Diet, cellular, and systemic homeostasis control the cycling of potassium stable isotopes in endothermic vertebrates

The naturally occurring stable isotopes of potassium (41K/39K, expressed as δ41K) have the potential to make significant contributions to vertebrate and human biology. The utility of K stable isotopes is, however, conditioned by the understanding of the dietary and biological factors controlling natural variability of δ41K. This paper reports a systematic study of K isotopes in extant terrestrial endothermic vertebrates. δ41K has been measured in 158 samples of tissues, biofluids, and excreta from 40 individuals of four vertebrate species (rat, guinea pig, pig and quail) reared in two controlled feeding experiments. We show that biological processing of K by endothermic vertebrates produces remarkable intra-organism δ41K variations of ca. 1.6‰. Dietary δ41K is the primary control of interindividual variability and δ41K of bodily K is +0.5-0.6‰ higher than diet. Such a trophic isotope effect is expected to propagate throughout trophic chains, opening promising use for reconstructing dietary behaviors in vertebrate ecosystems. In individuals, cellular δ41K is related to the intensity of K cycling and effectors of K homeostasis, including plasma membrane permeability and electrical potential. Renal and intestinal transepithelial transports also control fractionation of K isotopes. Using a box-modeling approach, we establish a first model of K isotope homeostasis. We predict a strong sensitivity of δ41K to variations of intracellular and renal K cycling in normal and pathological contexts. Thus, K isotopes constitute a promising tool for the study of K dyshomeostasis.

Single-Cell Metabolic Profiling: Metabolite Formulas from Isotopic Fine Structures in Heterogeneous Plant Cell Populations

Characterization of the metabolic heterogeneity in cell populations requires the analysis of single cells. Most current methods in single-cell analysis rely on cell manipulation, potentially altering the abundance of metabolites in individual cells. A small sample volume and the chemical diversity of metabolites are additional challenges in single-cell metabolomics. Here, we describe the combination of fiber-based laser ablation electrospray ionization (f-LAESI) with 21 T Fourier transform ion cyclotron resonance mass spectrometry (21TFTICR-MS) for in situ single-cell metabolic profiling in plant tissue. Single plant cells infected by bacteria were selected and sampled directly from the tissue without cell manipulation through mid-infrared ablation with a fine optical fiber tip for ionization by f-LAESI. Ultrahigh performance 21T-FTICR-MS enabled the simultaneous capture of isotopic fine structures (IFSs) for 47 known and 11 unknown compounds, thus elucidating their elemental compositions from single cells and providing information on metabolic heterogeneity in the cell population.

Concentration of trace elements in blood and feed of homebred animals in Southern Serbia

BACKGROUND, AIM AND SCOPE

The paper presents concentrations of trace elements in blood of homebred animals (cows and sheep) from Southern Serbia (Bujanovac) and the contents of natural and anthropogenic radionuclides and some heavy metals in feed. The region of Southern Serbia was exposed to contamination by depleted uranium ammunition during NATO attacks in 1999 and therefore, is of great concern to environmental pollution and human and animal health.

MATERIALS AND METHODS

Conventional instrumental and epithermal neutron activation analyses are used to measure trace elements in cow and sheep blood samples collected randomly at six locations in the region of Bujanovac (village of Borovac) in the spring of 2005. Samples of feed (grass and crops: corn, wheat and oats), collected on the same locations (households), are analysed for the contents of radionuclides on an HPGe detector (Ortec, relative efficiency 23%) by standard gamma spectrometry. The content of Hg, Pb and Cd in feed is determined by standard atomic absorption spectrometry on the VarianSpectra220/ThermoSolar GFS97 spectrometer.

RESULTS

Concentrations of 29 elements (Na, Al (P), Cl, K, Sc, Cr, Mn, Ni, Fe, Co, Zn, Se, As, Br, Sr, Rb, Sb, In, I, Ba, Cs, La, Nd, Eu, Sm, Tb, Hf, Ta and Th) are determined in blood of the examined animals. In feeds, natural (40)K is found in all of the samples, cosmogenic (7)Be and fission product (137)Cs are detected only in the grass samples, while heavy metals Hg, Cd and Pb are found in the range of 0.01-0.02, 0.84-1.15 and 0.74-7.34 mg/kg, respectively. Calculated soil-to-blood transfer factors are in a wide range of 8 x 10(-6) to 64, as a result of varying significance of the elements in animal metabolism and feeding habits.

