Molecular determinants of drug access to the receptor site for antiarrhythmic drugs in the cardiac Na+ channel

The clinical efficacy of local anesthetic and antiarrhythmic drugs is due to their voltage- and frequency-dependent block of Na+ channels. Quaternary local anesthetic analogs such as QX-314, which are permanently charged and membrane-impermeant, effectively block cardiac Na+ channels when applied from either side of the membrane but block neuronal Na+ channels only from the intracellular side. This difference in extracellular access to QX-314 is retained when rat brain rIIA Na+ channel alpha subunits and rat heart rH1 Na+ channel alpha subunits are expressed transiently in tsA-201 cells. Amino acid residues in transmembrane segment S6 of homologous domain IV (IVS6) of Na+ channel alpha subunits have important effects on block by local anesthetic drugs. Although five amino acid residues in IVS6 differ between brain rIIA and cardiac rH1, exchange of these amino acid residues by site-directed mutagenesis showed that only conversion of Thr-1755 in rH1 to Val as in rIIA was sufficient to reduce the rate and extent of block by extracellular QX-314 and slow the escape of drug from closed channels after use-dependent block. Tetrodotoxin also reduced the rate of block by extracellular QX-314 and slowed escape of bound QX-314 via the extracellular pathway in rH1, indicating that QX-314 must move through the pore to escape. QX-314 binding was inhibited by mutation of Phe-1762 in the local anesthetic receptor site of rH1 to Ala whether the drug was applied extracellularly or intracellularly. Thus, QX-314 binds to a single site in the rH1 Na+ channel alpha subunit that contains Phe-1762, whether it is applied from the extracellular or intracellular side of the membrane. Access to that site from the extracellular side of the pore is determined by the amino acid at position 1755 in the rH1 cardiac Na+ channel.

Structure and function of the beta 2 subunit of brain sodium channels, a transmembrane glycoprotein with a CAM motif

Voltage-gated sodium channels in brain neurons are complexes of a pore-forming alpha subunit with smaller beta 1 and beta 2 subunits. cDNA cloning and sequencing showed that the beta 2 subunit is a 186 residue glycoprotein with an extracellular NH2-terminal domain containing an immunoglobulin-like fold with similarity to the neural cell adhesion molecule (CAM) contactin, a single transmembrane segment, and a small intracellular domain. Coexpression of beta 2 with alpha subunits in Xenopus oocytes increases functional expression, modulates gating, and causes up to a 4-fold increase in the capacitance of the oocyte, which results from an increase in the surface area of the plasma membrane microvilli. beta 2 subunits are unique among the auxiliary subunits of ion channels in combining channel modulation with a CAM motif and the ability to expand the cell membrane surface area. They may be important regulators of sodium channel expression and localization in neurons.

Modulation of cardiac Na+ channel expression in Xenopus oocytes by beta 1 subunits

Voltage-gated Na+ channels consist of a large alpha subunit of 260 kDa associated with beta 1 and/or beta 2 subunits of 36 and 33 kDa, respectively. alpha subunits of rat cardiac Na+ channels (rH1) are functional when expressed alone in Xenopus oocytes or mammalian cells. beta 1 subunits are present in the heart, and localization of beta 1 subunit mRNA by in situ hybridization shows expression in the perinuclear cytoplasm of cardiac myocytes. Coexpression of beta 1 subunits with rH1 alpha subunits in Xenopus oocytes increases Na+ currents up to 6-fold in a concentration-dependent manner. However, no effects of beta 1 subunit coexpression on the kinetics or voltage dependence of the rH1 Na+ current were detected. Increased expression of Na+ currents is not observed when an equivalent mRNA encoding a nonfunctional mutant beta 1 subunit is coexpressed. Our results show that beta 1 subunits are expressed in cardiac muscle cells and that they interact with alpha subunits to increase the expression of cardiac Na+ channels in Xenopus oocytes, suggesting that beta 1 subunits are important determinants of the level of excitability of cardiac myocytes in vivo.

Molecular determinants of high affinity phenylalkylamine block of L-type calcium channels

The high affinity phenylalkylamine (-)D888 blocks ion currents through L-type Ca2+ channels containing the alpha 1C subunit with an apparent Kd of 50 nM, but N-type Ca2+ channels in the pheochromocytoma cell line PC12 are blocked with a 100-fold higher Kd value of 5 microM. L-type Ca2+ channels containing alpha 1C subunits with the site-directed mutations Y1463A, A1467S, or I1470A in the putative transmembrane segment S6 in domain IV (IVS6) were 6-12 times less sensitive to block by (-)D888 than control alpha 1C. Ca2+ channels containing paired combinations of these mutations were even less sensitive to block by (-)D888 than the single mutants, and channels containing all three mutations were > 100 times less sensitive to (-)D888 block, similar to N-type Ca2+ channels. In addition, the Y1463A mutant and all combination mutants including the Y1463A mutation had altered ion selectivity, suggesting that Tyr-1463 faces the pore and is involved in ion permeation. Since these three critical amino acid residues are aligned on the same face of the putative IVS6 alpha-helix, we propose that they contribute to a receptor site in the pore that confers a high affinity block of L-type channels by (-)D888.

