Characteristics of single Na+ channels of adult human skeletal muscle

The patch-clamp technique was used to study Na⁺ channels of human skeletal muscle. Preparations were from biopsies of quadriceps muscle from adults who were not suffering from neuromuscular diseases. Activity of Na⁺ channels was recorded from inside-out patches when the membrane potential was stepped from a holding potential of ±110mV to potential above a threshold of about ±65 mV. Single channel activity increased within minutes after hyperpolarizing the patch due to recovery from ultraslow inactivation. Up to ten Na⁺ channels were active in individual patches. Macroscopic currents were reconstructed by averaging single channel currents. The time-to-peak current declined from 1.6 ms at ±60 mV to 0.5 ms at +10 mV. The currents decayed mono-exponentially with time constants between 12.1 ms at ±60 mV and 0.4 ms at +10 mV (21°C). The conductance of single Na⁺ channels was 1.65 pS and the mean open time was voltage-dependent. At ±50 mV, the mean open time was 0.4 ms, while positive to ±10 mV it increased to values above 1 ms. In the threshold potential range, the number of openings per depolarizing pulse was larger than the number of channels under the patch-clamp pipette, indicating reopening of Na⁺ channels at this potential. Openings could be observed only rarely 10 ms after onset of depolarization and the macroscopic current produced by late openings was less than 0.1% of the peak current. Human skeletal muscle is thus suitable for investigation with the patch-clamp technique and the determination of properties of Na⁺ channels with this technique could be the basis for an assessment of possible defects of these channels in diseased muscle.     

Genetic variation in LINGO-1 (rs9652490) and risk of Parkinson's disease: twelve studies and a meta-analysis

Mutations of the voltage gated sodium channel gene (SCN4A) are responsible for non-dystrophic myotonia including hyperkalemic periodic paralysis, paramyotonia congenita, and sodium channel myotonia, as well as congenital myasthenic syndrome. In vitro functional analyses have demonstrated the non-dystrophic mutants to show a gain-of-function defect of the channel; a disruption of fast inactivation, an enhancement of activation, or both, while the myasthenic mutation presents a loss-of function defect. This report presents a case of non-dystrophic myotonia that is incidentally accompanied with acquired myasthenia. The patient presented a marked warm-up phenomenon of myotonia but the repeated short exercise test suggested mutations of the sodium channel. The genetic analysis identified a novel mutation, G1292D, of SCN4A. A functional study of the mutant channel revealed marked enhancement of activation and slight impairment of fast inactivation, which should induce muscle hyperexcitability. The effects of the alteration of channel function to the myasthenic symptoms were explored by using stimulation of repetitive depolarization pulses. A use-dependent channel inactivation was reduced in the mutant in comparison to normal channel, thus suggesting an opposing effect to myasthenia.     

Confirmation of linkage of hyperkalaemic periodic paralysis to chromosome 17.

Linkage studies were performed in six European families with hyperkalaemic periodic paralysis (PPII) with myotonia, an autosomal dominantly inherited disorder characterised by episodic weakness. The weakness is caused by non-inactivating sodium channels of reduced single channel conductance of the muscle fibre membrane. Recently, portions of the gene coding for the alpha subunit of the sodium channel of the adult human skeletal muscle (h-Na2) have been cloned and localised on chromosome 17q with no recombinants to the human growth hormone locus (GH1). Linkage between these two chromosome 17 markers and the disease was shown in our families (Z = 7.14, 0 = 0.00). These results, combined with the linkage data of a single large American family, suggest that the disease is caused by dominant mutations of the adult sodium channel, and that it is probably a genetically homogeneous disorder. Hyperkalaemic periodic paralysis is the first non-progressive myotonic disorder to be localised on the human genome.     

Na+ selective channels in the apical membrane of rabbit late proximal tubules (pars recta)

