Potassium fluxes in dialyzed squid axons

Mechanisms Underlying Poststimulation Block Induced by High-Frequency Biphasic Stimulation

OBJECTIVE

To reveal the possible mechanisms underlying poststimulation block induced by high-frequency biphasic stimulation (HFBS).

MATERIALS AND METHODS

A new axonal conduction model is developed for unmyelinated axons. This new model is different from the classical axonal conduction model by including both ion concentrations and membrane ion pumps to allow analysis of axonal responses to long-duration stimulation. Using the new model, the post-HFBS block phenomenon reported in animal studies is simulated and analyzed for a wide range of stimulation frequencies (100 Hz-10 kHz).

RESULTS

HFBS can significantly change the Na+ and K+ concentrations inside and outside the axon to produce a post-HFBS block of either short-duration (<500 msec) or long-duration (>3 sec) depending on the duration of HFBS. The short-duration block is due to the fast recovery of the Na+ and K+ concentrations outside the axon in periaxonal space by diffusion of ions into and from the large extracellular space, while the long-duration block is due to the slow restoration of the normal Na+ concentration inside the axon by membrane ion pumps. The 100 Hz HFBS requires the minimal electrical energy to achieve the post-HFBS block, while the 10 kHz stimulation is the least effective frequency requiring high intensity and long duration to achieve the block.

CONCLUSION

This study reveals two possible ionic mechanisms underlying post-HFBS block of axonal conduction. Understanding these mechanisms is important for improving clinical applications of HFBS block and for developing new nerve block methods employing HFBS.

Intracellular sodium concentration and membrane potential oscillation in axonal conduction block induced by high-frequency biphasic stimulation

OBJECTIVE

A new axonal conduction model was used to analyze the interaction between intracellular sodium concentration and membrane potential oscillation in axonal conduction block induced by high-frequency (kHz) biphasic stimulation (HFBS).

APPROACH

The model includes intracellular and extracellular sodium and potassium concentrations and ion pumps. First, the HFBS (1 kHz, 5.4 mA) was applied for a duration (59.4 seconds) long enough to produce an axonal conduction block after terminating the stimulation, i.e., a post-stimulation block. Then, the intensity of HFBS was reduced to a lower level for 4 seconds to determine if the axonal conduction block could be maintained.

MAIN RESULTS

The block duration was shortened from 1363 ms to 5 ms as the reduced HFBS intensity was increased from 0 mA to 4.1 mA. The block was maintained for the entire tested period (4000 ms) if the reduced intensity was above 4.2 mA. At the low intensity (<4.2 mA) the membrane potential oscillation disrupted the post-stimulation block caused by the increased intracellular sodium concentration, while at the high intensity (>4.2 mA) the membrane potential oscillation was strong enough to maintain the block and further increased the intracellular sodium concentration.

SIGNIFICANCE

This study indicates a possibility to develop a new nerve block method to reduce the HFBS intensity, which can extend the battery life for an implantable nerve stimulator in clinical applications to block pain of peripheral origin.

High-frequency stimulation induces axonal conduction block without generating initial action potentials

The purpose of this modeling study is to develop a novel method to block nerve conduction by high frequency biphasic stimulation (HFBS) without generating initial action potentials. An axonal conduction model including both ion concentrations and membrane ion pumps is used to analyze the axonal response to 1 kHz HFBS. The intensity of HFBS is increased in multiple steps while maintaining the intensity at a sub-threshold level to avoid generating an action potential. Axonal conduction block by HFBS is defined as the failure of action potential propagation at the site of HFBS. The simulation analysis shows that step-increases in sub-threshold intensity during HFBS can successfully block axonal conduction without generating an initial response because the excitation threshold of the axon can be gradually increased by the sub-threshold HFBS. The mechanisms underlying the increase in excitation threshold involve changes in intracellular and extracellular sodium and potassium concentration, change in the resting potential, partial inactivation of the sodium channel and partial activation of the potassium channel by HFBS. When the excitation threshold reaches a sufficient level, an acute block occurs first and after additional sub-threshold HFBS it is followed by a post-stimulation block. This study indicates that step-increases in sub-threshold HFBS intensity induces a gradual increase in axonal excitation threshold that may allow HFBS to block nerve conduction without generating an initial response. If this finding is proven to be true in human, it will significantly impact clinical applications of HFBS to treat chronic pain.

Model Analysis of Post-Stimulation Effect on Axonal Conduction and Block

OBJECTIVE

To reveal the possible contribution of changes in membrane ion concentration gradients and ion pump activity to axonal conduction/block induced by long-duration electrical stimulation.

METHODS

A new model for conduction and block of unmyelinated axons based on the classical Hodgkin-Huxley (HH) equations is developed to include changes in Na+ and K+ concentrations and ion pumps. The effects of long-duration stimulation on axonal conduction/block is analyzed by computer simulation using this new model.

