Some properties of the external activation site of the sodium pump in crab nerve

Polyploidy in Xenopus lowers metabolic rate by decreasing total cell surface area

Although polyploidization is frequent in development, cancer, and evolution, impacts on animal metabolism are poorly understood. In Xenopus frogs, the number of genome copies (ploidy) varies across species and can be manipulated within a species. Here, we show that triploid tadpoles contain fewer, larger cells than diploids and consume oxygen at a lower rate. Drug treatments revealed that the major processes accounting for tadpole energy expenditure include cell proliferation, biosynthesis, and maintenance of plasma membrane potential. While inhibiting cell proliferation did not abolish the oxygen consumption difference between diploids and triploids, treatments that altered cellular biosynthesis or electrical potential did. Combining these results with a simple mathematical framework, we propose that the decrease in total cell surface area lowered production and activity of plasma membrane components including the Na+/K+ ATPase, reducing energy consumption in triploids. Comparison of Xenopus species that evolved through polyploidization revealed that metabolic differences emerged during development when cell size scaled with genome size. Thus, ploidy affects metabolism by altering the cell surface area to volume ratio in a multicellular organism.

Physiological adaptation of an Antarctic Na+/K+-ATPase to the cold

Because enzymatic activity is strongly suppressed by the cold, polar poikilotherms face significant adaptive challenges. For example, at 0°C the catalytic activity of a typical enzyme from a temperate organism is reduced by more than 90%. Enzymes embedded in the plasma membrane, such as the Na(+)/K(+)-ATPase, may be even more susceptible to the cold because of thermal effects on the lipid bilayer. Accordingly, adaptive changes in response to the cold may include adjustments to the enzyme or the surrounding lipid environment, or synergistic changes to both. To assess the contribution of the enzyme itself, we cloned orthologous Na(+)/K(+)-ATPase α-subunits from an Antarctic (Pareledone sp.; -1.8°C) and a temperate octopus (Octopus bimaculatus; ∼18°C), and compared their turnover rates and temperature sensitivities in a heterologous expression system. The primary sequences of the two pumps were found to be highly similar (97% identity), with most differences being conservative changes involving hydrophobic residues. The physiology of the pumps was studied using an electrophysiological approach in intact Xenopus oocytes. The voltage dependence of the pumps was equivalent. However, at room temperature the maximum turnover rate of the Antarctic pump was found to be 25% higher than that of the temperate pump. In addition, the Antarctic pump exhibited a lower temperature sensitivity, leading to significantly higher relative activity at lower temperatures. Orthologous Na(+)/K(+) pumps were then isolated from two tropical and two Arctic octopus. The temperature sensitivities of these pumps closely matched those of the temperate and Antarctic pumps, respectively. Thus, reduced thermal sensitivity appears to be a common mechanism driving cold adaptation in the Na(+)/K(+)-ATPase.

Regulation of Na+/K+ ATPase transport velocity by RNA editing

Editing of Na⁺/K⁺ ATPase mRNAs modulates the Na⁺/K⁺ pump's turnover rate by selectively targeting the release of the final sodium to the outside.

Because firing properties and metabolic rates vary widely, neurons require different transport rates from their Na⁺/K⁺ pumps in order to maintain ion homeostasis. In this study we show that Na⁺/K⁺ pump activity is tightly regulated by a novel process, RNA editing. Three codons within the squid Na⁺/K⁺ ATPase gene can be recoded at the RNA level, and the efficiency of conversion for each varies dramatically, and independently, between tissues. At one site, a highly conserved isoleucine in the seventh transmembrane span can be converted to a valine, a change that shifts the pump's intrinsic voltage dependence. Mechanistically, the removal of a single methyl group specifically targets the process of Na⁺ release to the extracellular solution, causing a higher turnover rate at the resting membrane potential.

In order for excitable cells like neurons and muscles to generate electrical signals, they require ion gradients across their plasma membranes. For example, sodium concentrations are much lower inside a cell than outside, and for potassium it is the opposite case. The job of maintaining these ion gradients falls squarely on a single protein: the Na⁺/K⁺ pump. During each transport cycle, this enzyme moves three sodium ions out of the cell and imports two of potassium. Because this process is the foundation for so many physiological processes, the Na⁺/K⁺ pump is costly to operate, using ∼30% of the ATP generated by an organism. Proper regulation of its turnover rate is vital. In this work, we use the giant nerve cell of squid as a model to show that the Na⁺/K⁺ pump can be regulated by an unsuspected mechanism. Although the gene that codes for this enzyme can make a perfectly functional pump, sometimes its information changes as it passes through the messenger RNA. This is achieved by editing RNA and as a result subtly different versions of the pump can be made, differing at only three amino acids out of more than a thousand. We demonstrate that RNA editing modulates the Na⁺/K⁺ pump's turnover rate and sodium release.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

A physiologically based biokinetic model for cesium in the human body

A physiologically descriptive model of the biological behavior of cesium in the human body has been constructed around a detailed blood flow model. The rate of transfer from plasma into a tissue is determined by the blood perfusion rate and the tissue-specific extraction fraction of Cs during passage from arterial to venous plasma. Information on tissue-specific extraction of Cs is supplemented with information on the Cs analogues, K and Rb, and known patterns of discrimination between these metals by tissues. The rate of return from a tissue to plasma is estimated from the relative contents of Cs in plasma and the tissue at equilibrium as estimated from environmental studies. Transfers of Cs other than exchange between plasma and tissues (e.g. secretions into the gastrointestinal tract) are based on a combination of physiological considerations and empirical data on Cs or related elements. Model predictions are consistent with the sizable database on the time-dependent distribution and retention of radiocesium in the human body.

Light-induced oxygen consumption in Limulus ventral photoreceptors does not result from a rise in the intracellular sodium concentration

Illumination of Limulus ventral photoreceptors leads to an increase in the intracellular concentration of sodium, [Na+]i, and to an increase in the consumption of O2 (delta QO2). After a 1-s light flash, it takes approximately 480 s for [Na+]i to return to within 10% of its preillumination level, whereas delta QO2 takes approximately 90 s. Thus, the delta QO2 is complete long before [Na+]i has returned to its resting level. Pressure injection of Na+ into the cell in order to elevate [Na+]i to the same levels as attained by illumination causes a rise in [Na+]i that returns to baseline with the same time course as the light-induced rise in [Na+]i. However, the injection of Na+ does not lead to an increase of the consumption of O2. We conclude that activation of the Na pump by a rise in [Na+]i is not a factor involved in the light-induced activation of O2 consumption in these cells.

[3H]noradrenaline release from rabbit pulmonary artery: sodium-pump-dependent sodium-calcium exchange

1. The release of [3H]noradrenaline ([3H]NA) from the isolated main pulmonary artery of the rabbit has been measured in the presence of neuronal (cocaine, 3 X 10(-5) M) and extraneuronal (corticosterone, 5 X 10(-5) M) uptake blockers. 2. K+ removal from the external medium increased the release of [3H]NA, an action transiently inhibited by Ca2+-free (+1 mM-EGTA) solution, i.e. after Ca2+ removal transmitter release was first abolished and then started to increase again after a delay lasting about 90-120 min. 3. Ca2+ readmission to arteries which had been kept in Ca2+- and 'K+-free' solution, markedly increased the [3H]NA release. The rate of transmitter release was dependent on the preceding perfusion period with 'K+-free' solution, being greater for longer exposure times. 4. When Ca2+ and K+ were readmitted together to K+-depleted and Na+-enriched preparations, the release of [3H]NA transiently increased. 5. If K+ was readmitted first, the subsequently applied Ca2+ was ineffective in producing transmitter release. 6. Different alkali metal ions (Rb+, Cs+ or Li+) were also readmitted as K+ substitutes together with Ca2+. In all cases the release of neurotransmitter transiently increased; however, the rate of release was dependent on the monovalent cation used. Thus, Rb+ ions were as effective as, Cs+ about one-third as effective as, and Li+ about one-fifth as effective as K+ in activating the Na+ pump. 7. It is concluded that in the absence of external Ca2+, and in response to Na+-pump inhibition, the release of Ca2+ from internal stores is responsible for the NA release observed. On readmission of Ca2+ the rate of transmitter release is dependent on the Na+ previously gained inside. Furthermore, the activity of the Na+ pump determines the rate of transmitter release through the Na-Ca exchange mechanism.