Using the patch-clamp technique, Na⁺ selective channels were observed in the luminal membrane of rabbit straight proximal tubule segments. In the cell-attached configuration (NaCl-Ringers in pipette and bath) influx of Na⁺ ions from the pipette into the cell through fluctuating channels was observed was observed. The current-voltage curve of these Na⁺ channels yielded a zero-current potential of 84.3±30.9 mV (n=10), reflecting the electrochemical driving force for Na⁺ influx under resting conditions. The single channel conductance was 12.0±2.1 pS (n=13). In inside-out oriented cell-excised patches the single channel conductance was not significantly different with NaCl-Ringers on both sides. At clamp potentials ranging from +50 mV to –50 mV the single channel current was ohmic and channel kinetics were independent of the voltage. With KCl-Ringers on the bath side (corresponding to cell interior), the zero current potential was 62±19 mV (n=4), indicating a high selectivity of Na⁺ over K⁺ ions. Addition of 10^–5 mol/l amiloride to the bathing solution decreased the mean channel open time slightly. This effect was more pronounced with 10^–4 mol/l amiloride, whereas the single channel conductance was unaffected by the diuretic. 10^–3 mol/l amiloride caused a complete block of the channel. It is concluded that amiloride sensitive Na⁺ channels, with similar properties to those observed in tight epithelia, contribute to Na⁺ reabsorbtion in the straight portion of proximal tubules.     

Effects of S906T polymorphism on the severity of a novel borderline mutation I692M in Nav1.4 cause periodic paralysis

Hyperkalemic periodic paralysis (HyperPP) is a dominantly inherited muscle disease caused by mutations in SCN4A gene encoding skeletal muscle voltage gated Na_v1.4 channels. We identified a novel Na_v1.4 mutation I692M in 14 families out of the 104 genetically identified HyperPP families in the Neuromuscular Centre Ulm and is therefore as frequent as I693T (13 families out of 14 HyperPP families) in Germany. Surprisingly, in 13 families, a known polymorphism S906T was also present. It was on the affected allele in at least 10 families compatible with a possible founder effect in central Europe. All affected members suffered from episodic weakness; myotonia was also common. Compared with I692M patients, I692M‐S906T patients had longer weakness episodes, more affected muscles, CK elevation and presence of permanent weakness. Electrophysiological investigation showed that both mutants had incomplete slow inactivation and a hyperpolarizing shift of activation which contribute to membrane depolarization and weakness. Additionally, I692M‐S906T significantly enhanced close‐state fast inactivation compared with I692M alone, suggesting a higher proportion of inactivated I692M‐S906T channels upon membrane depolarization which may facilitate the initiation of weakness episodes and therefore clinical manifestation. Our results suggest that polymorphism S906T has effects on the clinical phenotypic and electrophysiological severity of a novel borderline Na_v1.4 mutation I692M, making the borderline mutation fully penetrant.     

Channelopathies: ion channel defects linked to heritable clinical disorders

Electrical signals are critical for the function of neurones, muscle cells, and cardiac myocytes. Proteins that regulate electrical signalling in these cells, including voltage gated ion channels, are logical sites where abnormality might lead to disease. Genetic and biophysical approaches are being used to show that several disorders result from mutations in voltage gated ion channels. Understanding gained from early studies on the pathogenesis of a group of muscle diseases that are similar in their episodic nature (periodic paralysis) showed that these disorders result from mutations in a gene encoding a voltage gated Na⁺ channel. Their characterisation as channelopathies has served as a paradigm for other episodic disorders. For example, migraine headache and some forms of epilepsy have been shown to result from mutations in voltage gated Ca²⁺ channel genes, while long QT syndrome is known to result from mutations in either K⁺ or Na⁺ channel genes. This article reviews progress made in the complementary fields of molecular genetics and cellular electrophysiology which has led to a better understanding of voltage gated ion channelopathies in humans and mice.


Keywords: ion channel genetics; ion channel physiopathology; channelopathies; hereditary diseases     

Effects of veratridine on single neuronal sodium channels expressed inXenopus oocytes

(1) Chick neuronal Na⁺ channels were expressed inXenopus laevis oocytes after injection with total messenger ribonucleic acid (mRNA) isolated from chick brain. The currents were investigated with the whole cell voltage clamp and with the patch clamp technique. Activation and inactivation of the induced current, and its sensitivity towards tetrodotoxin (TTX) and veratridine were reminiscent of vertebrate neuronal Na⁺ channels. (2) In the presence of veratridine normal single channel openings often converted into small amplitude openings of long duration. These small amplitude openings persisted for hundreds of milliseconds after return to the holding potential. (3) The slope conductance of the veratridine modified open channel state was 5–6 pS as compared to the normal state with 21–25 pS in the voltage range between –35 and +5 mV. (4) The modified channel showed saturation behaviour towards Na⁺ ions. Half saturation of the single channel amplitude was observed at 330 mM Na⁺ at a membrane potential of –100 mV. (5) Final closure of the modified channel after return to the holding potential followed an exponential time course. Its potential dependence was similar to that of the time course of the veratridine induced tail currents in the whole cell configuration. (6) The properties of the Na⁺ channel derived from chick forebrain are compared with the properties of the same channel derived from chick skeletal muscle. Both were expressed in the same membrane environment, theXenopus oocyte plasma membrane. While earlier results with Na⁺ channels of muscle origin showed two channel populations, one with short and another with long mean open times, Na⁺ channels of neuronal origin were homogeneous and characterized by short open times.     

