Secretion of sodium ions by the frog's sartorius

EFFECTS OF EXTERNAL POTASSIUM AND STROPHANTHIDIN ON SODIUM FLUXES IN FROG STRIATED MUSCLE

Unidirectional Na fluxes in isolated fibers from the frog's semitendinosus muscle were measured in the presence of strophanthidin and increased external potassium ion concentrations. Strophanthidin at a concentration of 10^-5M inhibited about 80 per cent of the resting Na efflux without having any detectable effect on the resting Na influx. From this it is concluded that the major portion of the resting Na efflux is caused by active transport processes. External potassium concentrations from 2.5 to 7.5 mM had little effect on resting Na efflux. Above 7.5 mM and up to 15 mM external K, the Na efflux was markedly stimulated; with 15 mM K the Na influx was 250 to 300 per cent greater than normal. On the other hand, Na influx was unchanged with 15 mM K. The stimulated Na efflux with the higher concentrations was not appreciably reduced when choline or Li was substituted for external Na, but was completely inhibited by 10^-5M strophanthidin. From these findings it is concluded that the active transport of Na is stimulated by the higher concentrations of K. It is postulated that this effect on the Na "pump" is produced as a result of the depolarization of the muscle membranes and is related to the increased metabolism and heat production found under conditions of high external K.

EFFECTS OF SODIUM AZIDE ON SODIUM FLUXES IN FROG STRIATED MUSCLE

Unidirectional Na fluxes from frog's striated muscle were measured in the presence of 0 to 5 mM sodium azide. With azide concentrations of 2 and 5 mM the Na efflux was markedly stimulated; the Na efflux with 5 mM azide was about 300 per cent greater than normal. A similar increase was present when all but the 5.0 mM sodium added with azide was replaced by choline. 10^-5M strophanthidin abolished the azide effect on Na²⁴ efflux. Concentrations of azide of 1.0 mM or less had no effect on Na efflux. The Na influx, on the other hand, was only increased by 41 per cent in the presence of 5 mM NaN₃. From these findings it is concluded that the active transport of Na is stimulated by the higher concentrations of azide. The hypothesis is advanced that the active transport of Na is controlled by the transmembrane potential and that the stimulation of Na efflux is produced as a consequence of the membrane depolarization caused by the azide.

THE CONTROL OF THE MEMBRANE POTENTIAL OF MUSCLE FIBERS BY THE SODIUM PUMP

Frog sartorius muscles were made Na-rich by immersion in K-free sulfate Ringer's solution in the cold. The muscles were then loaded with Na²⁴ and the extracellular space cleared of radioactivity. When such Na-rich muscles were transferred to lithium sulfate Ringer's solution at 20°C, Na efflux was observed to increase with time, to reach a maximum about 15 minutes after the transfer of the muscles to Li₂SO₄, and then to decline. The decline in efflux from these muscles was proportional to ([Na]_i)⁸ over a considerable range of [Na]_i. The membrane potential of Na-rich muscles was about -48 mv in K-free sulfate Ringer's at 4°C but changed to -76 mv in the same solution at 20°C and to -98 mv in Li₂SO₄ Ringer's at 20°C. By contrast, muscles with a normal [Na]_i showed a fall in membrane potential when transferred from K-free sulfate Ringer's to Li₂SO₄ Ringer's solution. The general conclusions from this study are (a) that Na extrusion is capable of generating an electrical potential, and (b) that increases in [Na]_i lead to reversible increases in P_Na of muscle fibers.

Sodium-azide-evoked noradrenaline and catecholamine release from peripheral sympathetic nerves and chromaffin cells

1. The spontaneous release of [3H]noradrenaline [( 3H]NA) has been measured from rabbit pulmonary arteries and bovine chromaffin cells in the presence of neuronal uptake blocker cocaine (3 x 10(-5) M). 2. The Na+-pump inhibitor sodium-azide (NaN3, 2mM) produced a moderate increase of [3H]NA release from both preparations and relaxed the arteries. The [3H]releasing action of NaN3 was accompanied by a 30% inhibition of 86Rb-uptake into chromaffin cells. 3. In both preparations, ouabain (10(-4) M) markedly increased the release of [3H], contracted the arteries and inhibited the 86Rb-uptake of chromaffin cells by about 75%. A combined application of NaN3 and ouabain produced a similar inhibition of 86Rb-uptake of chromaffin cells and failed to increase further the release of [3H] in comparison to that found in response to ouabain alone. 4. Removal of K+ from the external medium increased both the release of [3H]NA and the tone of pulmonary arteries. NaN3 further increased the transmitter release in "K+-free" solution but relaxed the muscle. In the absence of external K+ and in the presence of azide, ouabain further enhanced the transmitter release but failed to produce significant contraction. 5. Reactivation of the Na+-pump by readmission of K+ (5.9 mM) to the external medium abolished the transmitter releasing action of NaN3 in arteries. 6. It is concluded that in peripheral sympathetic nerves and chromaffin cells, NaN3 inhibits the Na+-pump producing NA and CA release respectively and in nerves even if NA release had already been increased by K+-removal.(ABSTRACT TRUNCATED AT 250 WORDS)