Origin of 5-hydroxytryptamine-induced hyperpolarization of the rat superior cervical ganglion and vagus nerve

1 5-Hydroxytryptamine (5-HT)-induced membrane potential changes were recorded extracellularly from rat superior cervical ganglia (SCG) and cervical vagus nerves in vitro. 2 On the SCG, low concentrations of 5-HT (1 X 10(-8)-3 X 10(-7) M) induced concentration-related hyperpolarization responses. Higher concentrations of 5-HT (1 X 10(-6) 1 X 10(-4) M) induced complex responses which typically consisted of an initial hyperpolarization, followed by a depolarization and subsequent after-hyperpolarization. The depolarization, but not the initial hyperpolarization, was blocked by metoclopramide (3 X 10(-5) M), quipazine (1 X 10(-6) M) or MDL 72222 (1 X 10(-5) M). 3 5-HT-induced hyperpolarization of the SCG was potentiated when the amount of calcium chloride added to the superfusion medium was reduced from 2.5 to 0.15 mmol l-1. Hyperpolarization responses recorded from SCG preparations superfused with this low-calcium medium were unaffected by the substitution of lithium chloride for sodium chloride and were potentiated by the omission of potassium ions. Ouabain (1 X 10(-3) M) abolished both the hyperpolarization and the depolarization induced by 5-HT. 4 On the vagus nerve, 5-HT (1 X 10(-7) - 3 X 10(-5)M) did not induce initial hyperpolarization in either normal or low-calcium Krebs-Henseleit medium. However, in the latter solution only, depolarization responses induced by 5-HT at concentrations of 1 X 10(-6)M or greater were followed by hyperpolarization. Both the depolarization and the post-5-HT hyperpolarization were blocked by metoclopramide (3 X 10(-5)M) but were unaffected by spiperone (1 X 10(-7)M). 5 On the vagus nerve, post-5-HT hyperpolarization responses were selectively and reversibly inhibited by ouabain, and by superfusion with Krebs-Henseleit medium that was either potassium-free or contained lithium chloride in place of sodium chloride. 7 These results demonstrate the generation in the rat SCG of a 5-HT-induced hyperpolarization response that is not mediated through 5-HT3 receptors and is unlikely to be a consequence of depolarization. In contrast, on the rat vagus nerve, the post-5-HT hyperpolarization observed in the present study had the characteristics expected of depolarization-dependent activation of a sodium ion pump.

The electrogenic Na-pump and spontaneous contraction of the hypokalemic rat duodenum

The effects of the electrogenic Na-pump on spontaneous contraction in the isolated, longitudinal muscle of the duodenum of rats which had been on a potassium-deficient diet for 7 weeks, have been investigated. Intracellular levels of Na+ are increased by this diet. The spontaneous contraction of the duodenal muscle was stopped, transiently, by 0.5 to 120 mM-K+ Krebs solution. The period of decrease of tone and amplitude occurring immediately after adding K+ was shortened when the external K+ concentration ([K]o) was increased from 0.5 to 120 mM. The decrease in tone and amplitude induced by K+ was abolished by exposure of the tissue to 0 mM [K]o, by exposure to a temperature below 14 degrees C, and in the presence of ouabain (3 X 10(-5)-10(-4) M). The spontaneous contraction of 'Na-rich' duodenum in bathing medium containing 15 mM K+ and following inhibition of the electrogenic Na-pump with cooling or ouabain was much the same as in the duodenum from rats fed balanced diets: i.e., increase of contractile tone immediately after adding K+. To activate the Na-pump in 'Na-rich' duodenum, the external K+ could be replaced by Rb+, Cs+, NH4+ and Tl3+. The effectiveness was in the order K+ greater than Rb+ greater than Cs+ greater than NH4+ greater than Tl3+. The possible existence of a neuronal or hormonal inhibitory mechanism affecting the active Na-K transport in rat smooth muscle in situ, under conditions of hypokalemia, is discussed.

The action of excess potassium and calcium on ouabain-evoked [3H]-noradrenaline release from the rabbit pulmonary artery

[3H]-noradrenaline [( 3H]-NA) release from the main pulmonary artery of the rabbit has been measured in the presence of neuronal (cocaine, 3 X 10(-5) M) and extraneuronal (corticosterone, 5 X 10(-5) M) uptake blockers. Removal of K from the external medium increased the [3H]-NA release. In the absence of external K, ouabain (10(-4) M) further enhanced the neurotransmitter release. The 'K-free' stimulated [3H]-NA release was inhibited by an increase of external Ca (7.5 mM), an action antagonized by ouabain. After preperfusion of the preparations for 30 min with either excess K (23.6 mM) or excess Ca (7.5 mM), the ouabain-stimulated [3H]-NA release was inhibited by about 50%; the rates of inhibition did not differ significantly from each other. However, the characteristic initial delay before ouabain-evoked neurotransmitter release was shortened in excess K, and prolonged in excess Ca-containing solution. When both excess K and Ca were applied together 30 min before ouabain perfusion, the action of ouabain in releasing neurotransmitter was also inhibited but the rate of inhibition did not differ significantly from that seen when K or Ca were applied separately. The action of K in shortening the initial delay was partly antagonized by Ca. Excess Ca antagonized the inhibition of ouabain-stimulated [3H]-NA release caused by excess K when Ca and ouabain were applied together after 30 min preperfusion with excess K-containing solution. Again excess Ca failed to inhibit the ouabain-evoked neurotransmitter release if ouabain and excess K were applied together after excess Ca preperfusion (30 min). In both cases the initial delay of ouabain action was greatly shortened. 6 The results suggest a Na-Ca competition at the external activation site of the nerve terminal sodium-pump similar to that of Na-K competition. Furthermore it seems that there is a sort of K-Ca competition as well, suggested by the finding that excess Ca prevented the inhibition caused by excess K of ouabain-evoked noradrenaline release and vice versa.

Presynaptic autoinhibition during rest and sodium-pump inhibition in isolated rat portal vein preparation

In the presence of cocaine and corticosterone low-frequency (2 Hz) nerve stimulation evoked release of [3H]noradrenaline measured from isolated rat portal vein preparation. In normal Krebs solution exogenously applied l-noradrenaline (3 X 10(-8)-10(-6) M) significantly reduced the nerve-evoked [3H]noradrenaline release. The IC50 value of L-noradrenaline proved to be 1.8 X 10(-7) M. Yohimbine (3 X 10(-7) M) maximally blocked the alpha 2-adrenoceptors and enhanced nerve-evoked [3H]noradrenaline release. In the presence of 5.9 mM external K+, ouabain up to 10(-4) M did not affect either the resting or the stimulation-evoked release of radioactivity from tissues. In the absence of external K+ both the resting and the nerve-evoked release of [3H]noradrenaline increased markedly. When K+ was readmitted to preparations which had been kept in K+-free solution both the resting and the stimulation-evoked [3H]noradrenaline release were greatly reduced temporarily. In K+-free solution L-noradrenaline (10(-6) M) and yohimbine (3 X 10(-7) M) failed to significantly alter the nerve-evoked release. However, 3 X 10(-6) M yohimbine in K+-free solution significantly increased the stimulation-evoked release of [3H]noradrenaline. It is concluded that presynaptic alpha 2-adrenoceptor-mediated "negative feed-back" is present in rat portal vein preparations which can be inhibited by the preferential alpha 2-adrenoceptor blocker, yohimbine. However, if the Na+-pump is inhibited (which by itself enhanced the transmitter release), presynaptic autoinhibition is more pronounced, since a high concentration of yohimbine is required to block it.

