Multiple calcium currents in a thyroid C-cell line: biophysical properties and pharmacology

Regulation of AQP4 protein expression in rat brain astrocytes: role of P2X7 receptor activation

ATP has been recognized as an important extracellular signaling molecule and P2X receptors are membrane ion channels activated by the binding of extracellular ATP. Since both AQP4 and P2X7 receptor (P2X7R) are known to be present in astrocytes, we examined whether P2X7R activation plays a role in the regulation of AQP4 expression in astrocytes. Immunoblotting and immunocytochemistry confirmed the expression of both P2X7R and AQP4 in primary cultured rat astrocytes. Co-immunoprecipitation assays of the HEK293 cells expressing both proteins revealed no protein-protein interaction. An activation of P2X7R in primary cultured astrocytes by a P2X7R agonist significantly decreased the AQP4 protein expression, which was abolished by the pre-treatment of a P2X7R antagonist. In addition, AQP4 expression was not affected by high extracellular copper, zinc, or iron concentrations. In a rat model with anoxia-induced brain injury where extracellular ATP levels could be increased, whole brain AQP4 expression was significantly decreased, whereas P2X7R expression was unchanged. Importantly, pre-treatment of P2X7R antagonist in rats significantly inhibited the AQP4 down-regulation in anoxic brain injury, consistent with the in vitro results observed in astrocytes. In conclusion, P2X7R activation in astrocytes was associated with down-regulation of AQP4 in rat brain astrocytes in vitro and in vivo, and this was prevented by P2X7 receptor blockade. Thus, an activation of P2X7R in astrocytes in response to brain injury is likely to play a role in the protective down-regulation of AQP4, which might inhibit water influx to the cells and attenuate the acute cytotoxic brain edema after acute brain injury.

Availability of low-threshold Ca2+ current in retinal ganglion cells

Spiking in central neurons depends on the availability of inward and outward currents activated by depolarization and on the activation and priming of currents by hyperpolarization. Of these processes, priming by hyperpolarization is the least described. In the case of T-type Ca2+ current availability, the interplay of hyperpolarization and depolarization has been studied most completely in expression systems, in part because of the difficulty of pharmacologically separating the Ca2+ currents of native neurons. To facilitate understanding of this current under physiological conditions, we measured T-type current of isolated goldfish retinal ganglion cells with perforated-patch voltage-clamp methods in solutions containing a normal extracellular Ca2+ concentration. The voltage sensitivities and rates of current activation, inactivation, deactivation, and recovery from inactivation were similar to those of expressed alpha1G (CaV3.1) Ca2+ channel clones, except that the rate of deactivation was significantly faster. We reproduced the amplitude and kinetics of measured T currents with a numerical simulation based on a kinetic model developed for an alpha1G Ca2+ channel. Finally, we show that this model predicts the increase of T-type current made available between resting potential and spike threshold by repetitive hyperpolarizations presented at rates that are within the bandwidth of signals processed in situ by these neurons.

Molecular physiology of low-voltage-activated t-type calcium channels

T-type Ca2+ channels were originally called low-voltage-activated (LVA) channels because they can be activated by small depolarizations of the plasma membrane. In many neurons Ca2+ influx through LVA channels triggers low-threshold spikes, which in turn triggers a burst of action potentials mediated by Na+ channels. Burst firing is thought to play an important role in the synchronized activity of the thalamus observed in absence epilepsy, but may also underlie a wider range of thalamocortical dysrhythmias. In addition to a pacemaker role, Ca2+ entry via T-type channels can directly regulate intracellular Ca2+ concentrations, which is an important second messenger for a variety of cellular processes. Molecular cloning revealed the existence of three T-type channel genes. The deduced amino acid sequence shows a similar four-repeat structure to that found in high-voltage-activated (HVA) Ca2+ channels, and Na+ channels, indicating that they are evolutionarily related. Hence, the alpha1-subunits of T-type channels are now designated Cav3. Although mRNAs for all three Cav3 subtypes are expressed in brain, they vary in terms of their peripheral expression, with Cav3.2 showing the widest expression. The electrophysiological activities of recombinant Cav3 channels are very similar to native T-type currents and can be differentiated from HVA channels by their activation at lower voltages, faster inactivation, slower deactivation, and smaller conductance of Ba2+. The Cav3 subtypes can be differentiated by their kinetics and sensitivity to block by Ni2+. The goal of this review is to provide a comprehensive description of T-type currents, their distribution, regulation, pharmacology, and cloning.

High-voltage-activated calcium current in developing neurons is insensitive to nifedipine

We have analyzed the effect of nifedipine on the macroscopic high-threshold, voltage-activated (HVA) calcium current in four cell types: postnatal rat Purkinje and dorsal root ganglion (DRG) neurons, embryonic chick DRG neurons, and adult cat ventricular myocytes. As is consistent with previous reports, nifedipine reduced HVA current in myocytes in a voltage-sensitive manner. Analysis of nifedipine actions on neurons, however, was compromised by slow inactivation of the current at holding potentials between -80 mV and -40 mV. The slow inactivation was voltage-dependent, irreversible after 5 min, and contributed to "rundown" of the current. At -40 mV, slow inactivation displayed two time constants: 12 +/- 8 s and 7 +/- 4 min. When slow inactivation was taken into account, we found no evidence for a nifedipine-sensitive component of the HVA current in these neurons. Consistent with previous studies, DRG neurons were reduced irreversibly by omega-conotoxin, whereas cardiac and Purkinje cells were unaffected. Our biophysical and pharmacological results are consistent with two types of neuronal HVA currents (N type and P type) in developing neurons that are distinct from cardiac HVA currents (L type).

