Sodium azide stimulated potassium efflux from striated muscles

Summary Sodium azide (NaN₃) in concentrations as low as 0.25 mM was capable of altering potassium fluxes in frog sartorius muscle fibers. Influx of potassium was maximally reduced in the presence of 2.0 mM NaN₃ to between 32 and 39% of control. All concentrations of azide tested increased potassium efflux, but above 2.0 mM NaN₃ the rate constant for potassium (K-42) efflux increased continuously. After the addition of 3.0 mM NaN₃ to the normal Ringer's solution the rate constant for potassium efflux increased from 267% to 358% over a 2 h period. The stimulation of potassium efflux by sodium azide was always greater than the reduction of potassium influx. Net intracellular sodium was unaltered by axide concentrations below 2.0 mM, but markedly increased in the presence of higher azide concentrations. Muscles were rapidly depolarized by sodium azide in both 2.5 mM KCl and 5.0 mM KCl Ringer's solution. At higher azide concentrations (>2.0 mM) the depolarization was constant and did not reflect the continuous loss of potassium ions. The loss of intracellular potassium which was not completely compensated by the gain of sodium was probably due to an increase in potassium permeability.     

Transport of caesium in frog muscle

1. The entry of caesium into sartorius muscle cells is strongly suppressed by the presence of 10(-5)M strophanthidin in Ringer solution.2. The amount by which caesium entry is reduced in the presence of strophanthidin is dependent on the intracellular sodium concentration and is greater the higher the intracellular sodium concentration.3. The magnitude of caesium influx in the absence of strophanthidin is highly dependent on the intracellular sodium concentration.4. Caesium uptake by muscles in which sodium has been largely replaced by lithium is reduced to very low values.5. Caesium can promote the extrusion of sodium from muscles with high intracellular sodium concentrations. The effects of 25 mM caesium and 5 mM potassium on sodium extrusion are roughly the same.6. External potassium inhibits the entry of caesium ions into muscle cells presumably by competing for transport sites.7. The drug strophanthidin has no effect on (134)Cs efflux provided that muscles have been loaded with tracer ions for a long period of time. Caesium efflux from the intracellular compartment appears to occur by a process not mediated by metabolism.8. The action of strophanthidin on (134)Cs efflux from muscles exposed to tracer for short times suggests that caesium ions are transported inwardly by an active process after first accumulating in a superficial reservoir.     

Increase of potassium flux by valinomycin in embryonic chick heart

Summary The effect of different concentrations of the antibiotic, valinomycin, was determined on⁴²K efflux and Na, K content of embryonic chick hearts. Valinomycin produces an increase of K efflux which is progressive in time and markedly dependent on the concentration of external K (0–5 mM) and valinomycin (10^–8 to 10^–5M). The change in K efflux is not due to a reversal of the Na–K pump mechanism, secondary to ATP depletion: i) the increase of K efflux by valinomycin persists in the absence of external Na ions. ii) analyses of Na and K content and⁴²K influx measurements with and without valinomycin indicate that active K influx is not inhibited in a solution containing 0.5 mM K and only slightly decreased in a solution containing 5 mM K. Valinomycin, acting as a K carrier, presumably increases K conductance of the cell membrane resulting in a rise in K efflux.     

Effect of amiloride on sodium transport of frog skin

Summary The effect of Amiloride on intracellular sodium content of the isolated frog skin was investigated. The maximal effect on intracellular exchangeable sodium concentration (–10 meq/kg cell water) was found at concentrations of Amiloride in the incubation solution between 0.3 and 1.0×10^–4 M/l, and at an incubation time of 6–15 min. Since total intracellular sodium concentration is also reduced by approximately 10 meq/kg cell water, it follows that the intracellular non-exchangeable sodium concentration was not affected by Amiloride. Water content, extracellular volume, and intracellular potassium concentration remained constant. The short circuit current reached a new steady state within a few seconds after addition of Amiloride. It is concluded that Amiloride affects a sodium transport pool, which contains about 10% of the exchangeable intracellular sodium.Supported by the Deutsche Forschungsgemeinschaft.     