DISCUSSION

The results of trace elements' concentrations in animal blood are in good agreement with available data for K, Ni, Zn, Se and Rb. Higher Br concentrations in animal blood are most probably caused by large biomass burning events during blood sampling. Very low concentration of Fe in cows and sheep confirms the results of previous biochemical studies on animal anaemia in the region. High concentration of As correlates with geochemical peculiarities of the Balkans and is also likely influenced by the use of pesticides in the agricultural production. For some of the elements (La, Nd, Eu, Sm, Tb, Sb, Hf, Ta, Th, In, Ba, Sr, Sc and Cs), there are few or no literature data. Therefore, some of the presented data are significant not only for the country and the region, but on a wider scale. Activities of natural radionuclides in feeds are within the average values reported for the region, while the activities of (210)Pb and (235/238)U are below the limit of detection. This is in accordance with previous investigations showing no widespread contamination by depleted uranium in the area. Contents of Hg and Pb in feeds are below the nationally permissible levels, unlike the content of Cd which exceeds it, probably caused by the use of phosphate fertilisers and fossil fuel combustion in the area.

CONCLUSIONS

In general, the concentrations of trace elements in blood of homebred cows and sheep are in good agreement with reference materials, available literature data and the results of previous studies in the area. The exceptions are Fe, As and Br. The contents of natural and anthropogenic radionuclides in feeds are within the expected levels, and there are no signs of contamination by depleted uranium or other fission products. Apart from Cd, there are no signs of pollution by heavy metals in feeds. The highly sensitive method of instrumental neutron activation analysis provides data on the concentration of some elements in animal blood not previously reported for the region and elsewhere.

RECOMMENDATIONS AND PERSPECTIVES

The presented study is a part of the long term ongoing project on the health risk assessment on animals and humans in the region. The collected data is intended to provide a base for the animal and human risk assessment as well as an estimate of the general pollution status of the environment in the region. Since some of the investigated elements are classified as important trace elements for livestock, the results could also be used to balance and improve the animal diet and thus, improve the growth and reproduction rate.

In vivo(39)K, (23)Na and (1)H MR imaging using a triple resonant RF coil setup

The maintenance of a gradient of potassium and sodium ions across the cell membranes is essential for the physiological function of the mammal organism. The measurement of the spatial distribution of pathologically changing ion concentrations of (23)Na and (39)K with magnetic resonance imaging offers a promising approach in clinical diagnostics to measure tissue viability. Existing studies were focused mainly on (23)Na imaging as well as spectroscopy with only one post-mortem study for (39)K imaging. In this paper a triple resonant RF coil setup for the rat head at 9.4T is presented for imaging of both nuclei ((23)Na and (39)K) and the acquisition of anatomical proton images in the same experiment without moving the subject or the RF coil. In vivo MR images of (39)K and (23)Na in the rat brain were acquired as well as anatomical proton images in the same scanning session.

Imaging nutrient distributions in plant tissue using time-of-flight secondary ion mass spectrometry and scanning electron microscopy

A new approach to trace the transport routes of macronutrients in plants at the level of cells and tissues and to measure their elemental distributions was developed for investigating the dynamics and structure-function relationships of transport processes. Stem samples from Phaseolus vulgaris were used as a test system. Shock freezing and cryo-preparation were combined in a cryogenic chain with cryo-time-of-flight secondary ion mass spectrometry (cryo-ToF-SIMS) for element and isotope-specific imaging. Cryo-scanning electron microscopy (cryo-SEM) was integrated into the cryogenic workflow to assess the quality of structural preservation. We evaluated the capability of these techniques to monitor transport pathways and processes in xylem and associated tissues using supplementary sodium (Na) and tracers for potassium (K), rubidium (Rb), and (41)K added to the transpiration stream. Cryo-ToF-SIMS imaging produced detailed mappings of water, K, calcium, magnesium, the K tracers, and Na without quantification. Lateral resolutions ranged from 10 microm in survey mappings and at high mass resolution to approximately 1 microm in high lateral resolution imaging in reduced areas and at lower mass resolution. The tracers Rb and (41)K, as well as Na, were imaged with high sensitivity in xylem vessels and surrounding tissues. The isotope signature of the stable isotope tracer was utilized for relative quantification of the (41)K tracer as a fraction of total K at the single pixel level. Cryo-SEM confirmed that tissue structures had been preserved with subcellular detail throughout all procedures. Overlays of cryo-ToF-SIMS images onto the corresponding SEM images allowed detailed correlation of nutrient images with subcellular structures.