RESULTS

The new model successfully simulates initiation, propagation, and block of action potentials induced by short-duration (multiple milliseconds) stimulations that do not significantly change the ion concentrations in the classical HH model. In addition, the activity-dependent effects such as action potential attenuation and broadening observed in animal studies are also successfully simulated by the new model. Finally, the model successfully simulates axonal block occurring after terminating a long-duration (multiple seconds) direct current (DC) stimulation as observed in recent animal studies and reveals 3 different mechanisms for the post-DC block of axonal conduction.

CONCLUSION

Ion concentrations and pumps play an important role in post-stimulation effects and activity-dependent effects on axonal conduction/block. The duration of stimulation is a determinant factor because it influences the total charges applied to the axon, which in turn determines the ion concentrations inside and outside the axon.

SIGNIFICANCE

Despite recent clinical success of many neurostimulation therapies, the effects of long-duration stimulation on axonal conduction/block are poorly understood. This new model could significantly impact our understanding of the mechanisms underlying different neurostimulation therapies.

ATP hydrolysis associated with an uncoupled sodium flux through the sodium pump: evidence for allosteric effects of intracellular ATP and extracellular sodium

1. A method has been developed for regenerating [gamma(32)P]ATP of constant specific activity within resealed red cell ghosts, and for measuring its hydrolysis. The method may be used to follow the hydrolysis of ATP at concentrations down to 1 muM, and for periods long enough for the ATP at these very low concentrations to turn over several hundred times.2. Using this method we have been able to show that the ;uncoupled' efflux of Na caused by the Na pump when resealed red cell ghosts are incubated in (Na + K)-free media is associated with a hydrolysis of ATP. The stoicheiometry is roughly 2-3 Na ions expelled per molecule of ATP hydrolysed.3. Measurements of ATP hydrolysis and Na efflux as functions of intracellular ATP concentration have shown that uncoupled Na efflux, and its associated ATP hydrolysis, are saturated at intracellular ATP concentrations in the region of 1 muM.4. Measurement of ATP hydrolysis as a function of ATP concentration in resealed ghosts incubated in a K-containing medium gave a complicated activation curve suggesting the involvement of high-affinity (K(m)ca. 1 muM) and low-affinity (K(m)ca. 100 muM) sites.5. When resealed ghosts containing about 1 muM-ATP were incubated in a Na-free or in a high-Na medium, the addition of K to the medium reduced the rate of ouabain-sensitive ATP hydrolysis.6. Ouabain-sensitive ATP hydrolysis in resealed ghosts incubated in K-free choline media was inhibited by external Na at low concentrations (K(i) < 1 mM), but this inhibition was reversed as the external Na concentration was further increased.7. The results show that uncoupled Na efflux may be thought of as the transport mode associated with Na-ATPase activity, just as Na-K exchange is the transport mode associated with (Na + K)-ATPase activity. The significance of the differences between uncoupled Na efflux and Na-ATPase activity, on the one hand, and Na-K exchange and (Na + K)-ATPase activity, on the other, is discussed.

Different approaches to the mechanism of the sodium pump

The way in which the sodium pump uses energy from the hydrolysis of ATP to perform osmotic and electrical work is not yet understood. We attempt to bring together the results of a number of different approaches to this problem. One approach has been to correlate biochemical changes and ionic fluxes, both when the pump operates normally and when it operates in various abnormal 'modes' in particular unphysiological conditions. A second approach has been to expose fragments of cell membrane to (gamma-32P)ATP and to study the properties of components of the membrane that become labelled. It is now clear that 32P can be transferred to the beta-carboxy group of an aspartyl residue in a pump polypeptide, but there is controversy about the interrelations of different forms of this polypeptide and its role, if any, in the normal functioning of the pump. A third approach has been to attempt to purify the pump and to determine the properties of the pure enzyme. It seems that the pump contains a polypeptide (molecular weight about 100,000), which bears the phosphorylation site, and a smaller glycopeptide, but there is disagreement about the molecular ratios. The results of these and other approaches cannot yet be fitted into a satisfactory model for the sodium pump, but we shall consider some of the problems involved in this task.

Energy utilization for control

When, on addition of a suitable substrate, a chemical potential is applied to an enzymic process such as glycolysis or respiration, whether in solution or membrane-bound, all components of the process pass into a nonequilibrium state, which might be steady or non-steady and which produces the following phenomena: (1) The reactants of each enzymic reaction are displaced from their equilibrium concentration, and energy is dissipated; (2) Part of each enzyme is transferred to a transition state of its catalytic function as well as isosteric and allosteric controlling functions, displaying local and gross conformation changes, and a rate-controlling state is generated; (3) In cyclic portions of a process futile events and chemical interconversion may occur; (4) In self- and cross-coupled portions of a process, oscillation with periodic changes of states and spatial propagation as well as instabilities may be observed; (5) At each step of a process, depending on the rate of flux and the specific enzymic function, a varying proportion of the free energy changes--which are concentration-dependent and derived from the overall potential of the system-is contributed to the control of flux rates. This will be exemplified for enzymes of bioenergetic pathways.

