Long duration responses in squid giant axons injected with 134cesium sulfate solutions

Giant axons from the squid were injected with 1.5 M cesium sulfate solutions containing the radioactive isotopes 42K and 134Cs. These axons, when stimulated, gave characteristic long duration action potentials lasting between 5 and 45 msec. The effluxes of 42K and 134Cs were measured both under resting conditions and during periods of repetitive stimulation. During the lengthened responses there were considerable increases in potassium efflux but only small increases in cesium efflux. The selectivity of the delayed rectification process was about 9 times greater for potassium ions than for cesium ions. The data suggest that internal cesium ions inhibit the outward potassium movement occurring during an action potential. The extra potassium effluxes taking place during excitation appear to be reduced in the presence of cesium ions to values between 7 and 22% of those expected in the absence of cesium inhibition.     

Potassium flux ratio in voltage-clamped squid giant axons

The potassium flux ratio across the axolemma of internally perfused, voltage-clamped giant axons of Loligo pealei has been evaluated at various membrane potentials and internal potassium concentrations ([K]i). Four different methods were used: (a) independent measurement of one-way influx and efflux of 42K; (b) simultaneous measurement of net K current (IK) and 42K influx; (c) simultaneous measurement of IK and 42K efflux; and (d) measurement of potassium conductance and 42K influx at the potassium equilibrium potential. The reliability of each of these methods is discussed. The average value of the exponent n' in the Hodgkin-Keynes equation ranged from 1.5 at -4mV and 200 mM [K]i to 3.3 at -38 mV and 350 mM [K]i and appeared to be a function of membrane potential and possibly of [K]i. It is concluded that the potassium channel of squid giant axon is a multi-ion, single-file pore with three or more sites.     

Activation of potassium channels in erythrocytes of marine teleost Scorpaena porcus

To assess the possibility of stimulating Ca2+-activated K+ channels, marine fish erythrocytes were incubated at 20-22 degrees C in saline containing a Ca2+-ATPase inhibitor (orthovanadate), a Ca2+ ionophore (A23187), propranolol or Pb2+. Incubation of the cells for up to 2 h under control conditions or in the presence of 5 mM NH4VO3 and 1 mM Ca2+ did not affect the intracellular K+ and Na+ concentrations. About 50% cellular K+ was lost from erythrocytes incubated in the presence of 0.01 mM A23187, 1 mM EGTA and 0.4-1.0 mM Ca2+. There was a significant loss of cellular K+ after the addition of 0.05-0.2 mM propranolol to the incubation medium. The stimulatory effect of propranolol on the K+ efflux was independent of external Ca2+. Blockers of Ca2+ transport, verapamil and Co2+, caused only a small decrease in the K+ loss induced by propranolol. The treatment of erythrocytes with 1-2 microM Pb2+ led to a minor K+ loss, but at a Pb2+ concentration of 20-50 microM, about 70% cellular K+ was lost. The K+ efflux induced by propranolol or Pb2+ was completely blocked by 1 mM quinine. The induced K+ loss from the erythrocytes was accompanied by a slight increase in the intracellular Na+ concentration. These data indicate the possibility of inducing Ca2+- and Pb2+-activated potassium channels in erythrocytes of S. porcus. A distinctive feature of the cells is a high sensitivity to propranolol, which activates K+ channels in the absence of external Ca2+.     

Unidirectional sodium and potassium fluxes through the sodium channel of squid giant axons.

Unidirectional 22Na-traced sodium influx or 42K-traced potassium efflux across the membranes of voltage-clamped squid giant axons was measured at various membrane potentials under bi-ionic conditions. Tetrodotoxin almost entirely eliminated the extra K+ efflux induced by short repetitive depolarizations in the presence of tetraethylammonium or 3,4-diaminopyridine. A method of determining the voltage dependence of the unidirectional flux through voltage-gated channels is described. This technique was used to obtain the unidirectional flux-voltage relation for the sodium channel in bi-ionic and single-ion conditions. It allows the determination of the unidirectional flux at the zero-current potential which, for influx, was found to be approximately 20% of the value measured 80 mV negative to the zero-current potential. The unidirectional flux ratio under bi-ionic conditions was also measured and the flux ratio exponent found to average 1.15 with an external sodium and an internal potassium solution. A three-barrier, two-site, multi-occupancy model previously obtained for other conditions was found to predict a similar non-unity average for the flux ratio exponent. It is also shown that some single-occupancy models can predict non-unity values for the flux ratio exponent in bi-ionic conditions.     

