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

Spatial patterns of neuronal activity in rat cerebral cortex during non-rapid eye movement sleep

It is commonly assumed that cortical activity in non-rapid eye movement sleep (NREMS) is spatially homogeneous on the mesoscopic scale. This is partly due to the limited observational scope of common metabolic or imaging methods in sleep. We used the recently developed technique of thallium-autometallography (TlAMG) to visualize mesoscopic patterns of activity in the sleeping cortex with single-cell resolution. We intravenously injected rats with the lipophilic chelate complex thallium diethyldithiocarbamate (TlDDC) during spontaneously occurring periods of NREMS and mapped the patterns of neuronal uptake of the potassium (K+) probe thallium (Tl+). Using this method, we show that cortical activity patterns are not spatially homogeneous during discrete 5-min episodes of NREMS in unrestrained rats-rather, they are complex and spatially diverse. Along with a relative predominance of infragranular layer activation, we find pronounced differences in metabolic activity of neighboring neuronal assemblies, an observation which lends support to the emerging paradigm that sleep is a distributed process with regulation on the local scale.

Single-cell resolution mapping of neuronal damage in acute focal cerebral ischemia using thallium autometallography

Neuronal damage shortly after onset or after brief episodes of cerebral ischemia has remained difficult to assess with clinical and preclinical imaging techniques as well as with microscopical methods. We here show, in rodent models of middle cerebral artery occlusion (MCAO), that neuronal damage in acute focal cerebral ischemia can be mapped with single-cell resolution using thallium autometallography (TlAMG), a histochemical technique for the detection of the K(+)-probe thallium (Tl(+)) in the brain. We intravenously injected rats and mice with thallium diethyldithiocarbamate (TlDDC), a lipophilic chelate complex that releases Tl(+) after crossing the blood-brain barrier. We found, within the territories of the affected arteries, areas of markedly reduced neuronal Tl(+) uptake in all animals at all time points studied ranging from 15 minutes to 24 hours after MCAO. In large lesions at early time points, areas with neuronal and astrocytic Tl(+) uptake below thresholds of detection were surrounded by putative penumbral zones with preserved but diminished Tl(+) uptake. At 24 hours, the areas of reduced Tl(+)uptake matched with areas delineated by established markers of neuronal damage. The results suggest the use of (201)TlDDC for preclinical and clinical single-photon emission computed tomography (SPECT) imaging of hyperacute alterations in brain K(+) metabolism and prediction of tissue viability in cerebral ischemia.

Disrupted cross-laminar cortical processing in β amyloid pathology precedes cell death

Disruption of neuronal networks in the Alzheimer-afflicted brain is increasingly recognized as a key correlate of cognitive and memory decline in Alzheimer patients. We hypothesized that functional synaptic disconnections within cortical columnar microcircuits by pathological β-amyloid accumulation, rather than cell death, initially causes the cognitive impairments. During development of cortical β-amyloidosis with still few plaques in the transgenic 5xFAD mouse model single cell resolution mapping of neuronal thallium uptake revealed that electrical activity of pyramidal cells breaks down throughout infragranular cortical layer V long before cell death occurs. Treatment of 5xFAD mice with the glutaminyl cyclase inhibitor, PQ 529, partially prevented the decline of pyramidal cell activity, indicating pyroglutamate-modified forms, potentially mixed oligomers of Aβ are contributing to neuronal impairment. Laminar investigation of cortical circuit dysfunction with current source density analysis identified an early loss of excitatory synaptic input in infragranular layers, linked to pathological recurrent activations in supragranular layers. This specific disruption of normal cross-laminar cortical processing coincided with a decline of contextual fear learning.

