Inhibition of T-type calcium currents by dihydropyridines in mouse embryonic dorsal root ganglion neurons

Modulators of calcium influx regulate membrane excitability in rat dorsal root ganglion neurons

BACKGROUND

Chronic neuropathic pain resulting from neuronal damage remains difficult to treat, in part, because of incomplete understanding of underlying cellular mechanisms. We have previously shown that inward Ca2+ flux (I(Ca)) across the sensory neuron plasmalemma is decreased in a rodent model of chronic neuropathic pain, but the direct consequence of this loss of I(Ca) on function of the sensory neuron has not been defined. We therefore examined the extent to which altered membrane properties after nerve injury, especially increased excitability that may contribute to chronic pain, are attributable to diminished Ca2+ entry.

METHODS

Intracellular microelectrode measurements were obtained from A-type neurons of dorsal root ganglia excised from uninjured rats. Recording conditions were varied to suppress or promote I(Ca) while biophysical variables and excitability were determined.

RESULTS

Both lowered external bath Ca2+ concentration and blockade of I(Ca) with bath cadmium diminished the duration and area of the after-hyperpolarization (AHP), accompanied by decreased current threshold for action potential (AP) initiation and increased repetitive firing during sustained depolarization. Reciprocally, elevated bath Ca2+ increased the AHP and suppressed repetitive firing. Voltage sag during neuronal hyperpolarization, indicative of the cation-nonselective H-current, diminished with decreased bath Ca2+, cadmium application, or chelation of intracellular Ca2+. Additional recordings with selective blockers of I(Ca) subtypes showed that N-, P/Q, L-, and R-type currents each contribute to generation of the AHP and that blockade of any of these, and the T-type current, slows the AP upstroke, prolongs the AP duration, and (except for L-type current) decreases the current threshold for AP initiation.

CONCLUSIONS

Taken together, our findings show that suppression of I(Ca) decreases the AHP, reduces the hyperpolarization-induced voltage sag, and increases excitability in sensory neurons, replicating changes that follow peripheral nerve trauma. This suggests that the loss of I(Ca) previously demonstrated in injured sensory neurons contributes to their dysfunction and hyperexcitability, and may lead to neuropathic pain.

Contribution of calcium channel subtypes to the intracellular calcium signal in sensory neurons: the effect of injury

BACKGROUND

Although the activation-induced intracellular Ca signal is disrupted by sensory neuron injury, the contribution of specific Ca channel subtypes is unknown.

METHODS

Transients in dissociated rat dorsal root ganglion neurons were recorded using fura-2 microfluorometry. Neurons from control rats and from neuropathic animals after spinal nerve ligation were activated either by elevated bath K or by field stimulation. Transients were compared before and after application of selective blockers of voltage-activated Ca channel subtypes.

RESULTS

Transient amplitude and area were decreased by blockade of the L-type channel, particularly during sustained K stimulation. Significant contributions to the Ca transient are attributable to the N-, P/Q-, and R-type channels, especially in small neurons. Results for T-type blockade varied widely between cells. After injury, transients lost sensitivity to N-type and R-type blockers in axotomized small neurons, whereas adjacent small neurons showed decreased responses to blockers of R-type channels. Axotomized large neurons were less sensitive to blockade of N- and P/Q-type channels. After injury, neurons adjacent to axotomy show decreased sensitivity of K-induced transients to L-type blockade but increased sensitivity during field stimulation.

CONCLUSIONS

All high-voltage-activated Ca current subtypes contribute to Ca transients in sensory neurons, although the L-type channel contributes predominantly during prolonged activation. Injury shifts the relative contribution of various Ca channel subtypes to the intracellular Ca transient induced by neuronal activation. Because this effect is cell-size specific, selective therapies might potentially be devised to differentially alter excitability of nociceptive and low-threshold sensory neurons.

Variation of T-type calcium channel protein expression affects cell division of cultured tumor cells

In this study we investigated the T-type calcium channel and its involvement in the cell division of U87MG cultured glioma cells and N1E-115 neuroblastoma cells. Using Western blot analysis, we found that expression of both alpha1G and alpha1H subunits of the T-type calcium channel decreased during conditions associated with a decrease in proliferation as evidenced by increased expression of cyclin D1, a marker for non-proliferating cells. Both serum starvation and application of mibefradil, a selective T-type calcium channel antagonist, resulted in a 50% decrease in the expression of alpha1G and alpha1H and a 700-900% increase in levels of cyclin D1 in U87MG and N1E-115 cells, respectively. Furthermore, overexpression of the alpha1H subunit resulted in a two-fold increase in cell proliferation compared to control cultures or cultures receiving an empty vector. In contrast, blocking expression of the alpha1G subunit using antisense oligonucleotides lead to a 70% decrease in proliferation of U87MG and N1E-115 cells compared to control cultures or cultures receiving a scrambled oligonucleotide. Our findings suggest that proliferation of U87MG glioma cells and N1E-115 is regulated by T-type calcium channel expression.

