The sodium and potassium content of cephalopod nerve fibers

Once upon a time the cell membranes: 175 years of cell boundary research

All modern cells are bounded by cell membranes best described by the fluid mosaic model. This statement is so widely accepted by biologists that little attention is generally given to the theoretical importance of cell membranes in describing the cell. This has not always been the case. When the Cell Theory was first formulated in the XIX(th) century, almost nothing was known about the cell membranes. It was not until well into the XX(th) century that the existence of the plasma membrane was broadly accepted and, even then, the fluid mosaic model did not prevail until the 1970s. How were the cell boundaries considered between the articulation of the Cell Theory around 1839 and the formulation of the fluid mosaic model that has described the cell membranes since 1972? In this review I will summarize the major historical discoveries and theories that tackled the existence and structure of membranes and I will analyze how these theories impacted the understanding of the cell. Apart from its purely historical relevance, this account can provide a starting point for considering the theoretical significance of membranes to the definition of the cell and could have implications for research on early life.

ON AXOPLASMIC PRESSURE WAVES AND THEIR POSSIBLE ROLE IN NERVE IMPULSE PROPAGATION

It is suggested that the propagation of the action potential is accompanied by an axoplasmic pressure pulse propagating in the axoplasm along the axon length. The pressure pulse stretch-modulates voltage-gated Na+ (Nav) channels embedded in the axon membrane, causing their accelerated activation and inactivation and increasing peak channel conductance. As a result, the action potential propagates due to mechano-electrical activation of Nav channels by straggling ionic currents and the axoplasmic pressure pulse. The velocity of such propagation is higher than in the classical purely electrical Nav activation mechanism, and it may be close to the velocity of propagation of pressure pulses in the axoplasm. Extracellular Ca2+ ions influxing during the voltage spike, or Ca2+ ions released from intracellular stores, may trigger a mechanism that generates and augments the pressure pulse, thus opposing its viscous decay. The model can potentially explain a number of phenomena that are not contained within the purely electrical Hodgkin–Huxley-type framework: the Meyer–Overton rule for the effectiveness of anesthetics, as well as various mechanical, optical and thermodynamic phenomena accompanying the action potential. It is shown that the velocity of propagation of axoplasmic pressure pulses is close to the measured velocity of the nerve impulse, both in absolute magnitude and in dependence on axon diameter, degree of myelination and temperature.

Sodium entry during action potentials of mammalian neurons: incomplete inactivation and reduced metabolic efficiency in fast-spiking neurons

We measured the time course of sodium entry during action potentials of mouse central neurons at 37 degrees C to examine how efficiently sodium entry is coupled to depolarization. In cortical pyramidal neurons, sodium entry was nearly completely confined to the rising phase of the spike: only approximately 25% more sodium enters than the theoretical minimum necessary for spike depolarization. However, in fast-spiking GABAergic neurons (cerebellar Purkinje cells and cortical interneurons), twice as much sodium enters as the theoretical minimum. The extra entry occurs because sodium channel inactivation is incomplete during the falling phase of the spike. The efficiency of sodium entry in different cell types is primarily a function of action potential shape and not cell-type-specific differences in sodium channel kinetics. The narrow spikes of fast-spiking GABAergic neurons result in incomplete inactivation of sodium channels; this reduces metabolic efficiency but likely enhances the ability to fire spikes at high frequency.

Presynaptic Na+ channels: locus, development, and recovery from inactivation at a high-fidelity synapse