The effects of cesium chloride on insulin release, ionic fluxes and membrane potential in pancreatic B-cells

Cs+ decreases K+ permeability in nerve and muscle cells. Its effects on the pancreatic B-cell function were studied with mouse islets. In the presence of 3 mM glucose, Cs+ substitution for K+ steadily inhibited 86Rb+ efflux and hyperpolarized the B-cell membrane. Addition of Cs+ to a K+-medium also inhibited 86Rb+ efflux, but depolarized the B-cell membrane. None of these changes altered insulin release. Substitution of Cs+ for K+ in a medium containing 10 mM glucose caused a Ca2+-dependent stimulation of insulin release and 45Ca2+ efflux, produced an initial fall and a secondary rise in 86Rb+ efflux and augmented the electrical activity in B-cells. Reintroduction of K+ to the medium was followed by a marked and transient inhibition of insulin release, that was blocked by ouabain and accompanied by an inhibition of 45Ca2+ and 86Rb+ efflux and by a hyperpolarization of the B-cell membrane. Addition of Cs+ to a K+ medium containing 10 mM glucose stimulated insulin release, 45Ca2+ efflux and 86Rb+ efflux. It also increased the electrical activity in B-cells. In the absence of Ca2+, however, Cs+ addition decreased the rate of 86Rb+ efflux. The effects of Cs+ on the B-cell function may be explained by its ability to decrease K+ permeability of the plasma membrane, by its inability to activate the sodium pump, and by a third unidentified effect likely brought about by the accumulation of intracellular Cs+.

Kinetic parameters and mechanism of active cation transport in HeLa cells as studied by Rb+ influx

On incubation of HeLa cells in chilled isotonic medium, intracellular Na+ (Nac+) increased and K+ (Kc+) decreased with time, reaching steady levels after 3 h. The steady levels varied in parallel with the extracellular cation concentrations ([Na+]e, [K+]e). The cell volumes and the protein and water contents, respectively, of cells kept for 3 h in chilled media of various [Na+]e and [K+]e were not significantly different. Ouabain-sensitive Rb+ influx took place at the initial rate for a certain period which depended on [Na+]c at the beginning of the assays. The existence of two external K+ loading sites per Na+/K+-pump was demonstrated. The affinities of the sites for Rb+ as a congener of K+ were almost the same. Na+e inhibited ouabain-sensitive Rb+ influx competitively, whereas K+ was not inhibitory. Kinetic parameters were determined: the K 1/2 for Rbe+ in the absence of Na+e was 0.16 mM and th Ki for Na+e was 36.8 mM; the K 1/2 for Na+c was 19.5 mM and the Ki for K+c seemed to be extremely large. The rate equation of the ouabain-sensitive Rb+ influx suggests that Na+ and K+ are exchanged alternately through the pump by a binary mechanism.

The influence of the permeant ions thallous and potassium on inward rectification in frog skeletal muscle

A three-electrode voltage-clamp method was used to investigate the inactivation of Tl currents through the inward rectifier of frog sartorius muscle fibres, and the interaction between the permeant ions Tl+ and K+. In 80 mM-Tl Ringer inward currents inactivated on hyperpolarization along an exponential time course, with time constants that initially increased and then fell with increasing hyperpolarization. Because of the inactivation process steady-state conductances were smaller than instantaneous conductances at all potentials in Tl Ringer. The steady-state conductance increased to a maximum value at around - 100 mV in 80 mM-Tl Ringer, and then fell with increasing hyperpolarization. In K Ringer the steady-state conductance was greater at all potentials than the instantaneous conductance because K currents activate (rather than inactivate) on hyperpolarization. Time constants of Tl inactivation were the same when measured from the decay of current during a single pulse, or from the rate of recovery from inactivation using either a two- or a three-pulse method, indicating that inactivation obeys first-order kinetics. In 80 mM-Tl Ringer steady-state inactivation increased with increasing hyperpolarization, e-fold every 48 mV. This would be consistent with the site at which inactivation occurs experiencing 0.5 of the membrane voltage field. Tl+ was more permeant than K+ through the inward rectifier, the permeability ratio PTl+/PK+ being 1.66. In solutions containing both Tl+ and K+ the membrane showed an anomalous mole-fraction dependence of conductance, the resting potential being more negative, and both instantaneous and steady-state conductances smaller than those recorded in solutions containing only Tl+ or only K+. The reduction in the amplitude of the instantaneous conductance in Tl-K mixtures was voltage-dependent, the block being initially increased and then falling with increasing hyperpolarization. Inward currents also inactivated on hyperpolarization in Tl-K mixtures. The time constants of inactivation, and the extent of inactivation which occurred, became less dependent on membrane potential in these solutions. When K+ is the major permeant ion in solution, Tl+ has a blocking effect on the currents carried by K+, and the degree of block is voltage-dependent. Increasing [Tl]o increased the block at all potentials. The results of our experiments in solutions containing both Tl+ and K+ are discussed in terms of an interaction between these ions within the channel.

Effect of sodium concentration and of atropine on the contractile response of the guinea-pig ileum to potassium ions

The possibility that the guinea-pig ileum's contractile response to K+-rich solutions is partly mediated by acetylcholine release from the intramural nervous tissue was examined by studying the inhibition of that response by atropine at different values of [Na+]o. In a medium in which the NaCl was replaced by iso-osmotic glucose, the response to high [K+]o was not greatly affected, while the responses to acetylcholine and to other agonists were significantly reduced. In control medium (149 mM Na+), atropine (10(-7) M) partly inhibited the responses to K+-rich solutions and to agonists such as histamine, 5-hydroxytryptamine and bradykinin. When [Na+]o was reduced to 12 mM, by iso-osmotic substitution of glucose for NaCl, the response to high K+ was no longer inhibited by atropine, which still partly inhibited the contractions elicited by the three agonists and totally blocked the response of acetylcholine. It is proposed that atropine's inhibition of the response to high K+ and to agonists is not due to its specific anti-muscarinic effect, but to an unspecific action, which in the case of the agonists is independent of [Na+]o. In addition, the inhibition of the response to high K+ would results from a different Na+-dependent mechanisms, possibly involving the stimulation of the Na-K pump by atropine. This is supported by the observation that this drug partly relaxed ileum preparations that were contracted by ouabain.

Kinetics of oxygen consumption after a single flash of light in photoreceptors of the drone (Apis mellifera)

The time course of the rate of oxygen consumption (QO2) after a single flash of light has been measured in 300-micrometers slices of drone retina at 22 degrees C. To measure delta QO2(t), the change in QO2 from its level in darkness, the transients of the partial pressure of O2 (PO2) were recorded with O2 microelectrodes simultaneously in two sites in the slice and delta QO2 was calculated by a computer using Fourier transforms. After a 40-ms flash of intense light, delta QO2, reached a peak of 40 microliters O2/g.min and then declined exponentially to the baseline with a time constant tau 1 = 4.96 +/- 0.49 s (SD, n = 10). The rising phase was characterized by a time constant tau 2 = 1.90 +/- 0.35 s (SD, n = 10). The peak amplitude of delta QO2 increased linearly with the log of the light intensity. Replacement of Na+ by choline, known to decrease greatly the light-induced transmembrane current, caused a 63% decrease of delta QO2. With these changes, however, the kinetics of delta QO2 (t) were unchanged. This suggest that the recovery phase is rate-limited by a single reaction with apparent first-order kinetics. Evidence is provided that suggests that this reaction may be the working of the sodium pump. Exposure of the retina to high concentrations of ouabain or strophanthidin (inhibitors of the sodium pump) reduced the peak amplitude of delta QO2 by approximately 80% and increased tau 1. The increase of tau 1 was an exponential function of the time of exposure to the cardioactive steroids. Hence, it seems likely that the greatest part of delta QO2 is used for the working of the pump, whose activity is the mechanism underlying the rate constant of the descending limb of delta QO2 (t).