Isolated cells of the frog sinus venosus: properties of the inward current activated during hyperpolarization

Single sinus venosus cells from frog, Rana esculenta, were isolated using an enzymic dispersion procedure, involving applications of collagenase and protease. About 40%–60% of the cells showed spontaneous contractions. Isolated cells were studied in the whole-cell configuration. Regenerative action potentials were tetrodotoxin-insensitive and similar to those recorded in multicellular preparations. Hyperpolarizing pulses in the voltage range negative to –50 mV induced the activation of a time-dependent inward current, which was blocked by 4 mM caesium but less affected by barium ions. A lower concentration of caesium (1 mM) exerted a voltage-dependent reduction of the current and decreased the spontaneous pacing rate. The activation range of the hyperpolarization-activated current approximately extended from –50 mV to –110 mV, but varied from cell to cell. A high variability was observed in the behaviour of the activation kinetics. The current had a reversal potential near –20 mV that was shifted positively by increasing the external potassium concentration (from 3 mM to 30 mM) and negatively by reducing the external sodium concentration (from 115 mM to 30 mM). The hyperpolarization-activated inward current of the frog sinus venosus cell appears to be carried by both sodium and potassium ions. It shows electrophysiological properties similar to those of the I _f current of the mammalian heart. The role of the current in the spontaneous activity is discussed.     

The influence of extracellular buffer concentration and propionate on lactate efflux from frog muscle

Lactate efflux from frog sartorius muscles was measured following a lactate load of about 18 mol · g^–1 induced by a 4-min period of stimulation. Lactate efflux rate was buffer concentration dependent. The initial efflux rate increased from about 150 nmol · g^–1 · min^–1 in 1 mM MOPS buffer to 400 nmol · g^–1 · min^–1 in 25 mM MOPS buffer. The addition of 20 mM propionate reduced mean intracellular pH by about 0.2 units and increased lactate efflux rate by 70% at the highest buffer concentration and 400% at the lowest buffer concentration. The observed results are in reasonable agreement with predictions based on a model in which net efflux is limited by diffusion of both buffer and lactate in the extracellular space. Transmembrane lactate efflux appears to consist of two components, one of which is proton linked and carried either by undissociated lactic acid or coupled proton-lactate transport, the other being carried by independent lactate ions.     

Na−K ATPase activity and intracellular ion concentrations in the lactating guinea pig mammary gland

Summary The intracellular sodium, potassium and chloride concentrations in slices of lactating guinea pig mammary gland have been determined by chemical analysis and the use of appropriate values for extracellular space. These ion concentrations after 1 hr incubation at 37° C in a Krebs-Ringer bicarbonate solution are 45 mM Na⁺, 138 mM K⁺ and 44 mM Cl^–, which values are in agreement with those found in fresh mammary gland slices. Inhibition of the Na–K activated ATPase cation pump system of the tissue by 10^–4 M ouabain, anoxia or cooling to 0°C causes a gain of Na⁺ and an equimolar loss of K⁺ without a significant change in chloride concentration. The effect of cooling (0°C) is reversible by reincubation at 37°C. Water content of the tissue (76.5% of wet weight) and extracellular space (40.5%) do not change under these conditions. The results permit the conclusion that the Na–K activated ATPase system is responsible for the maintenance of the intracellular Na⁺ and K⁺ concentrations, but do not support the presence of a chloride pump.     

The effects of ethacrynic acid and other sulphydryl reagents on sodium fluxes in frog muscle