A critical role for transmembrane segment IVS6 of the sodium channel alpha subunit in fast inactivation

Fast Na+ channel inactivation is thought to occur by the binding of an intracellular inactivation gate to regions around or within the Na+ channel pore through hydrophobic interactions. Previous studies indicate that the intracellular loop between domains III and IV of the Na+ channel alpha subunit (LIII-IV) forms the inactivation gate. A three-residue hydrophobic motif (IFM) is an essential structural feature of the gate and may serve as an inactivation particle that binds within the pore. In this study, we used alanine-scanning mutagenesis to examine the functional role of amino acid residues in transmembrane segment IVS6 of the Na+ channel alpha subunit in fast inactivation. Mutant F1764A, in the center of IVS6, and mutant V1774A, near its intracellular end, exhibited substantial sustained Na+ currents at the end of 30-ms depolarizations. The double mutation F1764A/V1774A almost completely abolished fast inactivation, demonstrating a critical role for these amino acid residues in the process of inactivation. Single channel analysis of these three mutants revealed continued reopenings late in 40-ms depolarizing pulses, indicating that the stability of the inactivated state was substantially impaired compared with wild type. In addition, the cumulative first latency distribution for the V1774A mutation contained a new component arising from opening transitions from the destabilized inactivated state. Substitution of multiple amino acid residues showed that the disruption of inactivation was not correlated with the hydrophobicity of the substitution at position 1774, in contrast to the expectation if this residue interacts directly with the IFM motif. Thermodynamic cycle analysis of simultaneous mutations in the IFM motif and in IVS6 suggested that mutations in these two regions independently disrupt inactivation, consistent with the conclusion that they do not interact directly. Furthermore, a peptide containing the IFM motif (acetyl-KIFMK-amide) restored inactivation to the F1764A/V1774A IVS6 mutant, indicating that the binding site for the IFM motif remains intact in these mutants. These results suggest that the amino acid residues 1764 and 1774 in IVS6 do not directly interact with the IFM motif of the inactivation gate but instead play a novel role in fast inactivation of the Na+ channel.

Functional co-expression of the beta 1 and type IIA alpha subunits of sodium channels in a mammalian cell line

Brain sodium channels are a complex of alpha (260 kDa), beta 1 (36 kDa), and beta 2 (33 kDa) subunits, alpha subunits are functional as voltage-gated sodium channels by themselves. When expressed in Xenopus oocytes, beta 1 subunits accelerate the time course of sodium channel activation and inactivation by shifting them to a fast gating mode, but alpha subunits expressed alone in mammalian cells activate and inactivate rapidly without co-expression of beta 1 subunits. In these experiments, we show that the Chinese hamster cell lines CHO and 1610 do not express endogenous beta 1 subunits as determined by Northern blotting, immunoblotting, and assay for beta 1 subunit function by expression of cellular mRNA in Xenopus oocytes. alpha subunits expressed alone in stable lines of these cells activate and inactivate rapidly. Co-expression of beta 1 subunits increases the level of sodium channels 2- to 4-fold as determined from saxitoxin binding, but does not affect the Kd for saxitoxin. Co-expression of beta 1 subunits also shifts the voltage dependence of sodium channel inactivation to more negative membrane potentials by 10 to 12 mV and shifts the voltage dependence of channel activation to more negative membrane potentials by 2 to 11 mV. These effects of beta 1 subunits on sodium channel function in mammalian cells may be physiologically important determinants of sodium channel function in vivo.

A mutation in segment IVS6 disrupts fast inactivation of sodium channels

Na(+)-channel inactivation is proposed to occur by binding of an intracellular inactivation gate to a hydrophobic inactivation gate receptor in the intracellular mouth of the pore. Amino acid residues in transmembrane segment S6 of domain IV (IVS6) that are critical for fast inactivation were identified by alanine-scanning mutagenesis. Mutant VIL1774-6AAA, in which three adjacent residues (Val-Ile-Leu) at the intracellular end of segment IVS6 were converted to alanine, had substantial (> 85%) sustained Na+ currents remaining 15 ms after depolarization, while a nearby mutation of three residues to alanine had no effect. Single-channel analysis revealed continued reopenings late in 40-ms depolarizing pulses indicating that inactivation was substantially impaired compared to wild type. The mean open time for VIL1774-6AAA was longer than wild type, suggesting that this mutation also decreases the rate of entry into the fast inactivated state. These results suggest that residues near the intracellular end of segment IVS6 are critical for fast Na(+)-channel inactivation and may form part of the hydrophobic receptor site for the fast inactivation gate.

