Inactivation of calcium channels

Age-dependent changes in the regulation mechanisms for intracellular calcium ions in ganglion cells of the mouse retina

The purpose of this study was to investigate the role of intracellular calcium buffering in retinal ganglion cells. We performed a quantitative analysis of calcium homeostasis in ganglion cells of early postnatal and adult mice by simultaneous patch-clamp recordings in sliced tissue and microfluorometric calcium measurements with Fura-2. Endogenous calcium homeostasis was quantified by using the 'added buffer' approach which uses amplitudes and decay time constants of calcium transients to give a standard for intracellular calcium buffering. The recovery phase of depolarization-induced calcium transients was well approximated by a mono-exponential function with a decay time constant that showed a linear dependence on dye concentration. Endogenous calcium binding ratios were found to be 575 (n = 18 cells) in early postnatal and 121 (n = 18 cells) in adult retinal ganglion cells. With respect to ganglion cell degeneration at early postnatal stages, our measurements suggest that neuroprotection of a majority of developing ganglion cells partially results from a specialized calcium homeostasis based on high buffering capacities. Furthermore, the dramatic decrease of the intracellular calcium buffering capacity during ganglion cell development may enhance their vulnerability to neurodegeneration.

Calcium feedback mechanisms regulate oscillatory activity of a TRP-like Ca2+ conductance in C. elegans intestinal cells

Inositol-1,4,5-trisphosphate (IP3)-dependent Ca2+ oscillations in Caenorhabditis elegans intestinal epithelial cells regulate the nematode defecation cycle. The role of plasma membrane ion channels in intestinal cell oscillatory Ca2+ signalling is unknown. We have shown previously that cultured intestinal cells express a Ca2+-selective conductance, I(ORCa), that is biophysically similar to TRPM7 currents. I(ORCa) activates slowly and stabilizes when cells are patch clamped with pipette solutions containing 10 mm BAPTA and free Ca2+ concentrations of approximately 17 nm. However, when BAPTA concentration is lowered to 1 mm, I(ORCa) oscillates. Oscillations in channel activity induced simultaneous oscillations in cytoplasmic Ca2+ levels. Removal of extracellular Ca2+ inhibited I(ORCa) oscillations, whereas readdition of Ca2+ to the bath caused a rapid and transient reactivation of the current. Experimental manoeuvres that elevated intracellular Ca2+ blocked current oscillations. Elevation of intracellular Ca2+ in the presence of 10 mm BAPTA to block I(ORCa) oscillations led to a dose-dependent increase in the rate of current activation. At intracellular Ca2+ concentrations of 250 nm, current activation was transient. Patch pipette solutions buffered with 1-4 mm of either BAPTA or EGTA gave rise to similar patterns of I(ORCa) oscillations. We conclude that changes in Ca2+ concentration close to the intracellular opening of the channel pore regulate channel activity. Low concentrations of Ca2+ activate the channel. As Ca2+ enters and accumulates near the pore mouth, channel activity is inhibited. Oscillating plasma membrane Ca2+ entry may play a role in generating intracellular Ca2+ oscillations that regulate the C. elegans defecation rhythm.

A cGMP-dependent cascade enhances an L-type-like Ca2+ current in identified snail neurons

We studied the impact of an NO-cGMP dependent signalling pathway on the high-voltage-activated (HVA) Ca(2+) current in identified neurons of the pulmonate snail, Helix pomatia, using Ba(2+) as charge carrier. The 3',5'-cyclic guanosine monophosphate (cGMP) analogues, dibutyryl-cGMP and 8-bromo-cGMP, consistently induced a biphasic response, consisting of an increase superseded by a decline of the Ba(2+) current. The NO donor, sodium nitroprusside (SNP), modulated only in a minority of neurons the Ba(2+) current. Blockade of protein kinase activity with 1-[5-isoquinolinesulfonyl]-2 methyl piperazine (H 7), a nonselective protein kinase inhibitor, or Rp-8-pCPT-cGMP, a selective protein kinase G (PKG) inhibitor, decreased, whereas Rp-cAMP, a selective protein kinase A (PKA) inhibitor, increased the Ba(2+) current upon application of cGMP analogues or SNP. Okadaic acid or calyculin, inhibitors of protein phosphatase 1 (PP1) and protein phosphatase 2A (PP2A), augmented the Ba(2+) current. Under these conditions, cGMP analogues or SNP had an additive-enhancing effect on the Ba(2+) current. When neurons were exposed to the nonselective phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine (IBMX), cGMP analogues induced a persistent increase of the Ba(2+) current, whereas SNP induced a biphasic response. These data suggest coexistence of cGMP-PKG and cGMP-PDE pathways as well as crosstalk between cGMP and 3',5'-cyclic adenosine monophosphate (cAMP) pathways, which converge on HVA Ca channels in Helix neurons. In this model, augmentation of the Ba(2+) current through HVA Ca channels is accomplished by PKA and PKG, whereas attenuation is mediated by PDEs, which prevent activation of protein kinases via hydrolysis of cyclic nucleotides.