Pump current and Na+/K+ coupling ratio of Na+/K+-ATPase in reconstituted lipid vesicles

A method is described for studying the coupling ratio of the Na+/K+ pump, i.e., the ratio of pump-mediated fluxes of Na+ and K+, in a reconstituted system. The method is based on the comparison of the pump-generated current with the rate of K+ transport. Na+/K+-ATPase from kidney is incorporated into the membrane of artificial lipid vesicles; ATPase molecules with outward-oriented ATP-binding site are activated by addition of ATP to the medium. Using oxonol VI as a potential-sensitive dye for measuring transmembrane voltage, the pump current is determined from the change of voltage with time t. In a second set of experiments, the membrane is made selectively K+-permeable by addition of valinomycin, so that the membrane voltage U is equal to the Nernst potential of K+. Under this condition, dU/dt reflects the change of intravesicular K+ concentration and thus the flux of K+. Values of the Na+/K+ coupling ratio determined in this way are close to 1.5 in the experimental range (10-75 mM) of extravesicular (cytoplasmic) Na+ concentrations.

Determination of ionic permeability coefficients of the plasma membrane of Xenopus laevis oocytes under voltage clamp

1. A method of estimating absolute ionic permeability coefficients which does not depend on the use of impermeant substitutes is reported. 2. The method is based on a pump leak model of the Xenopus laevis oocyte membrane. The procedure consists of measuring, in the same experiment, the pump current and the currents generated under voltage clamp by the partial substitution of one or two ions at a time. For each experimental condition, the measured currents are substituted in a Goldman-Hodgkin-Katz type equation with two unknowns (the permeability coefficients). The set of equations thus generated enables the computation of all the ionic permeability coefficients. 3. The Xenopus oocyte membrane (stages IV and V, Dumont, 1972) has been found to be permeable to conventional ion substitutes such as N-methyl-D-glucamine (NMG), sulphate, isethionate and gluconate. 4. The values for sodium, potassium and chloride permeability coefficients obtained from sixty-eight pooled experiments were, respectively, 5.44, 17.41 and 1.49 x 10(-8) cm s-1. 5. The diffusional currents for sodium, potassium and chloride computed from the experiments referred to above were, respectively, -1.16, 0.69 and -0.038 microA cm-2. 6. A stoichiometry of the Na+-K+ pump exchange of 3/1.8 was computed. 7. The intracellular concentrations of sodium, potassium and chloride ions, as determined by ion-selective microelectrodes, were, respectively, 10.1 +/- 0.66 mM (n = 12), 109.5 +/- 3.3 mM (n = 13) and 37.7 +/- 1.18 mM (n = 19), corresponding to equilibrium potentials of 61, -95 and -28 mV. 8. Since chloride is not at equilibrium across the membrane, we propose that there is an inward uphill Cl- transport.

Effects of internal Na and external K concentrations on Na/K coupling of Na,K-pump in frog skeletal muscle

To clarify the dependency of the Na/K coupling of the Na,K-pump on internal Na and external K concentrations in skeletal muscle, the ouabain-induced change in membrane potential, the ouabain-induced change in Na efflux and the membrane resistance were measured at various internal Na and external K concentrations in bullfrog sartorius muscle. Upon raising the internal Na concentration from 6 mmol/kg muscle water to 20 mmol/kg muscle water, the magnitude of the ouabain-induced change in membrane potential increased about eightfold and the magnitude of the ouabain-induced change in Na efflux increased about fivefold while the membrane resistance was not significantly changed. As the external K concentration increased from 1 to 10 mM, the magnitude of the ouabain-induced change in membrane potential decreased (1/5.5 fold), while the magnitude of the ouabain-induced change in Na efflux increased (about 1.5-fold). The membrane resistance decreased upon raising the external K concentration from 1 to 10 mM (1/2-fold). These observations imply that the values of the Na/K coupling of the Na,K-pump increases upon raising the internal Na concentration and decreases upon raising the external K concentration.

Relationship between ionic surroundings and insulin actions on glucose transport and Na,K-pump in muscles

1. It is well known that insulin has various effects on glucose transport and the Na,K-pump in muscles. It is also known to have some effects on the membrane potential--in general, insulin induces a hyperpolarization of the membrane in muscles. Furthermore, it is suggested that the actions of insulin are modified by changes in ionic surroundings. 2. In this review article, the actions of ionic surroundings and insulin on glucose transport in muscles are discussed; in particular, the effects of changes in extracellular and/or intracellular concentrations of Na, K, Ca and H ions will be mentioned. 3. The actions of ionic surroundings and insulin on the Na,K-pump in muscles are discussed; in particular, the effects of changes in extracellular an/or intracellular concentrations of Na, K, Ca and H ions will be examined. 4. The relationship between the actions of ionic surroundings and insulin are discussed. 5. In particular, the effects of changes in ionic surroundings on the insulin-induced hyperpolarization of the membrane are discussed by relating it to the Na,K-pump function. The relationship between the insulin-induced change in membrane potential and glucose transport will be also mentioned.