High-resolution mapping of neuronal activity using the lipophilic thallium chelate complex TlDDC: protocol and validation of the method

In neurons the rate of K(+)-uptake increases with increasing activity. K(+)-analogues like the heavy metal ion thallium (Tl(+)) can be used, therefore, as tracers for imaging neuronal activity. However, when water-soluble Tl(+)-salts are injected systemically only minute amounts of the tracer enter the brain and the Tl(+)-uptake patterns are influenced by regional differences in blood-brain barrier (BBB) K(+)-permeability. We here show that the BBB-related limitations in using Tl(+) for imaging neuronal activity are no longer present when the lipophilic Tl(+) chelate complex thallium diethyldithiocarbamate (TlDDC) is applied. We systemically injected rodents with TlDDC and mapped the Tl(+)-distribution in the brain using an autometallographic (AMG) technique, a histochemical method for detecting heavy metals. We find that Tl(+)-doses for optimum AMG staining could be substantially reduced, and regional differences attributable to differences in BBB K(+)-permeability were no longer detectable, indicating that TlDDC crosses the BBB. At the cellular level, however, the Tl(+)-distribution was essentially the same as after injection of water-soluble Tl(+)-salts, indicating Tl(+)-release from TlDDC prior to neuronal or glial uptake. Upon sensory stimulation or intracortical microstimulation neuronal Tl(+)-uptake increased after TlDDC injection, upon muscimol treatment neuronal Tl(+)-uptake decreased. We present a protocol for mapping neuronal activity with cellular resolution, which is based on intravenous TlDDC injections during ongoing activity in unrestrained behaving animals and short stimulation times of 5 min.

Silver enhancement of quantum dots resulting from (1) metabolism of toxic metals in animals and humans, (2) in vivo, in vitro and immersion created zinc–sulphur/zinc–selenium nanocrystals, (3) metal ions liberated from metal implants and particles

Autometallographic (AMG) silver enhancement is a potent histochemical tool for tracing a variety of metal containing nanocrystals, e.g. pure gold and silver nanoclusters and quantum dots of silver, mercury, bismuth or zinc, with sulphur and/or selenium. These nanocrystals can be created in many different ways, e.g. (1) by manufacturing colloidal gold or silver particles, (2) by treating an organism in vivo with sulphide or selenide ions, (3) as the result of a metabolic decomposition of bismuth-, mercury- or silver-containing macromolecules in cell organelles, or (4) as the end product of histochemical processing of tissue sections. Such nano-sized AMG nanocrystals can then be silver-amplified several times of magnitude by being exposed to an AMG developer, i.e. a normal photographic developer enriched with silver ions. The present monograph attempts to provide a review of the autometallographic silver amplification techniques known today and their use in biology. After achieving a stronghold in histochemistry by Timm's introduction of the "silver-sulphide staining" in 1958, the AMG technique has evolved and expanded into several different areas of research, including immunocytochemistry, tracing of enzymes at LM and EM levels, blot staining, retrograde axonal tracing of zinc-enriched (ZEN) neurons, counterstaining of semithin sections, enhancement of histochemical reaction products, marking of phagocytotic cells, staining of myelin, tracing of gold ions released from gold implants, and visualization of capillaries. General technical comments, protocols for the current AMG methods and a summary of the most significant scientific results obtained by this wide variety of AMG histochemical approaches are included in the present article.

High-resolution mapping of neuronal activity by thallium autometallography

Different methods are available for imaging neuronal activity in the mammalian brain with a spatial resolution sufficiently high to detect activation patterns at the level of individual functional modules such as cortical columns. Severe difficulties exist, however, in visualizing the different degree of activity of each individual neuron within such a module, and mapping neuronal activity with a spatial resolution of single axons has remained impossible thus far. Here, we present a novel method for mapping neuronal activity that is able to visualize activation patterns with light and electron microscopical resolution. The method is based on the tight coupling of neuronal activity and potassium (K(+)) uptake. We have injected Mongolian gerbils with the K(+) analogue thallium (Tl(+)), stimulated the animals with pure tones of different frequencies and analyzed, by an autometallographic method, the Tl(+) distribution in the auditory cortex (AC). We find tonotopically organized columns of increased Tl(+)-uptake in AC. Within columns, the spatial patterns of neuronal activity as revealed by thallium autometallography are highly elaborated. Tl(+)-uptake differs in different layers, sublayers, and cell types, being especially high in large multipolar inhibitory interneurons in layer IV. A prominent feature of the columnar activation pattern is the presence of vertical modules of minicolumnar dimensions. Clusters of layer Vb pyramidal cells and their apical dendrite bundles are clearly visible in the center of the columns.