Contrasting the effects of nifedipine on subtypes of endogenous and recombinant T-type Ca2+ channels

There is evidence that nifedipine (Nif) - a dihydropyridine (DHP) Ca(2+)-channel antagonist mostly known for its L-type-specific action--is capable of blocking low voltage-activated (LVA or T-type) Ca(2+) channels as well. However, the discrimination by Nif of either various endogenous T-channel subtypes, evident from functional studies, or cloned Ca(v)3.1, Ca(v)3.2 and Ca(v)3.3 T-channel alpha 1 subunits have not been determined. Here, we investigated the effects of Nif on currents induced by Ca(v)3.1, Ca(v)3.2 and Ca(v)3.3 expression in Xenopus oocytes or HEK-293 cells (I(alpha 1G), I(alpha 1H) and I(alpha 1I), respectively) and two kinetically distinct, "fast" and "slow", LVA currents in thalamic neurons (I(LVA,f) and I(LVA,s)). At voltages of the maximums of respective currents the drug most potently blocked I(alpha 1H) (IC(50)=5 microM, max block 41%) followed by I(alpha 1G) (IC(50)=109 microM, 23%) and I(alpha 1I) (IC(50)=243 microM, 47%). The mechanism of blockade included interaction with Ca(v)3.1, Ca(v)3.2 and Ca(v)3.3 open and inactivated states. Nif blocked thalamic I(LVA,f) and I(LVA,s) with nearly equal potency (IC(50)=22 microM and 28 microM, respectively), but with different maximal inhibition (81% and 51%, respectively). We conclude that Ca(v)3.2 is the most sensitive to Nif, and that quantitative characteristics of drug action on T-type Ca(2+) channels depend on cellular system they are expressed in. Some common features in the voltage- and state-dependence of Nif action on endogenous and recombinant currents together with previous data on T-channel alpha 1 subunits mRNA expression patterns in the thalamus point to Ca(v)3.1 and Ca(v)3.3 as the major contributors to thalamic I(LVA,f) and I(LVA,s), respectively.

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.

Characterization of action potential-evoked calcium transients in mouse postganglionic sympathetic axon bundles

1. Action potential-evoked Ca(2+) transients in postganglionic sympathetic axon bundles in mouse vas deferens have been characterized using confocal microscopy and Ca(2+) imaging. 2. Axonal Ca(2+) transients were tetrodotoxin sensitive. The amplitude depended on both the frequency of stimulation and the number of stimuli in a train. 3. Removal of extracellular Ca(2+) abolished the Ca(2+) transient. Cd(2+)(100 microM) inhibited the Ca(2+) transient by 78 +/- 10 %. The N-type Ca(2+) channel blocker omega-conotoxin GVIA (0.1 microM) reduced the amplitude by -35 +/-4 %, whereas nifedipine (10 microM; L-type) and omega-conotoxin MVIIC (0.1 microM; P/Q type) were ineffective. 4. Caffeine (10 mM), ryanodine (10 microM), cyclopiazonic acid (30 microM) or CCCP (10 microM) had no detectable effects. 5. Blockade of large and small conductance Ca(2+)-dependent K+ channels with iberiotoxin (0.1 microM) and apamin (1 microM), respectively, or Ca(2+)-dependent Cl(-) channels by niflumic acid (100 microM) did not alter Ca(2+) transients. 6. In contrast, the non-specific K+ channel blockers tetraethylammonium (10 mM) and 4-aminopyridine (10 mM) markedly increased the amplitude of the Ca(2+) transient. Blockade of delayed rectifiers and A-like K+ channels, by tityustoxin-K (alpha) (0.1 microM) and pandinustoxin-K (alpha) (10 nM), respectively, also increased the Ca(2+) transient amplitude. 7. Thus, Ca(2+) transients are evoked by Na(+)-dependent action potentials in axons. These transients originate mainly from Ca(2+) entry through voltage-dependent Ca(2+) channels (80 % Cd(2+) sensitive of which 40 % was attributable to N-type). Twenty per cent of the Ca(2+) transient was not due to Ca(2+) entry through voltage-gated Ca(2+) channels. Intracellular stores and mitochondria were not involved in the generation of the transient. Ca(2+) transients are modulated by A-like K+ channels and delayed rectifiers (possibly K(V)1.2) but not by Ca(2+)-activated ion channels.

Calcium currents from jellyfish striated muscle cells: preservation of phenotype, characterisation of currents and channel localisation

When striated muscle cells of the jellyfish Polyorchis penicillatus were dissociated at 30°C they retained their in vivo morphology and the integrity of ionic currents. This contrasted with cells dissociated at room temperature that rarely expressed any inward currents. Whole-cell, patch-clamp recordings from dissociated muscle cells revealed that the inward component of the total ionic current consisted of only one calcium current. This calcium current activated at –70 mV, peaked at –30 mV, and inactivated within 5 ms. In comparison with barium and strontium ions, calcium ions were the preferred current carriers. Calcium channels can be blocked by dihydropyridines and nickel ions at micromolar levels. Several properties of this current are reminiscent of T-type calcium currents. Localisation of this channel using the fluorescent channel blocker fDHP and the fluorescent dye RH414 indicated that myofibres had a higher density of these channels than the somata.