Distinct properties and differential beta subunit regulation of two C-terminal isoforms of the P/Q-type Ca(2+)-channel alpha(1A) subunit

Two C-terminal splice variants (BI-1 and BI-2, now termed Ca(v)2.1a and Ca(v)2.1b) of the neuronal voltage-gated P/Q-type Ca(2+) channel alpha(1A) pore-forming subunit have been cloned (Mori et al., 1991, Nature, 350, 398-402). BI-1 and BI-2 code for proteins of 2273 and 2424 amino acids, respectively, and differ only by their extreme carboxyl-termini sequences. Here, we show that, in Xenopus oocytes, the two isoforms direct the expression of channels with different properties. Electrophysiological analysis showed that BI-1 and BI-2 have peak Ba(2+) currents (I(Ba)) at a potential of +30 and +20 mV, respectively. The different C-terminal sequence (amino acids 2229-2273) of BI-1 caused a shift in steady-state inactivation by +10 mV and decreased the proportion of fast component of current inactivation twofold. Likewise, the biophysical changes in I(Ba) caused by coexpression of the beta(4) auxiliary subunit were substantially different in BI-1- and BI-2-containing channels in comparison to those induced by beta(3). Several of these differences in beta regulation were abolished by deleting the carboxyl-terminal splicing region. By creating a series of GST fusion proteins, we identified two locations in the C-terminal (Leu2090-Gly2229 for BI-1 and BI-2, and Arg2230-Pro2424 for BI-2 only) that determine the differential interaction of beta(4) with the distinct alpha(1A) isoforms. These interactions appear to favour the binding of beta(4) to the AID site, and also the plasma membrane expression of BI-2. These results demonstrate that the final segment of the C-terminal affects alpha(1A) channel gating, interaction and regulation with/by the beta subunits. The data will have several implications for the understanding of the biophysical effects of many channelopathies in which the carboxyl-termini of alpha(1A) and beta(4) are affected.

Calcium dynamics and buffering in oculomotor neurones from mouse that are particularly resistant during amyotrophic lateral sclerosis (ALS)-related motoneurone disease

Motoneurones are particularly vulnerable both in human forms of amyotrophic lateral sclerosis (ALS) and corresponding animal models of the disease. While most motoneurone populations are selectively impaired, oculomotor neurones are essentially resistant to ALS-related damage. Motoneurone vulnerability has been closely linked to disruptions of calcium signalling. To investigate underlying events, we performed a quantitative analysis of calcium homeostasis in oculomotor neurones from mice by simultaneous patch-clamp recordings in sliced tissue and microfluorometric-calcium measurements. Somatic calcium dynamics were investigated by using a computer-controlled microfluorometric system. In oculomotor neurones, basal calcium concentrations were around 80 nM and depolarisation-induced calcium responses were observed for membrane voltages positive to -40 u1u1u approximately mV1 approximately . Endogenous calcium homeostasis was quantified by using the 'added buffer' approach. The recovery phase of depolarisation-induced calcium transients was well approximated by a mono-exponential function with a decay time constant that showed a linear dependence on dye concentration. The extrapolated time constant in the absence of indicator dye was 1.7 +/- 0.2 s (n = 11 cells, 21C). Endogenous calcium binding ratios (kappa(s)) were found to be 264 +/- 25 (n = 11 cells), indicating that 99.6 % of cytosolic calcium ions were taken up by endogenous buffers. Recovery of calcium transients was characterised by an 'effective' extrusion rate gamma = 156 +/- 20 s-1 (n = 11 cells, 21 C). Endogenous calcium binding ratios in oculomotor neurones were 5- to 6-fold larger compared with those of more vulnerable motoneurones in the nucleus hypoglossus and spinal cord. In a first order approximation, they reduced the volume of local calcium elevations around open calcium channels, lowered peak amplitudes of global calcium transients for a given influx and prolonged calcium recovery times for a given set of uptake and extrusion mechanisms. With respect to motoneurone degeneration, our measurements suggest that the exceptional stability of oculomotor neurones partially results from a specialised calcium homeostasis based on high buffering capacities. Furthermore, they indicate that cellular adaptations that account for rapid calcium signalling in hypoglossal and spinal motoneurones enhance their vulnerability during ALS-related motoneurone disease.