Prenatal and postnatal thallium exposure in rats: effect on development of vasomotor reactivity in pups

Vasomotor reactivity has been evaluated in rats exposed perinatally and postnatally to thallium sulphate (1 mg/dl in their drinking water ad libitum). Prenatal and postnatal exposure to thallium did not modify the values of the systolic arterial blood pressure on the 30th and 60th day in pups of normotensive and DOCA-hypertensive rats. The hypertensive responses induced by endosinusal carotid hypotension and by 1-noradrenaline in pups of normotensive and DOCA-hypertensive rats, exposed or not exposed to thallium sulphate, were more intensive on the 60th than on the 30th day. Similar effects were observed for the hypotensive responses induced by 1-isoprenaline and acetylcholine. Prenatal exposure to thallium did not modify hypertensive responses induced by endosinusal carotid hypotension on the 30th and 60th days, but it caused a decrease of hypertensive responses induced by 1-noradrenaline on the 30th and 60th days and hypotensive responses induced by 1-isoprenaline and acetylcholine exclusively on the 60th day. Postnatal exposure to thallium did not modify hypertensive responses induced by endosinusal carotid hypotension and hypotensive responses induced by acetylcholine, but it caused a decrease of hypertensive responses induced by 1-noradrenaline on the 30th and 60th days in pups of normotensive rats and exclusively on the 60th day in pups of DOCA-hypertensive rats. Moreover, postnatal exposure to thallium caused a decrease of the hypotensive response induced by 1-isoprenaline exclusively on the 60th day. Our findings show that prenatal and postnatal exposure to thallium sulphate modifies the rat's developing vascular autonomic nervous system with a reduction of the alpha, beta-adrenergic and muscarinic vasomotor reactivity.

Thallium(I) secretion across the isolated mucosa of rat descending colon

Unidirectional Tl+-fluxes across the isolated mucosa of rat descending colon were measured under short circuit (SCC) or voltage clamp conditions (VCC). Under SCC a serosal-to-mucosal net flux (Jnet = -0.22 nmol X cm-2 X h-1) was observed that agrees with the voltage-independent component measured under VCC. At 7 degrees C the secretory net flux was abolished. In controls both unidirectional fluxes were unsaturable between 0.1 and 500 mumol/l thallium (I). After furosemide or withdrawal of Cl- or Na+ Jnet was abolished. Ouabain decreased serosal-to-mucosal Tl+ flux and an energy-dependent absorptive net flux of Tl+ was observed, characterized by a rather low Km (10.2 mumol/l), Vmax (3.8 nmol X cm-2 X h-1) and apparent activation energies (delta E = 10.7 kcal X mol-1) typically for enzyme catalaysed reactions or narrow channel interactions. The data suggest that in the mucosa of rat descending colon Tl+ ions share, at least in part, the same transport systems at K+.

Impairment of Na+ transport across frog skin by Tl+: effects on turnover, area density and saturation kinetics of apical Na+ channels

Na+ transport across abdominal skin of the frogs, Rana temporaria and Rana esculenta was followed by measuring Na+-dependent short-circuit, current (INa) kinetics and INa fluctuations induced by triamterene, a diuretic. Exposure of the skin to serosal Tl+ led to a pronounced and irreversible drop in INa and INa-blocker noise. At low serosal Tl+ concentrations, we observed mainly a decrease in the apparent Michaelis constant for INa saturation while, at larger [Tl+], the maximal INa dropped irreversibly. Tl+ acts even when serosal Tl+ "transporters" like the Na+-K+ pump, or the K+ channel are nonfunctional. The rate constants for the triamterene/Na+ channel reaction were unchanged after Tl+ whereas the relaxation noise from channel blockage decreased in amplitude. Noise analysis in terms of a two-state blocking model suggested that Tl+ poisoning results in a small decrease in single-channel current through apical Na+ pathways, as well as in a drastic and irreversible drop in channel density. The impairment of Na+ transport by Tl+ can be attributed to the above cited concerted events at the level of the apical membrane.