Modulation of brain Na+ channels by a G-protein-coupled pathway

Na+ channels in acutely dissociated rat hippocampal neurons and in Chinese hamster ovary (CHO) cells transfected with a cDNA encoding the alpha subunit of rat brain type IIA Na+ channel (CNaIIA-1 cells) are modulated by guanine nucleotide binding protein (G protein)-coupled pathways under conditions of whole-cell voltage clamp. Activation of G proteins by 0.2-0.5 mM guanosine 5'-[gamma-thio]triphosphate (GTP[gamma S]), a nonhydrolyzable GTP analog, increased Na+ currents recorded in both cell types. The increase in current amplitude was caused by an 8- to 10-mV negative shift in the voltage dependence of both activation and inactivation. The effects of G-protein activators were blocked by treatment with pertussis toxin or guanosine 5'-[beta-thio]diphosphate (GDP[beta S]), a nonhydrolyzable GDP analog, but not by cholera toxin. GDP[beta S] (2 mM) alone had effects opposite those of GTP[gamma S], shifting Na(+)-channel gating 8-10 mV toward more-positive membrane potentials and suggesting that basal activation of G proteins in the absence of stimulation is sufficient to modulate Na+ channels. In CNaIIA-1 cells, thrombin, which activates pertussis toxin-sensitive G proteins in CHO cells, caused a further negative shift in the voltage dependence of Na(+)-channel activation and inactivation beyond that observed with GTP alone. The results in CNaIIA-1 cells indicate that the alpha subunit of the Na+ channel alone is sufficient to mediate G protein effects on gating. The modulation of Na+ channels via a G-protein-coupled pathway acting on Na(+)-channel alpha subunits may regulate electrical excitability through integration of different G-protein-coupled synaptic inputs.

Voltage-dependent potentiation of L-type Ca2+ channels in skeletal muscle cells requires anchored cAMP-dependent protein kinase

Skeletal muscle L-type Ca2+ channels respond to trains of brief depolarizations with a strong shift of the voltage dependence of channel activation toward more negative membrane potentials and slowing of channel deactivation. Increased Ca2+ entry resulting from this potentiation of channel activity may increase contractile force in response to tetanic stimuli. This voltage-dependent Ca2+ channel potentiation requires phosphorylation by cAMP-dependent protein kinase (PKA) at a rate that suggests that kinase and channel may be maintained in close proximity through kinase anchoring. A peptide derived from the conserved kinase-binding domain of a PKA-anchoring protein (AKAP) prevents potentiation by endogenous PKA as effectively as inhibition of PKA by a specific peptide inhibitor or by omission of ATP from the intracellular solution. In contrast, a proline-substituted mutant of AKAP peptide has no effect. Potentiation in the presence of 2 microM exogenous catalytic subunit of PKA is unaffected, indicating that kinase anchoring is specifically blocked by the AKAP peptide. No effects of these agents were observed on the level or voltage dependence of basal Ca2+ channel activity before potentiation, suggesting that close physical proximity between the skeletal muscle Ca2+ channel and PKA is critical for voltage-dependent potentiation of Ca2+ channel activity but not for basal activity.

Molecular determinants of state-dependent block of Na+ channels by local anesthetics

Sodium ion (Na+) channels, which initiate the action potential in electrically excitable cells, are the molecular targets of local anesthetic drugs. Site-directed mutations in transmembrane segment S6 of domain IV of the Na+ channel alpha subunit from rat brain selectively modified drug binding to resting or to open and inactivated channels when expressed in Xenopus oocytes. Mutation F1764A, near the middle of this segment, decreased the affinity of open and inactivated channels to 1 percent of the wild-type value, resulting in almost complete abolition of both the use-dependence and voltage-dependence of drug block, whereas mutation N1769A increased the affinity of the resting channel 15-fold. Mutation I1760A created an access pathway for drug molecules to reach the receptor site from the extracellular side. The results define the location of the local anesthetic receptor site in the pore of the Na+ channel and identify molecular determinants of the state-dependent binding of local anesthetics.

Modulation of skeletal muscle sodium channels in a satellite cell line by protein kinase C

Adult vertebrate skeletal muscle sodium channels are responsible for the spread of excitation from the end-plate through the muscle membrane and transverse tubular system that ultimately leads to contraction. These channels can be distinguished from other sodium channels by their sensitivity to both mu-conotoxin and TTX. The mouse satellite muscle cell line MM14 expresses only TTX- and mu-conotoxin-sensitive sodium channels having the physiological characteristics of adult skeletal muscle channels in both undifferentiated myoblasts and differentiated myotubes. Using undifferentiated and differentiated MM14 cells as well as primary cultures of rat skeletal muscle, we have examined modulation of adult skeletal muscle sodium channels by activators of protein kinase C (PKC). Stimulation of PKC by 1-oleoyl-2-acetyl-sn-glycerol (OAG) slows sodium current macroscopic inactivation rate by up to 70% and reduces the peak sodium current as much as 88%. Single-channel analysis reveals prolonged single channel openings and greatly increased probability of multiple channel openings during sustained depolarizations. These effects are due to PKC activation since they are blocked by a specific peptide inhibitor of PKC. The two effects of OAG are sequential. Low OAG concentrations can cause slowed macroscopic sodium current inactivation in the absence of peak current reduction, and intermediate concentrations of OAG cause slowing of inactivation followed by reduction of peak current. The separation of these two effects indicates that PKC modulation of the skeletal muscle sodium channel may occur by phosphorylation at two independent sites. PKC modulation of muscle sodium channels is expected to have important effects on muscle excitability and resultant contractile activity. Detection of adult skeletal muscle ion channels in replicating MM14 cells suggest that satellite cells may express a distinct subset of muscle-specific genes prior to activation of the terminal differentiation program.

