Patch recordings from the electrocytes of Electrophorus electricus. Na currents and PNa/PK variability

Convergent patterns of evolution of mitochondrial oxidative phosphorylation (OXPHOS) genes in electric fishes

The ability to generate and detect electric fields has evolved in several groups of fishes as a means of communication, navigation and, occasionally, predation. The energetic burden required can account for up to 20% of electric fishes' daily energy expenditure. Despite this, molecular adaptations that enable electric fishes to meet the metabolic demands of bioelectrogenesis remain unknown. Here, we investigate the molecular evolution of the mitochondrial oxidative phosphorylation (OXPHOS) complexes in the two most diverse clades of weakly electric fishes-South American Gymnotiformes and African Mormyroidea, using codon-based likelihood approaches. Our analyses reveal that although mitochondrial OXPHOS genes are generally subject to strong purifying selection, this constraint is significantly reduced in electric compared to non-electric fishes, particularly for complexes IV and V. Moreover, analyses of concatenated mitochondrial genes show strong evidence for positive selection in complex I genes on the two branches associated with the independent evolutionary origins of electrogenesis. These results suggest that adaptive evolution of proton translocation in the OXPHOS cellular machinery may be associated with the evolution of bioelectrogenesis. Overall, we find striking evidence for remarkably similar effects of electrogenesis on the molecular evolution of mitochondrial OXPHOS genes in two independently derived clades of electrogenic fishes. This article is part of the theme issue 'Linking the mitochondrial genotype to phenotype: a complex endeavour'.

A model for studying the energetics of sustained high frequency firing

Regulating membrane potential and synaptic function contributes significantly to the energetic costs of brain signaling, but the relative costs of action potentials (APs) and synaptic transmission during high-frequency firing are unknown. The continuous high-frequency (200-600Hz) electric organ discharge (EOD) of Eigenmannia, a weakly electric fish, underlies its electrosensing and communication. EODs reflect APs fired by the muscle-derived electrocytes of the electric organ (EO). Cholinergic synapses at the excitable posterior membranes of the elongated electrocytes control AP frequency. Based on whole-fish O₂ consumption, ATP demand per EOD-linked AP increases exponentially with AP frequency. Continual EOD-AP generation implies first, that ion homeostatic processes reliably counteract any dissipation of posterior membrane E_Na and E_K and second that high frequency synaptic activation is reliably supported. Both of these processes require energy. To facilitate an exploration of the expected energy demands of each, we modify a previous excitability model and include synaptic currents able to drive APs at frequencies as high as 600 Hz. Synaptic stimuli are modeled as pulsatile cation conductance changes, with or without a small (sustained) background conductance. Over the full species range of EOD frequencies (200–600 Hz) we calculate frequency-dependent “Na⁺-entry budgets” for an electrocyte AP as a surrogate for required 3Na⁺/2K⁺-ATPase activity. We find that the cost per AP of maintaining constant-amplitude APs increases nonlinearly with frequency, whereas the cost per AP for synaptic input current is essentially constant. This predicts that Na⁺ channel density should correlate positively with EOD frequency, whereas AChR density should be the same across fish. Importantly, calculated costs (inferred from Na⁺-entry through Nav and ACh channels) for electrocyte APs as frequencies rise are much less than expected from published whole-fish EOD-linked O₂ consumption. For APs at increasingly high frequencies, we suggest that EOD-related costs external to electrocytes (including packaging of synaptic transmitter) substantially exceed the direct cost of electrocyte ion homeostasis.

An electric-eel-inspired soft power source from stacked hydrogels

Progress towards the integration of technology into living organisms requires electrical power sources that are biocompatible, mechanically flexible, and able to harness the chemical energy available inside biological systems. Conventional batteries were not designed with these criteria in mind. The electric organ of the knifefish Electrophorus electricus (commonly known as the electric eel) is, however, an example of an electrical power source that operates within biological constraints while featuring power characteristics that include peak potential differences of 600 volts and currents of 1 ampere. Here we introduce an electric-eel-inspired power concept that uses gradients of ions between miniature polyacrylamide hydrogel compartments bounded by a repeating sequence of cation- and anion-selective hydrogel membranes. The system uses a scalable stacking or folding geometry that generates 110 volts at open circuit or 27 milliwatts per square metre per gel cell upon simultaneous, self-registered mechanical contact activation of thousands of gel compartments in series while circumventing power dissipation before contact. Unlike typical batteries, these systems are soft, flexible, transparent, and potentially biocompatible. These characteristics suggest that artificial electric organs could be used to power next-generation implant materials such as pacemakers, implantable sensors, or prosthetic devices in hybrids of living and non-living systems.