(39)K NMR of solid potassium salts at 21 T: effect of quadrupolar and chemical shift tensors

39K Solid State NMR spectra (static and magic angle spinning (MAS)) on a set of potassium salts measured at 21.14 T show that the chemical shift range for K(+) ions in diamagnetic salts is well in excess of 100 ppm contrary to previous assumptions that it was quite small. Inequivalent potassium sites in crystals can be resolved through differences in chemical shifts, with chemically similar sites showing differences of over 10 ppm. The quadrupolar coupling constants obtained from MAS and solid echo experiments on powders cover the range from zero for potassium in cubic environments in halides to over 3 MHz for the highly asymmetric sites in K2CO3. Although the quadrupolar effects generally dominate the 39K spectra, in several instances, we have observed subtle but significant contributions of chemical shift anisotropy with values up to 45 ppm, a first such observation. Careful analysis of static and MAS spectra allows the observation of the various chemical shift and quadrupole coupling tensor components as well as their relative orientations, thereby demonstrating that high-field 39K NMR spectroscopy in the solid state has a substantial sensitivity to the local environment with parameters that will be of considerable value in materials characterization and electronic structure studies.

39K nuclear magnetic resonance and a mathematical model of K+ transport in human erythrocytes

(39)K nuclear magnetic resonance was used to measure the efflux of K(+) from suspensions of human erythrocytes [red blood cells (RBCs)], that occurred in response to the calcium ionophore, A23187 and calcium ions; the latter activate the Gárdos channel. Signals from the intra- and extracellular populations of (39)K(+) were selected on the basis of their longitudinal relaxation times, T (1), by using an inversion- recovery pulse sequence with the mixing time, tau(1), chosen to null one or other of the signals. Changes in RBC volume consequent upon efflux of the ions also changed the T (1) values so a new theory was implemented to obviate a potential artefact in the data analysis. The velocity of the K(+) efflux mediated by the Gárdos channel was 1.19+/-0.40 mmol (L RBC)(-1) min(-1) at 37 degrees C.

Muscle metabolism in fibromyalgia studied by P-31 magnetic resonance spectroscopy during aerobic and anaerobic exercise

OBJECTIVE

To investigate mechanisms underlying the reduced work capacity of fibromyalgia (FM) patients were compared to healthy controls at specified workloads, using P-31 magnetic resonance spectroscopy (MRS).

METHODS

The forearm flexor muscle group was examined with MRS at rest, at sub maximal and at maximal controlled dynamic work as well as at maximal isometric contraction. Aerobic fitness was determined by bicycle ergonometry.

RESULTS

Metabolite concentrations and muscle pH were similar for patients and controls at lower workloads. At maximal dynamic and static contractions the concentration of inorganic phosphate was lower and at static contractions the pH decrease was smaller in patients. The performed work by patients was only 50% compared to controls and the patients experienced more pain. Maximal oxygen uptake was lower in the fibromyalgia group. Expired gas-analysis in this group showed ventilatory equivalents at similar relative levels of maximal work capacity.

CONCLUSION

Fibromyalgia patients seem to utilise less of the energy rich phosphorous metabolites at maximal work despite pH reduction. They seemed to be less aerobic fitted and reached the anaerobic threshold earlier than the controls.