Changes in Ca(2+) channel expression upon differentiation of SN56 cholinergic cells

The SN56 cell line, a fusion of septal neurons and neuroblastoma cells, has been used as a model for central cholinergic neurons. These cells show increased expression of cholinergic neurochemical features upon differentiation, but little is known about how differentiation affects their electrophysiological properties. We examined the changes in Ca(2+) channel expression that occur as these cells undergo morphological differentiation in response to serum withdrawal and exposure to dibutyryl-cAMP. Undifferentiated cells expressed a T-type current with biophysical and pharmacological properties similar, although not identical, to those reported for the current generated by the alpha(1H) (CaV3.2) Ca(2+) channel subunit. Differentiated cells expressed, in addition to this T-type current, high voltage activated currents which were inhibited 38% by the L-type channel antagonist nifedipine (5 microM), 37% by the N-type channel antagonist omega-conotoxin-GVIA (1 microM), and 15% by the P/Q-type channel antagonist omega-agatoxin-IVA (200 nM). Current resistant to these inhibitors accounted for 15% of the high voltage activated current in differentiated SN56 cells. Our data demonstrate that differentiation increases the expression of neuronal type voltage gated Ca(2+) channels in this cell line, and that the channels expressed are comparable to those reported for native basal forebrain cholinergic neurons. This cell line should thus provide a useful model system to study the relationship between calcium currents and cholinergic function and dysfunction.

T-type Ca(2+) channels mediate neurotransmitter release in retinal bipolar cells

Transmitter release in neurons is thought to be mediated exclusively by high-voltage-activated (HVA) Ca(2+) channels. However, we now report that, in retinal bipolar cells, low-voltage-activated (LVA) Ca(2+) channels also mediate neurotransmitter release. Bipolar cells are specialized neurons that release neurotransmitter in response to graded depolarizations. Here we show that these cells express T-type Ca(2+) channel subunits and functional LVA Ca(2+) currents sensitive to mibefradil. Activation of these currents results in Ca(2+) influx into presynaptic terminals and exocytosis, which we detected as a capacitance increase in isolated terminals and the appearance of reciprocal currents in retinal slices. The involvement of T-type Ca(2+) channels in bipolar cell transmitter release may contribute to retinal information processing.

Modulation of glycine responses by dihydropyridines and verapamil in rat spinal neurons

Although glycine receptors (GlyRs) are responsible for the main spinal inhibitory responses in adult vertebrates, in the embryo they have been reported to mediate depolarizing responses, which can sometimes activate dihydropyridine-sensitive L-type calcium channels. However, these channels are not the only targets of dihydropyridines (DHPs), and we questioned whether GlyRs might be directly modulated by DHPs. By whole-cell recording of cultured spinal neurons, we investigated modulation of glycine responses by the calcium channel antagonists, nifedipine, nitrendipine, nicardipine and (R)-Bay K 8644, and by the calcium channel, agonist (S)-Bay K 8644. At concentrations between 1 and 10 microM, all these DHPs could block glycine responses, even in the absence of extracellular Ca2+. The block was stronger at higher glycine concentrations, and increased with time during each glycine application. Nicardipine blocked GABAA responses from the same neurons in a similar manner. In addition to their blocking effects, nitrendipine and nicardipine potentiated the peak responses to low glycine concentrations. Both effects of extracellular nitrendipine on glycine responses persisted when the drug was present in the intracellular solution. Thus, these modulations are related neither to calcium channel modulation nor to possible intracellular effects of DHPs. Another type of calcium antagonist, verapamil (10-50 microM), also blocked glycine responses. Our results suggest that some of the effects of calcium antagonists, including the neuroprotective and anticonvulsant effects of DHPs, might result partly from their interactions with ligand-gated chloride channels.

Molecular pharmacology of T-type Ca2+ channels

Over the past few years increasing attention has been focused on T-type calcium channels and their possible physiological and pathophysiological roles. Efforts toward elucidating the exact role(s) of these calcium channels have been hampered by the lack of T-type specific antagonists, resulting in the subsequent use of less selective calcium channel antagonists. In addition, the activity of these blockers often varies with cell or tissue type, as well as recording conditions. This review summarizes a variety of compounds that exhibit varying degrees of blocking activity towards T-type Ca2+ channels. It is designed as an aid for researchers in need of antagonists to study the biophysical and pathological nature of T-type channels, as well as a starting point for those attempting to develop potent and selective antagonists of the channel.

Biophysical characterization of rat caudal hypothalamic neurons: calcium channel contribution to excitability

