Comparison of tetrodotoxin and procaine in internally perfused squid giant axons

Squid giant axons were internally perfused with tetrodotoxin and procaine, and excitability and electrical properties were studied by means of current-clamp and sucrose-gap voltage-clamp methods. Internally perfused tetrodotoxin was virtually without effect on the resting potential, the action potential, the early transient membrane ionic current, and the late steady-state membrane ionic current even at very high concentrations (1,000–10,000 nM) for a long period of time (up to 36 min). Externally applied tetrodotoxin at a concentration of 100 nM blocked the action potential and the early transient current in 2–3 min. Internally perfused procaine at concentrations of 1–10 mM reversibly depressed or blocked the action potential with an accompanying hyperpolarization of 2–4 mv, and inhibited both the early transient and late steady-state currents to the same extent. The time to peak early transient current was increased. The present results and the insolubility of tetrodotoxin in lipids have led to the conclusion that the gate controlling the flow of sodium ions through channels is located on the outer surface of the nerve membrane.     

Sodium extrusion by internally dialyzed squid axons

A method has been developed which allows a length of electrically excitable squid axon to be internally dialyzed against a continuously flowing solution of defined composition. Tests showed that diffusional exchange of small molecules in the axoplasm surrounding the dialysis tube occurred with a half-time of 2–5 min, and that protein does not cross the wall of the dialysis tube. The composition of the dialysis medium was (mM): K isethionate 151, K aspartate 151, taurine 275, MgCI₂ 4–10, NaCl 80, KCN 2, EDTA 0.1, ATP 5–10, and phosphoarginine 0–10. The following measurements were made: resting Na influx 57 pmole/cm²sec (n = 8); resting potassium efflux 59 pmole/ cm²sec (n = 4); stimulated Na efflux 3.1 pmole/cm²imp (n = 9); stimulated K efflux 2.9 pmole/cm²imp (n = 3); resting Na efflux 48 pmole/cm²sec (n = 18); Q ₁₀ Na efflux 2.2 (n = 5). Removal of ATP and phosphoarginine from the dialysis medium (n = 4) or external application of strophanthidin (n = 1) reversibly reduced Na efflux to 10–13 pmole/cm²sec. A general conclusion from the study is that dialyzed squid axons have relatively normal passive permeability properties and that a substantial fraction of the Na efflux is under metabolic control although the Na extrusion mechanism may not be working perfectly.     

Sodium fluxes in internally dialyzed squid axons

The effects which alterations in the concentrations of internal sodium and high energy phosphate compounds had on the sodium influx and efflux of internally dialyzed squid axons were examined. Nine naturally occurring high energy phosphate compounds were ineffective in supporting significant sodium extrusion. These compounds were: AcP, PEP, G-3-P, ADP, AMP, GTP, CTP, PA, and UTP.¹ the compound d-ATP supported 25–50% of the normal sodium extrusion, while ATP supported 80–100%. The relation between internal ATP and sodium efflux was nonlinear, rising most steeply in the range 1 to 10 µM and more gradually in the range 10 to 10,000 µM. There was no evidence of saturation of efflux even at internal ATP concentrations of 10,000 µM. The relation between internal sodium and sodium efflux was linear in the range 2 to 240 mM. The presence of external strophanthidin (10 µM) changed the sodium efflux to about 8–12 pmoles/cm² sec regardless of the initial level of efflux; this changed level was not altered by subsequent dialysis with large concentrations of ATP. Sodium influx was reduced about 50 % by removal of either ATP or Na and about 70 % by removing both ATP and Na from inside the axon.     