Expression of highly polysialylated neural cell adhesion molecule in calcitonin-producing cells

Calcitonin-producing cells are endocrine derivatives of the neural crest and have several neuron-like properties. Expression of the neural cell adhesion molecule in calcitonin-producing cells was examined using two types of antibodies to neural cell adhesion molecule: monoclonal antibody 12E3 recognizes the polysialic acid portion of highly polysialylated neural cell adhesion molecule, and monoclonal antibody AF11 and polyclonal antiserum react with the polypeptide portion common to three major isoforms of neural cell adhesion molecule. An immunohistochemical study revealed that highly polysialylated neural cell adhesion molecule was expressed both in fetal rat thyroidal calcitonin-producing cells and in a calcitonin-producing cell line, rMTC 6-23, established from explantable neoplasm of rat calcitonin-producing cells. The neural cell adhesion molecule in the rMTC 6-23 cells was further characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblot analysis. Two anti-neural cell adhesion molecule monoclonal antibodies, 12E3 and AF11, revealed a broad positive band around 200,000-250,000 mol. wt in solubilized proteins. When the polysialic acids were eliminated by neuraminidase treatment, the immunoreactivity to monoclonal antibody 12E3 was completely abolished, and core polypeptide corresponding to neural cell adhesion molecule with a molecular weight of 120,000 was detected by monoclonal antibody AF11. These results suggest that cells of the calcitonin-producing cell line express on their surfaces highly polysialylated 120,000 mol. wt form of neural cell adhesion molecule polypeptide.

Block of current through T-type calcium channels by trivalent metal cations and nickel in neural rat and human cells

1. The effects of the trivalent cations yttrium (Y3+), lanthanum (La3+), cerium (Ce3+), neodymium (Nd3+), gadolinium (Gd3+), holmium (Ho3+), erbium (Er3+), ytterbium (Yb3+) and the divalent cation nickel (Ni2+) on the T-type voltage gated calcium channel (VGCC) were characterized by the whole-cell patch clamp technique using rat and human thyroid C cell lines. 2. All the metal cations (M3+) studied, blocked current through T-type VGCC (IT) in a concentration-dependent manner. Smaller trivalents were the best T-channel antagonists and potency varied inversely with ionic radii for the larger M3+ ions. Estimation of half-maximal blocking concentrations (IC50s) for IT carried by 10 mM Ca2+ resulted in the following potency sequence: Ho3+ (IC50 = 0.107 microM) approximately Y3+ (0.117) approximately Yb3+ (0.124) > or = Er3+ (0.153) > Gd3+ (0.267) > Nd3+ (0.429) > Ce3+ (0.728) > La3+ (1.015) >> Ni2+ (5.65). 3. Tail current measurements and conditioning protocols were used to study the influence of membrane voltage on the potency of these antagonists. Block of IT by Ni2+, Y3+, La3+ and the lanthanides was voltage independent in the range from -200 to +80 mV. In addition, the antagonists did not affect macroscopic inactivation and deactivation of T-type VGCC. 4. Increasing the extracellular Ca2+ concentration reduced the potency of IT block by Ho3+, indicative of competitive antagonism between this blocker and the permeant ion for a binding site. 5. The results suggest that the mechanism of metal cation block of T-type VGCC is occlusion of the channel pore by the antagonist binding to a Ca2+/M3+ binding site, located out of the membrane electric field. 6. Block of T-type VGCC by Y3+, lanthanides and La3+ differ from the inhibition of high voltage-activated VGCC block in several respects: smaller cations are more potent IT antagonists; block is voltage independent and the antagonists do not permeate T-type channels. These differences suggest corresponding structural dissimilarities in the permeation pathways of low and high voltage-activated Ca2+ channels.

Molecular cloning of the alpha-1 subunit of an omega-conotoxin-sensitive calcium channel

Of the four major types of Ca channel described in vertebrate cells (designated T, L, N, and P), N-type Ca channels are unique in that they are found specifically in neurons, have been correlated with control of neurotransmitter release, and are blocked by omega-conotoxin, a neuropeptide toxin isolated from the marine snail Conus geographus. A set of overlapping cDNA clones were isolated and found to encode a Ca channel alpha-1 subunit, designated rbB-I. Polyclonal antiserum generated against a peptide from the rbB-I sequence selectively immunoprecipitates high-affinity 125I-labeled omega-conotoxin-binding sites from labeled rat forebrain membranes. PCR analysis shows that, like N-type Ca channels, expression of rbB-I is limited to the nervous system and neuronally derived cell lines. This brain Ca channel may mediate the omega-conotoxin-sensitive Ca influx required for neurotransmitter release at many synapses.