1. Ethacrynic acid (2 mM) increased the sodium efflux from freshly dissected frog sartorius muscles. This increase was not observed in muscles previously treated with strophanthidin.2. In strophanthidin-treated muscles, the addition of ethacrynic acid (2 mM) caused a reduction of sodium efflux. The value of efflux reached in these muscles is similar to that observed in muscles immersed in sodium-free solutions containing strophanthidin.3. Ethacrynic acid reduced sodium influx into strophanthidin-treated muscles. Potassium influx was not affected by this substance. These findings suggest that the inhibitor blocks an exchange of sodium for sodium.4. The increase in sodium efflux caused by ethacrynic acid did not result from depolarization nor from an increase in [Na](i).5. Ethacrynic acid caused only a reduction of sodium efflux in muscles previously loaded with sodium by prolonged immersion in potassium-free solutions at low temperatures.6. A derivative of ethacrynic acid that lacks the ability to combine with sulphydryl (SH) groups did not reduce sodium efflux from muscles treated with strophanthidin.7. Para-chloromercuribenzoic acid (PCMB) reduced sodium efflux from control and strophanthidin-treated muscles. This reduction seems to be restricted to the sodium dependent component of the efflux.8. The inhibition of the sodium-dependent component of sodium efflux caused by ethacrynic acid and PCMB appears to have a similar mechanism, namely, the combination with SH groups.     

Intracellular ionic activities in the EDL muscle of the mouse

The intracellular ionic concentrations of sodium, potassium and chloride in the mouse EDL muscle were measured by chemical analysis using inulin as the extracellular marker. Cellular concentrations of 157±8, 38±3, 44±5 mmol kg^–1 intracellular water were estimated for potassium, sodium and chloride respectively.The resting membrane potential was measured by a conventional microelectrode filled with 3 mol KCl and found to be –76±0.5 mV.Ion-selective microelectrodes were used to measure the intracellular ionic activities of potassium, sodium and chloride. The activities measured were 117±5, 16±2, 5±0.1 mmol l^–1 for potassium, sodium and chloride respectively.Apparent activity coefficients for the intracellular ions were calculated. The observed discrepancies between the extracellular activity coefficient and the calculated apparent intracellular activity coefficients for sodium and chloride might be explained in terms of the binding to cellular macromolecules and/or the compartmentalisation of these ions. Potassium appears uniformly distributed throughout the cellular water.Intracellular chloride activity was similar to that predicted by the Donnan distribution and it is concluded, therefore, that chloride is distributed at electrochemical equilibrium.     

The sensitivity of the sodium pump to external sodium

1. When red cells are incubated in potassium-free solutions, ouabain-sensitive sodium efflux is nearly absent with 5 mM-Na externally, but increases as the external sodium concentration is reduced from 5 mM to zero. This increase suggests that the transport mechanism is very sensitive to small amounts of sodium at the outside surface of the cell membrane. Further evidence for such sensitivity has been obtained from the effects of external sodium on the relation between potassium influx and external potassium concentration.2. With 5 mM-[K](o), potassium influx is rather insensitive to [Na](o) but at low potassium concentrations even low levels of sodium inhibit.3. With 140 mM-[Na](o) the potassium influx curve is S-shaped below 1 mM [K](o). At much lower sodium concentrations, the S-shaped region and the value of [K](o) for which potassium influx is half-maximal are both shifted progressively towards zero. At 10 muM-[Na](o), potassium influx is half maximal at 0.14 mM-[K](o) and the curve is close to a rectangular hyperbola down to 22 muM-[K](o); there seems to be a trace of inflexion at about 15 muM-[K](o).4. When [Na](o) is reduced from 5 mM to zero, removal of the inhibitory effect of external sodium ions on sodium: potassium exchange could lead to an increase in sodium efflux into nominally potassium-free solutions if these solutions did in fact contain traces of potassium. Such traces could arise by leakage from the cells, but, in a number of experiments, direct measurements showed that [K](o) was too low to account in this way for all of the observed ouabain-sensitive sodium efflux. A further reason for rejecting this explanation is that ouabain-sensitive potassium loss into nominally (Na+K)-free solutions was unaffected by adding 5 mM-Na. (A slight increase in ouabain-resistant loss was observed.)5. The ouabain-sensitive efflux of sodium into (Na+K)-free solutions therefore seems to represent a mode of behaviour of the transport mechanism distinct both from the sodium: potassium exchange that occurs under physiological conditions and from the sodium: sodium exchange that occurs in K-free, Na-rich media.     