Inactivation of P2X2 purinoceptors by divalent cations

1. P2X2 channels are activated by extracellular ATP. Despite being commonly described as non-desensitizing, P2X2 receptors do desensitize or inactivate. In the unspliced, 472 amino acid isoform of the P2X2 receptor, inactivation required membrane disruption and the presence of extracellular Ca2+. 2. The ability to inactivate whole-cell currents developed slowly after breaking in. In contrast, currents from excised patches exhibited rapid (approximately 100 ms) inactivation with a dependence on extracellular Ca2+, ATP and voltage. 3. The inactivation rate increased with the fourth power of [Ca2+] suggesting that the functional channel may be a tetramer. Ca2+ had both a higher affinity and a larger Hill coefficient for inactivation than Mg2+, Ba2+ or Mn2+. Trivalent cations at concentrations up to the solubility product of ATP had no effect. The change in apparent co-operativity with ionic species suggests the presence of experimentally unresolved ligand-insensitive kinetic steps. 4. Based on the weak voltage dependence of inactivation and the lack of effect of intracellular Ca2+ buffers, the Ca2+-binding sites are probably located near the extracellular surface of the membrane. 5. The recovery from inactivation was slow, with a time constant of approximately 7 min. 6. Ca2+-sensitive inactivation only appeared when the membrane was disrupted in some manner. Treatment with actin and microtubule reagents did not induce inactivation, suggesting that an intact cytoskeleton is not necessary. 7. Inactivation rates observed in different patch configurations suggest that the induction of Ca2+-dependent inactivation was due to the loss of a diffusible cofactor located in the membrane or the cytoplasm.

Calcium dynamics and buffering in motoneurones of the mouse spinal cord

1. A quantitative analysis of endogenous calcium homeostasis was performed on 65 motoneurones in slices of the lumbar spinal cord from 2- to 8-day-old mice by simultaneous patch-clamp and microfluorometric calcium measurements. 2. Somatic calcium concentrations were monitored with a temporal resolution in the millisecond time domain. Measurements were performed by using a monochromator for excitation and a photomultiplier detection system. 3. Somatic calcium signalling was investigated during defined voltage-clamp protocols. Calcium responses were observed for membrane depolarizations positive to -50 mV. A linear relation between depolarization time and free calcium concentrations ([Ca2+]i) indicated that voltage-dependent calcium influx dominated the response. 4. Endogenous calcium homeostasis was quantified by using the 'added buffer' approach. In the presence of fura-2 and mag-fura-5, calcium transients decayed according to a monoexponential function. Decay-time constants showed a linear dependence on dye concentration and the extrapolated constant in the absence of indicator dye was 371 +/- 120 ms (n = 13 cells, 21 C). 5. For moderate elevations (< 1 microM), recovery kinetics of depolarization-induced calcium transients were characterized by a calcium-independent, 'effective' extrusion rate gamma = 140 +/- 47 s-1 (n = 13 cells, 21 C). 6. The endogenous calcium binding ratio for fixed buffers in spinal motoneurones was kappaB' = 50 +/- 17 (n = 13 cells), indicating that less than 2 % of cytosolic calcium ions contributed to [Ca2+]i. 7. Endogenous binding ratios in spinal motoneurones were small compared to those found in hippocampal or cerebellar Purkinje neurones. From a functional perspective, they provided motoneurones with rapid dynamics of cytosolic [Ca2+]i for a given set of influx, extrusion and uptake mechanisms. 8. With respect to pathophysiological conditions, our measurements are in agreement with a model where the selective vulnerability of spinal motoneurones during excitotoxic conditions and motoneurone disease partially results from low endogenous calcium buffering.