Single-channel current/voltage relationships of two kinds of Na+ channel in vertebrate sensory neurons

The electrical signals of nerve and muscle are fundamentally dependent on the voltage-gated Na⁺ channel, which is responsible for the rising phase of the action potential. At least two kinds of Na⁺ channel are expressed in the membrane of frog dorsal root ganglion (DRG) cells: Na⁺ channels with fast kinetics that are blocked by tetrodotoxin (TTX) at high affinity, and Na⁺ channels with slower kinetics that are insensitive to TTX. Recordings of single-channel currents from frog DRG cells, under conditions favoring Na⁺ as the charge carrier, reveal two distinct amplitudes of single-channel events. With 300 mM external Na⁺, single-channel events that can be measured in the presence of 1 M TTX have a slope conductance 7.5 pS. In the absence of TTX, events with a slope conductance of 14.9 pS dominate. Ensemble averages of the smaller single-channel events display the slower kinetics characteristic of the macroscopic TTX-insensitive Na⁺ currents, and ensemble averages of the larger events display the faster kinetics characteristic of the TTX-sensitive currents. The results are consistent with the idea that the toxin-binding site is sufficiently close to the pore to influence ion permeation.     

Intracellular Na+ modulates large conductance Ca2+-activated K+ currents in human umbilical vein endothelial cells

We studied the effects of Na⁺ influx on large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels in cultured human umbilical vein endothelial cells (HUVECs) by means of patch clamp and SBFI microfluorescence measurements. In current-clamped HUVECs, extracellular Na⁺ replacement by NMDG⁺ or mannitol hyperpolarized cells. In voltage-clamped HUVECs, changing membrane potential from 0 mV to negative potentials increased intracellular Na⁺ concentration ([Na⁺]_i) and vice versa. In addition, extracellular Na⁺ depletion decreased [Na⁺]_i. In voltage-clamped cells, BK_Ca currents were markedly increased by extracellular Na⁺ depletion. In inside-out patches, increasing [Na⁺]_i from 0 to 20 or 40 mM reduced single channel conductance but not open probability (NPo) of BK_Ca channels and decreasing intracellular K⁺ concentration ([K⁺]_i) gradually from 140 to 70 mM reduced both single channel conductance and NPo. Furthermore, increasing [Na⁺]_i gradually from 0 to 70 mM, by replacing K⁺, markedly reduced single channel conductance and NPo. The Na⁺–Ca²⁺ exchange blocker Ni²⁺ or KB-R7943 decreased [Na⁺]_i and increased BK_Ca currents simultaneously, and the Na⁺ ionophore monensin completely inhibited BK_Ca currents. BK_Ca currents were significantly augmented by increasing extracellular K⁺ concentration ([K⁺]_o) from 6 to 12 mM and significantly reduced by decreasing [K⁺]_o from 12 or 6 to 0 mM or applying the Na⁺–K⁺ pump inhibitor ouabain. These results suggest that intracellular Na⁺ inhibit single channel conductance of BK_Ca channels and that intracellular K⁺ increases single channel conductance and NPo. GH Liang and MY Kim contributed equally to this publication and therefore share the first authorship.     

Voltage-gated Na channels in chemoreceptor afferent neurons—Potential roles and changes with development

Carotid body chemoreceptors increase their action potential (AP) activity in response to a decrease in arterial oxygen tension and this response increases in the post-natal period. The initial transduction site is likely the glomus cell which responds to hypoxia with an increase in intracellular calcium and secretion of multiple neurotransmitters. Translation of this secretion to AP spiking levels is determined by the excitability of the afferent nerve terminals that is largely determined by the voltage-dependence of activation of Na⁺ channels. In this review, we examine the biophysical characteristics of Na⁺ channels present at the soma of chemoreceptor afferent neurons with the assumption that similar channels are present at nerve terminals. The voltage dependence of this current is consistent with a single Na⁺ channel isoform with activation around the resting potential and with about 60-70% of channels in the inactive state around the resting potential. Channel openings, due to transitions from inactive/open or closed/open states, may serve to amplify external depolarizing events or generate, by themselves, APs. Over the first two post-natal weeks, the Na⁺ channel activation voltage shifts to more negative potentials, thus enhancing the amplifying action of Na⁺ channels on depolarization events and increasing membrane noise generated by channel transitions. This may be a significant contributor to maturation of chemoreceptor activity in the post-natal period.     