Restoration of inactivation and block of open sodium channels by an inactivation gate peptide

Inactivation of sodium channels terminates the sodium current responsible for initiation of action potentials in excitable cells. A hydrophobic sequence (isoleucine-phenylalanine-methionine, IFM), located in the inactivation gate segment connecting homologous domains III and IV of the sodium channel alpha subunit, is required for fast inactivation. A synthetic peptide containing the IFM sequence (acetyl-KIFMK-amide) restores fast inactivation to mutant sodium channels having a defective inactivation gate and to wild-type sodium channels having inactivation slowed by alpha-scorpion toxin. This peptide also competes with the intrinsic inactivation particle and binds to and blocks open sodium channels in a voltage- and frequency-dependent manner. A peptide (acetyl-KIQMK-amide) containing a mutation that prevents fast inactivation is not effective in restoring inactivation or in blocking open sodium channels. The results support the hypothesis that the sequence IFM serves as the inactivation particle of the sodium channel and suggest that it enters the intracellular mouth of the pore and occludes it during the process of inactivation.

Modulation of cardiac Na+ channels expressed in a mammalian cell line and in ventricular myocytes by protein kinase C

Cardiac rH1 Na+ channel alpha subunits were expressed in cells of the Chinese hamster lung 1610 cell line by transfection, and a stable cell line expressing cardiac Na+ channels (SNa-rH1) was isolated. Mean Na+ currents of 2.2 +/- 1.0 nA were recorded, which corresponds to a cell surface density of approximately 1-2 channels active at the peak of the Na+ current per micron2. The expressed cardiac Na+ current was tetrodotoxin resistant (Kd = 1.8 microM) and had voltage-dependent properties similar to those of the Na+ current in neonatal ventricular myocytes. Activation of protein kinase C by 1-oleoyl-2-acetyl-sn-glycerol (OAG) (10 microM) decreased this current approximately 33% at a holding potential of -114 mV and 56% at -94 mV. This reduction in peak current was caused in part by an 8- to 14-mV shift of steady-state inactivation in the hyperpolarized direction. Na+ channel activation was unchanged. Effects of OAG in SNa-rH1 cells and in neonatal rat cardiac myocytes were similar, except that the time course of inactivation was slowed either transiently or persistently when protein kinase C was activated in myocytes bathed in low-Ca2+ (1 microM) or Ca(2+)-free solution but was unaffected in SNa-rH1 cells. The effects of OAG on cardiac Na+ current were blocked in cells that had been previously microinjected with a peptide inhibitor of protein kinase C but not with a peptide inhibitor of cAMP-dependent protein kinase, indicating that protein kinase C is responsible for the effect of OAG. Single-channel recordings from SNa-rH1 cells showed that the probability of channel opening was reduced by OAG, but the conductance was unaffected. OAG did not induce the late Na+ channel openings observed with PKC modulation of neuronal and skeletal muscle Na+ channels. Thus, the substantial reduction in Na+ current at normal diastolic depolarizations with 10 microM OAG is due to failure of channel opening in response to depolarization. Such Na+ current reductions may have profound effects on cardiac cell excitability.

Voltage-dependent potentiation of the activity of cardiac L-type calcium channel alpha 1 subunits due to phosphorylation by cAMP-dependent protein kinase

Barium currents mediated by the alpha 1 subunit of the cardiac L-type Ca channel expressed in Chinese hamster ovary (CHO) cells were increased up to 10-fold during dialysis of the cell with the catalytic subunit of cAMP-dependent protein kinase. After partial activation by exogenous kinase, the activity of the alpha 1 subunit was also reversibly potentiated up to 3.5-fold by prepulses to voltages in the range of 0 to +150 mV. Potentiation at +48 mV developed with a biphasic time course with time constants of 131 ms and 8 s. Reversal at -60 mV was biphasic with half-times of 12 ms and 100 ms and was blocked in the presence of the phosphatase inhibitor okadaic acid. Both the increase in calcium-channel activity during dialysis with kinase and the voltage-dependent potentiation were accompanied by shifts in the voltage dependence of activation to more negative membrane potentials. The increases in Ba current due to protein phosphorylation and to the dihydropyridine Ca channel agonist Bay K8644 were approximately additive. The results show that the alpha 1 subunit of the cardiac L-type Ca channel is sufficient for substantial modulation of Ca-channel activity by cAMP-dependent protein kinase and for potentiation by state-dependent protein phosphorylation. Voltage-dependent potentiation of the activity of the alpha 1 subunit may contribute to the increase in contractile force in response to increased rate of stimulation, the positive staircase effect in heart muscle.