Sodium and potassium fluxes and membrane potential of human neutrophils: evidence for an electrogenic sodium pump

Sodium and potassium ion contents and fluxes of isolated resting human peripheral polymorphonuclear leukocytes were measured. In cells kept at 37 degrees C, [Na]i was 25 mM and [K]i was 120 mM; both ions were completely exchangeable with extracellular isotopes. One-way Na and K fluxes, measured with 22Na and 42K, were all approximately 0.9 meq/liter cell water . min. Ouabain had no effect on Na influx or K efflux, but inhibited 95 +/- 7% of Na efflux and 63% of K influx. Cells kept at 0 degree C gained sodium in exchange for potassium ([Na]i nearly tripled in 3 h); upon rewarming, ouabain-sensitive K influx into such cells was strongly enhanced. External K stimulated Na efflux (Km approximately 1.5 mM in 140-mM Na medium). The PNa/PK permeability ratio, estimated from ouabain insensitive fluxes, was 0.10. Valinomycin (1 microM) approximately doubled PK. Membrane potential (Vm) was estimated using the potentiometric indicator diS-C3(5); calibration was based on the assumption of constant-field behavior. External K, but not Cl, affected Vm. Ouabain caused a depolarization whose magnitude dependent on [Na]i. Sodium-depleted cells became hyperpolarized when exposed to the neutral exchange carrier monensin; this hyperpolarization was abolished by ouabain. We conclude that the sodium pump of human peripheral neutrophils is electrogenic, and that the size of the pump-induced hyperpolarization is consistent with the membrane conductance (3.7-4.0 microseconds/cm2) computed from the individual K and Na conductances.     

Sodium and Potassium Ion Effluxes from Squid Axons under Voltage Clamp Conditions

Squid giant axons loaded with Na²⁴ were subjected to short duration (0.5 msec.) clamped depolarizations of about 100 mv at frequencies of 20/sec. and 60/sec. while in choline sea water. Under such conditions the early outward current was just about maximal at the time of termination of the clamping pulse. An integration of the early current versus time record gave 1.2 μcoulomb/cm² pulse, while a measurement of the extra Na²⁴ efflux resulting from repetitive pulsing gave a charge transfer of 1.4 μcoulomb/cm² pulse. In sodium-containing sea water and with pulses 50-75 mv more positive than E_Na the Na²⁴ efflux is about 3 times the measured charge transfer. The efflux of K⁴² from a previously loaded axon into normal sea water is only 50 per cent of the measured charge transfer when the membrane is held for about 5 msec. at a potential such that there is no early current, and such pulses are at 10-20/sec. The experiments appear to confirm the suggestion that the early current during bioelectric activity is sodium but provide unsatisfactory support for the identification of the delayed but sustained current solely with potassium ions. Resting Na⁺ efflux is 0.6 pmole/cm² sec. mmole [Na]₁, while the apparent K⁺ efflux is about 250 pmole/cm² sec. and is little affected by hyperpolarization.     

Changes in the composition of two molecular species of ethanolamine plasmalogen in normal human myelin during development

The effect of external and internal K+ on Na+o-dependent Ca2+ efflux was studied in dialyzed squid axons under constant membrane potential. With axons clamped at their resting potentials, external K+ (up to 70 mM) has no effect on Na+-Ca2+ exchange. Removal of Ki+ causes a marked inhibition in the Na+o-dependent Ca2+ efflux component. Internal K+ activates the Na+-Ca2+ exchange with low affinity (K 1/2 = 90 mM). Activation by Ki+ is similar in the presence or in the absence of Na+i, thus ruling out a displacement of Na+i from its inhibitory site. Axons dialyzed with ATP also show a dependency of Ca2+ efflux on Ki+. The present results demonstrate that Ki+ is an important cofactor (partially required) for the proper functioning of the forward Na+-Ca2+ exchange.     