Activation-inactivation of potassium channels and development of the potassium-channel spike in internally perfused squid giant axons

A spike that is the result of calcium permeability through potassium channels was separated from the action potential is squid giant axons internally perfused with a 30 mM NaF solution and bathed in a 100 mM CaCl2 solution by blocking sodium channels with tetrodotoxin. Currents through potassium channels were studied under voltage clamp. The records showed a clear voltage-dependent inactivation of the currents. The inactivation was composed of at least two components; one relatively fast, having a time constant of 20--30 ms, and the other very slow, having a time constant of 5--10 s. Voltage clamp was carried out with a variety of salt compositions in both the internal and external solutions. A similar voltage-dependent inactivation, also composed of the two components, was recognized in all the current through potassium channels. Although the direction and intensity of current strongly depended on the salt composition of the solutions, the time-courses of these currents at corresponding voltages were very similar. These results strongly suggest that the inactivation of the currents in attributable to an essential, dynamic property of potassium channels themselves. Thus, the generation of a potassium-channel spike can be understood as an event that occurs when the equilibrium potential across the potassium channel becomes positive.     

Calcium influx in internally dialyzed squid giant axons

A method has been developed to measure Ca influx in internally dialyzed squid axons. This was achieved by controlling the dialyzed segment of the axon exposed to the external radioactive medium. The capacity of EGTA to buffer all the Ca entering the fiber was explored by changing the free EGTA at constant [Ca++]i. At a free [EGTA]i greater than 200 microM, the measured resting Ca influx and the expected increment in Ca entry during electrical stimulation were independent of the axoplasmic free [EGTA]. To avoid Ca uptake by the mitochondrial system, cyanide, oligomycin, and FCCP were included in the perfusate. Axons dialyzed with a standard medium containing: [ATP] = 2 mM, [Ca++]i = 0.06 microM, [Ca++]o = 10 mM, [Na+]i = 70 mM, and [Na+]o = 465 mM, gave a mean Ca influx of 0.14 +/- 0.012 pmol.cm-2.s-1 (n = 12. Removal of ATP drops the Ca influx to 0.085 +/- 0.007 pmol.cm-2.s-1 (n = 12). Ca influx increased to 0.35 pmol.cm-2,s-1 when Nao was removed. The increment was completely abolished by removing Nai+ and (or) ATP from the dialysis medium. At nominal zero [Ca++]i, no Nai-dependent Ca influx was observed. In the presence of ATP and Nai [Ca++]i activates the Ca influx along a sigmoid curve without saturation up to 1 microM [Ca++]i. Removal of Nai+ always reduced the Ca influx to a value similar to that observed in the absence of [Ca++]i (0.087 +/- 0.008 pmol.cm-2.s-1; n = 11). Under the above standard conditions, 50-60% of the total Ca influx was found to be insensitive to Nai+, Cai++, and ATP, sensitive to membrane potential, and partially inhibited by external Co++.     

ATP-dependent chloride influx into internally dialyzed squid giant axons

Summary Measurements were made of³⁶Cl influx into squid giant axons whose internal solutes were controlled by means of internal dialysis. When the intracellular chloride concentration was 50mm and the internal concentration of adenosine 5-triphosphate (ATP) was 4mm, the average chloride influx was 11.6 pmoles/cm²×sec. When the axons were dialyzed with an ATP-free solution, the average influx fell to 5.1 pmoles/cm²×sec. The effect was fully reversible upon the return of ATP to the dialysis fluid. Chloride-36 influx in the presence and absence of ATP was found to be inversely related to the internal chloride concentration.     

Some factors influencing sodium extrusion by internally dialyzed squid axons

Squid giant axons were internally dialyzed by a technique previously described. In an axon exposed to cyanide seawater for 1 hr and dialyzed with an ATP-free medium, the Na efflux had a mean value of 1.3 pmole/cm²sec when [Na]_i was 88 mM, in quantitative agreement with flux ratio calculations for a purely passive Na movement. When ATP at a concentration of 5–10 mM was supplied to the axoplasm by dialysis, Na efflux rose almost 30-fold, while if phosphoarginine, 10 mM, was supplied instead of ATP, the Na efflux rose only about 15-fold. The substitution of Li for Na in the seawater outside did not affect the Na efflux from an axon supplied with ATP, while a change to K-free Na seawater reduced the Na efflux to about one-half. When special means were used to free an axon of virtually all ADP, the response of the Na efflux to dialysis with phosphoarginine (PA) at 10 mM was very small (an increment of ca. 3 pmole/cm²sec) and it can be concluded that more than 96% of the Na efflux from an axon is fueled by ATP rather than PA. Measurements of [ATP] in the fluid flowing out of the dialysis tube when the [ATP] supplied was 5 mM made it possible to have a continuous measurement of ATP consumption by the axon. This averaged 43 pmole/cm²sec. The ATP content of axons was also measured and averaged 4.4 mM. Estimates were made of the activities of the following enzymes in axoplasm: ATPase, adenylate kinase, and arginine phosphokinase. Values are scaled to 13°C.     