Convergent regulation of sodium channels by protein kinase C and cAMP-dependent protein kinase

The function of voltage-gated sodium channels that are responsible for action potential generation in mammalian brain neurons is modulated by phosphorylation by adenosine 3',5'-monophosphate (cAMP)-dependent protein kinase (cA-PK) and by protein kinase C (PKC). Reduction of peak sodium currents by cA-PK in intact cells required concurrent activation of PKC and was prevented by blocking phosphorylation of serine 1506, a site in the inactivation gate of the channel that is phosphorylated by PKC but not by cA-PK. Replacement of serine 1506 with negatively charged amino acids mimicked the effect of phosphorylation. Conversion of the consensus sequence surrounding serine 1506 to one more favorable for cA-PK enhanced modulation of sodium currents by cA-PK. Convergent modulation of sodium channels required phosphorylation of serine 1506 by PKC accompanied by phosphorylation of additional sites by cA-PK. This regulatory mechanism may serve to integrate neuronal signals mediated through these parallel signaling pathways.

Voltage-dependent potentiation of L-type Ca2+ channels due to phosphorylation by cAMP-dependent protein kinase

The force of contraction of motor units in skeletal muscle is graded by changing the discharge rate of motor neurons, and cytosolic calcium transients are similarly increased. During single twitches, contraction is not dependent on extracellular calcium, and L-type Ca2+ channels may only function as voltage sensors for initiating Ca2+ release from the sarcoplasmic reticulum. In contrast, forceful tetanic contractions triggered by action potentials at high frequency (20 to 200 Hz) are dependent on extracellular Ca2+ concentration and sensitive to L-type Ca2+ channel antagonists, but the mechanism of regulation of contractile force is unknown. Here we report a large, voltage- and frequency-dependent potentiation of skeletal muscle L-type Ca2+ currents by trains of high-frequency depolarizing prepulses, which is caused by a shift in the voltage-dependence of channel activation to more negative membrane potentials and requires phosphorylation by cyclic AMP-dependent protein kinase in a voltage-dependent manner. This potentiation would substantially increase Ca2+ influx and contractile force in skeletal muscle fibres in response to tetanic stimuli.

Voltage-dependent calcium and potassium conductances in striated muscle fibers from the scorpion, Centruroides sculpturatus

Ionic currents responsible for the action potential in scorpion muscle fibers were characterized using a three-intracellular microelectrode voltage clamp applied at the fiber ends (8-12 degrees C). Large calcium currents (ICa) trigger contractile activation in physiological saline (5 mM Ca) but can be studied in the absence of contractile activation in a low Ca saline (< or = 2.5 mM). Barium (Ba) ions (1.5-3 mM) support inward current but not contractile activation. Ca conductance kinetics are fast (time constant of 3 msec at 0 mV) and very voltage dependent, with steady-state conductance increasing e-fold in approximately 4 mV. Half-activation occurs at -25 mV. Neither ICa nor IBa show rapid inactivation, but a slow, voltage-dependent inactivation eliminates ICa at voltages positive to -40 mV. Kinetically, scorpion channels are more similar to L-type Ca channels in vertebrate cardiac muscle than to those in skeletal muscle. Outward K currents turn on more slowly and with a longer delay than do Ca currents, and K conductance rises less steeply with voltage (e-fold change in 10 mV; half-maximal level at 0 mV). K channels are blocked by externally applied tetraethylammonium and 3,4 diaminopyridine.

Inhibition of Na+ channels by the novel blocker PD85,639

This study examined the actions of the novel Na+ channel blocker PD85,639. In whole-cell voltage-clamp recordings from Chinese hamster ovary cells transfected with a cDNA encoding the rat brain type IIA Na+ channel and from dissociated rat brain neurons, PD85,639 attenuated Na+ currents when applied either in the external bath or in the internal pipette solution. Block had a tonic component that occurred in the absence of stimulus pulses and an additional use-dependent component that developed during a train of pulses. The EC50 for tonic block was 30 microM and was not strongly dependent on the holding potential. Use-dependent block was first detectable at 1 microM and was pronounced at higher concentrations, even at stimulus frequencies as low as 1 pulse/2 min. The marked use-dependent block was due to rapid drug binding during depolarizing pulses and very slow recovery of drug-bound channels between the pulses (tau = 11 min at -85 mV). Use-dependent block was greater at more depolarized potentials, suggesting that the drug binding site was partway across the membrane electric field. The block that developed with strong depolarizations was rapidly reversed by opening channels with trains of unblocking pulses to more negative potentials. These characteristics of block by PD85,639 suggest that it is a local anesthetic drug with novel properties.

A cluster of hydrophobic amino acid residues required for fast Na(+)-channel inactivation

The inward Na+ current underlying the action potential in nerve is terminated by inactivation. The preceding report shows that deletions within the intracellular linker between domains III and IV remove inactivation, but mutation of conserved basic and paired acidic amino acids has little effect. Here we show that substitution of glutamine for three clustered hydrophobic amino acids, Ile-1488, Phe-1489, and Met-1490, completely removes fast inactivation. Substitution of Met-1490 alone slows inactivation significantly, substitution of Ile-1488 alone both slows inactivation and makes it incomplete, and substitution of Phe-1489 alone removes inactivation nearly completely. These results demonstrate an essential role of Phe-1489 in Na(+)-channel inactivation. It is proposed that the hydrophobic cluster of Ile-1488, Phe-1489, and Met-1490 serves as a hydrophobic latch that stabilizes the inactivated state in a hinged-lid mechanism of Na(+)-channel inactivation.

