EEL ELECTROPLAQUES: SPIKE ELECTROGENESIS WITHOUT POTASSIUM ACTIVATION

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.

Analysis of Spike Electrogenesis and Depolarizing K Inactivation in Electroplaques of Electrophorus electricus, L

Voltage clamp analyses, combined with pharmacological tools demonstrate the independence of reactive Na and K channels in electrically excitable membrane of eel electroplaques. Spike electrogenesis is due to Na activation and is eliminated by tetrodotoxin or mussel poison, or by substituting choline, K, Cs, or Rb for Na in the medium. The K channels remain reactive, but K activation is always absent, the electroplaques responding only with K inactivation. This is indicated by an increased resistance when the membrane is depolarized by more than about 30 mv. The resting resistance (1 to 5 ohm cm²) is dependent upon the ionic conditions, but when K inactivation occurs the resistance becomes about 10 ohm cm² in all conditions. K inactivation does not change the EMF significantly. The transition from low to high resistance may give rise to a negative-slope voltage current characteristic, and to regenerative inactivation responses under current clamp. The further demonstration that pharmacological K inactivation (by Cs or Rb) leaves Na activation and spike electrogenesis unaffected emphasizes the independence of the reactive processes and suggests different chemical compositions for the membrane structures through which they operate.

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.

ADAPTATION IN STRETCH RECEPTOR NEURONS OF CRAYFISH

Two factors involved in the adaptation in stretch receptor cells of crayfish were separated and studied: (i) the decline of the rate of discharge during intracellular application of constant current and (ii) the decline of generator potential during sustained stretch. The change in generator potential with time was essentially identical in both rapidly and slowly adapting cells. The slowly adapting cells continued to discharge throughout the application of depolarizing currents, whereas the rapidly adapting cells stopped discharging while the current was applied. The different rates of adaptation are therefore attributable to the difference in the properties of the electrically excitable membranes rather than the properties which produce generator potentials.

Pharmacologic actions of tetrodotoxin studied by direct perfusion of the sinus node

The effects of tetrodotoxin on sinus node activity were studied by direct perfusion through its nutrient artery. Tetrodotoxin suppresses action potentials by blocking sodium entry into the cell. In the open-chest anesthetized dog, direct perfusion of the sinus node artery with tetrodotoxin, 1.0 µg/ml, caused immediate sinus slowing lasting 12 to 60 minutes. This slowing was not affected by intranodal atropinization. Tetrodotoxin also produced complete blockade of sinus node response to vagal stimulation. Blockade of sinus node responses to stellate stimulation was less profound. Tetrodotoxin had no effect on chronotropic responses to intranodal acetylcholine and norepinephrine. These findings suggest that blockade of transmembrane sodium transport (or the rapid entry channel) has a negative chronotropic effect which is not cholinergic and during which response to neurotransmitter substances is normal. Concomitant with this effect there is blockade of neurotransmitter release (or synthesis) in the sinus node.

Tetrodotoxin: effects on brain metabolism in vitro

A 3-micromolar concentration of tetrodotoxin completely inhibits the stimulation of respiration of rat brain cortex that takes place upon application of electrical impulses. It also inhibits increase in the rate of the respiration that occurs when calcium ions are omitted from the incubation medium. No effect of tetrodotoxin on brain respiration takes place when stimulation is brought about by the addition of 100 millimolar potassium chloride. Tetrodotoxin prevents the fall in the rate of oxidation of cerebral acetate that occurs during electrical stimulation but does not affect the increased rate of the oxidation that occurs in the presence of an increased concentration of potassium chloride. The data indicate that oxidation of cerebral acetate is diminished by influx of sodium ions, which is prevented by tetrodotoxin, and is increased by influx of potassium ions, which is unaffected by tetrodotoxin.

Differences in Na and Ca spikes as examined by application of tetrodotoxin, procaine, and manganese ions

The effects of tetrodotoxin, procaine, and manganese ions were examined on the Ca spike of the barnacle muscle fiber injected with Ca-binding agent as well as on the action potential of the ventricular muscle fiber of the frog heart. Although tetrodotoxin and procaine very effectively suppress the "Na spike" of other tissues, no suppressing effects are found on "Ca spike" of the barnacle fiber, while the initiation of the Ca spike is competitively inhibited by manganese ions. The initial rate of rise of the ventricular action potential is suppressed by tetrodotoxin and procaine but the plateau phase of the action potential is little affected. In contrast the suppressing effect of manganese ions is mainly on the plateau phase. The results suggest that the plateau phase of the ventricular action potential is related to the conductance increase in the membrane to Ca ions even though Na conductance change may also contribute to the plateau.

Behavior of delayed current under voltage clamp in the supramedullary neurons of puffer

Depolarizations applied to voltage-clamped cells bathed in the normal solution disclose an initial inward current followed by a delayed outward current. The maximum slope conductance for the peak initial current is about 30 times the leak conductance, but the maximum slope conductance for the delayed current is only about 10 times the leak conductance. During depolarizations for as long as 30 sec, the outward current does not maintain a steady level, but declines first exponentially with a time constant of about 6 msec; it then tends to increase for the next few seconds; finally, it declines slowly with a half-time of about 5 sec. Concomitant with the changes of the outward current, the membrane conductance changes, although virtually no change in electromotive force occurs. Thus, the changes in the membrane conductance represent two phases of K inactivation, one rapidly developing, the other slowly occurring, and a phase of K reactivation, which is interposed between the two inactivations. In isosmotic KCl solution after a conditioning hyperpolarization there occurs an increase in K permeability upon depolarization. When the depolarizations are maintained, the increase of K permeability undergoes changes similar to those observed in the normal medium. The significance of the K inactivation is discussed in relation to the after-potential of the nerve cells.

Tetrodotoxin and manganese ion: effects on action potential of the frog heart

Tetrodotoxin, which effectively suppresses the permeability of various tissues to sodium ions, has no effect on the calcium action potential of the fiber of barnacle muscle, which potential is produced by increase in permeability to calcium ions. Manganese ions, however, suppress the action potential. When applied to fiber of frog cardiac ventricle, tetrodotoxin suppresses the rate of rise of the action potential without affecting the overshoot; the suppressive effect of manganese ion is mainly on the overshoot of the action potential. This suggests that, in the action potential of the car-diac ventricle of the frog, the platealu phase is related primarily to the increase in permeability of the membrane to calcium ions.