Regulation of Ca-sensitive inactivation of a 1-type Ca2+ channel by specific domains of beta subunits

Ca2+ channel auxiliary beta subunits have been shown to modulate voltage-dependent inactivation of various types of Ca2+ channels. The beta1 and beta2 subunits, that are differentially expressed with the L-type alpha1 Ca2+ channel subunit in heart, muscle and brain, can specifically modulate the Ca2+-dependent inactivation kinetics. Their expression in Xenopus oocytes with the alpha1C subunit leads, in both cases, to biphasic Ca2+ current decays, the second phase being markedly slowed by expression of the beta2 subunit. Using a series of beta subunit deletion mutants and chimeric constructs of beta1 and beta2 subunits, we show that the inhibitory site located on the amino-terminal region of the beta2a subunit is the major element of this regulation. These results thus suggest that different splice variants of the beta2 subunit can modulate, in a specific way, the Ca2+ entry through L-type Ca2+ channels in different brain or heart regions.

Voltage and calcium use the same molecular determinants to inactivate calcium channels

During sustained depolarization, voltage-gated Ca2+ channels progressively undergo a transition to a nonconducting, inactivated state, preventing Ca2+ overload of the cell. This transition can be triggered either by the membrane potential (voltage-dependent inactivation) or by the consecutive entry of Ca2+ (Ca2+-dependent inactivation), depending on the type of Ca2+ channel. These two types of inactivation are suspected to arise from distinct underlying mechanisms, relying on specific molecular sequences of the different pore-forming Ca2+ channel subunits. Here we report that the voltage-dependent inactivation (of the alpha1A Ca2+ channel) and the Ca2+-dependent inactivation (of the alpha1C Ca2+ channel) are similarly influenced by Ca2+ channel beta subunits. The same molecular determinants of the beta subunit, and therefore the same subunit interactions, influence both types of inactivation. These results strongly suggest that the voltage and the Ca2+-dependent transitions leading to channel inactivation use homologous structures of the different alpha1 subunits and occur through the same molecular process. A model of inactivation taking into account these new data is presented.

Calcium-dependent inactivation of high-threshold calcium currents in human dentate gyrus granule cells

1. Dentate gyrus granule cells acutely dissociated from hippocampal slices obtained from chronic temporal lobe epilepsy (TLE) patients displayed a high-voltage activated (HVA) Ca2+ conductance with a pronounced Ca2+-dependent inactivation. 2. Inactivation time constants and peak HVA Ca2+ current (ICa) amplitudes did not differ between perforated patch and whole-cell recordings without added exogenous Ca2+ buffers, indicating that the Ca2+-dependent characteristics of ICa inactivation were well preserved in whole-cell recordings. 3. Inactivation time constants correlated with whole-cell ICa, and were increased when Ca2+ was replaced with Ba2+ in the external solution or 5 mM BAPTA was added to the pipette solution. 4. In recordings without added exogenous Ca2+ buffers, the time course of ICa inactivation was comparable between human TLE and kindled rat granule cells. Conversely, the time course of ICa in human TLE granule cells loaded with 5 mM intracellular BAPTA resembled that observed in buffer-free recordings from control rat neurones. 5. The loss of a putative intraneuronal Ca2+ buffer, the Ca2+-binding protein calbindin (CB), from human granule cells during TLE may result in the pronounced Ca2+-dependent ICa inactivation. This process could serve a neuroprotective role by significantly decreasing Ca2+ entry during prolonged trains of action potentials known to occur during seizures.

Effect of bay K 8644 (-) and the beta2a subunit on Ca2+-dependent inactivation in alpha1C Ca2+ channels

Ca2+ currents recorded from Xenopus oocytes expressing only the alpha1C pore-forming subunit of the cardiac Ca2+ channel show Ca2+-dependent inactivation with a single exponential decay. This current-dependent inactivation is not detected for inward Ba2+ currents in external Ba2+. Facilitation of pore opening speeds up the Ca2+-dependent inactivation process and makes evident an initial fast rate of decay. Facilitation can be achieved by (a) coexpression of the beta2a subunit with the alpha1C subunit, or (b) addition of saturating Bay K 8644 (-) concentration to alpha1C channels. The addition of Bay K 8644 (-) to alpha1Cbeta2a channels makes both rates of inactivation faster. All these maneuvers do not induce inactivation in Ba2+ currents in our expression system. These results support the hypothesis of a mechanism for the Ca2+-dependent inactivation process that is sensitive to both Ca2+ flux (single channel amplitude) and open probability. We conclude that the Ca2+ site for inactivation is in the alpha1C pore-forming subunit and we propose a kinetic model to account for the main features of alpha1Cbeta2a Ca2+ currents.