Identification of single transiently opening (“A-type”) K channels in guinea-pig colonic myocytes

Two K⁺ channel populations were identified in depolarized cell-attached membrane patches of myocytes freshly dispersed from the circular smooth muscle of guinea-pig proximal colon. First, a large-conductance (150 pS) Ca²⁺-activated K⁺ channel which was non-inactivating and sensitive to blockade by tetraethylammonium (TEA, 0.5–5 mM); and second, a smaller conductance K⁺ channel which opened and closed within 100 ms, was insensitive to TEA (0.5–5 mM), but was blocked by 5 mM 4-aminopyridine (4-AP) or maintained depolarization, and which had a unitary conductance of 12–13 pS. The averaged time course of these smaller conductance K⁺ channels closely resembled the time course of the 4-AP-sensitive, Ca²⁺-insensitive transient outward K⁺ current recorded in the whole-cell recording mode.     

Patch-clamp study of cultured human sweat duct cells: amiloride-blockable Na+ channel

The reabsorptive duct of the eccrine sweat gland has a large transepithelial conductance consisting mainly of a high conductance to Cl^– and a smaller, amiloride-blockable Na⁺ conductance (Bijman and Frömter 1986; Quinton 1985). Cells have been cultured from sweat ducts and their properties previously studied in Ussing chambers (Pedersen 1988) and with microelectrodes (Jones et al. 1988). We have now studied the ion channels present in excised, inside-out patches of human cultured sweat duct cells, and find a marked predominance of linear, 16 pS, amiloride-blockable, low selectivity, Na⁺ channels. Such channels were seen in 54/92 (59%) of the patches, with up to 7 channels recorded in a single patch.Other channel types were seen at much lower densities. The prevalence of an amiloride-blockable Na⁺ channel in cultured duct cells clearly distinguishes these cells from cultured sweat gland secretory cells, which lack such a channel.     

Inactivation modifiers of Na+ currents and the gating of rat brain Na+ channels in planar lipid membranes

Rat brain Na⁺ channels whose inactivation process had been removed either by batrachotoxin (BTX) or veratridine (VT) were reconstituted into planar lipid membranes. The voltage dependence of the open probability (P _o) of the channel, of the opening and closing rate constants, and the conductance and relative permeability for Na⁺ and K⁺ were studied in voltage-clamp conditions in the presence of agents known to modify the inactivation of Na⁺ currents. In relation to alkaloids (BTX, VT, and aconitine), it was found that once a Na⁺ channel was modified by BTX or VT, the addition of another alkaloid did not change further the gating and permeation properties of the channel over a period of about 1 h. Once the inactivation process of the channels is removed by BTX, the addition of a proteolytic enzyme (trypsin) or an halogenated compound (chloramine-T, CT) induced profound and specific modifications on the opening and closing events of Na⁺ channels: (1) the voltage dependence of the channel P _o shifted to more hyperpolarized potentials; (2) this voltage shift can be explained by equal hyperpolarizing voltage shifts of the opening and closing rate constants of the channel; (3) although the gating properties of the channel were modified by these compounds, the permeation properties of the channel, as evaluated by the conductance and the selectivity to Na⁺ and K⁺ ions, were unaltered; (4) trypsin and CT were active only in the intracellular side of the channel and were irreversible within the time course of the experiments, suggesting covalent modifications of the channel. Inactivation modifiers also affected the gating of toxin-activated single Na⁺ channels. This alteration is compatible with a simple increase in the intracellular potential as seen by the voltage sensor of the channel.     

Regulation of a non-selective cation channel of cultured porcine coronary artery smooth muscle cells by tyrosine kinase

A non-selective cation channel was found in primary cultured porcine coronary artery smooth muscle cells. In patch-clamp studies in the cell-attached mode, this channel was activated by bath application of genistein, a specific inhibitor of tyrosine kinase, but not by daidzein, which is similar in structure to genistein but has no inhibitory effect on tyrosine kinase. This channel discriminated poorly between Na⁺ and K⁺ (permeability ratio P _Na/P _K=1.03), and also transported Ca²⁺. The single-channel conductance measured with a pipette solution containing 150mM Na⁺ was 139±24 pS (mean ± SD, n=5), and that for the inward current measured with 100 mM Ca²⁺ solution was 25±9 pS (n=3). This non-selective cation channel was also activated by staurosporine, a non-specific protein kinase inhibitor, but not by H-7, an inhibitor of protein serine/ threonine kinase. These results suggest that the activity of the non-selective cation channel is negatively regulated by tyrosine kinase activity, and thus a decrease of the enzyme activity in porcine coronary artery smooth muscle cells may result in membrane depolarization and Ca²⁺ entry.     