Neurons in the caudal hypothalamus (CH) are responsible for the modulation of various processes including respiratory and cardiovascular output. Previous results from this and other laboratories have demonstrated in vivo that these neurons have firing rhythms matched to the respiratory and cardiovascular cycles. The goal of the present study was to characterize the biophysical properties of neurons in the CH with particular emphasis in those properties responsible for rhythmic firing behavior. Whole cell, patch-clamped CH neurons displayed a resting membrane potential of -58.0 +/- 1.1 mV and an input resistance of 319.3 +/- 16.6 MOmega when recorded in current-clamp mode in an in vitro brain slice preparation. A large proportion of these neurons displayed postinhibitory rebound (PIR) that was dependent on the duration and magnitude of hyperpolarizing current as well as the resting membrane potential of the cell. Furthermore these neurons discharged tonically in response to a depolarizing current pulse at a depolarized resting membrane potential (more positive than -65 mV) but switched to a rapid burst of firing to the same stimulus when the resting membrane potential was lowered. The PIR observed in these neurons was calcium dependent as demonstrated by the ability to block its amplitude by perfusion of Ca(2+)-free bath solution or by application of Ni(2+) (0.3-0.5 mM) or nifedipine (10 microM). These properties suggest that low-voltage-activated (LVA) calcium current is involved in the PIR and bursting firing of these CH neurons. In addition, high-voltage-activated calcium responses were detected after blockade of outward potassium current or in Ba(2+)-replacement solution. In addition, almost all of the CH neurons studied showed spike frequency adaptation that was decreased following Ca(2+) removal, indicating the involvement of Ca(2+)-dependent K(+) current (I(K,Ca)) in these cells. In conclusion, CH neurons have at least two different types of calcium currents that contribute to their excitability; the dominant current is the LVA or T-type. This LVA current appears to play a significant role in the bursting characteristics that may underlie the rhythmic firing of CH neurons.

Potent inhibition of a recombinant low voltage-activated Ca(2+) channel by SB-209712

T-type Ca(2+) currents were recorded in 2 mM Ca(2+) from HEK293 cells stably expressing the low voltage-activated Ca(2+) channel sub-unit alpha(1I). These currents were inhibited by the known Ca(2+) channel antagonist mibefradil with an IC(50) close to 1 microM. SB-209712 (1,6,bis¿1-[4-(3-phenylpropyl)piperidinyl]¿hexane), a compound originally developed as a high voltage-activated Ca(2+) channel blocker, proved to be a more potent T-type channel antagonist, exhibiting an IC(50) in the region of 500 nM. The antagonism produced by SB-209712 was reversed following drug removal and the observed antagonism exhibited little or no voltage-dependence with respect to either holding or test potential. These data indicate that SB-209712 is amongst the most potent known non-peptide T-type channel antagonists and thus may have some use in understanding the role of these channels in cellular function.

Regulation of the calcium channel alpha(1G) subunit by divalent cations and organic blockers

The pharmacological properties of the expressed murine T-type alpha(1G) channel were characterized using the whole cell patch clamp configuration. Ba(2+) or Ca(2+) were used as charge carriers. Both I(Ba) and I(Ca) were blocked by Ni(2+) and Cd(2+) with IC(50) values of 0.47+/-0.04 and 1.13+/-0.06 mM (Ni(2+)) and 162+/-13 and 658+/-23 microM (Cd(2+)), respectively. Ni(2+), but not Cd(2+), modified the gating of channel activation. Ni(2+) consistently accelerated channel deactivation while Cd(2+) had a similar effect only on I(Ca). The alpha(1G) channel was potently blocked by mibefradil in a dose- and voltage-dependent manner. I(Ba) was moderately blocked by phenytoin (IC(50) 73.9+/-1.9 microM) and was resistant to the block by valproate. Also 3 mM ethosuximide blocked 20 and 35% of the I(Ba) at a HP of -100 and -60 mV, respectively, while 5 mM amiloride inhibited I(Ba) by 38% and significantly slowed current activation. The alpha(1G) channel was not affected by 10 microM tetrodotoxin. Both 1 microM (+)isradipine and 10 microM nifedipine inhibited 18 and 14% of I(Ba) amplitude at a HP of -100 mV, and 23% and 29% of I(Ba) amplitude at a HP of -60 mV, respectively. The alpha(1G) current was minimally activated by 1 microM Bay K 8644.

Ca2+ influx in resting rat sensory neurones that regulates and is regulated by ryanodine-sensitive Ca2+ stores

1. Store-operated, voltage-independent Ca2+ channels are activated by depletion of intracellular Ca2+ stores and mediate Ca2+ influx into non-excitable cells at resting membrane potential. We used microfluorimetry, patch-clamp and Mn2+-quench techniques to explore the possibility that a similar mechanism exists in rat dorsal root ganglion (DRG) neurones in primary culture. 2. Following caffeine-induced depletion, ryanodine-sensitive Ca2+ stores refilled with Ca2+ at resting membrane potential. The refilling process required extracellular Ca2+, was blocked by 2 mM Ni2+, and was facilitated by membrane hyperpolarization from -55 to -80 mV, indicating a key role for Ca2+ influx. This influx of Ca2+ was not affected by the voltage-operated Ca2+ channel (VOCC) antagonists nicardipine (10 microM), nimodipine (10 microM) or omega-grammotoxin SIA (1 microM). 3. When ryanodine-sensitive Ca2+ stores were depleted in Ca2+-free media, a return to 2 mM external Ca2+ resulted in a pronounced [Ca2+]i overshoot, indicating an increased permeability to Ca2+. Depletion of Ca2+ stores also produced a 2-fold increase in the rate of Mn2+ influx. The [Ca2+]i overshoot and Mn2+ entry were both inhibited by Ni2+, but not by VOCC antagonists. 4. Caffeine induced periodic Ca2+ release from, and reuptake into, ryanodine-sensitive stores. The [Ca2+]i oscillations were arrested by removal of extracellular Ca2+ or by addition of Ni2+, but they were not affected by VOCC antagonists. Hyperpolarization increased the frequency of this rhythmic activity. 5. These data suggest the presence of a Ca2+ entry pathway in mammalian sensory neurones that is distinct from VOCCs and is regulated by ryanodine-sensitive Ca2+ stores. This pathway participates in refilling intracellular Ca2+ stores and maintaining [Ca2+]i oscillations and thus controls the balance between intra- and extracellular Ca2+ reservoirs in resting DRG neurones.