Na+ channel recovery from inactivation limits the maximal rate of neuronal firing. However, the properties of presynaptic Na+ channels are not well established because of the small size of most CNS boutons. Here we study the Na+ currents of the rat calyx of Held terminal and compare them with those of postsynaptic cells. We find that presynaptic Na+ currents recover from inactivation with a fast, single-exponential time constant (24 degrees C, tau of 1.4-1.8 ms; 35 degrees C, tau of 0.5 ms), and their inactivation rate accelerates twofold during development, which may contribute to the shortening of the action potential as the terminal matures. In contrast, recordings from postsynaptic cells in brainstem slices, and acutely dissociated, reveal that their Na+ currents recover from inactivation with a double-exponential time course (tau(fast) of 1.2-1.6 ms; tau(slow) of 80-125 ms; 24 degrees C). Surprisingly, confocal immunofluorescence revealed that Na+ channels are mostly absent from the calyx terminal but are instead highly concentrated in an unusually long (approximately 20-40 microm) unmyelinated axonal heminode. Outside-out patch recordings confirmed this segregation. Expression of Na(v)1.6 alpha-subunit increased during development, whereas the Na(v)1.2alpha-subunit was not present. Serial EM reconstructions also revealed a long pre-calyx heminode, and biophysical modeling showed that exclusion of Na+ channels from the calyx terminal produces an action potential waveform with a shorter half-width. We propose that the high density and polarized locus of Na+ channels on a long heminode are critical design features that allow the mature calyx of Held terminal to fire reliably at frequencies near 1 kHz.

Effects of potassium, sodium, and azide on the ionic movements that accompany activity in frog nerves

Stimulation of intact or desheathed frog sciatic nerves produced an increase in the sodium content and a decrease in the potassium content of this tissue. In desheathed preparations the magnitudes of the changes in ionic contents decreased as the concentration of the potassium in the bathing solution was increased, while changing the external sodium concentration produced small effects on the ionic shifts. During tetanization, the rate of decline of the compound action potential also decreased as the external potassium concentration increased. Eliminating the activity respiration with 0.2 mM azide did not greatly modify the changes in sodium and potassium distribution that accompanied activity in either intact or desheathed nerves.

The efflux of potassium from electroplaques of electric eels

1. The movement of labeled potassium ions has been measured across the innervated membranes of single isolated electroplaques, obtained from the organ of Sachs of Electrophorus electricus, mounted in an apparatus which allowed a separate washing of the two membranes. 2. Equations have been derived for a 3 compartment system in series in which tracer from a large pool in one outer compartment is collected in the other outer compartment. The amount of unlabeled ion in the middle compartment may be calculated and also the fluxes across the two membranes. 3. The flux of potassium across the innervated membranes of resting cells in a steady state was between 700 to 1000 µµmoles/cm.²/sec. and was unaffected by d-tubocurarine. 4. Direct stimulation of electroplaques with external electrodes caused an increase in the efflux of potassium from the innervated membrane of 5 to 8 µµmoles/cm.²/impulse, which was unaffected by d-tubocurarine; no change occurred in the efflux across the non-innervated membrane. 5. It is concluded that the discharge of electroplaques is accompanied by a small outward movement of potassium ions across the innervated membrane of the same order of magnitude as that found on excitation of squid giant axons. The data show a basic similarity of potassium movements across these two entirely different types of conducting membranes and suggest that this phenomenon may be a general feature of bioelectric currents propagating an action potential.

Sodium and potassium ion effluxes from squid axons under voltage clamp conditions

Squid giant axons loaded with Na(24) were subjected to short duration (0.5 msec.) clamped depolarizations of about 100 mv at frequencies of 20/sec. and 60/sec. while in choline sea water. Under such conditions the early outward current was just about maximal at the time of termination of the clamping pulse. An integration of the early current versus time record gave 1.2 mucoulomb/cm(2) pulse, while a measurement of the extra Na(24) efflux resulting from repetitive pulsing gave a charge transfer of 1.4 mucoulomb/cm(2) pulse. In sodium-containing sea water and with pulses 50-75 mv more positive than E(Na) the Na(24) efflux is about 3 times the measured charge transfer. The efflux of K(42) from a previously loaded axon into normal sea water is only 50 per cent of the measured charge transfer when the membrane is held for about 5 msec. at a potential such that there is no early current, and such pulses are at 10-20/sec. The experiments appear to confirm the suggestion that the early current during bioelectric activity is sodium but provide unsatisfactory support for the identification of the delayed but sustained current solely with potassium ions. Resting Na(+) efflux is 0.6 pmole/cm(2) sec. mmole [Na](1), while the apparent K(+) efflux is about 250 pmole/cm(2) sec. and is little affected by hyperpolarization.