Single calcium-activated potassium channel in cultured mammary epithelial cells

The properties of Ca²⁺-activated K⁺ channels in mouse mammary epithelial cells in primary culture were studied by the patch-clamp technique. In cell-attached patches, spontaneous channel openings were sometimes observed; the slope conductance of the currents was about served; the slope conductance of the currents was about 12 pS at negative membrane potentials with a physiological solution (152 mM Na⁺, 5.4 mM K⁺) in the pipette. External application of A23187, a calcium ionophore, activated this channel. In excised inside-out patches, the channel was activated by increasing the internal Ca²⁺ concentration (10^–7 to 10^–6 M). No voltage dependence of the channel activity was observed. Internal Na⁺ blocked the outward K⁺ current in a voltage dependent manner and this block led to the non-linear I–V relationship at positive membrane potentials. The channel was blocked by internal Ba²⁺ (0.1 mM) and tetracthylammonium (TEA⁺, 20–50 mM). Ba²⁺ reduced the open probability but not the single channel conductance, whereas TEA⁺ reduced the single channel conductance. The single channel conductance of this channel, measured from the inward current with a high-K⁺ solution (150 mM K⁺) in the pipette, was large (about 40 pS), and showed inward rectification. These results suggest that this channel is different from the usual small conductance Ca²⁺-activated K⁺ channels observed in many other cells.     

Changes in intracellular ion activities induced by adrenaline in human and rat skeletal muscle

To study the stimulating effect of adrenaline (ADR) on active Na⁺/K⁺ transport we used double-barrelled ion-sensitive micro-electrodes to measure the activities of extracellular K⁺ (aK_e) and intracellular Na⁺ (aNa_i) in isolated preparations of rat soleus muscle, normal human intercostal muscle and one case of hyperkalemic periodic paralysis (h.p.p.). In these preparations bath-application of ADR (10⁻⁶ M) resulted in a membrane hyperpolarization and transient decreasesaK_e andaNa_i which could be blocked by ouabain (3×10⁻⁴ M). In the h.p.p. muslce a continuous rise ofaNa_i induced by elevation ofaK_e to 5.2 mM could be stopped by ADR. In addition, the intracellular K⁺ activity (aK_i), the free intracellular Ca²⁺ concentration (pCa_i) and intracellular pH (pH_i) were monitored in rat soleus muscle. During ADRaK_i increased, pH_i remained constant and intracellular Ca²⁺ apparently decreased. In conclusion, our data show that ADR primarily stimulates the Na⁺/K⁺ pump in mammalian skeletal muscle. This stimulating action is not impaired in the h.p.p. muscle. Parts of the results have been presented to the German Physiological Society (Ballanyi and Grafe 1987)     

Amiloride-blockable Ca2+-activated Na+-permeant channels in the fetal distal lung epithelium

The Na⁺ transport function of alveolar epithelium represents an important mechanism for clearance of fluid in air space at birth. I observed the activity of two types of amiloride-blockable Na⁺-permeant cation channels in the apical membrane of fetal distal lung epithelium cultured on permeable filters for 2 days after harvesting of the cells from Wistar rats of 20 days' gestation (term = 22 days). One type was a nonselective cation (NSC) channel and had a linear current/voltage (I/V) relationship with a single-channel conductance of 26.9 ± 0.8 pS (n = 5). The other type was highly Na⁺ selective (i.e. Na⁺ channel) and had an inwardly rectifyingI/V relationship with a single-channel conductance of 11.8 ± 0.2 pS (n = 5) around resting membrane potential. The NSC channel was more frequently observed (1.37 ± 0.15 per patch membrane;n = 73) than the Na⁺ channel (0.15 ± 0.40 per patch membrane;n = 73). However, the open probability of the NSC channel was smaller than that of the Na⁺ channel. Both types of the channels were activated by cytosolic Ca²⁺, however the sensitivity to cytosolic Ca²⁺ was much higher in the Na⁺ channel than in the NSC channel. Furthermore, both types of the channels were blocked by amiloride or benzamil. The half-maximal inhibitory concentration (IC₅₀) of amiloride or benzamil of the Na⁺ channel was 1–2 M, while that of NSC channel was less than 1 M. Both channels were activated by insulin.     