The sodium pump in opossum vascular smooth muscle

Ouabain-sensitive Rb+ uptake and [3H]ouabain binding were used to measure rates of Na+ pumping and the number of pump sites, respectively, in thoracic aortae from opossums. From the number of Rb+ ions pumped per site per minute, estimates of pump turnover have been made. Values obtained are comparable to those of other species (see Table 1).

Sodium-selective micro-electrode study of apical permeability in frog skin: effects of sodium, amiloride and ouabain

The intracellular sodium ion activity (aiNa), apical membrane potential (psi ac) and apical sodium electrochemical driving force (delta mu Na) in Rana temporaria skin were measured using double-barrelled sodium-sensitive micro-electrodes, in the presence of various apical sodium activities (aoNa), amiloride, ouabain, and during voltage clamp of psi ac. The permeability and specific conductance of the apical cell membrane to sodium entry (PaNa and GaNa respectively) were calculated from the Goldman-Hodgkin-Katz equation and the Nernst-Planck (electrodiffusion) permeability equations respectively. The roles of aoNa and aiNa in the control of apical sodium entry were studied. PaNa increased linearly with log decrease in aoNa between 79 and 0.01 mM. Under short-circuit conditions, aiNa remained constant over the aoNa range of 10-79 mM, but decreased when aoNa was lower than 10 mM, due to a fall in delta mu Na and GaNa. Amiloride decreased PaNa, GaNa and aiNa, a result analogous to that observed in spontaneous low-transporting skins. Ouabain inhibited sodium transport and increased aiNa before any changes in PaNa occurred. The latter decreased only when aiNa rose above 15 mM. Increasing delta mu Na by hyperpolarizing voltage clamp of the apical cell membrane elicited a saturable increase in aiNa. The opposite effect was elicited by depolarizing psi ac. Electrodiffusion appears to be the sole mode of apical sodium entry.

Intracellular ion activities in frog skin in relation to external sodium and effects of amiloride and/or ouabain

Intracellular activities of sodium, potassium and chloride ions, aiNa, aiK, and aiCl were measured with ion-selective single-, double- and triple-barrelled micro-electrodes in skin and isolated epithelia of Rana temporaria bathed on both sides with normal or modified physiological saline. Apical and basolateral membrane potentials, psi ac and psi cs and resistance Ra and Rb respectively were also measured and from the latter the fractional resistance of the apical membrane, F(Ra) and voltage divider ratio, delta psi ac/delta psi cs were measured as criteria of satisfactory membrane penetration by the micro-electrodes. Under control conditions, aiNa was 12.3 +/- 0.8 mM, aiK was 70.3 +/- 22 mM and aiCl was 20.3 +/- 1.6 mM with psi ac averaging -38.0 +/- 3.2 mV. When 10(-4) M-amiloride was added to the apical bathing fluid aiNa fell within 10 min to 1.18 +/- 0.1 mM and aiCl to 5.2 +/- 0.9 mM, while aiK increased to 86.2 +/- 3.8 mM as measured from the basolateral border of isolated epithelia. The sodium transport pool of the skin was measured from the fall in aiNa in the presence of amiloride and could be expressed as 33 X 10(-9) mol cm-2 of epithelium. The mean rate of fall of aiNa under these conditions corresponded to an efflux rate at the basolateral border of 30.1 X 10(-9) mol cm-2 min-1 (48 microA cm-2) giving a half-time for turnover of the sodium transport pool of 33 s. Reduction of sodium concentration in the apical fluid from the normal 79 mM-Na to 10, 1 and 0.1 mM caused aiNa to fall in stages to 2 mM. Because psi ac increased in negativity to -101 mV in the process, this driving force for passive sodium accumulation, more than offset the increased sodium gradient opposing sodium influx across the apical border.