Ca2+- and voltage-dependent inactivation of Ca2+ channels in nerve terminals of the neurohypophysis

Ca2+ channel inactivation was investigated in neurohypophysial nerve terminals by using patch-clamp techniques. The contribution of intracellular Ca2+ to inactivation was evaluated by replacing Ca2+ with Ba2+ or by including BAPTA in the internal recording solution. Ca2+ channel inactivation during depolarizing pulses was primarily voltage-dependent. A contribution of intracellular Ca2+ was revealed by comparing steady-state inactivation of Ca2+ channels with Ca2+ current and with intracellular [Ca2+]. However, this contribution was small compared to that of voltage. In contrast to voltage-gated Ca2+ channels in other preparations, in the neurohypophysis Ba2+ substitution or intracellular BAPTA increased the speed of inactivation while reducing the steady-state level of inactivation. Ca2+ channel recovery from inactivation was studied by using a paired-pulse protocol. The rate of Ca2+ channel recovery from inactivation at negative potentials was increased dramatically by Ba2+ substitution or intracellular BAPTA, indicating that intracellular Ca2+ inhibits recovery. Stimulation with trains of brief pulses designed to mimic physiological bursts of electrical activity showed that Ca2+ channel inactivation was much greater with 20 Hz trains than with 14 Hz trains. Inactivation induced by 20 Hz trains was reduced by intracellular BAPTA, suggesting an important role for Ca2+-dependent inactivation during physiologically relevant forms of electrical activity. Inhibitors of calmodulin and calcineurin had no effect on Ca2+ channel inactivation, arguing against a mechanism of inactivation involving these Ca2+-dependent proteins. The inactivation behavior described here, in which voltage effects on Ca2+ channel inactivation predominate at positive potentials and Ca2+ effects predominate at negative potentials, may be relevant to the regulation of neuropeptide release.

On-line detection of glutamate release from cultured chick retinospheroids

A continuous fluorometric assay was adapted to measure the release of endogenous glutamate from cultured chick retinospheroids. The results obtained with this technique are compared with the release of [3H]D-aspartate from monolayer cultures of chick retina cells. It is shown that although excitatory amino acids may be released in a Ca(2+)-dependent manner, most of the neurotransmitter release from cultured retina cells occurs by reversal of the glutamate transporter. The presence of extracellular Ca2+ may actually inhibit glutamate release by the cells present in the retinospheroids, or the [3H]D-aspartate release by cells in monolayers, when veratridine is the depolarizing agent.

Voltage gated calcium channels in molluscs: classification, Ca2+ dependent inactivation, modulation and functional roles

Molluscan neurons and muscle cells express transient (T-type like) and sustained LVA calcium channels, as well as transient and sustained HVA channels. In addition weakly voltage sensitive calcium channels are observed. In a number of cases toxin or dihydropyridine sensitivity justifies classification of the HVA currents in L, N or P-type categories. In many cases, however, pharmacological characterization is still preliminary. Characterization of novel toxins from molluscivorous Conus snails may facilitate classification of molluscan calcium channels. Molluscan preparations have been very useful to study calcium dependent inactivation of calcium channels. Proposed mechanisms explain calcium dependent inactivation through direct interaction of Ca2+ with the channel, through dephosphorylation by calcium dependent phosphatases or through calcium dependent disruption of connections with the cytoskeleton. Transmitter modulation operating through various second messenger mediated pathways is well documented. In general, phosphorylation through PKA, cGMP dependent PK or PKC facilitates the calcium channels, while putative direct G-protein action inhibits the channels. Ca2+ and cGMP may inhibit the channels through activation of phosphodiesterases or phosphatases. Detailed evidence has been provided on the role of sustained LVA channels in pacemaking and the generation of firing patterns, and on the role of HVA channels in the dynamic changes in action potentials during spiking, the regulation of the release of transmitters and hormones, and the regulation of growth cone behavior and neurite outgrowth. The accessibility of molluscan preparations (e.g. the squid giant synapse for excitation release studies, Helisoma B5 neuron for neurite and synapse formation) and the large body of knowledge on electrophysiological properties and functional connections of identified molluscan neurons (e.g. sensory neurons, R15, egg laying hormone producing cells, etc.) creates valuable opportunities to increase the insight into the functional roles of calcium channels.