The effects of alterations in the external sodium concentration on human leucocyte sodium and potassium transport in vitro

Human leucocytes incubated in tissue culture fluid of low-sodium concentration (2 mM; iso-osmolarity maintained with choline chloride) reached a new equlibrium within 1 hour and lost approximately 25% of intracellular potassium and 70% of intracellular sodium. The rate constant for ouabainsensitive sodium efflux fell by more than 50% and the ouabain-insensitive rate constant increased nearly threefold in the low-sodium medium. Total sodium efflux fell in proportion to internal sodium whereas ouabain-insensitive sodium efflux remained unchanged. A reduction in external sodium from 140 to 2 mM was associated with a 75% fall in sodium influx. In the low-sodium medium ouabainsensitive potassium influx exceeded ouabain-sensitive sodium efflux and no ouabain-sensitive potassium efflux could be demonstrated. Ouabain-insensitive potassium influx and that portion of potassium efflux which is dependent on external potassium fell in parallel in low-sodium cells, suggesting reduced activity of a ouabain-insensitive K:K exchange system.     

Vanadate selectively inhibits the Ko+-activated Na+ efflux in squid axons.

The effects of internally applied 1 mM vanadate on the Na+ efflux in dialysed squid axons were found to depend on the presence of external K+. In K+-free artificial sea water, vanadate did not produce any change in the rate of Na+ efflux, whereas in the presence of 10 mM K+ the Na+ efflux was reduced to values even lower than those observed in the absence of K+ (inversion of the K+-free effect). In vanadate-poisoned axons, K+ and NH+4 at low concentrations activated Na+ efflux, but at high concentrations both cations were inhibitory. However, NH+4 was always a better activator and a poorer inhibitor than K+.     

Calcium content and net fluxes in squid giant axons

Axons freshly dissected from living specimens of the tropical squid Dorytheutis plei have a calcium content of 68 mumol/kg of axoplasm. Fibers stimulated at 100 impulses/s in 100 mM Ca seawater increase their Ca content by 150 mumol/kg.min; axons placed in 3 Ca (choline) seawater increase their Ca content by 12 mumol/kg.min. Axons loaded with 0.2--1.5 mmol Ca/kg of axoplasm extruded Ca with a half time of 15--30 min when allowed to recover in 3 Ca (Na) seawater. The half time for recovery of loaded axons poisoned with carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) and iodoacetic acid (IAA) is about the same as control axons. Axons placed in 40 mM Na choline seawater (to reduce chemical gradient for Na) or in 40 mM Na, 410 mM K seawater to reduce the electrochemical gradient for Na to near zero either fail to lose previously loaded Ca or gain further Ca.     

A TEA-insensitive flickering potassium channel active around the resting potential in myelinated nerve

A novel potassium-selective channel which is active at membrane potentials between -100 mV and +40 mV has been identified in peripheral myelinated axons of Xenopus laevis using the patch-clamp technique. At negative potentials with 105 mM-K on both sides of the membrane, the channel at 1 kHz resolution showed a series of brief openings and closings interrupted by longer closings, resulting in a flickery bursting activity. Measurements with resolution up to 10 kHz revealed a single-channel conductance of 49 pS with 105 mM-K and 17 pS with 2.5 mM-K on the outer side of the membrane. The channel was selective for K ions over Na ions (PNa/PK = 0.033). The probability of being within a burst in outside-out patches varied from patch to patch (> 0.2, but often > 0.9), and was independent of membrane potential. Open-time histograms were satisfactorily described with a single exponential (tau o = 0.09 msec), closed times with the sum of three exponentials (tau c = 0.13, 5.9, and 36.6 msec). Sensitivity to external tetraethylammonium was comparatively low (IC50 = 19.0 mM). External Cs ions reduced the apparent unitary conductance for inward currents at Em = -90 mV (IC50 = 1.1 mM). Ba and, more potently, Zn ions lowered not only the apparent single-channel conductance but also open probability. The local anesthetic bupivacaine with high potency reduced probability of being within a burst (IC50 = 165 nM). The flickering K channel is clearly different from the other five types of K channels identified so far in the same preparation. We suggest that this channel may form the molecular basis of the resting potential in vertebrate myelinated axons.     