K(+) transport and volume regulatory response by NKCC in resting rat hindlimb skeletal muscle

This study tested the hypothesis that the NKCC is involved in volume regulation, specifically regulatory volume increase (RVI), in resting skeletal muscle. Neurally and vascularly isolated rat hindlimbs were perfused with a bovine erythrocyte perfusate containing (42)K or (86)Rb as markers of unidirectional K(+) flux across the sarcolemma. Compared to controls, perfusion with 120 microM bumetanide (a specific inhibitor of the NKCC) decreased J(in)K by 15+/-2%, indicating the functional presence of the NKCC. Experiments with ouabain (to block active K(+) transport by the Na,K ATPase) showed that the bumetanide-sensitive component of J(in)K comprised 35% of the total ouabain-sensitive J(in)K. Inhibition of NKCC resulted in a net loss of water by muscle. When hindlimbs were perfused with hypertonic (380 mOsm/L by addition of sucrose) perfusate for 20 min, after initially blocking K(+) channels with 1 mM barium, J(in)K rapidly (2-3 min) increased 2-fold followed by a rapid decline. This rapid, transient increase in J(in)K was abolished with bumetanide, confirming that perfusion with hypertonic perfusate stimulated NKCC activity and RVI. The hypertonic perfusate also resulted in temporally associated decreases in net water uptake by muscle. It is concluded that a functional NKCC is present in mammalian skeletal muscle and that it is involved in cell volume regulation.

Three-dimensional (87)Rb NMR imaging and spectroscopy of K(+) fluxes in normal and postischemic pig hearts

K(+) uptake rates were measured in the anterior (An) and posterior (Pos) LV walls of pig hearts before and after regional ischemia and reperfusion using Rb(+) as a K(+) congener and 3D (87)Rb NMR imaging and spectroscopy as detection methods. The hearts were perfused by the Langendorff method with Krebs-Henseleit (KH) buffer and loaded with Rb(+) (4.7 mM, Rb-KH) after 120-min ischemia and 60-min reperfusion. A second protocol involved Rb(+) loading prior to ischemia. Ischemia was produced by occlusion of the left anterior descending artery, which after 110 min of reperfusion resulted in infarction in the An wall (24 +/- 6% of the LV mass) determined by triphenyltetrazolium chloride staining. At the end of reperfusion pressure-rate product and oxygen consumption rate decreased to 58 +/- 10 and 74 +/- 4% of their preischemic values, respectively. Phosphocreatine, ATP, and intracellular pH (pHi), measured by (31)P NMR spectroscopy in the infarcted area, decreased to 59 +/- 17, 32 +/- 6%, and 6.7 +/- 0.36 (from 7.05 +/- 0.13), respectively. Serial (87)Rb images were acquired according to both protocols. Rate constants (k x 10(3), min(-1)), relative amount of intracellular Rb(+) (A, %) and relative fluxes (F = kA, %/min) for the An and Pos walls were determined from the images. Before ischemia, F and k were comparable in the Pos and An walls. Ischemia + reperfusion decreased F in the An wall (from 4.4 +/- 0.3 to 1.4 +/- 0.85) due to a decrease in A (20 vs. 73) and increased F in Pos wall (from 3.2 +/- 0.6 to 6.6 +/- 0.23) due to an increase in k (from 42 +/- 3 to 93 +/- 6). The intensities of the Rb images correlated with the Rb(+) content measured in tissue samples. Magn Reson Med 44:83-91, 2000. Published 2000 Wiley-Liss, Inc.

Recent developments in solid-state nuclear magnetic resonance of quadrupolar nuclei and applications to biological systems

Recent advances in nuclear magnetic resonance (NMR) methodology and improvements in high-field NMR instrumentation have generated a new wave of research interests in the application of solid-state NMR to the study of quadrupolar nuclei. These developments now permit increasingly complex biological systems to be probed by quadrupolar NMR. In this review I describe a few recent developments in NMR studies of quadrupolar nuclei and demonstrate the potential of solid-state quadrupolar NMR in the study of biological systems. In particular, I discuss the application of solid-state NMR of (17)O, 67Zn, 59Co, 23Na, and 39K nuclei with a prognosis for future work.