A tail of two voltages: Proteomic comparison of the three electric organs of the electric eel

The electric eel (Electrophorus electricus) is unusual among electric fishes because it has three pairs of electric organs that serve multiple biological functions: For navigation and communication, it emits continuous pulses of weak electric discharge (<1 V), but for predation and defense, it intermittently emits lethal strong electric discharges (10 to 600 V). We hypothesized that these two electrogenic outputs have different energetic demands reflected by differences in their proteome and phosphoproteome. We report the use of isotope-assisted quantitative mass spectrometry to test this hypothesis. We observed novel phosphorylation sites in sodium transporters and identified a potassium channel with unique differences in protein concentration among the electric organs. In addition, we found transcription factors and protein kinases that show differential abundance in the strong versus weak electric organs. Our findings support the hypothesis that proteomic differences among electric organs underlie differences in energetic needs, reflecting a trade-off between generating weak voltages continuously and strong voltages intermittently.

Voltage-Gated Na+ Channel Isoforms and Their mRNA Expression Levels and Protein Abundance in Three Electric Organs and the Skeletal Muscle of the Electric Eel Electrophorus electricus

This study aimed to obtain the coding cDNA sequences of voltage-gated Na+ channel (scn) α-subunit (scna) and β-subunit (scnb) isoforms from, and to quantify their transcript levels in, the main electric organ (EO), Hunter's EO, Sach's EO and the skeletal muscle (SM) of the electric eel, Electrophorus electricus, which can generate both high and low voltage electric organ discharges (EODs). The full coding sequences of two scna (scn4aa and scn4ab) and three scnb (scn1b, scn2b and scn4b) were identified for the first time (except scn4aa) in E. electricus. In adult fish, the scn4aa transcript level was the highest in the main EO and the lowest in the Sach's EO, indicating that it might play an important role in generating high voltage EODs. For scn4ab/Scn4ab, the transcript and protein levels were unexpectedly high in the EOs, with expression levels in the main EO and the Hunter's EO comparable to those of scn4aa. As the key domains affecting the properties of the channel were mostly conserved between Scn4aa and Scn4ab, Scn4ab might play a role in electrogenesis. Concerning scnb, the transcript level of scn4b was much higher than those of scn1b and scn2b in the EOs and the SM. While the transcript level of scn4b was the highest in the main EO, protein abundance of Scn4b was the highest in the SM. Taken together, it is unlikely that Scna could function independently to generate EODs in the EOs as previously suggested. It is probable that different combinations of Scn4aa/Scn4ab and various Scnb isoforms in the three EOs account for the differences in EODs produced in E. electricus. In general, the transcript levels of various scn isoforms in the EOs and the SM were much higher in adult than in juvenile, and the three EOs of the juvenile fish could be functionally indistinct.

Energetics of Sensing and Communication in Electric Fish: A Blessing and a Curse in the Anthropocene?

Weakly electric freshwater fish use self-generated electric fields to image their worlds and communicate in the darkness of night and turbid waters. This active sensory/communication modality evolved independently in the freshwaters of South America and Africa, where hundreds of electric fish species are broadly and abundantly distributed. The adaptive advantages of the sensory capacity to forage and communicate in visually-unfavorable environments and outside the detection of visually-guided predators likely contributed to the broad success of these clades across a variety of Afrotropical and neotropical habitats. Here we consider the potentially high and limiting metabolic costs of the active sensory and communication signals that define the gymnotiform weakly electric fish of South America. Recent evidence from two well-studied species suggests that the metabolic costs of electrogenesis can be quite high, sometimes exceeding one-fourth of these fishes' daily energy budget. Supporting such an energetically expensive system has shaped a number of cellular, endocrine, and behavioral adaptations to restrain the metabolic costs of electrogenesis in general or in response to metabolic stress. Despite a suite of adaptations supporting electrogenesis, these weakly electric fish are vulnerable to metabolic stresses such as hypoxia and food restriction. In these conditions, fish reduce signal amplitude presumably as a function of absolute energy shortfall or as a proactive means to conserve energy. In either case, reducing signal amplitude compromises both sensory and communication performance. Such outcomes suggest that the higher metabolic cost of active sensing and communication in weakly electric fish compared with the sensory and communication systems in other neotropical fish might mean that weakly electric fish are disproportionately susceptible to harm from anthropogenic disturbances of neotropical aquatic habitats. Fully evaluating this possibility, however, will require broad comparative studies of metabolic energetics across the diverse clades of gymnotiform electric fish and in comparison to other nonelectric neotropical fishes.