In squid axons intracellular Mg2+ is essential for ATP-dependent Na+ efflux in the absence and presence of strophanthidin

The effect on Na+ efflux of removal of intracellular Mg2+ was studied in squid giant axons dialyzed without internal Ca2+. In the absence of Mg2i+, ATP was unable to stimulate any efflux of Na+ above the baseline of about 1 pmol . cm-2 . s-1. This behavior was observed in otherwise normal axons and in axons poisoned with 50 microM strophanthidin in the sea water. Reinstatement of 4 mM MgCl2 in excess to ATP in the dialysis solution brought about the usual response of Na+ efflux to ATP, external K+ and strophanthidin. The present experiments show that, regardless of the mechanism for the ATP-dependent Na+ efflux in strophanthidin-poisoned axons, this type of flux shares with the active Na+ extrusion the need for the simultaneous presence of intracellular ATP and Mg2+.     

Formyl peptide stimulation of superoxide anion release from lung macrophages: sodium and potassium involvement

We examined the role of the monovalent cations Na+ and K+ in the events encompassing the release of O-2 by alveolar macrophages after stimulation with formyl methionyl phenylalanine (FMP). This was accomplished by determining the effect of changing the extracellular [Na+] and/or [K+] on FMP-stimulated O-2 production; and measuring 22Na+, 42K+ and 86Rb+ influx and efflux and intracellular [K+] for control and FMP-stimulated alveolar macrophages. Stimulated O-2 production was relatively insensitive to changes in extracellular K+ or Na+ concentrations until the [Na+] was decreased below 35 mM. At 4 mM [Na+], the rate of O-2 production remained at 75% of the maximal rate observed at physiological concentrations of [Na+]. Both influx and efflux of 22Na+ were stimulated above control rates by FMP. The increased rates of fluxes lasted for a few minutes suggesting a transient increase in membrane permeability to Na+. Ouabain partially inhibited 22Na+ efflux but had no effect on O-2 release. The influx of 86Rb+ and 42K+ was not altered by the addition of FMP but was virtually abolished in the presence of 10 microM ouabain or 1 mM quinine. In the presence of extracellular calcium, FMP-stimulated a prolonged (greater than 20 minutes) increase in 86Rb+ or 42K+ efflux which was inhibitable by 1 mM quinine. In the absence of extracellular calcium, FMP stimulation of K+ efflux was greatly diminished and was not affected by quinine, although quinine still inhibited O-2 production under these conditions. It was also observed that there was a loss of intracellular K+ when cells were stimulated by FMP in the presence of Ca+2, but not in the absence of Ca+2. Taken together, these results suggest a minimal direct role, if any, for K+ in the events that lead to FMP-stimulated O-2 release by alveolar macrophages.     

Dramatic magnesium efflux induced by high potassium in rat thymocytes

When incubated in 150 mM KCl, rat thymocytes exhibited a very important magnesium efflux (11.4 +/- 0.7 mmoles/liter cells/20 min, n = 29), about 90 times higher than the physiological magnesium efflux catalyzed by the Na-Mg exchanger (0.126 +/- 0.093 mmoles/liter cells/20 min). Cells remained viable (trypan blue test) and membrane integrity was shown by the absence of an increase in sodium permeability. K(+)-induced magnesium efflux exhibited the following properties: (i) it required the presence of external chloride; (ii) it was fully blocked by DIOA, a selective KCl-cotransporter inhibitor (IC(50) = 35 microm); and (iii) it was associated to a progressive increase in cell volume via the DIOA-sensitive K-Cl cotransporter. Such cell swelling seems to play a causal role, because (i) hypertonic media (+400 mM sucrose) abolished K(+)-induced magnesium efflux and (ii) hypotonic Ringer media (205 mOsm) increased both cell volume and magnesium efflux (from a basal value of 0.35 +/- 0.03 mmoles/liter cells/20 min up to 1.44 +/- 0.24 mmoles/liter cells/20 min), even in the presence of DIOA. In conclusion, high potassium induced a dramatic release of intracellular magnesium from rat thymocytes. Such a phenomenon was, at least in part, caused by cell swelling via the DIOA-sensitive K-Cl cotransporter. The nature of the magnesium transport mechanism and its role in the transduction signal of K-Cl cotransporter activation by cell swelling deserve further investigation.     

The independence of membrane potential and potassium activation of the sodium pump in muscle.

The membrane potential (Em) of sartorius muscle fibers was made insensitive to [K+] by equilibration in a 95 mM K+, 120 mM Na+ Ringer solution. Under these conditions a potassium-activated, ouabain-sensitive sodium efflux was observed which had characteristics similar to those seen in muscles with Em sensitive to [K+]. In addition, in the presence of 10 mM K+, these muscles were able to produce a net sodium extrusion against an electrochemical gradient which was also inhibited by 10- minus 4 M oubain. This suggests that the membrane potential does not play a major role in the potassium activation of the sodium pump in muscles.     