Asymmetry currents in intracellularly perfused squid giant axons.

Asymmetry currents were recorded from intracellularly perfused squid axons subjected to exactly equal positive and negative voltage clamp pulses at a temperature close to 0 degrees C. The voltage and time dependence of the asymmetry currents was studied at a holding potential of minus 80 to minus 100 mV. The effect of varying the holding potential was investigated. The latter experiments showed that the voltage dependence of the asymmetrical charge movement is different from the voltage dependence of the m system.     

K+-selective microelectrode study of internally dialyzed squid giant axons.

Intracellular potassium activity, (aK)i, and axoplasmic K+ concentration, [K+]i, were measured by means of K+-selective microelectrodes and atomic absorption spectroscopy, respectively, in squid giant axons dialyzed with K+-free dialysis solution and bathed in K+-free artificial sea water. (aK)i measurements indicated that axoplasmic free K+ could be depleted by dialysis, whereas [K+]i measurements on axoplasm extruded from these axons suggest substantial retention of K+ (15.5 +/- 1.7 mmol/kg axoplasm K+; n = 9). In comparison, [K+]i in axoplasm extruded from freshly dissected axons was 330 +/- 16 mmol/kg axoplasm (n = 6). These data suggest that approximately 5% of the axoplasmic K+ ions are not easily removed by dialysis and that these ions are either bound to macromolecular sites or sequestered into membrane-enclosed organelles.     

Sodium-calcium exchange and calcium-calcium exchange in internally dialyzed squid giant axons.

The influx and efflux of calcium (as 45Ca) and influx of sodium (as 24Na) were studied in internally dialyzed squid giant axons. The axons were poisoned with cyanide and ATP was omitted from the dialysis fluid. The internal ionized Ca2+ concentration ([Ca2+]i) was controlled with Ca-EGTA buffers. With [Ca2+]i greater than 0.5 muM, 45Ca efflux was largely dependent upon external Na and Ca. The Nao-dependent Ca efflux into Ca-free media appeared to saturate as [Ca2+]i was increased to 160 muM; the half-saturation concentration was about 8 muM Ca2+. In two experiments 24Na influx was measured; when [Ca2+]i was decreased from 160 muM to less than 0.5 muM, Na influx declined by about 5 pmoles/cm2 sec. The Nao-dependent Ca efflux averaged 1.6 pmoles/cm2 sec in axons with a [Ca2+]i of 160 muM, and was negligible in axons with a [Ca2+]i of less than 0.5 muM. Taken together, the Na influx and Ca efflux data may indicate that the fluxes are coupled with a stoichiometry of about 3 Na+-to-1 Ca2+. Ca efflux into Na-free media required the presence of both Ca and an alkali metal ion (but not Cs) in the external medium. Ca influx from Li-containing media was greatly reduced when [Ca2+]i was decreased from 160 to 0.23 muM, or when external Li was replaced by choline. These data provide evidence for a Ca-Ca exchange mechanism which is activated by certain alkali metal ions. The observations are consistent with a mobile carrier mechanism which can exchange Ca2+ ions from the axoplasm for either 3 Na+ ions, or one Ca2+ and an alkali metal ion (but not Cs) from the external medium. This mechanism may utilize energy from the Na electrochemical gradient to help extrude Ca against an electrochemical gradient.     

Geographical distribution and inactivation kinetics in internally perfused Myxicola giant axons.