A highly polarized excitable cell separates sodium channels from sodium-activated potassium channels by more than a millimeter

The bioelectrical properties and resulting metabolic demands of electrogenic cells are determined by their morphology and the subcellular localization of ion channels. The electric organ cells (electrocytes) of the electric fish Eigenmannia virescens generate action potentials (APs) with Na(+) currents >10 μA and repolarize the AP with Na(+)-activated K(+) (KNa) channels. To better understand the role of morphology and ion channel localization in determining the metabolic cost of electrocyte APs, we used two-photon three-dimensional imaging to determine the fine cellular morphology and immunohistochemistry to localize the electrocytes' ion channels, ionotropic receptors, and Na(+)-K(+)-ATPases. We found that electrocytes are highly polarized cells ∼ 1.5 mm in anterior-posterior length and ∼ 0.6 mm in diameter, containing ∼ 30,000 nuclei along the cell periphery. The cell's innervated posterior region is deeply invaginated and vascularized with complex ultrastructural features, whereas the anterior region is relatively smooth. Cholinergic receptors and Na(+) channels are restricted to the innervated posterior region, whereas inward rectifier K(+) channels and the KNa channels that terminate the electrocyte AP are localized to the anterior region, separated by >1 mm from the only sources of Na(+) influx. In other systems, submicrometer spatial coupling of Na(+) and KNa channels is necessary for KNa channel activation. However, our computational simulations showed that KNa channels at a great distance from Na(+) influx can still terminate the AP, suggesting that KNa channels can be activated by distant sources of Na(+) influx and overturning a long-standing assumption that AP-generating ion channels are restricted to the electrocyte's posterior face.

Na+/K+-ATPase α-subunit (nkaα) isoforms and their mRNA expression levels, overall Nkaα protein abundance, and kinetic properties of Nka in the skeletal muscle and three electric organs of the electric eel, Electrophorus electricus

This study aimed to obtain the coding cDNA sequences of Na⁺/K⁺-ATPase α (nkaα) isoforms from, and to quantify their mRNA expression in, the skeletal muscle (SM), the main electric organ (EO), the Hunter’s EO and the Sach’s EO of the electric eel, Electrophorus electricus. Four nkaα isoforms (nkaα1c1, nkaα1c2, nkaα2 and nkaα3) were obtained from the SM and the EOs of E. electricus. Based on mRNA expression levels, the major nkaα expressed in the SM and the three EOs of juvenile and adult E. electricus were nkaα1c1 and nkaα2, respectively. Molecular characterization of the deduced Nkaα1c1 and Nkaα2 sequences indicates that they probably have different affinities to Na⁺ and K⁺. Western blotting demonstrated that the protein abundance of Nkaα was barely detectable in the SM, but strongly detected in the main and Hunter’s EOs and weakly in the Sach’s EO of juvenile and adult E. electricus. These results corroborate the fact that the main EO and Hunter’s EO have high densities of Na⁺ channels and produce high voltage discharges while the Sach’s EO produces low voltage discharges. More importantly, there were significant differences in kinetic properties of Nka among the three EOs of juvenile E. electricus. The highest and lowest V_max of Nka were detected in the main EO and the Sach’s EO, respectively, with the Hunter’s EO having a V_max value intermediate between the two, indicating that the metabolic costs of EO discharge could be the highest in the main EO. Furthermore, the Nka from the main EO had the lowest K_m (or highest affinity) for Na⁺ and K⁺ among the three EOs, suggesting that the Nka of the main EO was more effective than those of the other two EOs in maintaining intracellular Na⁺ and K⁺ homeostasis and in clearing extracellular K⁺ after EO discharge.

Direct recording and molecular identification of the calcium channel of primary cilia

A primary cilium is a solitary slender non-motile protuberance of structured microtubules (9+0) enclosed by plasma membrane¹. Housing components of the cell division apparatus between cell divisions, they also serve as specialized compartments for calcium signaling² and Hedgehog (Hh) signaling pathways³. Specialized sensory cilia such as retinal photoreceptors and olfactory cilia employ diverse ion channels^4-7. An ion current has been measured from primary cilia of kidney cells⁸ but the responsible genes have not been identified. The polycystin proteins (PC, PKD), identified in linkage studies of polycystic kidney disease⁹, are candidate channels divided into two structural classes: 11-transmembrane (TM) proteins (PKD1, PKD1-L1 and PKD1-L2) remarkable for a large extracellular N-terminus of putative cell adhesion domains and a GPCR proteolytic site, and the 6-TM channel proteins (PKD2, PKD2-L1, PKD2-L2; TRPPs). Evidence suggests that the PKD1s associate with the PKD2s via coiled-coil domains^10-12. Here, we employ a transgenic mouse in which only cilia express a fluorophore and employ it to directly record from primary cilia and demonstrate that PKD1-L1 and PKD2-L1 form ion channels at high densities in several cell types. In conjunction with the companion manuscript², we show that the PKD1-L1/PKD2-L1 heteromeric channel establishes the cilia as a unique calcium compartment within cells that modulates established Hedgehog pathways.