Inhibition of 2,4-dinitrophenol-induced potassium efflux by adenine nucleotides in mitochondria

The influence of nucleotides on 2,4-dinitrophenol (DNP)-induced K+ efflux from intact rat liver mitochondria has been studied. ATP and ADP at micromolar concentrations were found to inhibit mitochondrial potassium transport, whereas GTP, GDP, CTP, and UTP did not show tha same effect. The values of half-maximal inhibition (IC50) were approximately 20 microM for ATP and approximately 60 microM for ADP. It is suggested that adenine nucleotides exert their inhibitory action at the matrix side of the inner mitochondrial membrane since the inhibitor of adenine nucleotide translocase atractyloside at concentration of 1 microM completely removed the inhibitory effect of ATP and ADP. The mitochondrial ATPase inhibitor oligomycin (2 microg/ml) was found to reduce slightly the rate of DNP-induced K+ efflux and had no effect on inhibition by adenine nucleotides; the latter was insensitive to Mg2+ and the changes in pH. It seems likely that the regulation of potassium transport is not due to phosphorylation of the channel-forming protein but to binding of the nucleotides in specific regulatory sites. The possibility of potassium efflux from mitochondria in the presence of uncoupler via the ATP-dependent potassium channel is discussed.     

Active potassium transport coupled to active sodium transport in vesicles reconstituted from purified sodium and potassium ion-activated adenosine triphosphatase from the rectal gland of Squalus acanthias.

Vesicles containing a purified shark rectal gland (sodium + potassium)-activated adenosine triphosphatase-(NaK ATPase) were prepared by dialyzing for 2 days egg lecithin, cholate, and the NaK ATPase purified from the rectal gland of Squalus acanthias. These vesicles were capable of both Na+ and K+ transport. Studies of K+ transport were made by measuring the ATP-stimulated transport outward of 42K+ or 86Rb+. Vesicles were preloaded with isotope by equilibration at 4 degrees for 1 to 3 days. Transport of 42K+ or 86Rb+ was initiated by addition of MgATP to the vesicles. The ATP-dependent exit of either isotope was the same. Experiments are presented which show that this loss of isotope was not due to changes in ion binding but rather due to a loss in the amount of ion trapped in the vesicular volume. The transport of K+ was dependent on external Mg2+. CTP was almost as effective as ATP in stimulating K+ transport, while UTP was relatively ineffective. These effects of nucleotides parallel their effects on Na+ accumulation and their effectiveness as substrates for the enzyme. Potassium transport was inhibited by ouabain and required the presence of Na+. The following asymmetries were seen: (a) addition of external Mg2+ supported K+ transport; (b) ouabain inhibited K+ transport only if it was present inside the vesicles; (c) addition of external Na+ to the vesicles stimulated K+ transport. External Li+ was ineffective as a Na+ substitute. The specific requirement of external Na+ for K+ transport indicates that K+ exit is coupled to Na+ entry. Changes in the internal vesicular ion concentrations were studied with vesicles prepared in 20 mM NaCl and 50 mM KCl. After 1 hour of transport at 25 degrees, a typical Na+ concentration in the vesicles in the presence of ATP was 72 mM. A typical K+ concentration in the vesicles was 10 mM as measured with 42K+ or 6 mM as measured with 86Rb+. The following relationships have been calculated for Na+ transport, K+ transport and ATP hydrolysis: Na+/ATP = 1.42, K+/ATP =1.04, and Na+/K+ = 1.43. The ratio of 2.8 Na+ transported in to 2 K+ transported out is very close to the value reported for the red cell membrane. Potassium-potassium exchange similar to that observed in the red cell membrane and attributed to the Na+-K+ pump (stimulated by ATP and orthophosphate and inhibited by ouabain) was observed when vesicles were prepared in the absence of Na+. The results reported in this paper prove that the shark rectal gland NaK ATPase, which is 90 to 95% pure, is the isolated pump for the coupled transports of Na+ and K+.     

Caclium uptake and associated adenosine triphosphatase activity in fragmented sarcoplasmic reticulum. Requirement for potassium ions.