Activation of electrogenic Na+/K+ exchange by extracellular K+ in canine cardiac Purkinje fibers

Transient increments in sodium pump current were elicited in small voltage-clamped Purkinje fibers suspended in a fast flow system by briefly exposing them to K+-free fluid, to temporarily inhibit the pump, and then suddenly returning them to K+-containing fluid. The exponential time course of decay of the current increment provides a measure of the pump rate constant for Na+ extrusion. The dependence of that rate constant, and of the peak amplitude of the increment in pump current, on the extracellular K+ concentration was determined. The results indicate: that in cardiac Purkinje cells, as in many other cells, the pump is half-maximally activated by about 1 mM K+; that the coupling ratio for Na+/K+ exchange is independent of either intracellular Na+ concentration or external K+ concentration; and that a simple model in which intracellular Na+ concentration is determined by a passive "leak," and an active extrusion of Na+, seems sufficient to account for moderate changes in cellular Na+ concentration.

Characterization of the electrogenic sodium pump in cardiac Purkinje fibres

1. The Na pump is examined in sheep cardiac Purkinje fibres using a two micro-electrode voltage clamp technique.2. After reducing the external K concentration, [K](o), to zero for 2 min or more, subsequent addition of an ;activator cation' (known to activate the Na pump in other preparations) produces a transient increase of outward current. This outward current transient is abolished by 10(-5)M-strophanthidin (cf. Gadsby & Cranefield, 1979a).3. It is concluded that this transient increase of outward current is a result of a transient stimulation of the sodium pump by the raised [Na](i) following exposure to 0-K(o). Although this current transient may reflect the activity of an electrogenic Na pump, it is difficult to use K as the activator cation to establish this point. This is due to the extracellular K depletion that occurs during Na pump reactivation and the subsequent change that this K depletion produces in the current-voltage relationship of the Purkinje fibre.4. Rb(o) or Cs(o) have been used instead of K(o) to reactivate the Na pump when examining the transient increase of outward current. On adding either of these cations after exposing a preparation to a solution without such ;activator cations', the outward current transient is relatively voltage independent over a wide range of potentials (-90 to +10 mV). It is concluded that, following the addition of Rb(o) or Cs(o), the transient increase of outward current is a direct measure of the transient increase of the electrogenic Na pump current.5. Increasing [Rb](o) or [Cs](o) over the range of 0-40 mM increases the rate of decay of the electrogenic Na pump current transient. Using a simple model (cf. Rang & Ritchie, 1968), it is shown that the decay rate constant of the electrogenic Na pump current transient is a good measure of the degree of activation of the external site of the Na pump. At a given concentration of activator cation, Rb(o) produces a greater activation of the Na pump than does Cs(o). The K(0.5) for Rb(o) is 6.3 mM and for Cs(o) is 14.2 mM. Li(o) activates the Na pump more weakly than Rb(o) and Cs(o).6. The coupling ratio of the Na pump is shown to be independent of Rb(o) or Cs(o) over the range 2-40 mM. Furthermore, consistent with the results of Gadsby & Cranefield (1979a), the coupling ratio is independent of Na(i) over the range considered.7. The Q(10) for the electrogenic Na pump current transient varies between 1.6 and 2.3 over the range of temperature 26-46 degrees C.8. A maximum Na pump current of about 0.78 muA cm(-2) is obtained. Assuming a coupling ratio of 3Na/2K, the rate of Na ion transport into the cell is estimated to be about 23 p-mole cm(-2) sec(-1). Assuming a Na pump turnover of 150 sec(-1), we estimate that there are about 1000 Na pump sites per mum(2) of cell surface.9. We conclude that the electrogenic Na pump current transient provides a good measure of the activity of the Na pump when Rb or Cs are used as ;activator cations'. This measure can be used in the intact preparation to investigate the relationship between Na pump rate and other cellular events such as the regulation of tension (Eisner & Lederer, 1980).

The relationship between sodium pump activity and twitch tension in cardiac Purkinje fibres

1. Sheep cardiac Purkinje fibres were studied under voltage clamp conditions to examine the relationship between the Na pump rate and twitch tension.2. The Na pump activity was measured following a period of decreased Na pump rate and increased internal Na activity, Na(i), by examining the electrogenic Na pump current transient produced by reactivating the Na pump (see Eisner & Lederer, 1980).3. Various concentrations of Cs or Rb were used to reactivate the Na pump in the absence of extracellular K, K(o). Results from such experiments showed that these ;activator cations' produced a monotonic increase in Na pump rate with increasing concentration while producing monotonic decreases in steady-state twitch tension. A given concentration of Rb was more potent than the same concentration of Cs in its effects on both Na pump rate and tension. Nevertheless, varying [Rb](o) or [Cs](o) produced the same relationship between Na pump activity and tension.4. After the preparation was exposed to low K(o) (below 4 mM), the Na pump was reactivated with 10 mM-Rb(o) (0-K(o)). The area under the resulting electrogenic Na pump current transient gives a measure of the increase of [Na](i) that occurred when the preparation was exposed to the test solution compared to the steady-state Na(i) in 10 mM-Rb(o). Increasing the duration of exposure to low K(o), further augments the twitch tension achieved at the end of the test period and the area under the electrogenic Na pump current transient. Similarly, for equal periods of exposure, the lower the [K](o) in the test solution, the greater the increase of twitch tension at the end of the test period and the greater the area under the electrogenic Na pump current transient.5. The relationship between tension and the area under the electrogenic Na pump current transient is the same for a variable duration exposure to 0-K(o) or for a constant exposure to various low K(o). It is concluded that a rise in [Na](i) is the rate limiting step linking Na pump activity and twitch tension.6. In an experiment similar to the one described in (4), the fibre was exposed to a test solution containing a variable concentration of one of the activator cations of the Na pump for a fixed period. The effects of different cations were compared with the effects of a test solution containing no activator cation. Increasing the concentration of a particular activator cation from zero reduced the twitch tension augmentation in the test solution and also reduced the area under the electrogenic Na pump current transient on subsequently reactivating the Na pump with 10 mM-Rb(o). It is concluded that the ability of a test solution to reduce the area of the electrogenic Na pump current transient reflects the ability of the test solution to activate the Na pump. Equivalent concentrations of the activator cations that are required to activate the Na pump are: 2 mM-K(o) = 2 mM-Rb(o) = 6 mM-Cs(o) = 6 mM-NH(4o) = 22 mM-Li(o). Tl(o) was more effective than K(o). The order of these cations to reduce twitch tension is found to be the same as that to reduce the area under the electrogenic Na pump current transient.7. We conclude that the effects of the activator cations on twitch tension are determined by their effects on the Na pump and thereby on [Na](i).