Effects of thallium on membrane currents at diastolic potentials in canine cardiac Purkinje strands

A two-micro-electrode voltage-clamp technique was used to record membrane currents from canine cardiac Purkinje strands during hyperpolarizing steps to potentials between -70 and -150 mV in Tyrode solutions containing K+ and/or Tl+. Complete replacement of external K+ by equimolar Tl+ increases the instantaneous inwardly rectifying current. The inwardly rectifying region of the instantaneous I-V relation is shifted to more positive potentials and its slope is increased. The diastolic time-dependent current is reduced or reversed. Partial substitution of equimolar Tl+ for K+ reduces the diastolic time-dependent current. The instantaneous I-V relation is shifted inward for molar fractions of Tl+ (YTl) greater than 0.5, and is slightly more inward or unchanged for YTl less than or equal to 0.5. Addition of small amounts of Tl+ shifts the instantaneous I-V relation inward and reduces the diastolic time-dependent current. Addition of Tl+ in solutions containing Ba2+ to block the background inward rectifier has no effect on the instantaneous I-V relation; the diastolic time-dependent (pace-maker) current is reduced. Block of the pace-maker current by Tl+ is largely independent of potential in Ba2+ Tyrode solution. Since Tl+ has opposite effects on the pace-maker current and the inward rectifier, these findings support other evidence that the pace-maker current is not part of the background inward rectifier.

Kinetics of thallium exchange in cultured rat myocardial cells

The kinetics of thallium exchange in cultured rat myocardial cells were studied and compared to those of potassium in the same tissue. Studies were carried out using low concentrations (10 nM to 5 microM) of thallium-204, approximating those likely to be encountered during clinical myocardial scintigraphy. Both thallium uptake and release could be described by a single exponential with a half-time of exchange which was approximately half that of potassium and which was largely independent of extracellular thallium concentration. Some 60% of thallium uptake occurred via an "active" or ouabain-inhibitable mechanism which, in the absence of extracellular potassium, could be activated by low concentrations (10 nM to 5 microM) of thallium. The apparent Km for thallium on this active transport mechanism was 2-7 microM. Increasing extracellular potassium from 0-10 mM caused significant, concentration-dependent decreases in both the total and the active component of the thallium influx. Similarly nonradioactive thallium (0.10 microM to 0.10 mM) caused a concentration-dependent decrease in active potassium influx. Analysis of these results by both Lineweaver-Burk plots and Dixon plots confirmed competitive inhibition, potassium on thallium influx and vice versa, for the active component of the fluxes, and noncompetitive in the remainder. These findings indicate that active transport accounts for the greater portion of the influx of thallium and potassium, and that this active transport occurs via a common mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonin increases an anomalously rectifying K+ current in the Aplysia neuron R15

Previous work has shown that serotonin causes an increase in K+ conductance in the identified Aplysia neuron R15. This response is mediated by cAMP-dependent protein phosphorylation. The results presented here show that the K+ channel modulated by serotonin is an anomalous or inward rectifier (designated IR) that is present in R15 together with the three other distinct K+ channels previously described for this cell. Several lines of evidence indicate that this inward rectifier is partially activated in the resting cell and is further activated by serotonin. Voltage clamp analysis of resting and serotonin-evoked membrane currents at various external K+ concentrations shows that both currents have reversal potentials close to the potassium equilibrium potential, exhibit similar dependences in magnitude on external K+ concentration, and display marked anomalous rectification. The effects of particular monovalent and divalent cations are also similar on the resting and serotonin-evoked currents. Rb+, Cs+, and Ba2+ block both currents while Tl+ can substitute for K+ as a charge carrier and channel activator in both. These properties are characteristics of anomalous rectifiers in other systems. Furthermore, measurement of the voltage dependence of inactivation for the fast transient K+ current shows that this current cannot account for the anomalously rectifying K+ conductance in R15. The inward rectifier is therefore a separate current mediated by its own channels, the activity of which can be modulated by serotonin.