The effect of the internal Na concentration on the electrogenicity of the insulin-stimulated Na,K-pump in frog skeletal muscles

Insulin induced a hyperpolarization of the membrane by stimulating the Na,K-pump in frog skeletal muscles. The Na,K-pump activity was dependent on the internal Na concentration. As the internal Na concentration was raised from 5 mmol/kg muscle water to 18 mmol/kg muscle water, the magnitude of the insulin-induced increase in the ouabain-sensitive Na efflux (an index of the Na,K-pump activity) rose by 5-fold and the magnitude of the insulin-induced hyperpolarization rose by 8.5-fold. On the other hand, the specific membrane resistance was not significantly changed by a rise in the internal Na concentration. The Na/K coupling of the Na,K-pump was calculated at low, normal or high internal Na concentration by using the values of the insulin-induced changes in the ouabain-sensitive Na efflux and the membrane potential. As a result of the calculation, it was suggested that in frog skeletal muscles the Na/K coupling would increase with a rise of the internal Na concentration.

Transmembrane ion balance in slowly and rapidly adapting lobster stretch receptor neurones

The transmembrane exchange of Na+, K+, and Cl- in slowly and rapidly adapting lobster stretch receptor neurones was studied using ion-sensitive microelectrodes in combination with conventional electrophysiological techniques. The investigation was founded on the assumption that the transmembrane ion exchange is accomplished by active and passive transports which add up to zero in steady state for each ion involved. The active transports are assumed to include Na+ and K+ transports driven by an electrogenic Na-K pump. To these transports are also added equimolar fluxes of K+ and Cl- leaking from the impaling micro-electrode. The passive transports are assumed to pass through membrane channels in accordance with constant field kinetics. For a quantitative evaluation of the transmembrane ion exchange in resting conditions measurements were made of the resting concentrations of Na+, K+ and Cl-; the voltage dependence of the ungated leak current; and ouabain-induced changes in resting membrane current and intracellular ion concentrations. From the results it follows that both the resting pump current and the leak permeabilities for the ions investigated have values which do not seem to differ between slowly and rapidly adapting receptor neurones. For a quantitative evaluation of the relation between internal Na+ and pump current production, measurements were made of the outward membrane current as a function of internal Na+ and K+ following a shift of these ions by means of prolonged repetitive impulse activation. It was found that the investigated relation is compatible with Garay-Garrahan kinetics (Garay & Garrahan, 1973) in both receptor neurones, but the results imply a larger maximum Na+-extrusion capacity in slowly than in rapidly adapting cells. From recordings of the time course of post-tetanic normalization of both the membrane current and intracellular Na+ concentration, cell volume values could be deduced which were closely similar in slowly and rapidly adapting receptors. A corresponding similarity was also found for the cell area which was derived from membrane capacitance measurements.

Effects of external K concentration on the electrogenicity of the insulin-stimulated Na,K-pump in frog skeletal muscle

Insulin hyperpolarized the membrane of frog skeletal muscle by stimulating the electrogenic Na,K-pump. At external K concentrations of 1,2,5 and 10 mM, both the insulin-induced hyperpolarization and the insulin-stimulated ouabain-sensitive Na efflux (an index of Na,K-pump activity) were observed. By increasing the external K concentration, the insulin-stimulated Na efflux increased, but the magnitude of the insulin-induced hyperpolarization decreased; i.e., although the activity of the insulin-stimulated Na,K-pump increased, on the contrary, the magnitude of the hyperpolarization decreased. To clarify the causes of this phenomenon, the specific membrane resistance was measured and found to decrease upon increasing the external K concentration. One of the reasons for the decrease in magnitude of the hyperpolarization is the decrease in the specific membrane resistance. However, the decrease in magnitude of the hyperpolarization with a rise of the external K concentration, which increased the insulin-stimulated Na,K-pump activity, cannot be explained only by the decrease in the specific membrane resistance. It is suggested that the decrease in magnitude of the hyperpolarization is mainly caused by a decrease in the electrogenicity of the insulin-stimulated Na,K-pump upon an increase in the external K concentration. The conclusion of the present study is that the electrogenicity of the insulin-stimulated Na,K-pump in muscles is variable and decreases with increasing the external K concentration.