Functional modulation of brain sodium channels by cAMP-dependent phosphorylation

Voltage-gated Na+ channels, which are responsible for the generation of action potentials in brain, are phosphorylated by cAMP-dependent protein kinase in vitro and in intact neurons. Phosphorylation by cAMP-dependent protein kinase reduces peak Na+ currents 40%--50% in membrane patches excised from rat brain neurons or from CHO cells expressing type IIA Na+ channels. Inhibition of basal cAMP-dependent protein kinase activity by transfection with a plasmid encoding a dominant negative mutant regulatory subunit increases Na+ channel number and activity, indicating that even the basal level of kinase activity is sufficient to reduce Na+ channel activity significantly. Na+ currents in membrane patches from kinase-deficient cells were reduced up to 80% by phosphorylation by cAMP-dependent protein kinase. These effects could be blocked by a specific peptide inhibitor of cAMP-dependent protein kinase and reversed by phosphoprotein phosphatases. Convergent modulation of brain Na+ channels by neurotransmitters acting through the cAMP and protein kinase C signaling pathways may result in associative regulation of electrical activity by different synaptic inputs.

Tetrodotoxin-insensitive sodium channels in a cardiac cell line from a transgenic mouse

The electrophysiological properties of a cardiac cell line (MCM1) originating from a transgenic mouse were characterized. The dominant current in these cells is a sodium current that is insensitive to concentrations of tetrodotoxin (TTX) up to 100 microM. It activates and inactivates rapidly with half-maximal activation at -40 mV and half-maximal inactivation at -79 mV. This sodium current is reduced by agents that increase intracellular adenosine 3',5'-cyclic monophosphate (cAMP) and activate cAMP-dependent protein kinase including isoproterenol, 8-bromo-cAMP, and isobutylmethylxanthine. The phenylalkylamine desmethoxyverapamil blocks the TTX-insensitive sodium current in MCM1 cells in both tonic and use-dependent fashion. Membrane depolarization enhances this block. It is proposed that the TTX-insensitive sodium current in these cells may be similar in origin to the embryonic type of TTX-insensitive sodium current described in other cardiac and skeletal muscle preparations.

Efficient expression of rat brain type IIA Na+ channel alpha subunits in a somatic cell line

Type IIA rat brain Na+ channel alpha subunits were expressed in CHO cells by nuclear microinjection or by transfection using a vector containing both metallothionein and bacteriophage SP6 promoters. Stable cell lines expressing Na+ channels were isolated, and whole-cell Na+ currents of 0.9-14 nA were recorded. The mean level of whole-cell Na+ current (4.5 nA) corresponds to a cell surface density of approximately 2 channels active at the peak of the Na+ current per microns 2, a density comparable to that observed in the cell bodies of central neurons. The expressed Na+ channels had the voltage dependence, rapid activation and inactivation, and rapid recovery from inactivation characteristic of Na+ channels in brain neurons, bound toxins at neurotoxin receptor sites 1 and 3 with normal properties, and were posttranslationally processed to a normal mature size of 260 kd. Expression of Na+ channel cDNA in CHO cells driven by the metallothionein promoter accurately and efficiently reproduces native Na+ channel properties and provides a method for combined biochemical and physiological analysis of Na+ channel structure and function.

A phosphorylation site in the Na+ channel required for modulation by protein kinase C

Voltage-gated sodium channels are responsible for generation of action potentials in excitable cells. Activation of protein kinase C slows inactivation of sodium channels and reduces peak sodium currents. Phosphorylation of a single residue, serine 1506, that is located in the conserved intracellular loop between domains III and IV and is involved in inactivation of the sodium channel, is required for both modulatory effects. Mutant sodium channels lacking this phosphorylation site have normal functional properties in unstimulated cells but do not respond to activation of protein kinase C. Phosphorylation of this conserved site in sodium channel alpha subunits may regulate electrical activity in a wide range of excitable cells.

Frequency and voltage-dependent inhibition of type IIA Na+ channels, expressed in a mammalian cell line, by local anesthetic, antiarrhythmic, and anticonvulsant drugs

This study examined the actions of phenytoin, carbamazepine, lidocaine, and verapamil on rat brain type IIA Na+ channels functionally expressed in mammalian cells, using the whole-cell voltage-clamp recording technique. The drugs blocked Na+ currents in both a tonic and use-dependent manner. Tonic block was more pronounced at depolarized holding potentials and reduced at hyperpolarized membrane potentials, reflecting an overall negative shift in the relationship between membrane potential and steady state inactivation. Dose-response relationships with phenytoin supported the hypothesis that the voltage dependence of tonic block resulted from the higher affinity of the drugs for inactivated than for resting channels. At -62 mV, approximately 50% of the Na+ channels were blocked by phenytoin at 13 microM, compared with therapeutic brain levels of 4-8 microM. The use-dependent component of block developed progressively during a 2-Hz train of 40-msec-long stimulus pulses from -85 mV to 0 mV. At 2 Hz, verapamil was the most potent use-dependent blocker, lidocaine and phenytoin had intermediate potencies, and carbamazepine was least effective. The use-dependent block resulted from drug binding to open and inactivated channels during the depolarizing pulses and the slow repriming of drug-bound channels during the interpulse intervals. Verapamil, lidocaine, and phenytoin all bound preferentially to open channels, but this open channel block was most striking for verapamil. Use-dependent block was less pronounced at hyperpolarized membrane potentials, due to more rapid repriming of drug-bound channels. The results indicate that type IIA Na+ channels expressed in a mammalian cell line retain the complex pharmacological properties characteristic of native Na+ channels. These channels are likely to be an important site of the anticonvulsant action of phenytoin and carbamazepine. Lidocaine and verapamil, drugs with well characterized effects on peripheral Na+ and Ca2+ channels, are also effective blockers of these brain Na+ channels.