The effects of monovalent cations on calcium uptake by fragmented sarcoplasmic reticulum have been clarified. Homogenization of muscle tissue in salt-containing solutions leads to contamination of this subcellular fraction with actomyosin and mitochondrial membranes. When, in addition, inorganic cations are contributed by the microsomal suspension and in association with nucleotide triphosphate substrates there is an apparent inhibition of the calcium transport system by potassium and other cations. However, when purified preparations were obtained after homogenization in sucrose medium followed by centrifugation on a sucrose density gradient in a zonal rotor, calcium uptake and the associated adenosine triphosphatase activity were considerably activated by potassium and other univalent cations. When plotted against the log of the free calcium concentration there was only a slight increase in calcium uptake and ATPase activity in the absence of potassium ions but sigmoid-shaped curves were obtained in 100 mM K+ with half-maximal stimulation occurring at 2 muM Ca2+ for both calcium uptake and ATPase activity. The augmentation in calcium uptake was not due to an ionic strength effect as Tris cation at pH 6.6 was shown to be inactive in this respect. Other monovalent cations were effective in the order K+ greater than Na+ greater than NH4+=Rb+=Cs+ greater than Li+ with half-maximal stimulation in 11 mM K+, 16 mM Na+, 25 mM NH4+, Rb+, and Cs+ and in 50 mM Li+. There was nos synergistic action between K+ AND Na+ ions and both calcium uptak and associated ATPase were insensitive to ouabain. Thallous ions stimulate many K+-requiring enzymes and at one-tenth the concentration were nearly as effective as K+ ions in promoting calcium uptake. The ratio of Ca2+ ions transported to P1 released remained unchanged at 2 after addition of K+ ions indicating an effect on the rate of calcium uptake rather than an increased efficiency of uptake. In support of this it was found that during the stimulation of calcium uptake by Na+ ions there was a reduction in the steady state concentration of phosphorylated intermediate formed from [gamma-32P]ATP. It is considered that there is a physiological requirement for potassium ions in the relaxation process.     

Control of ion distribution in isolated smooth muscle cells. I. Potassium

We describe a technique for examining unidirectional ion movements in suspensions of enzymatically disaggregated smooth muscle cells derived from stomach muscle of the toad. This technique has been used to analyze the movement of 42K across these cells. This analysis was greatly simplified by the finding that the cells were in a steady state with respect to K+ distribution after isolation. The potassium contents of the isolated cells were identical to those of intact smooth muscle (131 mM/liter intracellular fluid) and stable for over 4 h; moreover, the unidirectional influx and efflux rates were equal. An additional simplification was provided by the finding that virtually all the K+ exchanges in a manner predicted for a simple two-compartment system consisting of an extracellular and an intracellular space. Transmembrane K+ flux in these cells averaged 1.2 pmol.cm-2.s-1 at room temperature. A large portion (approximately 80%) of 42K influx appeared to be mediated by a saturable transport system with an apparent Km of 0.6 mM and an apparent Vmax of 1.3 pmol.cm-2.s-1. The calculated resting membrane permeability to K+ in these isolated smooth muscle cells, assuming a membrane potential of -50 mV, was 2.9 X 10(-8) cm/s. The calculated gK+ was 2.7 mumho/cm2 constituting only a small fraction of the total membrane conductance as measured electrophysiologically. The latter finding suggests that the resting membrane potential in the isolated cells must be determined by ions in addition to K+. We propose that these methods for studying ion movements in smooth muscle should aid in unraveling the mechanisms responsible for controlling the distribution of ions both at rest, as in the present study, as well as in response to neurotransmitters.     

Carrier-mediated Potassium Efflux Across the Cell Membrane of Red Beet

The flux ratio (influx/efflux) of K⁺ across the plasmalemma of beet cells at an external potassium concentration of 0.6 mm does not respond to changes of membrane potential in the manner expected for the free diffusion of ions. The K⁺ efflux is affected by the presence of adsorbed Ca²⁺, but is apparently unrelated to the electrical potential or to the net uptake of potassium. The K⁺ efflux is greater than the efflux of the sulfate and organic anions which are accumulated with potassium, and is partially dependent on the presence of external potassium. Thus the loss of ⁴²K from the cell does not appear to be a leakage of freely diffusing K⁺ ions, nor a leakage of ion pairs, but a carrier-mediated transport or exchange of potassium across the cell membrane.