Electronic measurement of the intracellular concentration and net flux of sodium in the squid axon

A unique, rapid, and non-destructive determination of the intracellular sodium concentration of a squid axon may be provided by the "voltage clamp" technique, in which the potential across the axon membrane is under electronic control. The potential at which the early component of ionic current reverses following a membrane potential step was used as an index of the intracellular sodium concentration. Several types of experiments were used to test the applicability of this method for measurement of intracellular sodium and its net flux. The concentration was found to increase from 38 mM for a fresh axon to 50 mM in about an hour. From this change, the net flux for a fresh resting axon was estimated to be 40 pmoles/cm² sec. Rapid stimulation of an unclamped axon produced a marked increase in the rate of sodium accumulation. Rapid pulsing of the membrane in a voltage clamp to potentials more positive than the sodium potential moved sodium out fast enough to produce a definite decrease in internal concentration. The agreement between the results with this method and those with more direct methods is quite satisfactory. An attractive feature of this method of intracellular sodium determination is that the physiological function of the axon is maintained and other measurements may be made concurrently.

THEORETICAL POTASSIUM LOSS FROM SQUID AXONS AS A FUNCTION OF TEMPERATURE

Theoretical net ionic movements have been calculated for the propagated impulse of the squid axon from the Hodgkin-Huxley equations. The computed potassium movements agree approximately with the experimental data of Shanes, but vary too much with temperature (Q(10) = 1/2.75 from computation, 1/1.91 from experiment). Theoretical corrections providing higher ionic conductances increasing with temperature (according to J. W. Moore's experiments) give a Q(10) of 1/2.24, but the incorporation of the higher values of the maximum conductances, as observed under improved environmental conditions, leads to potassium movements that are considerably higher than Shanes's values.

THE EFFECTS OF VARIOUS IONS ON RESTING AND SPIKE POTENTIALS OF BARNACLE MUSCLE FIBERS

Effects of monovalent cations and some anions on the electrical properties of the barnacle muscle fiber membrane were studied when the intra- or extracellular concentrations of those ions were altered by longitudinal intra-cellular injection. The resting potential of the normal fiber decreases linearly with increase of logarithm of [K⁺]_out and the decrement for a tenfold increase in [K⁺]_out is 58 mv when the product, [K⁺]_out ·[Cl^-]_out, is kept constant. It also decreases with decreasing [K⁺]_in but is always less than expected theoretically. The deviation becomes larger as [K⁺]_in increases and the resting potential finally starts to decrease with increasing [K⁺]_in for [K⁺]_in > 250 mM. When the internal K⁺ concentration is decreased the overshoot of the spike potential increases and the time course of the spike potential becomes more prolonged. In substituting for the internal K⁺, Na⁺ and sucrose affect the resting and spike potentials similarly. Some organic cations (guanidine, choline, tris, and TMA) behave like sucrose while some other organic cations (TEA, TPA, and TBA) have a specific effect and prolong the spike potential if they are applied intracellularly or extracellularly. In all cases the active membrane potential increases linearly with the logarithm of [Ca⁺⁺]_out/[K⁺]_in and the increment is about 29 mv for tenfold increase in this ratio. The fiber membrane is permeable to Cl^- and other smaller anions (Br^- and I^-) but not to acetate^- and larger anions (citrate^-, sulfate^-, and methanesulfonate^-).

Large conductance Ca(2+)-activated K+ channels are involved in both spike shaping and firing regulation in Helix neurones