Electrocyte physiology: 50 years later

Weakly electric gymnotiform and mormyrid fish generate and detect weak electric fields to image their worlds and communicate. These multi-purpose electric signals are generated by electrocytes, the specialized electric organ (EO) cells that produce the electric organ discharge (EOD). Just over 50 years ago the first experimental analyses of electrocyte physiology demonstrated that the EOD is produced and shaped by the timing and waveform of electrocyte action potentials (APs). Electrocytes of some species generate a single AP from a distinct region of excitable membrane, and this AP waveform determines EOD waveform. In other species, electrocytes possess two independent regions of excitable membrane that generate asynchronous APs with different waveforms, thereby increasing EOD complexity. Signal complexity is further enhanced in some gymnotiforms by the spatio-temporal activation of distinct EO regions with different electrocyte properties. For many mormyrids, additional EOD waveform components are produced by APs that propagate along stalks that connect postsynaptic regions to the main body of the electrocyte. I review here the history of research on electrocyte physiology in weakly electric fish, as well as recent discoveries of key phenomena not anticipated during early work in this field. Recent areas of investigation include the regulation of electrocyte activity by steroid and peptide hormones, the molecular evolution of electrocyte ion channels, and the evolutionary selection of ion channels expressed in excitable cells. These emerging research areas have generated renewed interest in electrocyte function and clear future directions for research addressing a broad range of new and important questions.

A sodium-activated potassium channel supports high-frequency firing and reduces energetic costs during rapid modulations of action potential amplitude

We investigated the ionic mechanisms that allow dynamic regulation of action potential (AP) amplitude as a means of regulating energetic costs of AP signaling. Weakly electric fish generate an electric organ discharge (EOD) by summing the APs of their electric organ cells (electrocytes). Some electric fish increase AP amplitude during active periods or social interactions and decrease AP amplitude when inactive, regulated by melanocortin peptide hormones. This modulates signal amplitude and conserves energy. The gymnotiform Eigenmannia virescens generates EODs at frequencies that can exceed 500 Hz, which is energetically challenging. We examined how E. virescens meets that challenge. E. virescens electrocytes exhibit a voltage-gated Na(+) current (I(Na)) with extremely rapid recovery from inactivation (τ(recov) = 0.3 ms) allowing complete recovery of Na(+) current between APs even in fish with the highest EOD frequencies. Electrocytes also possess an inwardly rectifying K(+) current and a Na(+)-activated K(+) current (I(KNa)), the latter not yet identified in any gymnotiform species. In vitro application of melanocortins increases electrocyte AP amplitude and the magnitudes of all three currents, but increased I(KNa) is a function of enhanced Na(+) influx. Numerical simulations suggest that changing I(Na) magnitude produces corresponding changes in AP amplitude and that K(Na) channels increase AP energy efficiency (10-30% less Na(+) influx/AP) over model cells with only voltage-gated K(+) channels. These findings suggest the possibility that E. virescens reduces the energetic demands of high-frequency APs through rapidly recovering Na(+) channels and the novel use of KNa channels to maximize AP amplitude at a given Na(+) conductance.

Molecular evolution of Na+ channels in teleost fishes

Voltage-dependent sodium channels are critical for electrical excitability. Invertebrates possess a single sodium channel gene; two rounds of genome duplication early in vertebrates increased the number to four. Since the teleost-tetrapod split, independent gene duplications in each lineage have further increased the number of sodium channel genes to 10 in tetrapods and 8 in teleosts. Here we review how the occurrence of multiple sodium channel paralogs has influenced the evolutionary history of three groups of fishes: pufferfish, gymnotiform and mormyriform electric fish. Pufferfish (tetraodontidae) produce a neurotoxin, tetrodotoxin, that binds to and blocks the pore of sodium channels. Pufferfish evolved resistance to their own toxins by amino acid substitutions in the pore of their sodium channels. These substitutions had to occur in parallel across multiple paralogs for organismal resistance to evolve. Gymnotiform and mormyriform fishes independently evolved electric organs to generate electricity for communication and object localization. Two sodium channel genes are expressed in muscle in most fishes. In both groups of weakly electric fishes, one gene lost its expression in muscle and became compartmentalized in the evolutionary novel electric organ, which is a muscle derivative. This gene then evolved at elevated rates, whereas the gene that is still expressed in muscle does not show elevated rates of evolution. In the electric organ-expressing gene, amino acid substitutions occur in parts of the channel involved in determining how long the channel will be open or closed. The enhanced rate of sequence evolution of this gene likely underlies the species-level variations in the electric signal.