The role of the sodium pump in the effects of potassium-depleted solutions on mammalian cardiac muscle

1. Mammalian Purkinje fibres and ventricular muscle are significantly affected by exposure to low K solutions (Eisner & Lederer, 1979). Such exposure produces two classes of effects. ;Early' effects, developing over tens of seconds include (in ventricular muscle) a more negative resting potential and a lengthening of the action potential. In Purkinje fibres the principal ;early' effect is a decrease in slope conductance. ;Late' effects develop over minutes. In ventricular muscle such effects include a shortening of the action potential, an increase in twitch and tonic tension, and the development of transient depolarizations and aftercontractions. The late effects in Purkinje fibres are the increase in twitch tension and voltage dependent tonic tension, the development of transient depolarizations and the underlying oscillatory transient inward currents, the appearance of aftercontractions accompanying the transient depolarizations or transient inward currents, and the development of a slow ;creep' in both current and tension.2. The rate of development of early effects is consistent with the time taken to change the bathing K concentration, K(o). However the time course of onset of the late effects (including the positive inotropy) is too slow to be explained by the time taken to change K(o).3. The late effects of reducing K(o) from 4 to 0 mM can be prevented by including appropriate concentrations of the activator cations of the Na pump (Tl, Rb, Cs, NH(4) or Li) in the 0 K(o) bathing solution. Similarly the late effects of 0 K(o), once established, can be reversed by adding these cations to the 0 K(o) superfusing solution.4. The order of potency of these cations to remove the effects of 0 K(o) was found to be: Tl > K approximately Rb > NH(4) approximately Cs > Li. This is similar to the order of efficacy shown to activate the external K site of the Na pump in nerve and other tissue (Rang & Ritchie, 1968).5. Strophanthidin (10(-5)M) produces qualitatively similar electrical and mechanical effects as those seen in 0 K(o). However, the effects of strophanthidin are not reversed by the activator cations. Furthermore, in the presence of strophanthidin (10(-5)M), these cations do not reverse the effects of 0 K(o).6. In voltage-clamped Purkinje fibres, returning to a solution of 4 mM-K(o) after exposure to 0 K(o) produces a transient increase in outward current. Similarly, during exposure to 0 K(o) the addition of activator cations also produces a transient increase of outward current. The ability of these ions to develop this outward transient current is correlated with their ability to remove the inotropic and arrhythmogenic effects of 0 K(o).7. The transient outward current produced by activator cations in 0 K(o) is blocked by strophanthidin (10(-5)M). We conclude that the outward current transient reflects activation of an electrogenic Na pump. Furthermore, we find that, as in other tissues, the activator cations can substitute for K(o) in activating the Na-K pump.8. The reversal of inotropic and arrhythmogenic effects of 0 K(o) by activator cations indicates that such effects result from Na pump blockade. No additional explanation (e.g. Ca/K exchange) need be invoked.

Phosphate efflux and oxygen consumption in small non-myelinated nerve fibres at rest and during activity

1. The oxygen consumption and the movements of labelled phosphate were measured in garfish olfactory nerve at rest and during activity.2. In solutions with 2.5 mM-K and 0.2 mM-phosphate the resting oxygen consumption was 0.206 m-mole/kg.min; activity at 2 sec(-1) produced an extra oxygen consumption of 2.46 mumole/kg.impulse. The extra oxygen consumption declined exponentially with a time constant of 2.62 min at 22-26 degrees C.3. The phosphate efflux, measured simultaneously, had a resting efflux rate constant of 1.24 x 10(-3) min(-1); activity at 2 sec(-1) produced an extra fractional loss of 9.38 x 10(-6) impulse(-1). The increase in phosphate efflux followed almost the same time course as the increase in oxygen consumption.4. Increasing the frequency of stimulation from 2 sec(-1) to 3 or 5 sec(-1) decreased both the extra oxygen consumption and the extra fractional loss of phosphate. When the frequency was decreased to 0.5 or 1 sec(-1) the extra oxygen consumption per impulse increased, while the extra phosphate liberation was lowered.5. Changing the phosphate concentration did not much affect the extra oxygen consumption; on the other hand, lowering or increasing the phosphate from the standard 0.2 mM decreased both the resting and the stimulated phosphate efflux.6. Lowering the K from the standard 2.5 mM did not affect the extra oxygen consumption, but increased both the resting and the extra loss of phosphate. At higher K concentrations the extra oxygen consumption and the extra fractional loss of phosphate decreased without much change in the resting phosphate efflux.7. Application of 1-20 muM-strophanthidin produced a transient decrease in the resting phosphate efflux without much change in resting oxygen consumption. With 10 or 20 muM-strophanthidin the extra fractional loss of phosphate and the extra oxygen consumption were both lowered in approximately the same proportions.8. The findings are consistent with the hypothesis that the increase in intracellular inorganic phosphate that results from increased break-down of ATP after activity, is the main cause for the increased phosphate efflux. A fraction of the increase in intracellular phosphate only appears to be liberated to the outside, the value of the fraction depending on the resting phosphate efflux before activity.9. The initial increase in intracellular inorganic phosphate after an impulse, estimated from the oxygen consumption or the phosphate fluxes, appears to be about 12-19 mumole/kg nerve, remarkably close to the value known from chemical analysis.

Ouabain-sensitive ion fluxes in the smooth muscle of the guinea-pig's taenia coli

1. Tissues with raised intracellular Na levels, produced by incubation in K-free media, were used throughout. The uptake of 42K by these Na-loaded tissues was followed for 10 min in the presence and absence of 1-37 X 10(-4) M ouabain, this being sufficient to inhibit Na pumping maximally. Subtraction of the uptake seen in the presence from that seen in the absence of ouabain gave estimates of the pumped ouabain-sensitive K uptake. 2. In Na-free (MgCl2) medium this depended on the [K]0 in a sigmoidal fashion with a half maximal [K]0 for activation of some 4mM. The maximal uptake of K was 3 m-mole/kg.min corresponding to a transmembrane flux of some 12-5 p-mole. cm-2.sec-1. 3. In the presence of Na the K activation curve became more obviously sigmoid and higher concentrations of K were needed to achieve a given active K influx. The results were well fitted by assuming that Na and K competed for two identical, non-interacting sites on the external pump face. 4. Addition of K during the efflux of 24Na into a Na-free (MgCl2) medium led to an increased rate of tracer loss. The magnitude of this increase depended on the [K] used in a hyperbolic fashion and it was abolished by addition of ouabain. The [K] causing half-maximal activation of ouabain-sensitive Na efflux was in the order of 1-2 mM. 5. When the [K] in the uptake media was 1-5 mM; Na, Li, Rb and Cs all inhibited ouabain-sensitive K uptake, the order of effectiveness being Rb greater than Cs greater than Na greater than Li. With a E1TKA10 OF 0-15 MM low concentrations of Cs and Rb were shown to stimulate K uptake. Such an effect is predicted by assuming two ion binding sites on the pump's outer face, and that the pump can translocate mixtures of K and either Rb or Cs...

The effect of lowering external sodium on the intracellular sodium activity of crab muscle fibres

1. Intracellular Na activity, aiNa, was continuously measured in crab (Carcinus maenas) muscle fibres using a recessed-tip Na+ -sensitive glass micro-electrode. Experiments could last up to several hours. AiNa remained stable during prolonged experiments. The mean resting aiNa was 8-4 +/- 0-02 mM (S.E. of mean for eighty-nine fibres) and the mean resting membrane potential was 65-3 mV +/- 0-3 (S.E. of mean for eighty-nine fibres). 2. Reducing [Na]o to 1/10 normal (maintaining ionic strength with equivalent amounts of either Li or Tris) caused a large and rapid fall of aiNa. There appeared to be two components of the effect, a fast and slow. The initial fast rate of decrease was about 3-5 m-mole/min decreasing to half this value in about 1 min. The rate of decrease of aiNa was not linearly related to aiNa. The size of the fast change of aiNa was related to the magnitude of the Na gradient across the membrane. 3. High concentrations (2 x 10-4m) of ouabain caused a very slow rise of aiNa by 1 or 2 mn/hr. This was equivalent to a net Na influx of between 1 and 10 p-mole/cmi. sec, depending on whether or not a correction was applied to account for the increased surface area of the fibre caused by the invaginating cleft system. 4. The response to low Nso was virtually insensitive to the removal of Ko or to prolonged reatment with high concentrations of ouabain (2 x 10-4 m; 100 min) and so could not readily be attributed to active Na/K pumping. 5. The response of aiNa to low Nao was reversibly inhibited by high concencentrations of Mn (50 mm) and by low concentrations of La (3-1 mm). La itself stimulated a rapid fall of aiNa in normal Nao.