Small vessel membrane potential, sympathetic input, and electrogenic pump rate in SHR

Comparative measurements of transmembrane potential (Em) were made in situ in vascular smooth muscle cells (VSM) of mesenteric small principal arteries and veins with innervation and circulation intact. Vessels were in an externalized, topically suffused jejunal loop in 4- to 5-wk-old (initial hypertension) and 12- to 15-wk-old (established hypertension) anesthetized, spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) normotensive control rats. Comparable in vitro measurements of Em were also made in VSM of isolated intact small mesenteric vessel segments (from the 12- to 15-wk-old animals) maintained at their in situ lengths and suffused with physiological salt solution (PSS). During suffusion in situ with control PSS, VSM of both small veins and arteries in older (but not younger)SHR were less polarized than in WKY. Local chemical sympathetic denervation in situ (with 6-hydroxydopamine) hyperpolarized VSM of both vessel types in older (but not younger) SHR to the same Em levels measured in situ in respective WKY vessels. After local denervation, VSM of small arteries (but not veins) of both SHR and WKY remained less polarized in situ than in vitro, suggesting the presence of one or more circulating factors with a specific depolarizing action on the arterial side in both animal types. In vitro, VSM of both small arteries and veins from WKY but not SHR were depolarized immediately by 10(-3) M ouabain. In contrast, reduction of the PSS suffusate temperature to 16 degrees C caused a significantly greater depolarization in VSM of SHR vessels.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in the noradrenergic innervation of the area dentata after axotomy of coeruleohippocampal projections or unilateral lesion of the locus coeruleus

Lesions of the septal region result in a significant decrease in the norepinephrine content of the area dentata. Over a period of one year, norepinephrine levels return to normal, presumably due to the proliferation of remaining locus coeruleus fibers. Unilateral lesions of the locus coeruleus produce reductions in [3H]norepinephrine uptake values of about 70% and 30% in the ipsilateral and contralateral area dentata, respectively. By 12 weeks after lesion, the noradrenergic fiber density in the contralateral area dentata is within the range of control measurements, whereas the area dentata ipsilateral to the lesion remains significantly depleted of noradrenergic fibers. At 26 weeks postlesion, no further change was observed in the noradrenergic fiber density of either area dentata. These results are interpreted in light of the hypothesis that locus coeruleus axon proliferation is induced by axotomy and represents the expansion of intact collaterals of damaged fibers.

Na+ and K+ transport at basolateral membranes of epithelial cells. I. Stoichiometry of the Na,K-ATPase

The stoichiometry of pump-mediated Na/K exchange was studied in isolated epithelial sheets of frog skin. 42K influx across basolateral membranes was measured with tissues in a steady state and incubated in either beakers or in chambers. The short-circuit current provided estimates of Na+ influx at the apical membranes of the cells. 42K influx of tissues bathed in Cl- or SO4-Ringer solution averaged approximately 8 microA/cm2. Ouabain inhibited 94% of the 42K influx. Furosemide was without effect on pre-ouabain-treated tissues but inhibited a ouabain-induced and Cl--dependent component of 42K influx. After taking into account the contribution of the Na+ load to the pump by way of basolateral membrane recycling of Na+, the stoichiometry was found to increase from approximately 2 to 6 as the pump-mediated Na+ transport rate increased from 10 to 70 microA/cm2. Extrapolation of the data to low rates of Na+ transport (less than 10 microA/cm2) indicated that the stoichiometry would be in the vicinity of 3:2. As pump-mediated K+ influx saturates with increasing rates of Na+ transport, Na+ efflux cannot be obligatorily coupled to K+ influx at all rates of transepithelial Na+ transport. These results are similar to those of Mullins and Brinley (1969. Journal of General Physiology. 53:504-740) in studies of the squid axon.

Kinetics of (Na+ + K+)-ATPase: analysis of the influence of Na+ and K+ by steady-state kinetics

The influence of Na+ and K+ on the steady-state kinetics at 37 degrees C of (Na+ + K+)-ATPase was investigated. From an analysis of the dependence of slopes and intercepts (from double-reciprocal plots or from Hanes plots) of the primary data on Na+ and K+ concentrations a detailed model for the interaction of the cations with the individual steps in the mechanism may be inferred and a set of intrinsic (i.e. cation independent) rate constants and cation dissociation constants are obtained. A comparison of the rate constants with those obtained from an analogous analysis of Na+-ATPase kinetics (preceding paper) provides evidence that the ATP hydrolysis proceeds through a series of intermediates, all of which are kinetically different from those responsible for the Na+-ATPase activity. The complete model for the enzyme thus involves two distinct, but doubly connected, hydrolysis cycles. The model derived for (Na+ + K+)-ATPase has the following properties: The empty, substrate free, enzyme form is the K+-bound form E2K. Na+ (Kd = 9 mM) and MgATP (Kd = 0.48 mM), in that order, must be bound to it in order to effect K+ release. Thus Na+ and K+ are simultaneously present on the enzyme in part of the reaction cycle. Each enzyme unit has three equivalent and independent Na+ sites. K+ binding to high-affinity sites (Kd = 1.4 mM) on the presumed phosphorylated intermediate is preceded by release of Na+ from low-affinity sites (Kd = 430 mM). The stoichiometry is variable, and may be Na:K:ATP = 3:2:1. To the extent that the transport properties of the enzyme are reflected in the kinetic ATPase model, these properties are in accord with one of the models shown by Sachs ((1980) J. Physiol. 302, 219-240) to give a quantitative fit of transport data for red blood cells.