1. The role of BK-type calcium-dependent K+ channels (K+Ca) in cell firing regulation was evaluated by performing whole-cell voltage clamp and patch clamp experiments on the U cell neurones in the snail Helix pomatia. These cells were selected because most of the repolarizing K+ current flowed through K+Ca channels. 2. U cells generated overshooting Ca(2+)-dependent spikes in Na(+)-free saline. In response to prolonged depolarizing current, they fired a limited number of spikes of decreasing amplitude, and behaved like fast-adapting or phasic neurones. 3. Under voltage clamp conditions, the K+Ca current had a slow onset at voltages that induced small Ca2+ entries. By manipulating the Ca2+ entry (either with appropriate voltage programmes or by changing the Ca2+ content of the bath), the K+Ca channel opening was found to be rate limited by the Ca2+ binding step and not by the voltage-dependent conformational change to the open state. 4. Despite the slow activation rate observed in voltage-clamped cells, 25-30% of the available K+Ca current was found to be active during isolated spikes. These data were based on patch clamp, spike-like voltage clamp and hybrid current clamp-voltage clamp experiments. 5. The fact that spikes led the slowly rising K+Ca current to shift into a fast activating mode was accounted for by the large surge of Ca2+ current concomitant with spike upstroke. The early calcium surge resulted in local increases in cytosolic calcium, which speeded up the binding of calcium ions to the closed K+Ca channels. From changes in the null Ca2+ current voltage, it was calculated that the submembrane [Ca2+]i increase to 50-80 microM during the spike. 6. Due to their fast voltage dependence, K+Ca channels appeared to play no role in shaping the interspike trajectory. 7. Even in the fast activating mode, the K+Ca current had a finite rate of rise and was not involved in repolarizing short duration Na(+-dependent action potentials. The current became more and more active, however, when voltage-gated K+ channels were progressively inactivated during firing. 8. The fast adaptation exhibited by U cells upon sustained depolarization was not paralleled by a recruitment of K+Ca channels because of the cumulative Ca2+ entries. During a spike burst, the K+Ca current progressively overlapped the depolarizing Ca2+ current, which ultimately stopped the firing. The early opening of K+Ca channels was ascribed to residual Ca2+ accumulation that kept part of the channels in the Ca(2+)-bound state ready to be opened quickly by cell depolarization.(ABSTRACT TRUNCATED AT 400 WORDS)

The interactions of ouabain with post-tetanic and facilitatory drug potentiations at cat soleus neuromuscular junctions in vivo

Cat soleus motor nerve terminals, after high frequency conditioning, generate a post-tetanic repetition (PTR) which leads to a post-tetanic (PTP) of the muscle response. This property enables quantitative assessment of enhancement or depression of this nerve terminal excitability in vivo. The present study focuses on ionic mechanisms underlying the PTRs produced in this neuromuscular system either by high frequency stimulation or edrophonium. Ouabain was used as a specific probe for inhibition of Na(+)-K+ ATPase and its known consequences on Na+ and Ca2+ translocation. Ouabain pretreatment doubled the duration over which single stimuli, following either high frequency or edrophonium conditioning produced PTR. Ouabain in the doses used had no effect per se but as a function of dose augmented the frequency dependent responses. This pointed to Na+ loading of nerve terminals via high frequency stimulation plus ouabain inhibition of Na(+)-K+ ATPase. Ouabain potentiation of PTR responses evidently depends on exchange of intra-terminal sodium for external calcium. Thus, calcium entry blockers, Mn2+, and Co2+ suppressed or abolished the potentiations both before and after ouabain. Diphenylhydantoin, a Na+ and Ca2+ blocker, acted similarly. The effects of stimulation frequency, ouabain and the sequence of events leading to PTR in the soleus neuromuscular system appeared in general no different from those derived from the many in vitro microphysiologic studies of this phenomenon. Thus, EPPs were augmented and prolonged. It was concluded that intracellular Ca2+ is critical for regulating the stability of systems in which repetitive firing is both a normal and abnormal function.