Old gene duplication facilitates origin and diversification of an innovative communication system--twice

The genetic basis of parallel innovation remains poorly understood due to the rarity of independent origins of the same complex trait among model organisms. We focus on two groups of teleost fishes that independently gained myogenic electric organs underlying electrical communication. Earlier work suggested that a voltage-gated sodium channel gene (Scn4aa), which arose by whole-genome duplication, was neofunctionalized for expression in electric organ and subsequently experienced strong positive selection. However, it was not possible to determine if these changes were temporally linked to the independent origins of myogenic electric organs in both lineages. Here, we test predictions of such a relationship. We show that Scn4aa co-option and rapid sequence evolution were tightly coupled to the two origins of electric organ, providing strong evidence that Scn4aa contributed to parallel innovations underlying the evolutionary diversification of each electric fish group. Independent evolution of electric organs and Scn4aa co-option occurred more than 100 million years following the origin of Scn4aa by duplication. During subsequent diversification of the electrical communication channels, amino acid substitutions in both groups occurred in the same regions of the sodium channel that likely contribute to electric signal variation. Thus, the phenotypic similarities between independent electric fish groups are also associated with striking parallelism at genetic and molecular levels. Our results show that gene duplication can contribute to remarkably similar innovations in repeatable ways even after long waiting periods between gene duplication and the origins of novelty.

Designing artificial cells to harness the biological ion concentration gradient

Cell membranes contain numerous nanoscale conductors in the form of ion channels and ion pumps that work together to form ion concentration gradients across the membrane to trigger the release of an action potential. It seems natural to ask if artificial cells can be built to use ion transport as effectively as natural cells. Here we report a mathematical calculation of the conversion of ion concentration gradients into action potentials across different nanoscale conductors in a model electrogenic cell (electrocyte) of an electric eel. Using the parameters extracted from the numerical model, we designed an artificial cell based on an optimized selection of conductors. The resulting cell is similar to the electrocyte but has higher power output density and greater energy conversion efficiency. We suggest methods for producing these artificial cells that could potentially be used to power medical implants and other tiny devices.

Voltage-gated potassium conductances in Gymnotus electrocytesAB

Electrocytes are muscle-derived cells that generate the electric organ discharge (EOD) in most gymnotiform fish. We used an in vitro preparation to determine if the complex EOD of Gymnotus carapo was related to the membrane properties of electrocytes. We discovered that in addition to the three Na(+)-mediated conductances described in a recent paper [Sierra F, Comas V, Buño W, Macadar O (2005) Sodium-dependent plateau potentials in electrocytes of the electric fish Gymnotus carapo. J Comp Physiol A 191:1-11] there were four K(+)-dependent conductances. Membrane depolarization activated a delayed rectifier (I(K)) and an A-type (I(A)) current. I(A) displayed fast voltage-dependent activation-inactivation kinetics, was blocked by 4-aminopyridine (1 mM) and played a major role in action potential (AP) repolarization. Its voltage dependence and kinetics shape the brief AP that typifies Gymnotus electrocytes. The I(K) activated by depolarization contributed less to AP repolarization. Membrane hyperpolarization uncovered two inward rectifiers (IR1 and IR2) with voltage dependence and kinetics that correspond to the complex "hyperpolarizing responses" (HRs) described under current-clamp. IR1 shows "instantaneous" activation, is blocked by Ba(2+) and Cs(+) and displays a voltage and time dependent inactivation that matches the hyperpolarizing phase of the HR. The activation of IR2 is slower and at more negative potentials than IR1 and is resistant to Ba(2+) and Cs(+). This current fits the depolarizing phase of the HR. The EOD waveform of Gymnotus carapo is more complex than that of other gymnotiform fish species, the complexity originates in the voltage responses generated through the interactions of three Na(+) and four K(+) voltage- and time-dependent conductances although the innervation pattern also contributes [Trujillo-Cenóz O, Echagüe JA (1989) Waveform generation of the electric organ discharge in Gymnotus carapo. I. Morphology and innervation of the electric organ. J Comp Physiol A 165:343-351].

Sodium-dependent plateau potentials in electrocytes of the electric fish Gymnotus carapo

The weakly electric fish Gymnotus carapo emits a triphasic electric organ discharge generated by muscle-derived electrocytes, which is modified by environmental and physiological factors. Two electrode current clamp recordings in an in vitro preparation showed that Gymnotus electrocytes fired repetitively and responded with plateau potentials when depolarized. This electrophysiological behavior has never been observed in electrocytes from related species. Two types of plateaus with different thresholds and amplitudes were evoked by depolarization when Na(+)-dependent currents were isolated in a K(+)- and Ca(2+)-free solution containing TEA and 4-AP. Two electrode voltage clamp recordings revealed a classical fast activating-inactivating Na+ current and two persistent Na(+)-dependent currents with voltage-dependencies consistent with the action potential (AP) and the two plateaus observed under current clamp, respectively. The three currents, the APs and the plateaus were reduced by TTX, and were absent in Na(+)-free solution. The different Na(+)-dependent currents in Gymnotus electrocytes may be targets for the modifications of the electric organ discharge mediated by environmental and physiological factors.