A microperfusion investigation of sodium resorption and potassium secretion by the main excretory duct of the rat submaxillary gland

Summary The ability of the main excretory duct of the rat submaxillary gland to transform a plasmalike, primary saliva into a low sodium, high potassium, final salvia, was investigated using a technique of continuous microperfusion.1. When the duct was perfused with Ringers solution, there was a nett efflux of sodium, and a smaller nett influx of potassium, until steady-state electrolyte concentrations of 2.8 mEq/l for sodium and 135 mEq/l for potassium developed in the luminal fluid. These changes were accompanied by a small nett water efflux which declined as the steady-state concentrations were approached. The flux rates of water and electrolytes observed were sufficient to account for the changes in salivary composition observed under free-flow conditions as precursor saliva passed along the main duct to the mouth.2. Perfusion of the duct with sodium-free solutions resulted in a nett influx of sodium to establish the same steady-state sodium concentration. Nett potassium influx also took place, although the rate of influx was reduced by more than 50% when compared with that observed during perfusion of the duct with Ringers solution.3. Measurement of trans-epithelial potential differences associated with various intraluminal sodium and potassium concentrations demonstrated an approximately logarithmic relationship between potential difference and the intraluminal sodium concentration. The trans-epithelial potential difference was about –70 mV (lumen negative) when the duct was filled with Ringers solution, and about –11 mV (lumen negative) under steady-state (high potassium, low sodium) conditions.4. Calculations of the electrochemical potential gradients for sodium and potassium under steady-state conditions demonstrated that both these cations underwent active transport. In addition, potassium secretion was enhanced by a favourable electrical gradient associated in some way with active sodium transport.C. J. Marin Travelling Research Fellow of the National Health and Medical Research Council of Australia. Present Address: Department of Physiology, University of Sydney, N.S.W. Australia.     

Further evidence for a potassium-like action of lithium ions on sodium efflux in frog skeletal muscle

1. The efflux of labelled sodium as well as net sodium and lithium changes were studied in aged high sodium sartorius muscles of the South American frog Leptodactilus ocelatus.2. In the presence of 2.5 mM potassium in the media, the replacement of external sodium with lithium or magnesium resulted in an increase in sodium efflux. The magnitude of such increase was always larger in lithium.3. With the absence of potassium in the media, the response of sodium efflux to replacement of external sodium varied with the cation used as a substitute. In lithium Ringer there was always a noticeable increase, whereas in magnesium there was always a marked reduction. The same results were observed when calcium was substituted for magnesium.4. The replacement of 60 mM external sodium with sucrose did not prevent the stimulating effect of 5 mM potassium on sodium efflux, nor the inhibitory action of 10(-4)M ouabain. This indicates that neither sucrose by itself, nor the lowering of the ionic strength, modified to an appreciable extent the function of the sodium pump.5. Net sodium extrusion took place against an electrochemical gradient in potassium-free - 50 mM sodium - mM lithium Ringer. About 75% of this efflux was ouabain sensitive.6. Muscles made both sodium and lithium rich and incubated in potassium-free - 60 mM sodium - 50 mM lithium Ringer also showed net sodium extrusion against an electrochemical gradient, which was 85% ouabain sensitive. This extrusion took place even under conditions where the changes in free energy favouring lithium entry were always lower than the changes in free energy opposing sodium going out. This indicates that a sodium-lithium exchange by a counter-transport process is unlikely.7. External potassium reduced the ouabain sensitive lithium influx in muscles incubated in lithium Ringer. The values found were 5.90 +/- 0.39 mu-mole/g.hr and 2.66 +/- 0.43 mumole/g.hr in potassium-free and 15 mM potassium respectively. At the same time potassium had no effect on the ouabain-insensitive lithium uptake.8. Muscles incubated in potassium-free-magnesium Ringer had a residual sodium efflux which could not be accounted for by passive movement. About 40% of it was abolished by 10(-4)M ouabain. This ouabain-sensitive part could be a consequence of some stimulation of the sodium pump by potassium leaking out of the cells. If this is correct it should be inhibited by external sodium and should not contribute to the total sodium efflux in potassium-free sodium media.9. Magnesium was used as the reference cation to study the sodium-stimulated sodium efflux under potassium-free conditions. The total sodium efflux amounted to 0.668 hr(-1) (rate constant) and was 71% ouabain sensitive.10. The present experiments demonstrated that lithium ions have a direct stimulating effect on sodium efflux in high sodium skeletal muscle, and strongly support the notion that this effect is produced by an activation of the sodium pump through a potassium-like action.     