Functional modulation of brain sodium channels by protein kinase C phosphorylation

Voltage-gated sodium channels, which are responsible for the generation of action potentials in the brain, are phosphorylated by protein kinase C (PKC) in purified form. Activation of PKC decreases peak sodium current up to 80 percent and slows its inactivation for sodium channels in rat brain neurons and for rat brain type IIA sodium channel alpha subunits heterologously expressed in Chinese hamster ovary cells. These effects are specific for PKC because they can be blocked by specific peptide inhibitors of PKC and can be reproduced by direct application of PKC to the cytoplasmic surface of sodium channels in excised inside-out membrane patches. Modulation of brain sodium channels by PKC is likely to have important effects on signal transduction and synaptic transmission in the central nervous system.

Functional properties of rat brain sodium channels expressed in a somatic cell line

Transfection of Chinese hamster ovary cells with complementary DNA encoding the RIIA sodium channel alpha subunit from rat brain led to expression of functional sodium channels with the rapid, voltage-dependent activation and inactivation characteristic of sodium channels in brain neurons. The sodium currents mediated by these transfected channels were inhibited by tetrodotoxin, persistently activated by veratridine, and prolonged by Leiurus alpha-scorpion toxin, indicating that neurotoxin receptor sites 1 through 3 were present in functional form. The RIIA sodium channel alpha subunit cDNA alone is sufficient for stable expression of functional sodium channels with the expected kinetic and pharmacological properties in mammalian somatic cells.

Inhibition of inactivation of single sodium channels by a site-directed antibody

The effects of site-directed antibodies on single sodium channel currents in excised membrane patches from rat brain neurons have been examined. Of six antibodies directed against different intracellular domains of the sodium channel alpha subunit, only an antibody directed against a highly conserved intracellular segment between homologous transmembrane domains III and IV induced late single channel openings and prolonged single channel open times during depolarizing test pulses, resulting in nearly complete inhibition of sodium channel inactivation. The antibody effect was not observed if the membrane patches were depolarized to inactivate sodium channels before exposure to the antibody, indicating that the intracellular sequence recognized by the antibody is rendered inaccessible by inactivation. The results show that a conformational change involving the intracellular segment between domains III and IV of the alpha subunit of the sodium channel molecule is required for fast sodium channel inactivation and suggest that this segment may be the fast inactivation gate of the sodium channel.

Identification of an intracellular peptide segment involved in sodium channel inactivation

Antibodies directed against a conserved intracellular segment of the sodium channel alpha subunit slow the inactivation of sodium channels in rat muscle cells. Of four site-directed antibodies tested, only antibodies against the short intracellular segment between homologous transmembrane domains III and IV slowed inactivation, and their effects were blocked by the corresponding peptide antigen. No effects on the voltage dependence of sodium channel activation or of steady-state inactivation were observed, but the rate of onset of the antibody effect and the extent of slowing of inactivation were voltage-dependent. Antibody binding was more rapid at negative potentials, at which sodium channels are not inactivated; antibody-induced slowing of inactivation was greater during depolarizations to more positive membrane potentials. The peptide segment recognized by this antibody appears to participate directly in rapid sodium channel inactivation during large depolarizations and to undergo a conformational change that reduces its accessibility to antibodies as the channel inactivates.

Identification of an intracellular peptide segment involved in sodium channel inactivation

Antibodies directed against a conserved intracellular segment of the sodium channel alpha subunit slow the inactivation of sodium channels in rat muscle cells. Of four site-directed antibodies tested, only antibodies against the short intracellular segment between homologous transmembrane domains III and IV slowed inactivation, and their effects were blocked by the corresponding peptide antigen. No effects on the voltage dependence of sodium channel activation or of steady-state inactivation were observed, but the rate of onset of the antibody effect and the extent of slowing of inactivation were voltage-dependent. Antibody binding was more rapid at negative potentials, at which sodium channels are not inactivated; antibody-induced slowing of inactivation was greater during depolarizations to more positive membrane potentials. The peptide segment recognized by this antibody appears to participate directly in rapid sodium channel inactivation during large depolarizations and to undergo a conformational change that reduces its accessibility to antibodies as the channel inactivates.