Permeability of squid axon membrane to various ions

The permeability of the squid axon membrane was determined by the use of radioisotopes of Na, K, Ca, Cs, and Br. Effluxes of these isotopes were measured mainly by the method of intracellular injection. Measurements of influxes were carried out under continuous intracellular perfusion with an isotonic solution of potassium sulfate. The Na permeability of the resting (excitable) axonal membrane was found to be roughly equal to the K permeability. The permeability to anion was far smaller than that to cations. It is emphasized that the axonal membrane has properties of a cation exchanger. The physicochemical nature of the "two stable states" of the excitable membrane is discussed on the basis of ion exchange isotherms.

Theoretical basis for sodium and potassium MRI of the human heart at 1.5 T

Knowledge of the extent and location of viable tissue is important to clinical diagnosis. In principle, sodium (23Na) and potassium (39K) MRI could noninvasively provide information about tissue viability. In practice, imaging of these nuclei is difficult because, compared with water protons (1H), 23Na and 39K have lower MR sensitivities (9.2 and 0.051%, respectively), and lower in vivo concentrations (ca. 1000-fold). On the other hand, the relatively short T1 relaxation times of 23Na and 39K (ca. 30 and 10 ms, respectively) suggest that optimized imaging pulse sequences may in part alleviate the weak signal of these nuclei. In this study, numerical simulations of high-speed imaging sequences were developed and used to maximize 23Na and 39K image signal-to-noise ratio (SNR) per unit time within the constraints of existing gradient hardware. The simulation demonstrated that decreasing receiver bandwidth at the expense of echo time (TE) results in a substantial increase in 23Na and 39K image SNR/time despite the short T2 and T2* of these nuclei. Referenced to the available 1H signal on existing 1.5 T scanners, the simulation suggested that it should be possible to acquire three-dimensional 23Na images of the human heart with 7 x 7 x 7 mm resolution and 39K images with 26 x 26 x 26 mm resolution in 30 min. Experimentally in humans at 1.5 T, three-dimensional 23Na images of the heart were acquired in 15 min with 6 x 6 x 12 mm resolution and signal-to-noise ratios of 11 and 7 in the left ventricular cavity and myocardium, respectively, which is very similar to the predicted result. The results demonstrate that by choosing imaging pulse sequence parameters that fully exploit the short relaxation times of 23Na and 39K, potassium MRI is improved but remains impractical, whereas sodium MRI improves to the point where 23Na imaging of the human heart may be clinically feasible on existing 1.5 T scanners.

Effect of metabolic acidosis on the potassium content of bone

Metabolic acidosis induces resorption of cultured bone, resulting in a net efflux of calcium (Ca) from the bone and an apparent loss of mineral potassium (K). However, in these organ cultures, there is diffusion of K between the medium and the crystal lattice, causing difficulty in interpretation of the acid-induced changes in mineral ion composition. To determine the effects of acidosis on bone mineral K, we injected 4-day-old neonatal mice with pure stable isotope 41K, equal to approximately 5% of their total body K. Calvariae were dissected 24 h later and then cultured for 24 h in medium without added 41K, either at pH approximately 7.4 (Ctl) or at pH approximately 7.1 (Ac), with or without the osteoclastic inhibitor calcitonin (3 x 10(-9) M, CT). The bone isotopic ion content was determined with a high-resolution scanning ion microprobe utilizing secondary ion mass spectrometry. 41K is present in nature at 6.7% of total K. The injected 41K raised the ratio of bone 41K/(39K+41K) to 9.8+/-0.5% on the surface (ratios of counts per second of detected secondary ions, mean+/-95% confidence interval) but did not alter the ratio in the interior (6.9+/-0.4%), indicating biological incorporation of the 41K into the mineral surface. The ratios of 41K/40Ca on the surface of Ctl calvariae was 14.4+/-1.2, indicating that bone mineral surface is rich in K compared with Ca. Compared with Ctl, Ac caused a marked increase in the net Ca efflux from bone that was blocked by CT. Ac also induced a marked fall in the ratio of 41K/40Ca on the surface of the calvariae (43+/-0.5, p < 0.01 vs. Ctl), which was partially blocked by CT (8.2+/-0.9, p < 0.01 vs. Ctl and vs. Ac), indicating that Ac causes a greater release of bone mineral K than Ca which is partially blocked by CT. Thus, bone mineral surface is rich in K relative to Ca, acidosis induces a greater release of surface mineral K than Ca, and osteoclastic function is necessary to support the enriched levels of surface mineral K in the presence of acidosis.