Saturation of the internal sodium site of the sodium pump can distort estimates of potassium affinity

The Na+/K+ exchange pump in cardiac Purkinje strands has been well studied with the voltage clamp and Na+-selective microelectrodes. Models describing the observed results suggest that the pump rate can be considered proportional to [Na+]i over the range examined and depends on external [K+] in accordance with Michaelis-Menten kinetics. Estimates of the external [K+] that achieves a half-maximal pump rate (Km) range from 0.9 to 6.3 mM depending on the preparation and method of estimation. Here we show that much of the variability in the estimates of the Km can be eliminated when saturation of the internal Na+ pump site is taken into account. If the half-activation concentration for saturation of this Na+ site is sufficiently high (greater than 20 mM), removal of intracellular Na+ in response to a Na+ load will approximate first-order kinetics. Under these conditions however, Na+ saturation will nevertheless cause large systematic errors in estimates of the K+ dependence of pump activity.

Cation-coupled chloride influx in squid axon. Role of potassium and stoichiometry of the transport process

Evidence is presented showing that the Cl- uptake process in the squid giant axon is tightly coupled not only to Na+ uptake but also to K+ uptake. Thus, removal of external K+ causes both Cl- and Na+ influxes to be reduced, particularly when [Cl-]i is low, that is, under conditions previously shown to be optimal for Cl-/Na+-coupled influx. In addition, there exists a ouabain-insensitive K+ influx, which depends on the presence of external Cl- and Na+, is inversely proportional to [Cl-]i, and is blocked by furosemide/bumetanide. Finally, this ouabain-insensitive K+ influx appears to require the presence of cellular ATP. The stoichiometry of the coupled transport process was measured using a double-labeling technique combining in the same axon either 36Cl and 42K or 22Na and 42K. The stoichiometry of the flux changes occurring in response either to varying [Cl-]i between 150 and 0 mM or to treatment with 0.3 mM furosemide is, in both cases, approximately 3:2:1 (Cl-/Na+/K+). Although these fluxes require ATP, they are not inhibited by 3 mM vanadate. In addition, treatment with DIDS has no effect on the fluxes.

Ca++ distribution after Na+ pump inhibition in cultured neonatal rat myocardial cells

The influence of inhibition of the Na+ pump, with secondary stimulation of Na+-Ca++ exchange, on cellular Ca++ distribution is examined using the on-line scintillation disk technique and cultured neonatal rat myocardial cells. Under control conditions, La+++ displaced 78.1 +/- 1.13% (SEM) of the total cell associated 45Ca. Application of 1 mm ouabain or reduction of [K+]o to 0.5 mM resulted in a net increase of 10.6 +/- 1.3% and 13.8 +/- 2%, respectively, in total cell-associated Ca++. Of this added 45Ca, 75.9 +/- 2.7% and 78.4 +/- 2.1%, respectively remained La+++-displaceable. The 45Ca-binding characteristics of isolated sarcolemma, prepared from the cultured neonatal rat myocardial cells using the gas dissection technique, were examined. When treated with either ouabain or low [K+]o solutions, sarcolemmal 45Ca binding did not change. This result indicates that functional, intact tissue is necessary to observe the Ca++ increase. Treatment of the cells with verapamil before and during ouabain exposure failed to inhibit the ouabain-induced increase in cell-associated 45Ca. The evidence indicates that inhibition of the Na+-pump, and secondary stimulation of Na+-Ca++ exchange, result in a net increase of 11-15% in cell-associated Ca++, 78% of which remains La+++-displaceable and is, therefore, localized to the sarcolemma-glycocalyx complex.

Relation between sodium-coupled amino acid and sugar transport and sodium/potassium pump activity in isolated intestinal epithelial cells

Cells isolated from the epithelium of the small intestine are used to study the relationship between amino acid or sugar-coupled sodium transport and potassium uptake through the sodium/potassium pump. Potassium influx is a saturable function of the external potassium concentration. Uptake in the presence of ouabain, a specific pump inhibitor, is greatly reduced. This remaining influx is linearly related to the concentration up to 6 mM potassium. Sugars and amino acids are actively accumulated by the intestinal cells. Their transport is accompanied by an initial extra influx of sodium. Although cells seem to regulate their internal sodium concentrations, this is not accompanied with a concomitant increase in potassium uptake through the pump. Thus L-alanine, 3-0-methyl-D-glucoside, and alpha-methyl-D-glucoside all fail to increase the rate of ouabain-sensitive potassium uptake. A very high coupling ratio of sodium efflux to potassium influx through the pump would be a likely explanation of the present results though they cannot be regarded as conclusive.

Modeling repetitive firing and bursting in a small unmyelinated nerve fiber

The Hodgkin-Huxley equations, originally developed to describe the electrical events in the squid giant axon, have been modified to simulate the ionic and electrical events in a small unmyelinated nerve fiber. The modified equations incorporate an electrogenic sodium-potassium pump, finite intra-axonal volume, a periaxonal space, a calcium current, and calcium-dependent potassium conductance (GKCa). The model shows that adaptation can occur in two ways: increased Na-K pump activity because of periaxonal K accumulation or intra-axonal Na accumulation; or from an increase in (GKCa) caused by calcium accumulating within the axon. Bursting is an extension of adaptation and occurs when the sensitivity of the Na-K pump or (GKCa) to changes in ionic concentration is increased.