Low-voltage activated calcium channels: achievements and problems

Low-voltage activated Ca2+ channels, which possess unique properties quite different from those of common (high-voltage activated) channels, were discovered 15 years ago but the first alpha1 subunit has only recently been identified which might provide their structural basis. However, simultaneously, extensive data are being accumulated on the functional diversity of low-voltage activated Ca2+ currents with regard to their pharmacological sensitivity, ionic selectivity, activation and inactivation kinetics. Such diversity corresponds to equally prominent heterogeneity in the location and function of the channels. This commentary summarizes the data available in an attempt to predict a possibly wider structural subdivision of low-voltage activated Ca2+ channels into subtypes.

Electrophysiological characterisation of the human N-type Ca2+ channel III: pH-dependent inhibition by a synthetic macrocyclic polyamine

The effects of a novel synthetic macrocyclic polyamine (LY310315) were investigated on recombinant human N-type Ca2+ channels stabley expressed in HEK293 cells. LY310315 proved to be a potent and reversible N-type Ca2+ channel antagonist. Inhibition by this compound was dose-dependent with an IC50 of approximately 0.4 microM at pH 7.35. LY310315 blocked very rapidly at all concentrations tested. Upon washout, recovery of the Ca2+ current developed with a time constant of approximately 30 s. Use-dependence in the development of block indicated that voltage-dependent transitions in the channel protein were required to permit significant inhibition. Application of > 100 times the IC50 dose of LY310315 to the interior of the cell produced no detectable Ca2+ current inhibition. LY310315 had no effects on the kinetics of channel activation or deactivation but did slightly slow the rate of macroscopic inactivation observed during a 300 ms test depolarisation. In the presence of LY310315 the activation curve was significantly shallower. This resulted in a shift in the activation midpoint voltage to a more depolarised levels. LY310315-induced inhibition of human N-type channels was strongly dependent on the extracellular pH, with increased potency seen upon extracellular acidification. Although most effective against N-type Ca2+ channels, LY310315 was also found to inhibit both P-type and L-type Ca2+ channels. LY310315 proved to be a weak blocker of Na+ currents, but produced approximately 50% of the K+ currents of AtT20 cells at a concentration of 0.5 microM.

Voltage-gated calcium channels in Pleurodeles oocytes: classification, modulation and functional roles

In unfertilised Pleurodeles oocytes, two distinct types of high voltage-activated Ca2+ channels are expressed: a slowly inactivating Ca2+ channel and a transient one. The first is dihydropyridine-sensitive and is referred to as the L-type Ca2+ channel. The transient channel is highly sensitive to Ni2+. Phosphorylation through protein kinases G and A facilitates and inhibits the L-type Ca2+ channel respectively. The transient type channel is insensitive to stimulation by protein kinases (A and G). The functional expression of L-type and transient Ca2+ channels is modulated by the two maturation seasons. The transient Ca2+ currents are only observed during the resting season, while the L-type current is observed either alone during the breeding season or in association with the transient current during the resting season. Moreover, the current density of the L-type Ca2+ channel is much greater during the breeding season than the resting season. Thus, the wide distribution of L-type Ca2+ channels in Pleurodeles oocytes during the two seasons suggests that the roles of these channels may be important in the regulation of the maturation process.

Different potencies of dihydropyridine derivatives in blocking T-type but not L-type Ca2+ channels in neuroblastoma-glioma hybrid cells

Evidence has accumulated that classic L-type Ca2+ channel blockers with a dihydropyridine structure also inhibit T-type Ca2+ channels in certain types of central and peripheral neurons and in smooth muscle cells, albeit with a lower potency. Thus beneficial therapeutic effects of dihydropyridines in cardiovascular and neurological diseases may not only be associated with L-type but also with T-type Ca2+ channel blockade. Little is known about the exact order of potency of dihydropyridine derivatives at T-type Ca2+ channels. Here we investigate the efficacy and potency of four therapeutically used compounds, i.e. nifedipine, nimodipine, nicardipine, niguldipine, in the neuroblastoma-glioma cell line NG108-15. For comparative purposes the Ca2+ channel agonist Bay K 8644 was included. Ca2+ channel currents were measured with the whole-cell voltage clamp technique. Subtype Ca2+ channel currents were separated by clamp protocol and selective blockers. T-type Ca2+ channel currents were inhibited with decreasing potency in the order niguldipine > nicardipine > nimodipine > nifedipine (IC50-values 244 nM, 2.5 microM, 9.8 microM, 39 microM), whereas L-type Ca2+ channel currents were blocked with similar potency (IC50 for nicardipine 75 nM). Bay K 8644 increased T-type Ca2+ channel current at nanomolar concentrations (i.e. 95 +/- 16% increase by 300 nM). T-type Ca2+ channel block was completely reversible with exception of the block by niguldipine. Our results indicate a variability of two orders of magnitude in potency of T-type Ca2+ channel block by the dihydropyridine derivatives investigated. It is speculated that the relation between the L- and T-type Ca2+ channel block may determine the therapeutic profile of a dihydropyridine derivative.