The electrical basis for enhanced potassium secretion in rat distal colon during dietary potassium loading

Previous studies in rat distal colon provide evidence for an active absorptive process for potassium under basal conditions, and for active potassium secretion during chronic dietary potassium loading. The present studies were performed with conventional and potassium-selective microelectrodes to determine the electrical basis for the increase in transcellular (active) potassium secretion observed during potassium loading. Compared to control tissues, potassium loading resulted in a 5-fold increase in transepithelial voltage (V _T) and a 52% decrease in total resistance (R _T) in the distal colon. The rise inV _T was due to a decrease in apical membrane resistance and an increase in basolateral membrane voltage from –45±2 mV (cell interior negative) in control to –56±2 mV (P<0.001) in potassium loaded tissues. This difference in basolateral membrane voltage reflected in increase in intracellular potassium activity from 86±4 mM to 153±12 mM (P<0.001). In control tissues, the sequential mucosal addition of the sodium channel blocker amiloride (0.1 mM) and the potassium channel blocker tetraethylammonium chloride (TEA: 30 mM) produced no effect on the electrical measurements. However, in potassium loaded tissues, amiloride and TEA produced transepithelial changes consistent with inhibition of apical membrane conductances for sodium and potassium, respectively, reflected by increases in the resistance ratio, (ratio of apical to basolateral membrane resistances). These data indicate that the decrease in apical membrane resistance during potassium loading was caused by an increase in apical membrane conductance for both potassium and sodium.     

Electrical activity in embryonic heart cell aggregates

Summary Aggregates were formed from dissociated heart cells of 7-day chick embryos. When spontaneous action potentials were blocked with 10^–8 to 10^–7 g/ml tetrodotoxin (TTX) oscillatory pacemaker potentials were sometimes seen. The emergence of these pacemaker potentials was critically dependent on the external potassium concentration. In 1.3 mM potassium medium TTX suppression of action potential generation always led to a stable resting potential close to the threshold level (–55 to –50 mV). In 4.3 mM potassium TTX suppression was followed by a train of pacemaker potentials which usually gave way to a stable resting potential of about –70 mV. Raising the calcium concentration from 1.8 to 5 mM often induced long lasting (3 hrs) pacemaker oscillations of 20 to 30 mV peak to peak amplitude. These were abolished by raising the potassium concentration to 8.3 mM or upon the addition of 1.5 mM Mn²⁺. The responses of TTX-treated aggregates are discussed in terms of Noble and Tsien's pacemaker theory for Purkinje fibers. The results are well described by assuming the existence of ani _{k 2}-like potassium current in embryonic heart cells. The role of calcium is unclear but it may help provide the inward current against which the outward potassium current can function.     

The dependence of the action potential of the frog's heart on the external and intracellular sodium concentration

1. The overshoot of the action potential of the frog's heart was reduced when external sodium chloride was replaced by sucrose. However, the potential decrement was only 17·3 mV for a 10-fold reduction of sodium as compared with 58 mV expected on the basis of the sodium hypothesis of excitation.

2. Replacement of up to 75% of the external sodium by choline did not reduce the overshoot, provided atropine was present in sufficient concentrations to suppress any parasympathomimetic action.

3. The maximum rate of rise of the action potential markedly declined in low sodium fluids whether sucrose or choline chloride was used to replace sodium chloride.

4. The maximum rate of rise was reduced to only a small extent when external sodium was replaced by lithium.

5. Increasing the intracellular sodium concentration in exchange for lost potassium caused overshoots to decline. The effects resembled those obtained in similar experiments with skeletal muscle fibres (Desmedt, 1953).

6. Action potentials occurring under certain conditions even in the presence of very low external sodium concentrations (≤ 5% normal) also declined in height when the intracellular sodium concentration was increased.

7. The behaviour of the action potential in low external sodium concentrations may be explained by an action of calcium on the excitable membrane.