Potassium channels of the lamprey erythrocyte membrane exhibit a high selectivity to K+ over Rb+: a comparative study of 86Rb and 41K transport

In order to assess the specificity of potassium channels, we examined the K+ transport pathways in the lamprey red blood cells using 86Rb and stable isotope 41K as tracers measured by the mass spectrometric method. Upon replacing 4 mmol/l K+ with 4 mmol/l Rb+ in the incubation medium, the rate coefficient for unidirectional 86Rb influx (Kin) was significantly reduced from 1.48 +/- 0.10 h-1 to 0.82 +/- 0.05 h-1 (P < 0.001). Addition of 1 mmol/l Ba2+ to the incubation medium significantly decreased 86Rb influx in K(+)- (by 53%) and Rb(+)-media (by 26%). Thus, the reduction in the rate coefficient for 86Rb influx in Rb(+)-medium as compared to K(+)-medium was mainly due to the Ba(2+)-sensitive component (0.23 +/- 0.04 vs. 0.78 +/- 0.10 h-1, P < 0.001). The ouabain-sensitive component of 86Rb influx was also higher in K(+)-(Kin = 0.62 +/- 0.05 h-1) than in Rb(+)-medium (Kin = 0.54 +/- 0.05 h-1, P < 0.05). In the presence of 4 mmol/l 41KCl, the average value of the rate coefficient for the total 41K influx was 8.1 +/- 0.5 h-1. The rate coefficient for 41K influx was reduced to 0.82 +/- 0.13 h-1 in the presence of 1 mmol/l Ba2+. Under the conditions of our assays, the rate of 86Rb uptake via potassium channels of the lamprey red cells was only about 11% in K(+)-medium and only about 3% in Rb(+)-medium compared to 41K influx. These data clearly demonstrate a high selectivity of potassium channels for K+ over Rb+, as already reported for some potassium channels in a variety of cells and tissues.

CHANGES IN THE MEMBRANE PERMEABILITY OF FROG'S SARTORIUS MUSCLE FIBERS IN CA-FREE EDTA SOLUTION

The changes in the membrane permeability to sodium, potassium, and chloride ions as well as the changes in the intracellular concentration of these ions were studied on frog sartorius muscles in Ca-free EDTA solution. It was found that the rate constants for potassium and chloride efflux became almost constant within 10 minutes in the absence of external calcium ions, that for potassium increasing to 1.5 to 2 times normal and that for chloride decreasing about one-half. The sodium influx in Ca-free EDTA solution, between 30 and 40 minutes, was about 4 times that in Ringer's solution. The intracellular sodium and potassium contents did not change appreciably but the intracellular chloride content had increased to about 4 times normal after 40 minutes. By applying the constant field theory to these results, it was concluded that (a) P_Cl did not change appreciably whereas P_K decreased to a level that, in the interval between 10 and 40 minutes, was about one-half normal, (b) P_Na increased until between 30 and 40 minutes it was about 8 times normal. The low value of the membrane potential between 30 and 40 minutes was explained in terms of the changes in the membrane permeability and the intracellular ion concentrations. The mechanism for membrane depolarization in this solution was briefly discussed.

TRACER AND NON-TRACER POTASSIUM FLUXES IN FROG SARTORIUS MUSCLE AND THE KINETICS OF NET POTASSIUM MOVEMENT

Experiments were performed to test the applicability of permeability kinetics to whole frog sartorius muscle using K⁴² ions as tracers of potassium flux. The whole muscle was found to obey closely the kinetic laws expected to hold for single cellular units in which the potassium fluxes are membrane-limited and intracellular mixing is rapid enough not to introduce serious error. In a 5 mM K Ringer's solution, potassium efflux was very nearly equal to influx when the rate constant for K⁴² loss was applied to the whole of the muscle potassium. Over a fairly wide range of external potassium concentration, the assumed unidirectional fluxes measured with tracer K⁴² showed good agreement with net potassium changes determined analytically. The specific activity of potassium lost from labeled muscles to an initially K-free Ringer's solution was measured as a test of the adequacy of intracellular mixing. The results were those expected for a population of cells with uniformly distributed intracellular K⁴². A small deviation was encountered which can be attributed either to a dispersion of fiber sizes in the sartorius or to a possible small additional cellular compartment in each individual fiber. The additional cellular compartment, should it exist, contains from 0.5 to 1 per cent of the muscle potassium. This is evidently not large enough to interfere seriously with the applicability of permeability kinetics to the whole muscle.