Molecular cloning and expression of a Kv1.1-like potassium channel from the electric organ of Electrophorus electricus

Electrocytes from the electric organ of Electrophorus electricus exhibited sodium action potentials that have been proposed to be repolarized by leak currents and not by outward voltage-gated potassium currents. However, patch-clamp recordings have suggested that electrocytes may contain a very low density of voltage-gated K(+) channels. We report here the cloning of a K(+) channel from an eel electric organ cDNA library, which, when expressed in mammalian tissue culture cells, displayed delayed-rectifier K(+) channel characteristics. The amino-acid sequence of the eel K(+) channel had the highest identity to Kv1.1 potassium channels. However, different important functional regions of eel Kv1.1 had higher amino-acid identity to other Kv1 members, for example, the eel Kv1.1 S4-S5 region was identical to Kv1.5 and Kv1.6. Northern blot analysis indicated that eel Kv1.1 mRNA was expressed at appreciable levels in the electric organ but it was not detected in eel brain, muscle, or cardiac tissue. Because electrocytes do not express robust outward voltage-gated potassium currents we speculate that eel Kv1.1 channels are chronically inhibited in the electric organ and may be functionally recruited by an unknown mechanism.

Coregulation of voltage-dependent kinetics of Na(+) and K(+) currents in electric organ

The electric organ cells of Sternopygus generate action potentials whose durations vary over a fourfold range. This variation in action potential duration is the basis for individual variation in a communication signal. Thus, action potential duration must be precisely regulated in these cells. We had observed previously that the inactivation kinetics of the electrocyte Na(+) current show systematic individual variation. In this study, using a two-electrode voltage clamp, we found that the voltage-dependent activation and deactivation kinetics of the delayed rectifying K(+) current in these cells covary in a graded and predictable manner across fish. Furthermore, when Na(+) and K(+) currents were recorded in the same cell, their voltage-dependent kinetics were highly correlated. This finding illustrates an unprecedented degree of coregulation of voltage-dependent properties in two molecularly distinct ionic channels. Such a coregulation of ionic channels is uniquely observable in a cell specialized to generate individual differences in electrical activity and in which the results of biophysical control mechanisms are evident in behaving animals. We propose that the precise coregulation of the voltage-dependent kinetics of multiple ionic currents may be a general mechanism for regulation of membrane excitability.

Coregulation of voltage-dependent kinetics of Na(+) and K(+) currents in electric organ

The electric organ cells of Sternopygus generate action potentials whose durations vary over a fourfold range. This variation in action potential duration is the basis for individual variation in a communication signal. Thus, action potential duration must be precisely regulated in these cells. We had observed previously that the inactivation kinetics of the electrocyte Na(+) current show systematic individual variation. In this study, using a two-electrode voltage clamp, we found that the voltage-dependent activation and deactivation kinetics of the delayed rectifying K(+) current in these cells covary in a graded and predictable manner across fish. Furthermore, when Na(+) and K(+) currents were recorded in the same cell, their voltage-dependent kinetics were highly correlated. This finding illustrates an unprecedented degree of coregulation of voltage-dependent properties in two molecularly distinct ionic channels. Such a coregulation of ionic channels is uniquely observable in a cell specialized to generate individual differences in electrical activity and in which the results of biophysical control mechanisms are evident in behaving animals. We propose that the precise coregulation of the voltage-dependent kinetics of multiple ionic currents may be a general mechanism for regulation of membrane excitability.

A novel voltage clamp technique for mapping ionic currents from cultured skeletal myotubes

The biophysical properties and cellular distribution of ion channels largely determine the input/output relationships of electrically excitable cells. A variety of patch pipette voltage clamp techniques are available to characterize ionic currents. However, when used by themselves, such techniques are not well suited to the task of mapping low-density channel distributions. We describe here a new voltage clamp method (the whole cell loose patch (WCLP) method) that combines whole-cell recording through a tight-seal pipette with focal extracellular stimulation through a loose-seal pipette. By moving the stimulation pipette across the cell surface and using a stationary whole-cell pipette to record the evoked patch currents, this method should be suitable for mapping channel distributions, even on large cells possessing low channel densities. When we applied this method to the study of currents in cultured chick myotubes, we found that the cell cable properties and the series resistance of the recording pipette caused significant filtering of the membrane currents, and that the filter characteristics depended in part upon the distance between the stimulating and recording pipettes. We describe here how we determined the filter impulse response for each loose-seal pipette placement and subsequently recovered accurate estimates of patch membrane current through deconvolution.