Effect of intracellular calcium on ATP-activated, GTP-dependent calcium channels in rat macrophages

1. In order to study the effect of intracellular free Ca2+ concentration ([Ca2+]i) on the activity of ATP-activated, GTP-dependent Ca2+ channels in rat macrophages, experiments were performed using the inside-out configuration of the patch-clamp technique. 2. Channel activity was observed in the cell-attached mode when 100 microM ATP was added to the pipette solution containing 105 mM Ba2+, but it disappeared rapidly after patch excision. The activity could be restored by the application of 100 microM GTP or GTP gamma S onto the internal surface of the plasma membrane. 3. The properties of the GTP gamma S-evoked channels are identical to those of channels activated by extracellular application of ATP. The channels exhibited four current sublevels with conductances of about 3.5, 7, 10 and 15 pS when 105 mM Ba2+ was the only permeant cation. The extrapolated reversal potentials were similar for all the sublevels and averaged about +40 mV. 4. Elevation of [Ca2+]i within the range 0.01-1 microM resulted in a decrease in mean inward current. The half-maximal value of the mean current was about 0.08 microM. 5. This decreases in mean current resulted from a redistribution of sublevel occupancies: the 1st sublevel tended to be come more abundant with elevation of [Ca2+]i, while the relative weights of the high-conductance 3rd and 4th sublevels decreased. 6. The open-channel current fell with an increase in [Ca2+]i as quickly as the mean current did, indicating that the sublevel redistribution alone is sufficient to produce the revealed decrease in net inward current. 7. It is concluded that [Ca2+]i elevation does not fix the channel in a closed state but rather decreases the ability of the channel to operate in high-conductance states.

Effects of cerebral ischemia on N-methyl-D-aspartate and dihydropyridine-sensitive calcium currents. An electrophysiological study in the rat hippocampus in situ

BACKGROUND AND PURPOSE

During cerebral ischemia, both promoting and limiting factors are present for activation of the N-methyl-D-aspartate (NMDA) receptor ion channel and the dihydropyridine (DHP)-sensitive Ca2+ channels. We investigated the activity of these channels during ischemia and reperfusion in the rat hippocampus in situ.

METHODS

Reversible ischemia was induced by bilateral carotid artery ligation. NMDA and BAY K8644 were applied by iontophoresis or pneumatic ejection, and extracellular field potential and resistance changes were recorded from the CA1 region of the rat hippocampus. Resting membrane potentials of the CA1 neurons were also recorded.

RESULTS

DC potential shifts produced by NMDA and BAY K8644 were reduced when ischemia depressed the evoked activity more than 50%. They disappeared on total failure of synaptic transmission and recovered during reperfusion. When the evoked activity was depressed less than 50%, DC shifts were greater than their preischemic values; however, BAY K8644-induced potentiation did not reach statistical significance. CA1 neurons were depolarized during ischemia.

CONCLUSIONS

These data suggest that ischemia severe enough to cause transmission failure inactivates NMDA and DHP-sensitive Ca2+ currents. During less intense ischemia and reperfusion, NMDA and DHP-sensitive Ca2+ channels are functional, and their overactivation may lead to neurotoxicity.

Impairment of GABAA receptor function by N-methyl-D-aspartate-mediated calcium influx in isolated CA1 pyramidal cells