Ethanol effects on active Na+ and K+ transport in cultured fetal rat hepatocytes

To define further the influence of ethanol on membranes, its effects on Na+ pump function were studied in monolayer cultures of fetal rat hepatocytes. The effects of ethanol (2 and 4 mg/ml) on total K+ influx, ouabain-sensitive K+ influx, Na+ pump density (from specific [3H]ouabain binding), pump turnover rates and intracellular Na+ were measured following exposure of the cells to ethanol for 1-24 hr. In parallel studies, the effects of ethanol (2 mg/ml) on cell water content and membrane fluidity were measured. Ethanol had no immediate effect on K+ influx, but after 1 hr ethanol in concentrations of 2 and 4 mg/ml decreased the total K+ influx (mumol/10(11) cells/sec) from a control of 8.5 +/- 0.64 to 4.46 +/- 0.50 and 4.09 +/- 0.26 respectively (N = 6 for each experiment; P less than 0.001). This represented the maximum effect of ethanol since after 6 and 24 hr of ethanol treatment the K+ influx had increased towards control levels but remained significantly (P less than 0.01 for 2 mg/ml and P less than 0.001 for 4 mg/ml) below that in control cells even at 24 hr. The decrease in K+ influx reflected a decrease in mean ouabain-sensitive K+ influx from a control of 5.87 to 3.24 and 2.70 (mumol/10(11) cells/sec) after a 1-hr treatment with 2 and 4 mg ethanol/ml medium respectively. Ethanol (2 mg/ml) treatment for 1-hr decreased Na+ pump density (x 10(5) molecules ouabain per cell) from a control of 2.80 +/- 0.30 to 1.70 +/- 0.11 (P less than 0.001). At 6 and 24 hr [3H]ouabain binding showed a pattern similar to that seen with the K+ influx, tending to return to pretreatment levels. There was no change in individual pump turnover rates in the presence of ethanol. Following exposure to ethanol, cellular Na+ content steadily increased over the first 6 hr and then returned to control levels. When corrected for parallel changes in cell volume, however, intracellular Na+ concentration increased by 17% (P less than 0.01) after 1 hr and thereafter remained at this higher level throughout the 24-hr period. Measurements of membrane fluidity showed that it was increased markedly by ethanol at a concentration of 2 mg/ml and that the effect bore a close temporal relationship to the changes in active K+ influx and Na+ pump density. We conclude that ethanol has a depressant effect on hepatic Na+ pump function, resulting in an increase in intracellular Na+ and an eventual gain in cell water.(ABSTRACT TRUNCATED AT 400 WORDS)

The role of giant axons in studies of the nerve impulse

The large size of the individual axons in the motor nerves of certain invertebrates has facilitated technical approaches that were not feasible elsewhere. A brief account is given of the way in which giant axons have taken and held the lead in research on the mechanism of conduction.

The effects of histamine on contraction frequency, sodium influx, and cyclic AMP in cultured rat heart cells

Histamine has been shown to have both positive inotropic and chronotropic effects. To evaluate the chronotropic effects, spontaneously contracting monolayers of cultured rat myocardial cells were treated with histamine, 10(-7) M-10(-4) M. This resulted in a dose-dependent increase in contraction frequency reaching a maximum in 10(-5) M histamine. Contraction frequency (mean +/- SEM) increased from a control of 121 +/- 5 contractions per minute to 153 +/- 4.5, 181 +/- 9, 212 +/- 4, and 216 +/- 1 in 10(-7) M, 10(-6) M, 10(-5) M, and 10(-4) M histamine, respectively (for each n = 10, p less than 0.001). The effect was time-dependent, taking 30 minutes to develop fully. Changes in contraction frequency were accompanied by parallel dose- and time-dependent increases in the verapamil-sensitive sodium influx. Verapamil-sensitive sodium influx (pmol/cm2/sec) increased from a control of 10.45 +/- 1.44 (mean +/- SEM) to 24.34 +/- 2.41 and 32.57 +/- 2.35 at 10- and 30-minute treatment with 10(-6) M histamine (n = 5, p less than 0.001). These data fit the previously described relation between verapamil-sensitive sodium influx and contraction frequency in these cells. Cimetidine (10(-4) M) but not diphenhydramine (10(-4) M) abolished both the contraction frequency and sodium influx response to histamine. Subsequent studies showed a dose- and time-dependent elevation of cyclic adenosine monophosphate (cAMP) with histamine treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Potassium and sodium shifts during in vitro isometric muscle contraction, and the time course of the ion-gradient recovery