The dependence of sodium pumping and tension on intracellular sodium activity in voltage-clamped sheep Purkinje fibres

1. Intracellular Na activity (aiNa) was measured in sheep cardiac Purkinje fibres using a recessed-tip Na+-sensitive micro-electrode. The membrane potentials was controlled with a two-micro-electrode voltage clamp. Tension was measured simultaneously. 2. Removing external K produced a rise of aiNa and both twitch and tonic tension. On adding 4-10 mM-[Rb]0 to reactivate the Na-K pump aiNa and tension declined. An electrogenic Na pump current transient accompanied the fall of aiNa. 3. The half-time of decay of the electrogenic Na pump current transient was similar to that of aiNa, (mean tNa0.5/tI0.5 = 0.97 +/- 0.03 (S.E.M.; n = 28)). Following the re-activation of the Na-K pump, the electrogenic Na pump current transient was linearly related to aiNa. 4. The duration of exposure to K-free, Rb-free solutions was varied to change the level of aiNa. On subsequently re-activating the Na-K pump with 10 mM-[Rb]0, the ratio of the charge extruded to the total change of aiNa was constant. It is concluded that the fraction of Na extruded electrogenically is unaffected by changes of aiNa. About 26% of the total Na extrusion appeared as charge transfer. 5. The relationship between tonic tension and aiNa was usually different during Na-K pump inhibition in a K-free, Rb-free solution compared with the relationship during Na-K pump re-activation. In general, a given aiNa was associated with a greater level of tonic tension during Na-K pump inhibition compared with that during pump re-activation. A similar hysteresis was often seen between twitch tension and aiNa.

The interaction of potassium ions and ATP on the sodium pump of resealed red cell ghosts

1. Ouabain-sensitive K or Rb influx was measure into ghosts resealed to contain ATP concentrations of 1 micrometers-3 mM and no K. 2. Increasing ATP from 1 to 100 micro M, at saturation external K, increased K influx about twentyfold while have no effect on the ratio of ouabain-sensitive K influx to ouabain-sensitive ATPase activity. 3. Increasing external K decreased the apparent affinity for ATP. Similarly increasing ATP decreased the apparent affinity for external K. 4. The K influx can be empirically described as: influx = VmaxK2/(K + Kapp)2. Increasing ATP increased Vmax and (Kapp)2 by the same amount. 5. These results are consistent with a consecutive model for the Na pump in which an ATP-dependent reaction follows a K-activated dephosphorylation.

Metabolic modulation of stoichiometry in a proton pump

The current-voltage characteristics of the ATP-dependent proton pump in the plasma membrane of Neurospora have been explored under varied metabolic conditions imposed by mutation and by differential respiratory inhibition. The reversal potential, or presumed equilibrium potential, for the pump was observed at about -400 mV under energy-replete conditions, and at about -200 mV during a stable metabolic downshift of 55 percent. Steady-state levels of adenine nucleotides and inorganic phosphate, however, were not affected by this partial energy restriction, so that under both normal and restricted conditions the apparent free energy of ATP hydrolysis remained near -500 mV. The results suggest that a normal pump stoichiometry of 1 H+ extruded/1 ATP split is modified to 2 H+/1 ATP, by chronic energy restriction.

Sodium and potassium fluxes across the dialyzed giant axon of Myxicola

Resting and stimulated fluxes of sodium and potassium across the giant axon of the marine annelid, Myxicola infundibulum, have been characterized using the technique of internal dialysis. In most respects the ion movements were found to be similar to those in squid axons. Sodium efflux and potassium influx were found to be active, cardiac glycoside-sensitive fluxes, with a variable coupling ratio. However, when [ATP]i was lowered to less than 20 microM by treatment with cyanide and continuous dialysis, or to less than 2 microM by dialysis with glucose following injection of hexokinase, Na efflux and K influx were unaltered. The maintained fluxes were not accounted for by an increased passive permeability of the axolemma, although 30-60% of the Na efflux appeared to be due to Na-Na exchange. An altered form of Na pump operation at low [ATP]i is a more likely explanation than an alternate energy source, or an ATP source proximate to the axolemma. The transient response of 22Na efflux to a change in [22Na]i was found to be much slower than in squid, tau = 360 sec. The efflux delay could only be accounted for by an extra-axonal diffusion barrier, which is probably the basement membrane surrounding the ventral nerve cord.