Regulation of epithelial ion channels by the cystic fibrosis transmembrane conductance regulator

In most epithelia ion transport is tightly regulated. One major primary target of such regulation is the modulation of ion channels. The present brief review focuses on one specific example of ion channel regulation by the cystic fibrosis transmembrane conductance regulator (CFTR). CFTR functions as a cAMP-regulated Cl^- channel. Its defect leads to the variable clinical pictures of cystic fibrosis (CF), which today is understood as a primary defect of epithelial Cl^- channels in a variety of tissues such as the respiratory tract, intestine, pancreas, skin, epididymis, fallopian tube, and others. Most recent findings suggest that CFTR also acts as a channel regulator. Three examples are discussed by which CFTR regulates other Cl^- channels, K⁺ channels, and epithelial Na⁺ channels. From this perspective it is evident that CFTR may play a major role in the integration of cellular function.Abbreviations CF Cystic fibrosis - CFTR Cystic fibrosis transmembrane conductance regulator - IBMX Isobutylmethylxanthine - ICOR Intermediate conductance outwardly rectifying - MDR Multidrug resistance protein Supported by DFG: Gr 480/11     

Distribution of (Na+ + K+)ATPase and sodium channels in skeletal muscle and electroplax

Summary The distributions of (Na⁺ + K⁺)ATPase and sodium channels in skeletal muscle fibres and electrocytes were determined by immunofluorescent and immunoelectron microscopic techniques using antibodies against rat and eel (Na⁺ + K⁺)ATPase and the eel electric organ sodium channel. The extrajunctional sarcolemma of skeletal muscle was uniformly stained by polyclonal antibodies against (Na⁺ + K⁺)ATPase and the sodium channel. The T-tubule system of skeletal muscle was also labelled heavily for both (Na⁺ + K⁺)ATPase and the sodium channel. The terminal cisternae of the sarcoplasmic reticulum was stained for (Na⁺ + K⁺)ATPase but not sodium channels. At the motor endplate, (Na⁺ + K⁺)ATPase-like immunoreactivity was present along the plasmalemma of motor nerve terminals but not along the postsynaptic junctional sarcolemma. Paradoxically, a monoclonal antibody that binds to the form of the catalytic subunit of (Na⁺ + K⁺)ATPase from rat hepatocytes and renal tubule cells did not label the enzyme in rat skeletal muscle. In electrocytes, (Na⁺ + K⁺)ATPase-like irnmunoreactivity was concentrated primarily along the plasmalemma and calveolae of the non-innervated face. In contrast, sodium channel-like immunoreactivity was concentrated along the plasmalemma of the innervated face except in the clefts of the postsynaptic membrane. Thus, we conclude that at endplates both the (Na⁺ + K⁺)ATPase of rat skeletal muscle and sodium channels of eel electrocytes are not concentrated in the juxtaneuronal postsynaptic membrane. We also interpret the failure of the monoclonal anti- (Na⁺ + K⁺)ATPase antibodies to bind to the enzyme in muscle to indicate that the catalytic subunit of skeletal muscle (Na⁺ + K⁺)ATPase displays different epitopes than does the a subunit of kidney and liver.     

Signaling complexes of voltage-gated sodium and calcium channels

Membrane depolarization and intracellular Ca²⁺ transients generated by activation of voltage-gated Na⁺ and Ca²⁺ channels are local signals, which initiate physiological processes such as action potential conduction, synaptic transmission, and excitation–contraction coupling. Targeting of effector proteins and regulatory proteins to ion channels is an important mechanism to ensure speed, specificity, and precise regulation of signaling events in response to local stimuli. This article reviews experimental results showing that Na⁺ and Ca²⁺ channels form local signaling complexes, in which effector proteins, anchoring proteins, and regulatory proteins interact directly with ion channels. The intracellular domains of these channels serve as signaling platforms, mediating their participation in intracellular signaling processes. These protein–protein interactions are important for regulation of cellular plasticity through modulation of Na⁺ channel function in brain neurons, for short-term synaptic plasticity through modulation of presynaptic Ca_V2 channels, and for the fight-or-flight response through regulation of postsynaptic Ca_V1 channels in skeletal and cardiac muscle. These localized signaling complexes are essential for normal function and regulation of electrical excitability, synaptic transmission, and excitation–contraction coupling.