Electrophorus electricus as a model system for the study of membrane excitability

The stunning sensations produced by electric fish, particularly the electric eel, Electrophorus electricus, have fascinated scientists for centuries. Within the last 50 years, however, electric cells of Electrophorus have provided a unique model system that is both specialized and appropriate for the study of excitable cell membrane electrophysiology and biochemistry. Electric tissue generates whole animal electrical discharges by means of membrane potentials that are remarkably similar to those of mammalian neurons, myocytes and secretory cells. Electrocytes express ion channels, ATPases and signal transduction proteins common to these other excitable cells. Action potentials of electrocytes represent the specialized end function of electric tissue whereas other excitable cells use membrane potential changes to trigger sophisticated cellular processes, such as myofilament cross-bridging for contraction, or exocytosis for secretion. Because electric tissue lacks these functions and the proteins associated with them, it provides a highly specialized membrane model system. This review examines the basic mechanisms involved in the generation of the electrical discharge of the electric eel and the membrane proteins involved. The valuable contributions that electric tissue continues to make toward the understanding of excitable cell physiology and biochemistry are summarized, particularly those studies using electrocytes as a model system for the study of the regulation of membrane excitability by second messengers and signal transduction pathways.

On the structural basis for ionic selectivity among Na+, K+, and Ca2+ in the voltage-gated sodium channel

Voltage-sensitive sodium channels and calcium channels are homologous proteins with distinctly different selectivity for permeation of inorganic cations. This difference in function is specified by amino acid residues located within P-region segments that link presumed transmembrane elements S5 and S6 in each of four repetitive Domains I, II, III, and IV. By analyzing the selective permeability of Na+, K+, and Ca2+ in various mutants of the mu 1 rat muscle sodium channel, the results in this paper support the concept that a conserved motif of four residues contributed by each of the Domains I-IV, termed the DEKA locus in sodium channels and the EEEE locus in calcium channels, determines the ionic selectivity of these channels. Furthermore, the results indicate that the Lys residue in Domain III of the sodium channel is the critical determinant that specifies both the impermeability of Ca2+ and the selective permeability of Na+ over K+. We propose that the alkylammonium ion of the Lys(III) residue acts as an endogenous cation within the ion binding site/selectivity filter of the sodium channel to tune the kinetics and affinity of inorganic cation binding within the pore in a manner analogous to ion-ion interactions that occur in the process of multi-ion channel conduction.

Anemone toxin II unmasks two conductance states in neuronal sodium channels

Anemone toxin II (ATX)-modified voltage-dependent neuronal sodium channels were studied in planar lipid bilayers. ATX-modified channels displayed two predominant conducting states: a short-lived (ms-s) high-conductance (approximately 65 pS) state and a long-lived (s-min) low-conductance (approximately 10 pS) state. The high-conductance state underwent brief closures (ms) and the low-conductance state underwent long closures (s). The probability of detecting these states was time- and voltage-dependent. The channel's fractional open time (fo) due to the high-conductance state increased with depolarization and had a midpoint potential (Va) of -36 mV and an apparent gating charge (Za) of 2.8. The channel's fo due to the low-conductance state increased with depolarization and had a Va of +13 mV and a Za of 1.4. At positive potentials, ATX-modified channels slowly (minutes) entered an absorbing non-conducting state. The permeability ratio of Na+/K+ was 2 and 4 for the low- and high-conductance states, respectively. The saxitoxin analog C3 blocked ATX-modified sodium channels with high affinity (Kd(60-90 mV) = 410 nM, 0.5 M NaCl). The data suggest that upon a depolarization step, ATX-modified channels enter rapidly (ms) into a high-conductance state and more slowly (s-min) into a low-conductance state. Also as the membrane potential becomes more positive, the equilibrium is shifted from the high- to the low-conductance state and from the conducting states to an absorbing non-conducting state.

Protein kinase A activation increases sodium current magnitude in the electric organ of Sternopygus

The inactivation kinetics of the Na+ current of the weakly electric fish Sternopygus are modified by treatment with androgens. To determine whether phosphorylation could play a role in this effect, we examined whether activation of protein kinase A by 8 bromo cyclic AMP (8 Br cAMP) altered voltage-dependent properties of the current. Using a two-electrode voltage-clamp procedure, we found no effect of 8 Br cAMP on inactivation kinetics or other voltage-dependent properties of the Na+ current of the electrocytes. However, treatment with 8 Br cAMP did produce a dose-dependent increase in the Na+ current compared with saline controls: 17.6% at 100 microM, 42.4% at 1 mM, and 43.1% at 5 mM. This effect was blocked by 30 microM H89, a PKA inhibitor, indicating that the observed effect was attributable to 8 Br cAMP activation of PKA. We conclude that androgen-induced changes in Na+ current inactivation are not mediated by PKA and suggest that PKA-mediated increases in Na+ current underlie increases in the amplitude of the electric organ discharge observed in social interactions or with changes in water conductance.