Toxin-resistant calcium currents in embryonic mouse sensory neurons

We characterized toxin-insensitive calcium currents expressed by acutely dissociated embryonic dorsal root ganglion neurons. In the presence of 3 microM omega-conotoxin-GVIA, 3 microM nitrendipine and either 500 nM omega-agatoxin-IVA or 500 nM omega-conotoxin-MVIIC to inhibit N-, L- and P/Q-type currents, respectively, all neurons expressed two residual currents: a T-type and another which we referred to as toxin-resistant current. The toxin-resistant current (i) consisted of an inactivating and a sustained components, (ii) had a threshold of activation and a steady-state inactivation comprised between that of the T-type current and that of the other high-voltage-activated currents, (iii) had the same permeability for barium and calcium used as charge carriers, (iv) was highly sensitive to both cadmium and nickel; and (v) was insensitive to 500 microM amiloride which abolished the T-type at this concentration. The properties of the toxin-resistant current are very similar to those of the currents expressed in oocytes following injection of alpha(1E) subunits which we demonstrated to be present in these neurons. Therefore a component of the toxin-resistant current calcium channels in sensory neurons may be closely related to those calcium channels formed by alpha(1E) subunits.

Contrasting biophysical and pharmacological properties of T-type and R-type calcium channels

In contrast to other kinds of voltage-gated Ca2+ channels, the underlying molecular basis of T-type and R-type channels is not well-understood. To facilitate comparisons with cloned Ca2+ channel subunits, we have carried out a systematic analysis of the properties of T-type currents in undifferentiated NG108-15 cells and R-type currents in cerebellar granule neurons. Marked differences were found in their biophysical and pharmacological features under identical recording conditions. T-type channels became activated at potentials approximately 25 mV more negative than R-type channels; however, T-type channels required potentials approximately 15 mV less negative than R-type channels to be available. Accordingly, T-type channels display a much larger overlap between the curves describing inactivation and activation, making them more suitable for generating sustained Ca2+ entry in support of secretion or pacemaker activity. In contrast, R-type channels are not equipped to provide a steady current, but are very capable of supplying transient surges of Ca2+ influx. In response to a series of increasingly strong depolarizations T-type and R-type Ca2+ channels gave rise to very different kinetic patterns. T-type current records crossed each other in a characteristic pattern not found for R-type currents. These biophysical distinctions were independent of absolute membrane potential and were, therefore, complementary to the conventional categorization of T- and R-type Ca2+ channels as low- and high-voltage activated. R-type channels deactivated approximately eight-fold more quickly than T-type channels, with clear consequences for the generation of divalent cation influx during simulated action potentials. Pharmacological comparisons revealed additional contrasts. R-type current was responsive to block by omega-Aga IIIA but not nimodipine, while the opposite was true for T-type current. Both channel types were potently inhibited by the non-dihydropyridine compound mibefradil. In all respects examined, R-type currents were similar to currents derived from expression of the alpha1E subunit whereas T-type currents were not.

T-type and L-type Ca2+ currents in canine bronchial smooth muscle: characterization and physiological roles

We examined the voltage-dependent Ca2+ currents in freshly dissociated smooth muscle cells obtained from canine bronchi (3rd to 5th order). When cells were depolarized from -40 mV, we observed an inward current that 1) exhibited threshold and peak activation at approximately -35 mV and +10 mV, respectively; 2) inactivated slowly with half-inactivation at -20 mV; 3) deactivated rapidly (time constant < 1 ms) upon repolarization; and 4) was abolished by nifedipine and suppressed by cholinergic agonist. These characteristics are typical of L-type Ca2+ current. During depolarization from -70 or -80 mV, however, many cells exhibited a second inward current superimposed on the L-type Ca2+ current. Activation of this other current was first noted at -60 mV, was maximal at approximately -20 mV, and was very rapid (reaching a peak within 10 ms). Inactivation of the other current was also rapid (time constant approximately 3 ms) and half-maximal at approximately -70 mV. There was a persistent "window" current over the physiologically relevant range of potentials (i.e., -60 to -30 mV). This current was also sensitive to nifedipine (although less so than the L-type current) and to Ni2+, but not to cholinergic agonist. Finally, the tail currents evoked upon repolarization to the holding potential decayed approximately 10 times more slowly than did L-type tail currents. These characteristics are all typical of T-type Ca2+ current. We conclude that there is a prominent T-type Ca2+ current in canine bronchial smooth muscle; this current may play a central role in excitation-contraction coupling, in refilling of the internal Ca2+ pool, and in electrical slow waves. Because airflow resistance is determined primarily by the smaller airways and not the trachea, these findings may have important implications with respect to airway physiology and the mechanisms underlying airway hyperreactivity and asthma.