In some preparations the time constant of Na current inactivation determined with two pulses (tau c) is larger over some range of potentials than that determined from the current decay during a single pulse (tau h), while in others tau c(V) and tau h(V) are the same. Myxicola giant axons obtained from specimens collected from coastal waters of northeastern North America display a tau c - tau h difference under all conditions we have tested. In these axons tau c(V) and tau h(V) are unchanged by reduction of Na current density, addition of K-channel blockers, or internal perfusion. Specimens of the same species, Myxicola infundibulum, collected from a different geographical location, the south coast of England, have been studied under internal perfusion with K as the major cation internally, with reduced external Na concentration and in the presence of K-channel blockers. In these axons tau c(V) and tau h(V) approximately superpose, raising the possibility that dramatic differences in Na current kinetics may not necessarily reflect basic differences in the organization of the Na channel gating machinery.     

Restoration of membrane excitability of squid giant axons by reagents activating tyrosine-tubulin ligase

Summary Using squid giant axon, an experimental survey was performed on restoration of the membrane excitability which had been partially suppressed. Among reagents examined, a combination of 400mm KF, 50 m tyrosine, 1mm ATP, 1mm Mg ions and 5 m cAMP was found to induce the restoration of the excitability to a large extent. Further addition of a small amount of either porcine brain microtubule proteins or the squid axoplasm was found to support complete restoration. The experiments suggest that tubulin-tyrosine ligase contained in the porcine brain microtubule protein fraction or the squid axoplasm maintains the coupling between cytoskeletal structures and the plasma membrane.     

Phosphorylation modulates potassium conductance and gating current of perfused giant axons of squid

The presence of internal Mg-ATP produced a number of changes in the K conductance of perfused giant axons of squid. For holding potentials between -40 and -50 mV, steady-state K conductance increased for depolarizations to potentials more positive than approximately -15 mV and decreased for smaller depolarizations. The voltage dependencies of both steady-state activation and inactivation also appears shifted toward more positive potentials. Gating kinetics were affected by internal ATP, with the activation time constant slowed and the characteristic delay in K conductance markedly enhanced. The rate of deactivation also was hastened during perfusion with ATP. Internal ATP affected potassium channel gating currents in similar ways. The voltage dependence of gating charge movement was shifted toward more positive potentials and the time constants of ON and OFF gating current also were slowed and hastened, respectively, in the presence of ATP. These effects of ATP on the K conductance occurred when no exogenous protein kinases were added to the internal solution and persisted even after removing ATP from the internal perfusate. Perfusion with a solution containing exogenous alkaline phosphatase reversed the effects of ATP. These results provide further evidence that the effects of ATP on the K conductance are a consequence of a phosphorylation reaction mediated by a kinase present and active in perfused axons. Phosphorylation appears to alter the K conductance of squid giant axons via a minimum of two mechanisms. First, the voltage dependence of gating parameters are shifted toward positive potentials. Second, there is an increase in the number of functional closed states and/or a decrease in the rates of transition between these states of the K channels.     

Asymmetrical properties of the Na-Ca exchanger in voltage-clamped, internally dialyzed squid axons under symmetrical ionic conditions

In this work we have investigated whether the asymmetrical properties of the Na/Ca exchange process found in intact preparations are intrinsic to the exchange protein(s) or the result of the asymmetric ionic environment normally prevailing in living cells. The activation of the Na/Ca exchanger by Ca2+ ions, monovalent cations, ATP gamma S and the effect of membrane potential on the different operational modes of the exchanger (Nao/Cai, Cao/Nai, Cao/Cai, and Nao/Nai) was studied in voltage-clamped squid giant axons externally perfused and internally dialyzed with symmetrical ionic solutions. Under these conditions: (a) Ca ions activate with higher affinity from the inside (K1/2 = 22 microM) than from the outside (K1/2 = 300 microM); (b) experiments measuring the Cao-dependent Ca efflux in the conditions Lio-Trisi, Lio-Lii, Triso-Trisi, and Triso-Lii, show that the activating monovalent cation site on the exchanger faces the external surface; (c) ATP gamma S activates the Cao-dependent Ca efflux (Cao/Cai exchange) only at nonsaturating [Ca2+]i. Its effect appears to be on the Ca transport site since no alteration in the apparent affinity of the activating monovalent cation site was observed. The above results show that the Na/Ca exchange process is indeed a highly asymmetric transport mechanism. Finally, the voltage dependence of the components of the different exchange modes was measured over the range of +20 to -40 mV. The voltage dependence (approximately 26% change/25 mV) was found to be similar for all modes of operation of the exchanger except Nao/Nai exchange, which was found to be voltage insensitive. The sensitivity of the Cao/Cai exchange to voltage was found to be the same in the presence and in the complete absence of monovalent cations. This finding does not support the proposition that the voltage sensitivity of the Cao/Cao exchange is induced by the binding and transport of an external monovalent cation.     