The interaction of "K+-like" cations with the apical K+ channel in frog skin

The apparent permeability of the apical K+ channel in the abdominal skin of the frog (Rana temporaria) for different monovalent cations was tested by comparing the short-circuit current (SCC) obtained after imposition of serosally directed ionic concentration gradients. Furthermore, the SCC was subjected to noise analysis. Of various cations tested, only the "K+-like" ions NH+4, Rb+ and Tl+, besides K+, were found to permeate the apical K+ channel, as reflected by SCC- and fluctuation analysis: (i) The SCC could be depressed by addition of the K+-channel blocker Ba2+ to the mucosal solution. (ii) With the K+-like ions (Ringer's concentration), a spontaneous Lorentzian noise was observed. Plateau values were similar for K+ and Tl+, and smaller for NH+4 and Rb+. The corner frequencies clearly increased in the order K+ less than NH+4 less than Tl+ much less than Rb+. The SCC dose-response relationships revealed a Michaelis-Menten-type current saturation only for pure K+- or Tl+-Ringer's solutions as mucosal medium, whereas a more complicated SCC behavior was seen with Rb+ and especially, NH+4. For K+-Tl+ mixtures an anomalous mole-fraction relationship was observed: At low [Tl+]/[K+] ratios, Tl+ ions appeared to inhibit competitively the K+ current while, at high [Tl+]/[K+] ratios, Tl+ seemed to be a permeant cation. This feature was also detected in the noise analysis of K+-Tl+ mixtures. Long-term exposure to mucosal Tl+ resulted in an irreversible deterioration of the tissue. The SCC depression by Ba2+ was of a simple saturation-type characteristic with, however, different half-maximal doses (NH+4 less than K+ less than Rb+). Ba2+ induced a "blocker noise" in presence of all permeant cations with corner frequencies that depended on the Ba2+ concentration. A linear increase of the corner frequencies of the Ba2+-induced noise with increasing Ba2+ concentration was seen for NH+4, Rb+ and K+. With the assumption of a pseudo two-state model for the Ba2+ blockade the on- and off-rate constants for the Ba2+ interaction with the NH+4/Rb+/K+ channel were calculated and showed marked differences, dependent on the nature of the permeant ion. The specific problems with Tl+ prevented such an analysis but SCC- and noise data indicated a comparably poor efficiency of Ba2+ as Tl+-current inhibitor. We attempted a qualitative analysis of our results in terms of a "two-sites, three-barriers" model of the apical K+ channel in frog skin.

Placental transfer and retention of 201Ti-thallium in the rat

Placental transfer of thallium was evidenced in rats treated with a single intraperitoneal dose of 2 micrograms 201T1-labelled thallium/rat on the 13th day of pregnancy. Both maternal and fetal organs showed remarkable thallium retention, approx. 10% of the dose being unexcreted 8 days after injection. The highest thallium accumulation was found in maternal muscle and brain tissues. Fetal brain exhibits higher thallium uptake and faster decay rate of thallium levels than maternal brain. It is suggested that the reduced activity of the mechanisms regulating ion movements and composition of nervous tissue and the immaturity of the blood-brain barrier play a role in the peculiar pattern of thallium kinetics in the developing rat brain.

Thallium interaction with the gastric (K, H)-ATPase

The gastric (K, H)-ATPase has been shown to catalyze an electroneutral H+ for K+ exchange. Tl+ is able to substitute for K+ as an activating cation in the hydrolytic reaction with an apparent dissociation constant of 90 microM as compared to about 870 microM for K+. The ability of Tl+ to participate in transport is shown by the development of pH gradients in the presence of Tl+ following addition of ATP to gastric vesicles and by the ATP-dependent efflux of Tl+ from gastric vesicles. Inhibition of hydrolysis is observed at pH 7.4 with external Tl+ concentrations above 3.0 mM. This inhibition of hydrolysis is correlated with inhibition of pH-gradient formation. The inhibition of transport activity is partially relieved by a decrease in medium pH. This inhibitory effect is attributed to Tl+ binding at an external, low affinity cation site. In contrast to rubidium chloride, at high Tl+ concentrations, following the initial Tl+ efflux, there is reuptake of the cation. This rapid uptake is attributed to lipid-dependent Tl+ entry pathways. The vesicles exhibit a high permeability to thallium nitrate demonstrating a half-time (t1/2) for uptake of about 1.0 min in contrast to 46 min for rubidium chloride. In both gastric vesicles or liposomes, external Tl+ concentrations in excess of 1 to 4 mM are able to dissipate intravesicular proton gradients. Thus, although Tl+ is able to activate the gastric ATPase by mimicking K+, the permeability of this cation in lipid bilayers tends to uncouple H+ transport at concentrations high enough to generate detectable proton gradients.