Functional properties of rat brain sodium channels lacking the beta 1 or beta 2 subunit

The sodium channel purified from rat brain is a heterotrimeric complex of alpha (Mr 260,000), beta 1 (Mr 36,000), and beta 2 (Mr 33,000) subunits. alpha and beta 2 are attached by disulfide bonds. Removal of beta 1 subunits by incubation in 1.0 M MgCl2 followed by reconstitution into phospholipid vesicles yielded a preparation of alpha beta 2 which did not bind [3H]saxitoxin, mediate veratridine-activated 22Na+ influx, or bind the 125I-labeled alpha-scorpion toxin from Leiurus quinquestriatus (LqTx). In contrast, removal of beta 2 subunits by reduction of disulfide bonds with 1.5 mM dithiothreitol followed by reconstitution into phospholipid vesicles yielded a preparation of alpha beta 1 that retained full sodium channel function. Alpha beta 1 bound [3H]saxitoxin with a KD of 4.1 nM at 36 degrees C. It mediated veratridine-activated 22Na+ influx at a comparable initial rate as intact sodium channels with a K0.5 for veratridine of 46 microM. Tetracaine and tetrodotoxin blocked 22Na+ influx. Like intact sodium channels, alpha beta 1 bound 125I-LqTx in a voltage-dependent manner with a KD of approximately 6 nM at a membrane potential of -60 mV and was specifically covalently labeled by azidonitrobenzoyl 125I-LqTx. When incorporated into planar phospholipid bilayers, alpha beta 1 formed batrachotoxin-activated sodium channels of 24 pS whose voltage-dependent activation was characterized by V50 = -110 mV and an apparent gating charge of 3.3 +/- 0.3. These results indicate that beta 2 subunits are not required for the function of purified and reconstituted sodium channels while a complex of alpha and beta 1 subunits is both necessary and sufficient for channel function in the purified state.

Charge movement and depolarization-contraction coupling in arthropod vs. vertebrate skeletal muscle

Voltage-dependent charge movement has been characterized in arthropod skeletal muscle. Charge movement in scorpion (Centuroides sculpturatus) muscle is distinguishable from that in vertebrate skeletal muscle by criteria of kinetics, voltage dependence, and pharmacology. The function of scorpion charge movement is gating of calcium channels in the sarcolemma, and depolarization-contraction coupling relies on calcium influx through these channels.

Contractile activation in scorpion striated muscle fibers. Dependence on voltage and external calcium

Excitation-contraction coupling was characterized in scorpion striated muscle fibers using standard microelectrode techniques as employed in studies on vertebrate skeletal muscle. The action potential of scorpion muscle consists of two phases of regenerative activity. A relatively fast, overshooting initial spike is followed by a prolonged after-discharge of smaller, repetitive spikes. This after-discharge is accompanied by a twitch that relaxes promptly upon repolarization. Twitches fail in Na-free, tetrodotoxin (TTX)-containing, or Ca-free media. However, caffeine causes contractures in muscles paralyzed by Na- and Ca-free solutions. Experiments on muscle fibers voltage-clamped at a point with two microelectrodes in Na-free or TTX-containing media indicate that: (a) the strength-duration relation for threshold contractions has a shape similar to that in frog muscle, but mean values are displaced approximately 20 mV in the positive direction; (b) tetracaine exerts a parallel effect on strength-duration curves from scorpion and frog; (c) contractile activation in scorpion is abolished in Ca-free media; and (d) the contractile threshold is highly correlated with the occurrence of inward Ca current for pulses of all durations. Thus, the voltage dependence of contractile activation in scorpion and frog muscle is similar. However, the preparations differ in their dependence on extracellular Ca for contraction. These results are discussed in relation to possible mechanisms coupling tubular depolarization to Ca release from the sarcoplasmic reticulum in vertebrate and invertebrate skeletal muscle.

Phenytoin reduces calcium current in the cardiac Purkinje fiber

We have investigated the effects of phenytoin on the electrical and mechanical activity of isolated calf and dog cardiac Purkinje fibers. Phenytoin (5-100 microM) lowers and shortens the plateau phase of the Purkinje fiber action potential and reduces twitch tension. We studied the ionic basis of these effects by combining a conventional two-microelectrode voltage clamp procedure with pharmacological techniques that allow separation of time-dependent plateau membrane currents. We find that phenytoin reduces voltage-dependent calcium current in the Purkinje fiber. In addition, this drug slightly reduces two time-dependent outward currents. These effects of phenytoin on membrane current account, at least in part, for its influence on the action potential and twitch tension.

Block of outward current in cardiac Purkinje fibers by injection of quaternary ammonium ions

We have studied the effects of iontophoretic injection of the quaternary ammonium compounds tetraethylammonium (TEA) and tetrabutylammonium (TBA) in cardiac purkinje fibers. We find that TBA(+) is a more effective blocker than TEA(+), but injection of either compound reduces the time-dependent outward plateau currents, transient outward current (I(to)), and the delayed rectifier (I(x)). Our findings provide evidence that these outward cardiac currents are carried by channels that in some respects are pharmacologically similar to squid axon potassium channels. We demonstrate that this procedure is a new tool that can be useful in the analysis of membrane currents in the heart.