     

Metabolic depression and myocardial potassium

Summary The action potential duration (APD) of guinea pig atrial muscle responded qualitatively to metabolic depression and altered glucose concentration as shown previously for papillary muscle. Both preparations lost potassium and gained sodium during 8 h anoxic incubations and these changes were partially prevented by 50 mM glucose. Experiments with potassium⁴² indicated that anoxia-induced loss of potassium was not primarily due to an increased efflux but to a decreased influx. Stimulation did not increase potassium⁴² efflux from atria but caused some increase in potassium loss. The ATP content of atria and ventricular muscle decreased rapidly during anoxic incubation but was maintained at a significantly higher level in the presence of 50 mM glucose. Since muscle potassium levels following 8 h of anoxic incubation were incompatible with observed resting potentials, the results support the concept of either an electrogenic sodium pump or the intracellular compartmentalization of potassium. In addition, the anoxia-induced reduction of action potential duration does not appear to be associated with an increase in potassium⁴² efflux.This work was supported by grants from the Medical Research Council of Canada and the Canadian Heart Foundation.     

Reversal of the potassium entry mechanism in red cells, with and without reversal of the entire pump cycle

1. (42)K has been used to study the ouabain-sensitive component of potassium efflux from intact human red cells.2. Ouabain-sensitive efflux of potassium was observed only in media containing either sodium ions in moderate or high concentration or potassium ions.3. The effects of sodium and potassium in the medium were not additive. Potassium ions always increased the ouabain-sensitive potassium efflux, but in media containing 4.2 mM-K an increase in sodium concentration from 4.5 to 131 mM had little effect.4. In potassium-free media, ouabain-sensitive potassium efflux increased roughly linearly as the external sodium concentration was increased from 35 to 155 mM.5. The sensitivity of potassium efflux to external potassium depended on the concentration of sodium in the medium. In a 5 mM-Na (choline) medium, ouabain-sensitive potassium efflux was half-maximal at about 0.27 mM-K. In a 150 mM-Na medium the effect of potassium was half-maximal at about 1 mM-K. The relation between external potassium concentration and ouabain-sensitive sodium efflux was similarly influenced by the concentration of sodium ions in the medium.6. Inosine greatly reduced the ouabain-sensitive efflux of potassium into both 5 mM-K and potassium-free media. It probably acted by reducing the intracellular phosphate concentration to a low level.7. Ouabain-sensitive potassium efflux was not affected by the concentration of inorganic phosphate outside the cells and was not associated with a ouabain-sensitive efflux of phosphate of comparable magnitude. A small associated efflux could not be excluded.8. Simultaneous measurements of sodium efflux and of potassium efflux from identical batches of cells incubated in potassium-free media showed that inosine reduced ouabain-sensitive potassium efflux at the same time as it increased ouabain-sensitive sodium efflux.9. When cells that had been largely depleted of energy stores by pre-incubation with 2-deoxyglucose were incubated in high-sodium media, with and without potassium, it was observed that potassium in the medium increased the ouabain-sensitive potassium efflux but reduced the ouabain-sensitive efflux of sodium.10. Simultaneous measurements of the ouabain-sensitive efflux of potassium and influx and efflux of sodium across the membranes of starved cells incubated in potassium-free media, with and without inosine, suggested that ouabain-sensitive potassium efflux was associated with a net ouabain-sensitive entry of sodium.11. The results are best explained by supposing that the ouabain-sensitive efflux of potassium does not reflect lack of discrimination by the mechanism responsible for sodium expulsion, but is brought about by the reversal of steps in the pump cycle normally responsible for potassium entry.     