POTASSIUM-40 CONTENT AS A BASIS FOR THE CALCULATION OF BODY CELL MASS IN MAN

On the assumption that the potassium content of the body cell mass is constant it should be possible to estimate body cell mass by measuring potassium-40 activity with a whole-body scintillation counter. Relations of body cell mass to weight, lean body mass, and total body water are demonstrated.

CAT HEART MUSCLE IN VITRO. VII. THE TEMPERATURE DEPENDENCE OF STEADY STATE K EXCHANGE IN PRESENCE AND ABSENCE OF NACL

In quiescent cat papillary muscles J_K, the rate of exchange of cellular K with K⁴² in the steady state, has been measured in the presence and absence of NaCl over a wide range of temperatures. J_K was found to be independent of the presence of external NaCl under the steady state conditions investigated. The Arrhenius plot for K exchange was linear over a range of temperatures from 2.5 to 37.5°C in the absence of NaCl, and from 17.5 to 37.5°C in the presence of NaCl. The corresponding apparent activation energies were, respectively, 10,200 and 8,800 calories/mole. J_K in the absence of NaCl was not affected by 10^-5M ouabain. These results are consistent with a passive distribution for the K of heart muscle cells. The observations suggest that a carrier-mediated forced exchange of K for Na does not occur during the steady state in mammalian heart muscle.

INFLUENCE OF LITHIUM IONS ON THE TRANSMEMBRANE POTENTIAL AND CATION CONTENT OF CARDIAC CELLS

The effect of lithium ions on cardiac cells was investigated by recording the changes in transmembrane potential and by following the movement of Li, Na, and K across the cell membrane. Isolated preparations of calf Purkinje fibers and cat ventricular muscles were used. Potentials were measured by intracellular microelectrodes; ion transport was estimated by flame photometric analysis and by using the radioactive isotopes of Na and K. It was shown (a) that Li ions can replace Na ions in the mechanism generating the cardiac action potential but that they also cause a marked depolarization and pronounced changes in action potential configuration; (b) that the resting permeability to Li ions is high and that these ions accumulate in the cell interior as if they were not actively pumped outwards. In Li-Tyrode [K]_i decreases markedly while the K permeability seems to be increased. In a kinetic study of net K and Na fluxes, the outward movement of each ion was found to be proportional to the second power of its intracellular concentration. The effect on the transmembrane potential is explained in terms of changes in ion movement and intracellular ion concentration.

POTASSIUM FLUXES IN DESHEATHED FROG SCIATIC NERVE

Desheathed frog (R. pipiens) sciatic nerves were soaked in Na-deficient solutions, and measurements were made of their Na and K contents and of the movements of K⁴². When a nerve is in Ringer's solution, the Na fluxes are equal to the K fluxes, and about 75 per cent of the K influx is due to active transport. The Na content and the Na efflux are linearly related to the Na concentration of the bathing solution, while the K content and the K fluxes are not so related. When a nerve is in a solution in which 75 per cent of the NaCl has been replaced by choline chloride or sucrose, the active K influx exceeds the active Na efflux, and the K content is maintained. When a nerve is soaked in a solution that contains Li, the K⁴² uptake is inhibited, and the nerve loses K and gains Li. When a Li-loaded nerve recovers in a Li-free solution, K is taken up in exchange for Li. This uptake of K requires Na in the external solution. It is concluded that the active transports of K and of Na may be due to different processes, that an accumulation of K occurs only in exchange for an intracellular cation, which need not be Na, and that Na plays a specific, but unknown, role in K transport.