Mechanisms of regulation of GABAA receptor function by intracellular calcium ([Ca2+]i) were examined in cell somata and apical dendrites of pyramidal cells, acutely dissociated from the CA1 hippocampal subfield of adult guinea-pigs. GABAA receptor-mediated currents were measured by whole-cell clamp recordings. N-methyl-D-aspartate receptor-mediated currents were used as conditioning source of calcium influx. Peak amplitudes of somatic GABAA whole-cell currents were reduced to about 15% of control values when net inward charge accumulation by N-methyl-D-aspartate currents reached 1.85 nC. A similar decline of GABAA currents was observed in dendritic recordings. The N-methyl-D-aspartate-mediated reduction of somatic and dendritic GABAA currents was accompanied by a well correlated decrease in peak and chord conductances. Pharmacological blockade of N-methyl-D-aspartate currents by 2-amino-5-phosphonopentanoic acid prevented the N-methyl-D-aspartate-mediated suppression of GABAA responses. The N-methyl-D-aspartate effect was mediated by the calcium component of N-methyl-D-aspartate receptor-mediated currents as demonstrated by a lack of effect in the absence of extracellular calcium and faster N-methyl-D-aspartate-mediated suppression of GABAA responses in lower intracellular 1,2-bis(2-aminophenoxy)ethane-N,N,N',N"-tetra-acetate. N-methyl-D-aspartate-mediated suppression of GABAA currents was significantly less expressed when intracellular ATP was replaced by its analog adenosine 5'-O-(3-thiotriphosphate) and when the specific phosphatase 2B inhibitor cypermethrin was added intracellularly. The reduction of GABAA responses persisted after cessation of N-methyl-D-aspartate-mediated calcium influx, indicating a long-term action of N-methyl-D-aspartate on GABAA responses. Voltage-activated calcium currents did not affect GABAA responses under the experimental conditions applied. In conclusion, the data presented show that calcium influxes through N-methyl-D-aspartate receptor channels result in long-term suppression of GABAA receptor function in CA1 pyramidal cells. Intracellular mechanisms of N-methyl-D-aspartate-mediated reduction of GABAA conductances involve activation of phosphatase 2B and consecutive dephosphorylation of the GABAA receptor or a closely associated GABAA receptor-regulating enzyme. Possible mechanisms of such a distinct N-methyl-D-aspartate-dependent calcium signalling pathway in the dephosphorylation-dependent suppression or GABAA receptor function are discussed.

Ca(2+)-dependent inactivation of a cloned cardiac Ca2+ channel alpha 1 subunit (alpha 1C) expressed in Xenopus oocytes

The alpha 1 subunit of cardiac Ca2+ channel, expressed alone or coexpressed with the corresponding beta subunit in Xenopus laevis oocytes, elicits rapidly inactivating Ca2+ currents. The inactivation has the following properties: 1) It is practically absent in external Ba2+; 2) it increases with Ca2+ current amplitudes; 3) it is faster at more negative potentials for comparable Ca2+ current amplitudes; 4) it is independent of channel density; and 5) it does not require the beta subunit. These findings indicate that the Ca2+ binding site responsible for inactivation is encoded in the alpha 1 subunit and suggest that it is located near the inner channel mouth but outside the membrane electric field.

Ca(2+)-dependent inactivation of Ca2+ current in Aplysia neurons: kinetic studies using photolabile Ca2+ chelators

1. The kinetics and sensitivity of the Ca(2+)-dependent inactivation of calcium current (ICa) were examined in intact cell bodies from the abdominal ganglion of Aplysia californica under two-electrode voltage clamp. 2. Rapid changes in the level of intracellular free calcium ([Ca2+]i) were generated at the cell surface by photolytic release of Ca2+ (nitr-5 and dimethoxy nitrophen) or Ca2+ buffer (diazo-4). 3. Diazo-4 increased ICa by 10-15% and slowed the rate of ICa decay when photolysed before a test pulse or between a prepulse and a test pulse. The predominant effect of further light flashes was to increase the amount of non-inactivating current (I infinity) remaining at the end of long (> 1 s) depolarizing pulses. 4. A rapid increase in [Ca2+]i buffering during ICa inactivation did not cause a rapid recovery of current but merely reduced the rate and extent of subsequent inactivation. This effect was not seen when Ba2+ was the charge carrier. 5. Photolytic release of Ca2+ from nitr-5 produced estimated Ca2+ jumps of 3-4 microM at the front surface of the cell but failed to augment inactivation either before or during ICa. In contrast, photolysis of DM-nitrophen 10-90 ms before the test pulse decreased peak ICa by about 30%. A flash given during ICa rapidly blocked 41 +/- 3% of peak current with a time constant of 3-4 ms at 17 degrees C. Similar results were seen with the barium current (IBa). 6. Microinjection of the potent phosphatase inhibitor microcystin-LR (5 microM) had variable effects on ICa inactivation and augmented the cyclic AMP-induced depression of the delayed rectifier (IK(V) by forskolin (100 microM) and 3-isobutyl-1-methylxanthine (IBMX; 200 microM). 7. Full recovery from inactivation measured in two-pulse experiments took at least 20 s. This slow recovery process was unaffected by increases in intracellular cyclic AMP elicited by direct injection or by bath application of forskolin and IBMX. It was also unaffected by decreases in cyclic AMP induced by injecting 2',5'-dideoxyadenosine (1 mM) or bath application of the Rp isomer of cyclic adenosine 3',5'-monophosphothioate (Rp-cAMPS; 200 microM). 8. A 'shell' model relating submembrane Ca2+ to inactivation was inconsistent with the experimental results since it greatly overestimated the effects of diazo-4 and predicted significant inactivation by nitr-5 photolysis. 9. A model linearly relating [Ca2+]i in a single Ca2+ channel 'domain' to inactivation more closely matched the experimental results with diazo-4 and DM-(ABSTRACT TRUNCATED AT 400 WORDS)