Intracellular potassium ([K+]i), interstitial potassium ([K+]inter), intracellular sodium ([Na+]i), and resting membrane potential (RMP) were measured before and after repetitive stimulation of mouse soleus and EDL (extensor digitorum longus) muscles. At rest, RMP was -69.8 mV for soleus and -74.9 mV for EDL (37 degrees C). [K+]i was 168 mM and 182 mM, respectively. In soleus, free [Na+]i was 12.7 mM. After repetitive stimulation (960 stimuli) RMP had decreased by 11.9 mV for soleus and by 18.2 mV for EDL. [K+]i was reduced by 32 mM and 48 mM, respectively, whereas [K+]inter was doubled. In soleus [Na+]i had increased by 10.6 mM, demonstrating that the [K+]i-decrease is three times higher than the [Na+]i-increase. It is concluded that this difference reflects different activity induced movements of Na and K, and that the difference is not due to the Na/K pumping ratio. The possible involvement of the potassium loss in muscle fatigue is discussed. After stimulation RMP recovered with a time constant of 0.9 min for soleus and 1.5 min for EDL. Within the first minutes after stimulation the intracellular potassium concentration increased by 20.4 mM/min for soleus and 21.7 mM/min for EDL. Free [Na+]i decreased with less than 10 mM/min. The mechanisms underlying the different rate of changes are discussed.

Effects of internal sodium and hydrogen ions and of external calcium ions and membrane potential on calcium entry in squid axons

Squid giant axons were impaled with electrodes to measure pNai, pHi, Em, and were injected with either aequorin or arsenazo III to measure [Ca]i or with phenol red to measure [H]i. Depolarization of such axons with elevated [K] in sea water leads to a Ca entry that is a function of [Ca]o, [Na]i, and [H]i. With saturating [Na]i half-maximal Ca entry is produced by a [Ca]o of 0.58 mM. With saturating [Ca]o, depolarization produced by 450 mM-K+ leads to half-maximal Ca entry when [Na]i is 25 mM; entry is virtually undetectable if [Na]i is 18 mM. If [Ca]o is 50 mM, Ca entry upon depolarization as measured with aequorin is phasic with a rapid phase of light emission and a plateau; Ca entry as measured with arsenazo III shows no such phasic behaviour, absorbance vs. time is a square wave that closely follows the depolarization vs. time trace. Both detectors of [Ca]i show a square-wave response if [Ca]o is 3 mM. The introduction of 2 mM-CN into the sea water bathing the axon does not affect the response to depolarization nor does the destruction of most of the ATP in the axon following the injection of apyrase. If axons are microinjected with phenol red rather than arsenazo, the entry of Ca produces an acidification in the peripheral parts of the axoplasm. Other experiments measuring [Ca]i show that Ca entry is strongly inhibited by a decrease in pHi. Making sea water alkaline with pH buffers scarcely affects the Ca entry induced by depolarization; making axoplasm alkaline by adding NH4+ to sea water greatly enhances Ca entry by Na/Ca exchange and also enhances the ability of axoplasmic buffers to absorb Ca.

Intracellular ion activities and equilibrium potentials in motoneurones and glia cells of the frog spinal cord

Intra-and extracellular ion activities were measured with ion sensitive microelectrodes in motoneurones and glia cells of the spinal cord of the frog. These data were corrected for cross sensitivities of the ion exchangers to intracellular interfering ions, and equilibrium potentials for K +, Na +, Ca2 + and C1- (EK, ENa, ECa and EC1) were calculated. In motoneurones with membrane potentials exceeding -60 mV the following mean equilibrium potentials were determined. ENa = + 29.4mV, EK = -87.9 mV, ECa = + 52.6 mV, EC1 = -34.1 mV. The corresponding values for glia cells were: ENa = + 40.5 mV, EK = -84.0 mV, ECa = + 35.7 mV, EC1 = -59.7 mV. The intracellular ionic milieu is probably disturbed by the impalement of the cells. This transiently decreases the intracellular K + and increases intracellular Na +. These effects were estimated and their origin is discussed. The results of the experiments suggest a non-passive transmembrane distribution of K +, Na + and Ca2 + in motoneurones and glia cells, a non-passive transmembrane distribution of C1- in motoneurones, and a passive transmembrane distribution of C1- in glia cells.