Cardiac Purkinje fibers: cesium as a tool to block inward rectifying potassium currents

When a cardiac Purkinje fiber is exposed to 20 mM Cs the membrane potential falls to about -60 mV within 1 min. In voltage clamp experiments, exposure to Cs blocks both the pacemaker current iK2 and the instantaneous outward current iK1, while the delayed outward rectifying potassium current ix is not affected. In the presence of 20 mM Cs, the steady state currents are related linearly to the clamp potential and are insensitive to alterations in [K]0. The Cs sensitive current was defined as the difference between control and membrane currents measured in the presence of 20 mM Cs. This current displays inward-going rectification and its reversal potential follows log E1K]0 with a slope of 60 mV per decade.

Effects of Na and K ions on the active Na transport in guinea-pig auricles

1. The effect of Na and K ions on active Na transport was studied in guinea-pig auricles by means of flame photometry. 2. The Na influx into preparations rewarmed in Tyrode's solution after cooling was estimated to be about 1.05 mmole/l fibre water - min (l.f.w.-min) or c. 8 pmole/cm2 - s. Intracellular Na ions enhanced the active Na efflux over a wide range of concentrations. A decrease in the extracellular Na concentration ([Na]o) had no major effect on the active Na efflux. 3. Extracellular K ions initiated an active Na efflux from rewarmed auricles with an elevated [Na[i over a narrow range of K concentrations ([K]o). 4. Assuming Michaelis-Menten kinetics the maximal active Na efflux activated by internal Na ions was calculated to be about 4 mmole/l.f.w. - min (30 pmole/cm2 - s). Half maximal Na efflux occurred at about 22 mmole/l.f.w. [Na]i. The maximal K-activated active - min (28 pmole/cm2 - s) and was half maximal at a [K]o of about 0.2 mM. 5. It is tentatively concluded that the maximal active Na efflux from guinea-pig atria is 3--4 times larger than the physiological flux. Under normal conditions active Na efflux in heart is mainly regulated by variations of [Na]i.

Components of O2-uptake by excised frog nerve dependent upon externally supplied sodium ions

The steady rate of oxygen uptake of excised frog nerves equilibrated in a solution having a very low concentration of sodium ions increases to a new high steady rate when equilibrated with a solution containing a high concentration of this ion. The increase is suppressed by ouabain, indicating participation of the sodium pump. Part of this sodium-activated increase in oxygen uptake is inhibited by tetrodotoxin, indicating that passive influx of sodium ions into axons is part of the total process. Thus, two pathways for passive sodium influx into axons are suggested by these experiments. Procedures known to increase the passive permeability of axons for sodium ions also increase this sodium-activated oxygen uptake. A mechanism is proposed to explain that part of the sodium-activated oxygen uptake that is inhibited by tetrodotoxin.

A comparison of radioactive thallium and potassium fluxes in the giant axon of the squid

1. The influx and the efflux of 204Tl and 42K were measured in intact squid giant axons. 2. The resting efflux of 204Tl was found to be about one half of 42K and to have a temperature coefficient (Q10) of 1-3 as compared to 1-1 for K. 3. The extra efflux of 204Tl associated with nerve impulses was 30% greater than 42K. 4. From either Cl or NO3 sea water, the resting influx of 204Tl was about three times that of 42K. Ouabain reduced the influx of either isotope by about two thirds without changing the Tl/K ratio of the fluxes. This indicates that the Na pump can transport Tl. 5. From NO3 sea water the extra influx of 204Tl assoicated with nerve impulses was about the same as 42K. From Cl sea water there was no detectable extra influx of 204Tl. 6. The flux ratio, ouabain-insensitive influx/efflux, was different for the two ions. The resting flux ratio for Tl was consistent with a passive non-interacting flux, whereas K movements were consistent with 'single file' passage through the membrane. 7. The extra flux associated with nerve impulses is different from the resting flux both in Tl/K selectivity and in the effect of anion in the sea water. There is also a much higher flux per unit time during the nerve impulse. These differences suggest differences in the mechanisms underlying ion permeability at rest and during nervous activity.

Aerobic and anaerobic metabolism in smooth muscle cells of taenia coli in relation to active ion transport

1. The O2 consumption and lactic acid production of the guinea-pig's taenia coli have been studied in relation to the active Na-K transport, in order to estimate the ratio: active Na extrusion/active K uptake/ATP hydrolysis. 2. By applying different procedures of partial metabolic ingibition, it was found that a reactivation of the active Na-K transport in K-depleted tissues could occur in an anaerobic medium, provided glucose was present and in an aerobic medium free of added metabolizable substrate. The active Na-K transport was rapidly blocked in an anaerobic-substrate free medium. 3. Readmission of K to K-depleted tissues under aerobic conditions stimulates both O2 consumption and lactic acid production. While the O2 consumption creeps up slowly and requires 50 min to reach control values, the aerobic lactic acid production increases to a maximum within 10 min and decreases again during the next 50 min to its steady-state value. 4. A reactivation of the Na-pump in K-depleted cells in a N2-glucose medium causes an immediate increase of the lactic acid production, which decreases to its control value after 60 min. The maximal increase in anaerobic lactic acid production during reactivation of the Na-K pump is a function of [K]O. The system can be cescribed with first order kinetics having a Vmax = 0-72 mumole.g-1 f. wt. min-1 and a Km = 1-1 mM. 5. By varying the glucose concentration of [K]O during reactivation of the Na-K pump, different Na-K pumping rates can be obtained. The ratios net Na extrusion/ATP or net K accumulation/ATP amount to -1-32 +/- 0-19 (36) and 1-02 +/- 0-11 (36), in the experiments with different glucose concentrations. Taking into account the interference by net passive fluxes, one can estimate a ratio:active Na transport/active K transport/ATP, of 1-7/0-8/1. This ratio is not very different from the values observed in other tissues.

Activation of electrogenic sodium pump in mammalian skeletal muscle by external cations

The effect of change of the external ionic composition on "Na-loaded" and "K-depleted" soleus muscle fibres of K-deficient rats was investigated by recording resting membrane potentials. The addition of K, Rb, Cs and NH4 ions to K-free Krebs solution bathing "Na-rich" muscles resulted in a rapid hyperpolarization. The hyperpolarization was abolished by removing the above cations, cooling to ca. 4 degrees C, and adding 0.1 mM ouabain. The effectiveness of cations for activating the electrogenic Na pump was Rb greater than or equal to K greater than NH4 greater than Cs, and NH4 ions seemed to be unique in their stimulating action. The resting cell membrane of "Na-rich" muscles is permeable to cations in the order of Rb = K greater than Cs greater than NH4. Reducing Na ions in Krebs solution had no effect on the rate of Na-pumping in "Na-rich" muscle fibres at a given K concentration. It is concluded that the external K ions could be replaced by Rb, Cs and NH4 ions in activating the electrogenic Na pump in "Na-rich" soleus muscle fibres, but that the electrogenic Na pump in this tissue does not require the external Na ions.