Dihydropyridine-sensitive and omega-conotoxin-sensitive calcium channels in a mammalian neuroblastoma-glioma cell line

1. Pharmacological and kinetic properties of high-voltage-activated (HVA) Ca2+ channel currents were studied using the whole-cell and perforated patch-clamp methods in a mouse neuroblastoma and rat glioma hybrid cell line, NG108-15, differentiated by dibutyryl cyclic AMP or by prostaglandin E1 and theophylline. 2. The HVA currents were separated into two components by use of two organic Ca2+ channel antagonists, omega-conotoxin GVIA (omega CgTX) and a dihydropyridine (DHP) compound, nifedipine. One current component, IDHP, was blocked by nifedipine (Kd = 8.2 nM) and was resistant to omega CgTX. Conversely, the other component, I omega CgTX, was irreversibly blocked by omega CgTX and was resistant to DHPs. Thus, IDHP could be studied in isolation by a short application of omega CgTX, while I omega CgTX could be studied in the presence of nifedipine. 3. The voltage for half-activation of IDHP was smaller than that of I omega CgTX by 13 mV. IDHP was activated at potentials that were subthreshold for voltage-dependent K+ currents of the cell, whereas I omega CgTX was not. 4. Time courses of activation and deactivation of IDHP were faster than those of I omega CgTX. 5. Voltage-dependent inactivation was small for both IDHP and I omega CgTX at any potential. 6. Ca(2+)-dependent inactivation of IDHP was faster and more prominent than that of I omega CgTX. The time course of the Ca(2+)-dependent inactivation of IDHP, but not I omega CgTX, was slowed as the membrane potential was made more positive between -20 and 30 mV, although amplitude of the current was increased. 7. Alkaline earth metal ions carried the two components of IHVA in the same order: Ba2+ greater than Sr2+ greater than Ca2+. 8. Metal ions blocked the two components of IHVA in the same order of potency: Gd3+ greater than La3+ greater than Cd2+ greater than Cu2+ greater than Mn2+ greater than Ni2+. 9. An alkylating agent, N-ethylmaleimide (NEM, 0.1 mM), selectively augmented IDHP by 30%. 10. During the course of cellular differentiation induced by dibutyryl cyclic AMP, IDHP appeared earlier than I omega CgTX. 11. These results indicate that two classes of Ca2+ channels contribute to the HVA currents of this cell line. The DHP-sensitive channel is more apt to generate Ca2+ spikes and Ca2+ plateau potentials than the omega CgTX-sensitive channel.

The influence of amlodipine and verapamil on ion and water transport in the nephron, skin and urinary bladder of amphibians

1. The addition of amlodipine or verapamil into the lumen of the newt distal tubule led to the decrease of reabsorption of Na, Cl, Ca and of fluid. 2. The application of amlodipine to the outside of the frog skin caused large increases in potential difference (PD) and short circuit (SCC) similar to what is seen with Co2+. If both amlodipine and Co2+ were applied simultaneously to the outer surface the increases in PD and SCC were additive. 3. Verapamil added to the outer surface of the skin caused a reduction in PD which could be overcome by subsequent addition of amlodipine. 4. After addition of amlodipine to serosal or mucosal surfaces of the frog urinary bladder, the ability of vasopressin to increase osmotic permeability was markedly attenuated. 5. It is likely that the calcium channel blockers used here not only affect intracellular calcium levels by inhibiting entry through calcium channels, but they may also alter calcium dependent processes within the plasma membranes which modulate sodium transfer across epithelia.