Effects of yohimbine on squid axons.

Yohimbine, an indolealkylamine alkaloid, reduces the amplitude of the sodium current in the squid giant axon. For doses that reduce sodium current amplitude by up to 50%, there is no significant change in the kinetics or in any of the voltage-dependent parameters associated with sodium channels. The effective equilibrium constant for yohimbine binding to the sodium channel is 3 x 10(-4) M. Repetitive depolarizing pulses increase the inhibition of squid axon sodium current by yohimbine. This use-dependent inhibition is enhanced by increasing the concentration of yohimbine, by increasing the frequency of pulsing, and by increasing the magnitude or the duration of depolarization. It is reduced by hyperpolarizing prepulses. This behavior can be explained by a model wherein yohimbine binds more readily to open sodium channels than to closed sodium channels and wherein the Hodgkin-Huxley kinetic parameters are modified by the binding of the drug. This type of model may also explain the tonic and use-dependent inhibition previously described by others for local anesthetics.     

Survival of the K+ channel in axons externally and internally perfused with K+-free media.

In perfused squid giant axons, potassium channels irreversibly deteriorate when the internal K+ is removed and replaced by impermeant ions. Under the same conditions in perfused Myxicola giant axons, the K+ conductance is also irreversibly lost with a time constant of 10-15 min. In contrast, the K+ conductance in Myxicola giant axons dialyzed with impermeant ions and bathed in monovalent cation free solutions does not deteriorate, even over 5-6 h. Thus we suggest that washout of some internal component may be an important additional factor in the deterioration of K+ channels in perfused giant axons.     

Separation of the action potential into a Na-channel spike and a K- channel spike by tetrodotoxin and by tetraethylammonium ion in squid giant axons internally perfused with dilute Na-salt solutions

Squid giant axons internally perfused with a 30 mM NaF solution and bathed in a 100 mM CaCl2 solution, which are known to produce long lasting action potentials in response to pulses of outward current, were investigated. The effects of tetrodotoxin (TTX) and of tetraethylammonium ion (TEA+) on such action potentials were studied. The results are summarized as follows: (a) An addition of 1--3 microM TTX to the external solution altered but did not block the action potentials; it increased the height of the action potential by approximately 15 mV, and it decreased the membrane conductance as the peak of excitation by about two-thirds. (b) Voltage-clamp experiments performed with both NaCl and TTX in the external CaCl2 solution revealed that the TTX-insensitive action potential does not involve a rise in gNa, whereas the experiments performed without TTX showed that the action potential is accompanied by a large rise in gNa. (c) Internally applied TEA+ was shown to selectively block the TTX- insensitive action potential, but it did not block the other component of the action potential, which is accompanied by a rise in gNa, and which is selectively suppressed by TTX. (d) The addition of a small amount of KCl to the external CaCl2 solution containing TTX greatly increased both the maximum peak inward current under voltage clamp and the maximum slope conductance. Furthermore, it was shown that K+ applied on both sides of the axon plays a dominant role in producing the membrane potential in the active state in the presence of TTX, even though a large amount of Ca2+ is presented in the bathing medium. These observations have led me to conclude that the sodium channel is responsible for the production of the TTX-sensitive component of the action potential under the ionic conditions of these experiments, and the potassium channel for the TTX-insensitive component of the action potential.