Ouabain-sensitive thallium fluxes in smooth muscle of rabbit uterus

1. Rabbit myometrium accumulates Tl in a time-dependent fashion and the majority of the uptake of Tl is ouabain-sensitive. 2. In normal chloride-containing media, the uptake of Tl, though ouabain-sensitive, is less than in chloride-free media and this difference is due to a greater ouabain-insensitive uptake. 3. The ouabain-insensitive uptake in normal chloride-containing media is reduced by furosemide and furosemide also reduces total uptake in this solution. In chloride-free media, however, furosemide is without effect on total or ouabain-sensitive uptake. 4. In chloride-free media, the uptake of Tl against Tl concentration is sigmoidal, suggesting that more than one Tl ion is being transported at a time. 5. Tl was capable of substituting for K in electrogenic Na pumping; it was approximately twice as effective as K and the inhibitory effect of Tl was blocked by ouabain. 6. Tl efflux can be explained by a simple two-compartment model in both normal and chloride-free solutions. 7. The uptake of Tl was inhibited by alkali cations with the order of potency being Tl+ > K+ > Rb+ > Cs+ at 10 mM and Tl+ > K4 = Rb+ > Cs+ at 5 mM ion concentrations. 8. It is concluded that Tl enters the smooth muscle of rabbit uterus by diffusion, active ouabain-sensitive transport and active chloride- and furosemide-sensitive transport.

Cationic selectivity and competition at the sodium entry site in frog skin

The cation selectivity of the Na entry mechanism located in the outer membrane of the bullfrog (Rana catesbeiana) skin epithelium was studied. This selectivity was determined by measuring the short-circuit current when all of the external sodium was replaced by another cation and, also, by noting the relative degree of inhibition that the alkali metal cations produced on Na influx. The ability of the Group Ia cations to permeate the apical membrane was determined from the tracer uptake experiments. The results demonstrate that (a) only Li and Na are actively transported through the epithelium; (b) the alkali cations K, Rb, and Cs do not enter the epithelium through the apical border and, therefore, Na and Li are the only alkali cations translocated through this membrane; (c) these impermeable cations are competitive inhibitors of Na entry; (d) the cations NH4 and Tl exhibit more complex behavior but, under well-defined conditions, also inhibit Na entry; and (e) the selectivity of the cation binding site is in the sequence Li congruent to Na > Tl > NH4 congruent to K > Rb > Cs, which corresponds to a high field strength site with tetrahedral symmetry.

Movement of thallous ion across the ascites cell membrane

The movement of thallous ion (Tl+) across the ascites cell membrane has been characterized. Analogous to previous findings for 86Rb (used as a tracer for K+), 204Tl+-influx could be resolved into three components: a ouabain-inhibitable "pump" flux, a passive flux, and a furosemide- or NO-3-sensitive "exchange" flux. Although Tl+ moved approximately nine times faster across the membrane than K+, the pump/leak ratio was equal for the two ions. This suggests that the pump- and leak-pathways share a common rate-limiting step. The exchange mechanism was shown to provide close coupling between the Tl+- and K+-gradients.

Accumulation of thallous ions (Tl+) as a measure of the electrical potential difference across the cytoplasmic membrane of bacteria

The accumulation of thallous ions (204Tl+) by intact bacteria was investigated. I conclude that Tl+ is a permeant cation, and that it therefore accumulates in response to the electrical potential difference (delta psi) across the cytoplasmic membrane (interior negative). A comparison with other methods shows that the distribution ratio of 204Tl+ serves as a reasonably satisfactory method for measuring the membrane potential of Streptococcus faecalis. Glycolyzing cells of this organism develop membrane potentials of up to 180 mV. Preliminary experiments with Escherichia coli, especially those with a mutant defective in the proton-translocating ATPase, indicate that the Tl+ distribution also serves as a measure of the membrane potential in this organism. The particular advantage of Tl+ over other indicators of the membrane potential is that the cells need not be pretreated in any way. By use of the Tl+ distribution, it was calculated that respiring cells of E. coli develop a membrane potential of 160 mV with D-lactate and 180 mV with glucose as a substrate, respectively.