Protein kinase A activation increases sodium current magnitude in the electric organ of Sternopygus

The inactivation kinetics of the Na+ current of the weakly electric fish Sternopygus are modified by treatment with androgens. To determine whether phosphorylation could play a role in this effect, we examined whether activation of protein kinase A by 8 bromo cyclic AMP (8 Br cAMP) altered voltage-dependent properties of the current. Using a two-electrode voltage-clamp procedure, we found no effect of 8 Br cAMP on inactivation kinetics or other voltage-dependent properties of the Na+ current of the electrocytes. However, treatment with 8 Br cAMP did produce a dose-dependent increase in the Na+ current compared with saline controls: 17.6% at 100 microM, 42.4% at 1 mM, and 43.1% at 5 mM. This effect was blocked by 30 microM H89, a PKA inhibitor, indicating that the observed effect was attributable to 8 Br cAMP activation of PKA. We conclude that androgen-induced changes in Na+ current inactivation are not mediated by PKA and suggest that PKA-mediated increases in Na+ current underlie increases in the amplitude of the electric organ discharge observed in social interactions or with changes in water conductance.

Stationarity of sodium channel gating kinetics in excised patches from neuroblastoma N1E 115

Na channel gating parameters in a number of preparations are translated along the voltage axis in excised patches compared to cell attached or whole cell recording. The aim of this study is to determine whether these changes in gating behavior continue over an extended period or, rather, develop rapidly on excision with stationary kinetics thereafter. Average currents were constructed from single-channel records from neuroblastoma N1E 115 at various times after excision, excluding the first 5 min, in eight inside-out excised patches. Single exponentials were fitted to the current decay of the average records, and the mean time constant for each patch was determined. Values were plotted as the percentage difference from these means for each patch against time from excision. Collected results show no obvious trend in values from 5 min to 2 h. Kinetics are stationary, and shifts in Na channel gating parameters along the voltage axis seen in excised as compared to whole cell configuration in neuroblastoma must be complete by the first few minutes after excision. Raising the internal Na concentration reduced the single channel current amplitude, confirming that these are Na channels.

Regulation of the eel electroplax Na channel and phosphorylation of residues on amino- and carboxyl-terminal domains by cAMP-dependent protein kinase

Previous studies have shown that the short-motif electroplax Na channel is phosphorylated in vitro by cyclic AMP-dependent protein kinase (PKA) at serines 6 or 7 and 1776 and threonine 17 (Emerick & Agnew, 1989). We here show that phosphatase treatment of solubilized, purified Na channels enhanced subsequent PKA labeling of four of five tryptic phosphopeptides, indicating that these sites are phosphorylated in vivo. Microsequencing and analysis of PTH-amino acid products revealed endogenous labeling of serines 6, 444, 1680, and 1776. Serines 1680 and 1776 lie in the carboxyl-terminal cytoplasmic domain, while serine 6 lies in the amino terminus and serine 444 is in the cytoplasmic loop between domains I and II. Endogenous phosphorylation of serine 6 establishes experimentally that the Na channel amino terminus is cytoplasmic. In electrophysiological experiments, brief exposure of inside-out membrane patches excised from Sachs-organ cells to MgATP and purified PKA catalytic subunit produced rapid, sustained reduction of Na current amplitude by approximately 80% and a hyperpolarizing shift in the conductance/voltage relation by 10-12 mV. The effect was absent in controls omitting either PKA or MgATP. Serines 6 and 1776 and threonine 17 are labeled rapidly and extensively in vitro, and only threonine 17 appears to be unphosphorylated in vivo. We suggest that phosphorylation of the amino and carboxyl domains, perhaps especially at threonine 17, underlies the demonstrated downregulation of the electroplax Na channel.

Conductances contributing to the action potential of Sternopygus electrocytes

In Sternopygus macrurus, electrocyte action potential duration determines the electric organ discharge pulse duration. Since the electric organ discharge is a sexually-dimorphic behavior under the control of steroid hormones, and because electrocyte action potential durations can range from 3-14 ms, the electrocytes provide a unique opportunity to study how sex steroids regulate membrane excitability. In this study, the voltage-sensitive ionic currents of electrocytes were identified under current- and voltage-clamp as a prelude to further studies on their regulation by sex steroid hormones. Bath application of TTX completely abolished the spike and eliminated an inward current under voltage clamp, indicating that the action potential is due primarily to a sodium current. Calcium-free saline had no effect on spike waveform or voltage-clamp currents, indicating that neither calcium nor calcium-dependent currents contribute to the action potential. Application of potassium channel blocking agents, such as tetraethylammonium and cesium ions, caused changes in the spike which, together with voltage-clamp results, indicate the presence of two potassium currents: an inward rectifier and a classical delayed rectifier. In addition, these cells have a large, presumably voltage-insensitive, chloride current. Differences in one or more of these currents could be responsible for the range of action potential durations found in these cells and for the steroid-mediated changes in spike duration.