The effect of metabolic inhibition on ion contents and sodium exchange in human lymphocytes

Lymphocytes depleted of ATP by incubation in iodoacetate (IAA) and nitrogen (N2) lost K and gained Na. Isotopic Na exchange showed a fast fraction and a slower exponential fraction, the latter conventionally assumed to reflect surface membrane properties. The gain of cell Na was not accounted for by a decrease in 22Na efflux in either the slow or the fast fraction. After 3-5 hours, Na efflux increased. These results led us to question the concept that normal cell ion levels are maintained by an ATPase pump and could not be explained by exchange diffusion, co-transport, countertransport, or other inherently dissipative mechanisms. The data are, on the other hand, consistent with the concept that cell ion contents are determined by their relative exclusion from cell water coupled with selective adsorption onto fixed macromolecular anionic sites within the cell. In this view, the IAA,N2-induced rise in cell Na is due to the occupancy of adsorption sites losing K, while the increased isotopic exchange is due to a decreased activation energy for ion-site interaction.

An analysis of the influence of membrane potential and metabolic poisoning with azide on the sodium pump in skeletal muscle

1. Activation of the Na pump in muscle by the external K concentration, [K]O, is independent of the membrane potential (Em) as shown by experiments in which Em was either stabilized during variation of [K]O or varied by application of azide at constant or zero [K]O. 2. Application of azide to Na-enriched muscles causes a transient increase in 22Na efflux which occurs either in the presence or in the absence of external K. 3. The increased 22Na efflux induced by azide is abolished by addition of ouabain and is greatly reduced by removal of almost all of the external Na concentration, [Na]o. 4. Azide-treated muscles show a rather normal K sensitivity of 22Na efflux and [K]O induces a net Na extrusion from Na-enriched muscles in the presence of azide. 5. Azide reduces ouabain-sensitive K influx to low values thus interfering with K pump but not with the ability of K to activate the Na pump. 6. The experiments provide evidence that azide promotes a ouabainsensitive Na-Na exchange in Na-enriched muscles and that it partially uncouples the Na-K exchange normally observed.

The sodium pump: a ghost story

Evidence that the sodium plus potassium activated adenosinetriphosphatase ((Na + K)-ATPase) is present and functions normally in a red blood cell ghost is summarised. The case is then argued that since ghost move neither sodium nor potassium against an electrochemical gradient, the (Na+ + K)-ATPase is not in itself sufficient to generate transmembrane gradients of sodium and potassium ions. If it is not sufficient in ghost, then it cannot be sufficient in intact cells, but most somehow work co-operatively with the cytoplasm. An alternative hypothesis to that of carrier-mediated transported is then proposed, and shown to be consistent with data on intact cells, membrane homogenates, ghosts, and membrane vesicles derived from bacteria.

Studies of cat heart muscle during recovery after prolonged hypothermia. Hyperpolarization of cell membranes and its dependence on the sodium pump with electrogenic characteristics

Cat heart muscles preserved in normal Tyrode's solution at 4°C for prolonged periods were investigated by recording the transmembrane potentials after rewarming to 37°C. The cell membranes were gradually hyperpolarized during rewarming, the maximum hyperpolarization being reached within a few hours. The amplitude of resting potentials obtained at a maximally hyperpolarized stage was largest for ventricular muscle that had been preserved for 20 hours at 4°C and for atrial muscle that had been preserved for about 50 hours at 4°C. The maximum potentials averaged 267.7 mv for the ventricles and 184.4 mv for the atria. KC1 at 10 x normal and epinephrine and norepinephrine in final concentrations of 2.5 x 10 -7 g/ml brought about a marked hyperpolarization of the cell membrane when the membrane potential of the muscles was declining. The cells of muscles loaded with Li had a greater than normal membrane potential for an hour during gradual equilibration over a range of 4 to 22°C before rewarming. When muscles were rewarmed to 37°C in Li-Tyrode's solution the membrane potential gradually decreased to 5 to 23 mv for the right atrium and 13 to 34 mv for the right ventricle. Ouabain 10 -5 and 10 -6 M abolished the hyperpolarization and subsequently depolarized the cell membrane. DNP (0.2 and 0.6 mM) and sodium azide (6 mM) also had an inhibiting effect on the hyperpolarization. The observations are consistent with a hypothesis of an electrogenic sodium pump, which produces hyperpolarization whose magnitude depends largely on the previous length of time the tissue was preserved in the hypothermic state.