Expression of a rapidly inactivating Ca2+ channel in Pleurodeles oocytes during the resting season

Two kinds of Ca2+ channel activities have been recorded in unfertilized Pleurodeles oocytes during the resting season: the previously described L-type and a transient one. The transient Ba current (I(Ba-t)) exhibited a voltage threshold of -36+/-7 mV, peaked at -18+/-8 mV and reversed around +50 mV. It showed a fast monoexponential decay with an inactivation time course of 31.4+/-1.7 ms at -20 mV. I(Ba-t) was insensitive to nifedipine and omega-conotoxin-GVIA but blocked by Ni2+ (50 microM). Moreover, Cd2+ also reduced I(Ba-t) but was less efficient than Ni2+. When using Ca2+ as the charge carrier, the amplitude and the decay of I(ca-t) were largely similar to those of I(Ba-t). These data demonstrated that the population of Ca2+ channels could be seasonally modulated in Pleurodeles oocytes.

Diversity of voltage-gated calcium currents in large diameter embryonic mouse sensory neurons

Voltage-gated Ca2+ currents were investigated in a subpopulation of dorsal root ganglion neurons (large diameter, neurofilament-positive) acutely isolated from 13-day-old mouse embryos and recorded using the whole-cell patch-clamp technique. Low- and high-voltage-activated calcium currents were recorded. These currents could be identified and separated by their distinct (i) threshold of activation, (ii) ability to run-up during the early phase of recording and (iii) decay kinetics using Ba2+ instead of Ca2+ as the charge carrier. Among high-voltage-activated currents, L-, N- and P-type Ca2+ currents were identified by their sensitivity to, respectively, the dihydropyridine agonist Bay K 8644 (5 microM) and antagonist nitrendipine (3 microM), omega-conotoxin GVIA (3 microM) and omega-agatoxin IVA (30 nM). In the combined presence of nitrendipine (3 microM), omega-conotoxin GVIA (3 microM) and omega-agatoxin IVA (30 nM), two additional high-voltage-activated components were detected. One, blocked by 500 nM omega-conotoxin MVIIC and 1 microM omega-agatoxin IVA, had properties similar to those of the Q-type Ca2+ current first reported in cerebellar granule cells. The other, defined by its resistance to saturating concentrations of all the blockers mentioned above applied in combination, resembles the R-type Ca2+ current also described in cerebellar granule cells. In conclusion, embryonic sensory neurons appear to express a large repertoire of voltage-activated Ca2+ currents with distinct pharmacological properties. This diversity suggests a great variety of pathways for Ca2+ signaling which may support different functions during development.

Dihydropyridines, phenylalkylamines and benzothiazepines block N-, P/Q- and R-type calcium currents

We compared the effects of representative members of three major classes of cardiac L-type channel antagonists, i.e. dihydropyridines (DHPs), phenylalkylamines (PAAs) and benzothiazepines (BTZs) on high-voltage-activated (HVA) Ca2+ channel currents recorded from a holding potential of -100 mV in rat ventricular cells, mouse sensory neurons and rat motoneurons. Nimodipine (DHP), verapamil (PAA) and diltiazem (BTZ) block the cardiac L-type Ca2+ channel current (EC50: 1 microM, 4 microM and 40 microM, respectively). At these concentrations, the drugs could also inhibit HVA Ca2+ channel currents in both sensory and motor neurons. Large blocking effects (> 50%) could be observed at 2-10 times these concentrations. The omega -conotoxin-GVIA-sensitive (omega -CTx-GVIA, N-type), omega -agatoxin-IVA-sensitive (omega -Aga-IVA, P- and Q-types) and non-L-type omega -CTx-GVIA-, omega -Aga-IVA-insensitive (R-types) currents accounted for more than 90% of the global current. Furthermore, our data showed that omega -CTx-GVIA and omega -Aga-IVA spare L-type currents and have only additive blocking effects on neuronal HVA currents. We conclude that DHPs, PAAs and BTZs have substantial inhibitory effects on neuronal non-L-type Ca2+ channels. Inhibitions occur at concentrations that are not maximally active on cardiac L-type Ca2+ channels.

Two high-voltage-activated, dihydropyridine-sensitive Ca2+ channel currents with distinct electrophysiological and pharmacological properties in cultured rat aortic myocytes

In smooth muscle cells, essentially two distinct types of voltage-gated Ca2+ channels have been shown, on the basis of their distinct electrophysiological and pharmacological properties, to coexist. Here we report that, in addition to a dihydropyridine (DHP)-sensitive, low-voltage-activated Ba2+ current (IBa,LVA), two types of high-voltage-activated Ba2+ currents with distinct waveforms were recorded in whole-cell clamped aortic myocytes; these were referred to as IBa,HVA1 and IBa,HVA2. They were investigated in cells where no IBa,LVA was detectable. IBa,HVA1 had a slow, monoexponential decay. In contrast, the decay of IBa,HVA2 was much faster and biexponential. In addition, IBa,HVA2 had more negative ranges of activation and steady-state inactivation than IBa,HVA1 and was more sensitive to the DHP antagonist nicardipine (concentrations for half maximum inhibition 0.2 microM and 2 microM, respectively). When using the physiological ion Ca2+ as the charge carrier, the decay of HVA1 currents was not altered, whereas both time constants of HVA2 current decay were accelerated five-fold. Moreover, permeability ratios (ICa/IBa) were also significantly different (0.2 and 0.6 for HVA1 and HVA2 respectively). IBa,HVA1 and IBa,HVA2 are consistent either with the existence and activation of two functionally distinct subtypes of the so-called "DHP-sensitive L-type" Ca2+ channel or with different gating behaviours of a single type of channel. Potentially, they may serve distinct biological functions and constitute distinct targets for neurotransmitters and drugs.

