins (LDL) indicated that the primary culture contained > 90% CMEC. Membrane currents were measured with the use of the whole cell arrangement of the patch-clamp technique with a Cs+ internal solution to prevent contamination by outward K+ currents. Voltage steps positive to -30 mV resulted in the activation of a fast, inward Na+ current (INa). In 20 cells examined, the peak inward current measured at 0 mV was 2.1 pA/pF. The half-maximal voltage required for inactivation of INa was -45 mV, and the current recovered from inactivation with a time constant of 10 ms. Inward currents were eliminated by replacement of external sodium with N-methylglucamine and were blocked by both tetrodotoxin (TTX) (dissociation constant = 5 nM) and saxitoxin (50 nM). Stimulation of protein kinase C, through application of phorbol 12,13-dibutyrate, resulted in an increase in the amplitude of INa without any change in the voltage dependence of current activation. Thus the endothelium of cardiac microvessels may be unique in expressing voltage gated, TTX-sensitive Na+ channels.