Movements of labelled sodium ions in isolated rat superior cervical ganglia

1. Isolated rat superior cervical ganglia were incubated in Krebs solution containing (24)Na and carbachol for 4 min at 25 degrees C. They were then washed at 3 degrees C for 15 min to remove extracellular (24)Na and the efflux of residual intracellular (24)Na stimulated by warming to 25 degrees C.2. During the 15 min wash at 3 degrees C desaturation curves became exponential with a rate constant of 0.012 +/- 0.001 min(-1) (n = 24). This was assumed to represent loss of intracellular (24)Na, and initial uptake of (24)Na was calculated therefrom by back-extrapolation to zero wash-time. After 4 min in (24)Na + 180 muM carbachol intracellular [(24)Na] so calculated was 61.6 +/- 3.1 mM (n = 18), representing 83% labelling of intracellular Na. In the absence of carbachol intracellular [(24)Na] was 10.0 +/- 0.5 mM, representing 49% labelling. Extracellular Na was labelled by > 90% after 4 min in (24)Na. The apparent rate constant for washout of extracellular (24)Na was 0.6 min(-1) at 3 degrees C and 0.95 min(-1) at 25 degrees C.3. The loss of the residual intracellular (24)Na during temperature stimulation was interpreted quantitatively in terms of an exponential decline of the bulk of intracellular (24)Na with an extrusion rate constant of 0.39 +/- 0.1 min(-1) (n = 18), efflux being delayed by passage through the extracellular space with an effective rate constant of 0.8-1.2 min(-1).4. The peak rate constant (k(C)) for the desaturation curve at 25 degrees C was 0.35 +/- 0.01 min(-1). An Arrhenius plot of log k(C)/T degrees K(-1) yielded a two-stage linear regression with a transition at 20 degrees C. Activation energies of 8 and 31 kcal. mole(-1) were calculated above and below this transition respectively.5. Omission of K from the 25 degrees C temperature-stimulating solution reduced k(C) by 62%. The K-sensitive component of extrusion rate constant was a hyperbolic function of [K](e) with half-saturation at 5.6 mM-[K](e) and maximum k(C) of 0.58 min(-1).6. Cyanide (2 mM), 2,4-dinitrophenol (1 mM) and ouabain (1.4 mM) reduced k(C) by 50-90%. The half-maximally inhibiting concentration of ouabain was about 60 muM.7. Substitution of sucrose, Li or choline for external Na did not reduce the extrusion rate of (24)Na in either 6 mM-[K](e) or 0 mM-[K](e). Li stimulated (24)Na extrusion in Na-free, K-free solution.8. The properties of the ganglionic Na pump deduced from rates of temperature-stimulated (24)Na extrusion accord with the view that the ganglion hyperpolarization observed after Na loading by exposure to nicotinic depolarizing agents results from electrogenic Na extrusion. A comparable hyperpolarization is observed after temperature stimulation following Na loading.

A kinetic description for sodium and potassium effects on (Na+ plus K+)-adenosine triphosphatase: a model for a two-nonequivalent site potassium activation and an analysis of multiequivalent site models for sodium activation

1. Dissociation constants for sodium and potassium of a site that modulates the rate of ouabain-(Na(+)+K(+))-ATPase interaction were applied to models for potassium activation of (Na(+)+K(+))-ATPase. The constants for potassium (0.213 mM) and for sodium (13.7 mM) were defined, respectively, as activation constant, K(a) and inhibitory constant, K(i).2. Tests of the one- and the two-equivalent site models, that describe sodium and potassium competition, revealed that neither model adequately predicts the activation effects of potassium in the presence of 100 or 200 mM sodium.3. The potassium-activation data, obtained at low potassium and high sodium, were explained by a two-nonequivalent site model where the dissociation constants of the first site are 0.213 mM for potassium and 13.7 mM for sodium. The second site was characterized by dissociation constants of 0.091 mM for potassium and 74.1 mM for sodium.4. The two-nonequivalent site model adequately predicted the responses to concentrations of potassium between 0.25 and 5 mM in the presence of 100-500 mM sodium. At lower sodium concentrations the predicted responses formed an upper limit for the function of observed activities. This limit was reached at lower concentrations of potassium and higher concentrations of sodium, which inferred saturation of the sodium-activation sites with sodium.5. Sodium-activation data were corrected for sodium interaction with potassium-activation sites by use of the two-nonequivalent site model for potassium activation. Tests of equivalent site models suggested that the corrected data for sodium activation may be most consistent with a model that has three-equivalent sites. Other multiequivalent site models (n = 2, 4, 5 or 6), however, cannot be statistically eliminated as possibilities. The three-equivalent site activation model was characterized by dissociation constants of 1.39 mM for sodium and 11.7 mM for potassium. The system theoretically would be half-maximally activated by 5.35 mM sodium in the absence of potassium.6. Derivation of the model for sodium activation assumed that the affinities of these sites for sodium and potassium are independent of cation interactions with the potassium-activation sites. Therefore, the kinetic descriptions for sodium and potassium effects form a composite model that is consistent with simultaneous transport of sodium and potassium.7. Predictions of the composite equation are in reasonable agreement with data obtained by variation of sodium (potassium = 10 mM), variation of potassium (sodium = 100 mM) and by simultaneous variation of sodium and potassium (sodium:potassium = 10). Sodium-activation data (2.5-20 mM sodium) also agree with predictions of the model in the presence of potassium concentrations which are thought to be present at the sodium-activation sites in vivo.8. The kinetic description for sodium (three-equivalent sites) and potassium (two-nonequivalent sites) activation of the transport-ATPase is in accord with the probable stoichiometric requirements of the sodium pump. The model is also in general agreement with other studies on intact transporting systems and (Na(+)+K(+))-ATPase in fragmented membrane preparations with respect to potassium activation, although there is a quantitative disagreement. The model for sodium activation, though consistent with data obtained by other studies on fragmented (Na(+)+K(+))-ATPase preparations, is in apparent variance with much of the data obtained for intact transporting systems. The description for potassium activation suggests that the rates of ouabain binding to (Na(+)+K(+))-ATPase are modulated by competition between sodium and potassium for one of the two potassium-activation sites.

Some further observations on the electrogenic sodium pump in non-myelinated nerve fibres

1. A study has been made of the hyperpolarization that follows a period of electrical activity (post-tetanic hyperpolarization) and of the hyperpolarization which develops when potassium is readmitted after bathing the desheathed vagus nerve of the rabbit in potassium-free Locke solution during 15 min (potassium-activated response).2. Reduction of the external chloride concentration increases the membrane resistance and the potassium-activated response without changing the time constant of the response. A linear relation between the amplitude of the potassium-activated response and the membrane resistance was found. When chloride was replaced completely by isethionate (sodium salt) and by sulphate (other salts) the potassium-activated response increased by a factor of 5.3. The membrane resistance is decreased during the post-tetanic hyper-polarization elicited in isethionate Locke solution: the decrease is more pronounced after a longer period of electrical stimulation of the nerve.4. A small increase of the membrane resistance was found during the potassium-activated response. The changed membrane potential during the response can account for the alteration of the membrane resistance observed.5. The amplitude of the potassium-activated response is increased during hyperpolarization and reduced during external depolarization of the nerve, whereas the time constant is not affected. The potassium-activated response appears to be independent of polarization of the membrane after correction for the changed membrane resistance.6. The maximum amplitude of the activated response and the external potassium concentration are related following Michaelis-Menten kinetics; the time constant of the response is inversely related to the external potassium concentration.7. The area of the electrogenic response activated by high potassium concentrations (5.6-20 mM) is almost constant, but is reduced at lower potassium concentrations. The amplitude and area of the thallium-activated response are increased (about 1.5 times) compared with the potassium-activated response.8. It was concluded that the electrogenic response, reflected by post-tetanic hyperpolarization, is not directly related to activity of the electrogenic pump, which is probably due to accumulation of potassium in the periaxonal space; that the potassium activated response is produced entirely by activity of the electrogenic sodium pump; and that the current produced by activity of the electrogenic sodium pump is independent of the electrochemical gradient and membrane resistance.