A ouabain-sensitive membrane conductance

1. Changes in membrane conductance and potential of sodium-loaded frog muscle fibres were found when the external recovery solution was changed: from cold to warm, to warm plus ouabain, to cold plus ouabain. Comparisons of these measurements for different external solutions were made by leaving the electrodes implanted in the same fibre during all solution changes. (The recovery solutions contained 10 mM-K and 82 mM-Cl.)2. The membrane potential became more negative on warming, less negative when ouabain was added, and still less negative when the ouabain-containing recovery solution was cooled. The membrane conductance increased on warming, increased further on addition of ouabain, and decreased when the ouabain-containing recovery solution was cooled.3. The increase of conductance which occurred on warming decreased with increasing periods in cold recovery. The increase of conductance which occurred on addition of ouabain decreased if the ouabain was added to the recovery solutions of muscles which were more fully recovered.4. The ouabain-sensitivity of the membrane conductance may be dependent upon the sodium-pump rate, or the extent of recovery of the sodium-loaded muscle fibre in the potassium- and chloride-containing recovery solutions.5. It is suggested that if the potassium conductance of the membrane increases with decreasing sodium-pump rates, then during the initial part of the recovery period a non-electrogenic mechanism must be producing a substantial part of the early net potassium influx.

Effects of sodium extrusion and local anaesthetics on muscle membrane resistance and potential

1. Membrane potentials and resistances of K-depleted muscles were measured in the cold and again after warming in K-containing media so that active ion movements occurred.2. On warming there was a fall of resistance and a gradual rise of potential which passed through a maximum. Later measurements of resistance in a chloride medium showed that values were, if anything, higher than initially in the warm.3. The excess potentials measured approximated to those required to induce passive inward movement of the K ions through the measured K resistance.4. Permeabilities for K(+) and Cl(-) were deduced. When cocaine, procaine, amytal or mepyramine were added or when K(+) was replaced by Rb(+) in the Cl(-)-free solution the K(+) permeability was eventually reduced. The same agents led to an enhanced initial response of potential to warming, but later the potentials in Cl(-)-free media fell to less than the K(+) equilibrium values.5. A method for obtaining the resistivity of the membrane from measurements made in conditions of non-linear voltage-current dependence was applied.

Active transport of sodium and potassium in mammalian skeletal muscle and its modification by nerve and by cholinergic and adrenergic agents

1. Active transport of Na(+) and K(+) by Na-rich extensor digitorum and soleus muscles of rat was found to be increased considerably when muscles were innervated during enrichment with Na(+) in K-free modified Krebs solution containing 160 mM-Na at 2 degrees C and recovery in a similar fluid with 10 mM-K and 137 mM-Na at 37 degrees C, bubbled with oxygen.2. Addition of acetylcholine (2.0 mug/ml.) to recovery fluid containing denervated extensors increased active transport, whereas addition of eserine (50 mug/ml.), decamethonium (0.1 mug/ml.) and to a lesser extent tubocurarine (0.26 mug/ml.) inhibited active transport. Blocking of nerve conduction in innervated extensor inhibited K(+) uptake more than Na(+) excretion.3. The membrane potential of Na-rich extensor muscles measured soon after re-immersion in recovery fluid was higher in denervated than in innervated muscles. In the latter it was close to the K-equilibrium potential (E(K)). It is suggested that denervation here makes the Na-pump electrogenic by decreasing K(+) uptake either by decreased permeability or by inactivating a K-pump. Evidence is presented that the latter is more likely.4. Addition of isoprenaline to Na-rich soleus muscles in recovery fluid increased active transport and reduced the membrane potential measured soon after re-immersion in recovery fluid. The Na-pump still remained electrogenic in the presence of isoprenaline. It was suggested that isoprenaline might also stimulate the Na-pump, perhaps through activation of lactic dehydrogenase.

Consumption of high-energy phosphates during active sodium and potassium interchange in frog muscle

1. Potassium-depleted muscles have been analysed for cations, phosphocreatine, adenosine triphosphate and lactate before or after an exposure to a medium with 10 mM potassium salt.2. The net movements of sodium out and potassium in when the system is anaerobic but not otherwise poisoned are accompanied by break-down of phosphocreatine and formation of lactate.3. In bicarbonate media oligomycin has little perceptible effect upon these observed changes, which is taken to indicate that mitochondrial phosphorylation is not essential. An inhibition by oligomycin was noted in media buffered with Tris.4. Dinitrofluorobenzene, which poisons creatine phosphotransferase, leads to the cation changes being accompanied by break-down of ATP and formation of lactate. This indicates that ATP is more directly concerned with energizing the ion movements than is phosphocreatine.5. Iodoacetate inhibits the glycolytic process and the ion movement is then accompanied by more phosphocreatine break-down than in the other conditions; the level of ATP also falls.6. The mean number of sodium ions moved out is closely equal to the number of potassium ions moved in. Conditions mentioned in (2) and (3) above lead to about 2.5 sodium ions being moved out per high-energy phosphate bond hydrolysed provided allowance is made for the glycolytic resynthesis of ATP.7. Some measurements of membrane potential under comparable conditions of ion movement are reported and these are used to calculate the energy requirement of the process of sodium-potassium interchange.