The relationship of K+ efflux at the inner surface of the isolated frog skin epithelium to the short circuit current

Isolated frog skins (without chorion) were incubated with 42K+ Ringers' solution, bathing the internal surface for 2 h. All the K+ contained in the frog skin was equilibrated in specific activity with external 42K+. The kinetics of the washout of 42K+ from the internal surface of the skin exhibits one fast and one slow exponential component. Amiloride reduces the release of 42K+ corresponding to both components without affecting the K+ content of the skin. Ouabain increases the loss of 42K+ of the slow component by 200%. Since the total K+ in the skin decreases to 25% of its original value both compartments are affected. The results suggest that two distinct functional compartments exist defined by two 42K+ release ratios and that because of the large K+ contents of these compartments both are intracellular. The relation with the transepithelial Na+ transport and the morphological identification of these compartments is discussed.

Mechanism of monensin-induced hyperpolarization of neuroblastoma-glioma hybrid NG108-15

Addition of the ionophore monensin to mouse neuroblastoma-rat glioma hybrid NG108-15 cells leads to a 20 to 30-mV increase in the electrical potential across the plasma membrane as shown by direct intracellular recording techniques and by distribution studies with the lipophilic cation [3H]-tetraphenylphosphonium+ (TPP+) [Lichtshtein, D., Kaback, H.R. & Blume, A.J. (1979) Proc. Natl. Acad. Sci. USA 76, 650-654]. The effect is not observed with cells suspended in high K+ medium, is dependent upon the presence of Na+ externally, and the concentration of monensin that induces half-maximal stimulation of TPP+ accumulation is approximately 1 microM. The ionophore also causes rapid influx of Na+, a transient increase in intracellular pH, and a decrease in extracellular pH, all of which are consistent with the known ability of monensin to catalyze the transmembrane exchange of H+ for Na+. Although ouabain has no immediate effect on the membrane potential, the cardiac glycoside completely blocks the increase in TPP+ accumulation observed in the presence of monensin. Thus, the hyperpolarizing effect of monensin is mediated apparently by an increase in intracellular Na+ that acts to stimulate the electrogenic activity of the Na+,K+-ATPase. Because monensin stimulates TPP+ accumulation in a number of other cultured cell lines in addition to NG108-15, the techniques described may be of general use for studying the Na+,K+ pump and its regulation in situ.

Effect of external potassium on the coupled sodium: potassium transport ratio of axons

1. Resting membrane potential and the current-voltage relation were measured in crayfish giant axons bathed in various potassium solutions with and without ouabain. 2. Ouabain caused a depolarization of the membrane at each [K]o used but did not affect membrane resistance. 3. The ouabain-sensitive transport current was least (3 microamperemeter/cm2) in 0 mM [K]o and greatest (7 microamperemeter/cm2) in 16.2 and 21.6 mM [K]o. 4. The assumption was made an some indirect evidence presented that axons equilibrated in various potassium solutions maintain constant internal sodium and potassium concentrations for up to 3 h. 5. On the basis of this assumption, the apparent ratio of coupled Na : K transport was calculated. It was found to be least (-1.3/1) in 0 mM [K]o and to approach infinity in 16.2 and 21.6 mM [K]o. 6. The data indicate that the apparent variability of the Na : K exchange ratio likely represents an intrinsic property of the exchange mechanism and is less likely to be explained by a fixed-ratio coupled Na : K transport operating in parallel with electro-neutral Na : Na or K : K exchange.

Magnesium influx in dialyzed squid axons

The influx of magnesium from seawater into squid giant axons has been measured under conditions where internal solute control in the axon was maintained by dialysis. Mg influx is smallest (1 pmol/cm2 sec) when both Na and ATP have been removed from the axoplasm by dialysis. The addition of 3 mM ATP to the dialysis fluid gives a Mg influx of 2.5 pmol/cm2 sec while the addition of [Na]i and [ATP]i gives 3 pmol/cm2 sec as a value for Mg influx; this corresponds well with fluxes measured in intact squid giant axons. The Mg content of squid axons is 6 mmol/kg axoplasm; this is unaffected by soaking axons in Li or Na seawater for periods of up to 100 min.

Electric current generated by squid giant axon sodium pump: external K and internal ADP effects

The operation of the sodium pump of giant axons of the squid, Loligo pealei, has been studied simultaneously in two independent ways: 1) by measuring sodium efflux with 22Na, and 2) by calculating the transmembrane current generated by the pump from measurements of membrane resistance and digitalis-sensitive membrane potential. In normal, untreated axons, the effect of increasing the external potassium concentration on both sodium efflux and pump current is similar, which suggests that Na:K pump stoichiometry remains relatively constant in the range of 0-20 mM external K. The data are compatible with a 3:2 Na:K ratio. In axons whose intracellular ADP level has been elevated by injection of L-arginine, a large, electrically silent, cardiotonic steroid-sensitive sodium efflux takes place in the absence of external potassium; this suggests that pump-mediated Na:Na exchange is 1:1 or electroneutral. Finally, elevation of external potassium levels causes the appearance, in high-ADP axons, of electrogenic pumping, with little effect on sodium efflux; hence, in contrast to what is seen in normal (low-ADP) axons, the charge translocated, per sodium ion extruded, increases sharply with increasing extracellular potassium levels.