The interaction of sodium and potassium with the sodium pump in red cells

1. At high internal K concentrations the efflux of Na from red cells increases with internal Na concentration following an S-shaped curve. As internal K is reduced the S-shaped region and the value of internal Na for which the Na efflux is half-maximal are both shifted progressively towards zero.2. The effects of internal Na on the shape of the Na efflux curves can be quantitatively accounted for if it is assumed that the rate of Na efflux is linearly related to the number of pump units having three identical and non-interacting sites occupied by Na.3. The effects of internal K on the shape of the Na efflux curves are fully explained if it is assumed that the inner sites for Na of the Na pump also behave as identical and non-interacting sites for internal K, being the K-carrier complexes unable to promote Na translocation. The apparent affinity of the Na pump for internal K is about 50 times less than for internal Na.4. Internal K not only alters the apparent affinity of the Na pump for Na, but also affects its turnover rate. The turnover rate of Na: K exchange increases with internal K following a curve which saturates at about 30 mM internal K. The turnover rate for Na:Na exchange increases linearly with internal K.5. The linear dependence of the rate of Na:Na exchange on internal K explains why, when internal Na is increased at the expense of internal K, the rate of Na:Na exchange progressively decreases after passing through a maximum.6. The effects of external Na on the rate of Na:Na exchange can be satisfactorily explained assuming that they are due to the occupation by external Na of three identical and non-interacting sites on each pump unit. The apparent affinity of the Na pump for external Na is about 160 times less than the apparent affinity for internal Na.7. Under all the experimental conditions tested, it was found that the relation between flux and cation concentration at one of the surfaces of the cell membrane is altered only by a constant factor by changes in the cation composition at the opposite surface of the cell membrane. This fact strongly suggests that there are no interactions between the inner and outer sites of the Na pump.8. The effects of inner and outer cations on both the Na:K and the Na:Na exchanges catalysed by the Na pump suggest that cation fluxes are proportional to the number of pump units having its inner and outer sites simultaneously occupied by the relevant cations. It seems therefore that sequential models for ion transort do not provide an adequate description of the molecular mechanism of active transport in red cells.

Effect of sodium and sodium-substitutes on the active ion transport and on the membrane potential of smooth muscle cells

1. The changes of the ion content and of the membrane potential of taenia coli cells have been studied during prolonged exposure to Na-deficient solutions containing either Li or choline.2. A K-free solution containing either 71 mM-Na-71 mM-Li or 71 mM-Na-71 mM choline causes a slower loss of cellular K than a 142 mM-Na solution. In both these Na-deficient solutions the membrane hyperpolarizes to about -100 mV for periods up to 6 hr. This hyperpolarization is partially abolished by 2 x 10(-5)M ouabain.3. Replacing all extracellular Na by Li and maintaining 5.9 mM-K causes a fast loss of all Na and a progressive replacement of K by Li. These changes of the intracellular ion content are accompanied by a depolarization of the cells, suggesting that intracellular Li cannot substitute for Na in activating the ion pump.4. Exposing K-depleted cells to a K-free 71 mM-Na-71 mM-Li solution results in a ouabain sensitive transport of Na and Li against their electro-chemical gradient.5. The K-uptake by K-depleted cells from a solution containing 0.59 mM-K is increased by reducing [Na](o) to half of its normal value. This finding indicates that external Na inhibits the active Na-K exchange.6. In Na-enriched tissues half of the Na efflux is due to a ouabain insensitive Na-exchange diffusion. If Li is used as a Na substitute, the Na-Li exchange compensates for the diminution of the Na-exchange diffusion unless ouabain is added.

Binding of the cardiac glycoside ouabain to intact cells

1. Measurements were made of the binding of [(3)H]ouabain to a variety of cell types.2. Two components of binding could usually be distinguished: a component that saturated at low glycoside concentrations and a component that increased up to the highest ouabain concentrations examined.3. Detailed studies with HeLa cells and kidney slices from guinea-pigs showed that the saturable component is probably associated with inhibition of the Na pump. The main evidence for this is (a) at low concentrations of ouabain there is a close correspondence between the concentration of ouabain giving half-maximum binding and the concentration giving half-maximum inhibition of the Na pump; (b) at low glycoside concentrations, binding precedes inhibition of the Na pump; (c) the rate of binding is very sensitive to external K ions, being highest in the absence of K; (d) binding is reversible and the release of ouabain is associated with reactivation of the Na pump, (e) binding is reduced in the absence of Na ions and in the presence of metabolic inhibitors; (f) binding has a Q(10) of about 4; and (g) in the presence of Na and ATP, lysed HeLa cells bind a similar amount of ouabain and the binding is sensitive to K ions.4. The linear component of binding does not seem to involve the Na pump and it may reflect uptake of ouabain into the cell interior. It has a Q(10) of 2.5 and is unaffected by K concentrations which have a large effect on the saturable component.5. Bound ouabain could be removed from HeLa cells by low pH, trichloroacetic acid, urea, high temperatures and 100% ethanol. These agents did not distinguish between the two components of binding.6. Criteria are developed for estimating the number of Na pumping sites in cells and the data for ouabain-binding to a number of cells is compared with the activity of the (Na + K)-activated ATPase in the same tissues. Although the number of pumping sites varies from less than 1/mu(2) to 1500/mu(2) of membrane, the turnover at these sites seems to be fairly constant between 3,500 and 15,000 min(-1) at 35 degrees C.

Origin of the after-hyperpolarization that follows removal of depolarizing agents from the isolated superior cervical ganglion of the rat

1. Potential changes in isolated rat superior cervical ganglia following addition and removal of depolarizing agents were recorded using a moving-fluid extracellular electrode system.2. Ganglionic negativity produced by carbachol was followed by a pronounced ganglionic positivity on washing. This after-positivity was attributed to hyperpolarization of the ganglion cells since it was unaffected by crushing the postganglionic trunk.3. The after-hyperpolarization was selectively depressed by (a) cooling (Q(10) 2.3), (b) metabolic inhibitors (cyanide, azide, 2,4-dinitrophenol), (c) reducing [K(+)](o) or substituting Cs(+) for K(+), (d) ouabain, and (e) substituting Li(+) for Na(+). This suggested a close dependence on active Na(+) transport.4. When K(+) was restored to K(+)-free solution, or the preparation was warmed rapidly, or when metabolic inhibitors were washed away, the hyperpolarization was rapidly regenerated. The effect of restoring K(+) indicated that the hyperpolarization was generated directly by the Na(+) pump.5. The hyperpolarization was not altered by replacing Cl(-) with isethionate, indicating that the voltage change produced by the Na(+) current was not modified by passive Cl(-) movements.6. Hexamethonium added to the washout fluid augmented the after-hyperpolarization, suggesting that there was a high (cationic) leak current due to continued receptor-activation on washing with normal Krebs solution.7. The hyperpolarization was reduced by omission of Ca(2+) and restored by addition of Mg(2+). This was considered to result from changes in passive membrane permeability.8. The time-course of post-carbachol hyperpolarization accorded with a Na(+) extrusion process whose rate was directly proportional to [Na(+)](i) with a rate constant of 0.38+/-0.02 min(-1) at 23-27 degrees C.9. With increasing concentrations of carbachol, the amplitude of the hyperpolarization increased in proportion to the preceding depolarization, but the rate constant of the hyperpolarization was unchanged.