The whole-cell calcium current in acutely dissociated magnocellular cholinergic basal forebrain neurones of the rat

1. The electrophysiological and pharmacological characteristics of the calcium current (ICa) in acutely dissociated magnocellular cholinergic basal forebrain neurones from 11- to 14-day-old post-natal rats were studied using the whole-cell patch-clamp technique. 2. All cells exhibited a small transient low-voltage-activated (LVA) current with half-activation and half-inactivation potentials of -40.2 and -49.3 mV and slope factors for activation and inactivation of 4.82 and 3.85 mV per e-fold change in membrane potential (Vm) respectively. Activation and inactivation rates for the LVA current were highly voltage dependent. For test potential changes from -50 to -20 mV, the time-to-peak of the current decreased from 39.1 to 6.4 ms, and the time constant of current decay decreased from 81.7 to 15.5 ms. 3. A high-voltage-activated (HVA) component of ICa could be elicited at threshold voltages between -46 and -30 mV from a holding potential (VH) of -80 mV. The HVA current peaked around 0 mV; a 10-fold increase in [Ca2+]o produced a 13 mV positive shift in the peak, whilst the amplitude of the current showed an approximately hyperbolic relationship to [Ca2+]o with half-saturation at 2.5 mM. The transient phase of the HVA current could be described by two exponential functions with time constants tau fast and tau slow of 16.2 and 301 ms. Steady-state inactivation of the transient and extrapolated true sustained (pedestal) components of HVA current were described by Boltzmann equations, with half-inactivation potentials (slope factors) of -47.3 mV, (9.04) and -29.2 mV (11.8) respectively. 4. omega-Conotoxin (omega-CgTX; 100 nM) irreversibly inhibited a kinetically distinct component of HVA current but had no effect upon the transient LVA current. The omega-CgTX-sensitive current could not be distinguished from the control HVA current by the voltage dependence of its activation or inactivation rates. 5. Low concentrations of amiloride (< or = 300 microM) or Ni2+ (< or = 5 microM) selectively inhibited the transient LVA current, with IC50 values of 97 and 5 microM respectively. Cd2+ (< or = 1 microM) selectively blocked a component of HVA current. At higher concentrations, Cd2+ and Ni2+ were non-selective and totally blocked all components of ICa. 6. The lanthanide ions Gd3+ and La3+ produced saturable incomplete block of the HVA current. Maximally effective concentrations of Gd3+ (100 microM) or La3+ (30 microM) inhibited 76.5 and 41.2% respectively of the sustained component of HVA current with corresponding IC50 values of 2.2 and 1.1 microM.(ABSTRACT TRUNCATED AT 400 WORDS)

P-type calcium channels in the somata and dendrites of adult cerebellar Purkinje cells

The pharmacological and single-channel properties of Ca2+ channels were studied in the somata and dendrites of adult cerebellar Purkinje cells. The Ca2+ channels were exclusively of the high threshold type: low threshold Ca2+ channels were not found. These high threshold channels were not blocked by omega-conotoxin GVIA and were inhibited rather than activated by BAY K 8644. They were therefore pharmacologically distinct from high threshold N- and L-type channels. Funnel web spider toxin was an effective blocker. The channels opened to conductance levels of 9, 14, and 19 pS (in 110 mM Ba2+). These slope conductances were in the range of those reported for N- and L-type channels. Our results are in agreement with previous reports suggesting that Ca2+ channels in Purkinje cells can be classified as P-type channels according to their pharmacology. The results also suggest that distinctions among Ca2+ channel types based on the single-channel conductance are not definitive.

Differential expression of voltage-gated Ca(2+)-currents in cultivated aortic myocytes

The expression of different types of Ca(2+)-channels was studied using the whole-cell patch-clamp technique in cultured rat aortic smooth-muscle myocytes. Ca(2+)-currents were identified as either low- or high voltage-activated (ICa,LVA or ICa,HVA, respectively) based on their distinct voltage-dependences of activation and inactivation, decay kinetics using Ba2+ as the charge carrier and sensitivity to dihydropyridines. The heterogeneity in the functional expression of the two types of Ca(2+)-channels in the cultured myocytes delineated four distinct phenotypes; (i), cells exhibiting only LVA currents; (ii), cells exhibiting only HVA currents; (iii), cells exhibiting both LVA and HVA currents and (iv), cells exhibiting no current. The myocytes exclusively expressed HVA currents both during the first five days in primary culture and after the cells had reached confluence (> 15 days). In contrast, LVA currents were expressed transiently between 5 and 15 days, during which time the cells were proliferating and had transient loss of contractility. Thus, both LVA and HVA Ca(2+)-current types contribute to Ca(2+)-signalling in cultured rat aortic myocytes. However, the differential expression of the two Ca2+ current types associated with differences in contractile and proliferative phenotypes suggest that they serve distinct cellular functions. Our results are consistent with the idea that LVA current expression is important for cell proliferation.