Anomalous permeabilities of the egg cell membrane of a starfish in K+-Tl+ mixtures

The electrical properties of "inward" rectifying egg cell membranes of the starfish mediastera aequalis have been studied in the presence of K(+)-Tl(+) mixtures. When the ratio of the external concentrations of these ions is changed while their sum is kept constant, both the conductance and the zero-current membrane potential go through a minimum, showing clear discrepancies from theoretical results based on conventional electrodiffusion models (E.g., Goldman's equation). By contrast, when the ration of the two concentrations is fixed and their sum varied, the potential follows an ideal Nernst slope, consistent with Goldman's equation. The membrane conductance which, according to previous studies on similar membranes, is to be viewed as a function of the displacement of the membrane potential from its resting value deltaV, shows marked differences between the cases in which K(+) or Tl(+) are the predominant ions: when K(+) is the predominant permeant ion in solution, the addition of small amounts of Tl(+) inhibits the current, while corresponding blocking effects of K(+) on the current are not observed when Tl(+) is the predominant permeant ion. Also, the time course of the conductance during voltage clamp is different in the two cases, being much faster in Tl(+) than in K(+) solution for comparable values of deltaV. Most of the above features are accounted for by a model in which it is assumed that the ionic channels have external binding sites for cations and that their permeability properties depend on the species of the cation bound (K(+)or Tl(+) in the present experiments).

Sodium efflux from voltage clamped squid giant axons

1. The efflux of radioactive sodium was measured from squid axons during simultaneous voltage clamp experiments such that it was possible to determine the efflux of sodium associated with a measured voltage clamp current. 2. The extra efflux of sodium associated with voltage clamp pulses increased linearly with the magnitude of the depolarization above 40 mV. A 100 mV pulse of sufficient duration to produce all of the sodium current increased the rate constant of efflux by about 10(-6). 3. Application of 100 nM tetrodotoxin eliminated the sodium current and the extra efflux of radioactive sodium. 4. Cooling the axon increased the extra efflux/voltage clamp pulse slightly with a Q10 of 1/1-1. On the same axons cooling increased the integral of the sodium current with a Q10 of 1/1-4. 5. Replacing external sodium with Tris, dextrose or Mg-mannitol reduced the extra efflux of sodium by about 50%. The inward sodium current was replaced with an outward current as expected. 6. Replacing external sodium with lithium also reduced the extra efflux by about 50% but the currents seen in lithium were slightly larger than those in sodium. 7. The effect of replacing external sodium was not voltage dependent. Cooling reduced the effect so that there was less reduction of efflux on switching to Tris ASW in the cold than in the warm. 8. The extra efflux of sodium into sodium-free ASW is approximately the same as the integral of the sodium current. Adding external sodium produces a deviation from the independence principle such that there is more exchange of sodium than predicted. Such a deviation from prediction was noted by Hodgkin & Huxley (1952c). 9. Using the equations of Hodgkin & Huxley (1952c) modified to include the deviation from independence reported in this paper and its temperature dependence, one can predict the temperature dependence of the sodium efflux associated with action potentials and obtain much better agreement than is possibly without these phenomena. 10. This deviation from independence in the sodium fluxes is the type expected from some kind of mixing and binding of sodium within the membrane phase.

The temperature dependence of the movement of potassium and chloride ions associated with nerve impulses

1. The influx and efflux of radioactive potassium and chloride across the membrane of the squid giant axon were measured in resting and in stimulated nerves. The measurements were made at room temperature and at 6-8 degrees C. 2. At room temperature all eight flux measurements were comparable to previously reported values. 3. When the axons were cooled the resting potassium influx decreased with a Q10 of 1-9 and the resting potassium efflux decreased with a Q10 of 1-2. 4. With cooling the resting chloride efflux decreased with a Q10 of 1-3 and the resting chloride influx decreased with a Q10 of 2-8. This latter value, together with anomalous flux ratios for resting chloride fluxes may indicate an active uptake of chloride ions into the axon. 5. Cooling increased the extra efflux of potassium associated with nerve impulses with a Q10 of 1/1-5 and increased the extra influx of potassium with a Q10 of 1/3-3. 6. No extra efflux of chloride was detected at either temperature. Cooling produced no statistically significant change in the extra chloride influx but there was considerable scatter in the data. 7. Fluxes were computed as a function of temperature for standard action potentials with a variety of temperature coefficients for the conductances and rate constants. No single curve could match either the influx or the efflux data.