Voltage-activated potassium,but not calcium currents in cultured bovine aortic endothelial cells

The electrophysiological properties of cultured bovine aortic endothelial cells were characterized using the patch clamp technique. Resting potentials were measured on passing to the whole cell recording configuration and were close to –65 mV in healthy cells. In cell-attached recordings with a high potassium pipette solution, inward single channel currents were observed with zero applied pipette potential. A linear slope conductance of 25 pS was found for a wide range of hyperpolarizing patch potentials and also for depolarizing patch potentials of up to 50–60 mV. A pronounced inward rectification was apparent as no reversal of these currents was seen for larger depolarizations. Whole cell recording in physiological solutions revealed the presence of a hyperpolarization-activated inward current with strong inward rectification and no voltage-dependent ionic current was observed upon depolarization in this subset of cells. Substitution of potassium for external sodium resulted in a shift in the zero current potential consistent with potassium being the main permeant ion. Together with the characteristic voltage-dependent blocking actions of external sodium ions and low concentrations of barium and caesium ions, our results indicate that this current is very similar to the classical inward rectifier as originally described in skeletal muscle and in tunicate eggs. In a second population of cells, a depolarization-activated outward current displaying characteristics of the fast transient A-type potassium current as first reported in molluscan neurones was also observed. No evidence for inward voltage dependent sodium or calcium currents was found.     

Comparison of the effects of potassium and pH on the calibre of cerebral veins and arteries

The vasomotor responses of individual pial veins and arteries on the convexity of the cerebral cortex to perivascular microinjection of mock cerebrospinal fluid (CSF) containing various concentrations of potassium (K⁺) and of various pH (achieved by altering the bicarbonate, HCO ₃ ^– concentration) have been examined in cats anaesthetised with -chloralose.Microapplication of CSF containing 0 mM HCO ₃ ^– (pH 4.80) effected significant increases in calibre of pial veins and arteries of 9.3±2.4% and 38.2±4% respectively (mean calibre change ±SE), whereas CSF containing 22 mM HCO ₃ ^– (pH 7.45) which constricted pial arteries significantly (–18.5±2.9%) minimally altered venous calibre (–4.3 ±2.2%).Microapplication of CSF containing 0 mM potassium resulted in a significant reduction in pial arterial calibre (–11.4±2.8%) but failed to alter pial venous calibre (–0.3 ±0.6%). Perivascular microapplication of CSF containing moderately elevated potassium concentrations (10 mM) which effected marked, significant increases in pial arterial calibre (49.3±3.9%) did not significantly alter the calibre of the pial veins (mean response –1.6±2.4%). The perivascular administration of CSF containing a high concentration of potassium (40 mM) resulted in the significant constriction of both pial veins (–13.5±0.9%) and pial arteries (–47.2±6.3%). The magnitude of the response was significantly smaller in the pial veins.The relative insensitivity to K⁺ and pH of the pial veins as compared to pial arteries suggests that alteration in the chemical composition of the perivascular fluid are of lesser importance in the control of cerebrovascular capacitance than for the regulation of cerebrovascular resistance.     

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

Intracellular potassium ([K⁺]_i), interstitial potassium ([K⁺]_inter), intracellular sodium ([Na⁺]_i), and resting membrane potential (RMP) were measured before and after repetitive stimulation of mouse soleus and EDL (extensor digitorum longus) muscles. At rest, RMP was –69.8 mV for soleus and –74.9 mV for EDL (37°C). [K⁺]_i was 168 mM and 182 mM, respectively. In soleus, free [Na⁺]_i was 12.7 mM. After repetitive stimulation (960 stimuli) RMP had decreased by 11.9 mV for soleus and by 18.2 mV for EDL. [K⁺]_i was reduced by 32 mM and 48 mM, respectively, whereas [K⁺]_inter was doubled. In soleus [Na⁺]_i had increased by 10.6 mM, demonstrating that the [K⁺]_i-decrease is three times higher than the [Na⁺]_i-increase. It is concluded that this difference reflects different activity induced movements of Na and K, and that the difference is not due to the Na/K pumping ratio. The possible involvement of the potassium loss in muscle fatigue is discussed. After stimulation RMP recovered with a time constant of 0.9 min for soleus and 1.5 min for EDL. Within the first minutes after stimulation the intracellular potassium concentration increased by 20.4 mM/min for soleus and 21.7 mM/min for EDL. Free [Na⁺]_i decreased with less than 10 mM/min. The mechanisms underlying the different rate of changes are discussed.Parts of this work have been published in preliminary form (Juel and Sjøgaard 1984)