Ouabain binding and cation transport in human erythrocytes

In the present studies we have explored the relation between ouabain binding and the inhibition of potassium influx in intact human erythrocytes. The rate at which bound ouabain molecules dissociate from the erythrocyte membrane is not altered by complete replacement of choline with sodium or by partial replacement with potassium. These findings indicate that the effects of these cations on ouabain binding reflect alterations in the rate of association of ouabain molecules with the erythrocyte membrane. Variations in the cation composition of the incubation solution did not alter the relation between the fraction of the glycosidebinding sites occupied by ouabain or the fraction of ouabain-sensitive potassium influx which was inhibited. That is, irrespective of the affinity of the erythrocyte membrane for ouabain molecules and irrespective of the magnitude of glycoside-sensitive potassium influx, occupation of a given fraction of the glycoside-binding sites by ouabain results in the inhibition of an equal fraction of the ouabain-sensitive potassium transport sites.

Efflux of potassium from isolated rod outer segments: a photic effect

1. Illumination of the isolated outer segments of rod photoreceptors loaded with (42)K or (86)Rb reduces the efflux of these ions.2. During the perfusion of the isolated rod outer segments with a solution containing only 2.36 mM-Na the effect of light is absent, and the amplitude of the photic effect is linearly related to the logarithm of the extracellular Na concentration.3. In darkness, raising the concentration of K in the fluid of perfusion gives an increase of the efflux of (86)Rb and increasing the extracellular concentration of Ca yields a retention. The efflux of (86)Rb and (42)K is greater in darkness when sucrose or choline substitute for Na.4. It is suggested that in darkness the isolated outer segments are permeable both to Na and to K. Light appears to decrease the permeability for Na ions. There is no evidence that the permeability for K ions is modified by light.

Nuclear magnetic resonance studies of sodium and potassium in etiolated pea stem

Based on nuclear magnetic resonance (NMR) spectroscopy and other evidence, it has been argued that tissues accumulate, and retain, ions in a binding process by a highly structured water-protoplasm system; thus active membrane transport need not be involved. Recent evidence has accounted for the loss of resonance intensity usually found when investigating quadrupolar ions in animal tissue. Using continuous wave NMR spectroscopy, we have examined two quadrupolar ions, Na(+) and K(+), in pea stem cells where about 90% of the ion content is in the largely aqueous vacuoles having a membrane barrier. The NMR resonances from these ions correspond to almost 100% of that expected from independent measurements of total ion content. This indicates that the ions are retained as free ions after accumulation. The small fraction which is NMR invisible may represent ions in an ordered, anisotropic environment, such as that in the wall or cytoplasm.

Effects of digoxin-specific antibodies on accumulation and binding of digoxin by human erythrocytes

THE PRESENT STUDIES INDICATE THAT ACCUMULATION OF DIGOXIN BY INTACT HUMAN ERYTHROCYTES IS THE RESULT OF TWO PROCESSES: binding of digoxin to the erythrocyte membrane and uptake of digoxin across the membrane into the cell. In contrast, accumulation of ouabain by human erythrocytes is entirely attributable to binding of this glycoside to the plasma membrane. Digoxin binding to the erythrocyte membrane involves a single class of binding sites, is a saturable function of the extracellular digoxin concentration, reversible, temperature-sensitive, dependent on the cation composition of the incubation medium, inhibited by other cardioactive steroids, and correlates with the inhibition of erythrocyte potassium influx. Digoxin uptake across the membrane into the cell is also temperature-sensitive and reversible but is a linear function of the extracellular digoxin concentration, not altered by changes in the cation composition of the incubation medium, not inhibited by other cardioactive steroids, and does not correlate with inhibition of erythrocyte potassium influx. Digoxinspecific antibodies can both prevent and reverse effects of digoxin on potassium influx in human erythrocytes by virtue of the capacity of the antibodies to decrease the amount of digoxin that is bound to the erythrocyte membrane. These antibodies also reduce uptake of digoxin across the plasma membrane into the erythrocyte; however, this portion of cellular digoxin is not responsible for the observed inhibition of potassium influx. In the presence of digoxin-specific antibodies, the changes in digoxin binding to the erythrocyte membrane and in digoxin uptake across the membrane into the cell reflect the ability of the antibodies to form complexes with "free" digoxin molecules in the incubation medium and thereby decrease the effective concentration of digoxin.

Mobility of potassium ion in frog muscle cells, both living and dead

The diffusion coefficient of potassium ion (DK) in frog muscles was studied by a new method. In normal cytoplasm DK averaged one-eighth of the value in the free solution. Arrest of metabolism or injury caused an increase in DK. In muscles killed with iodoacetate, DK rose to three-quarters of the value in free solution. The data support the association-induction hypothesis.

Kinetics of potassium transport across single distal tubules of rat kidney

1. The transport of potassium across the distal tubular epithelium was studied in vivo in rats on a normal potassium intake and in rats in which distal tubular potassium secretion was either stimulated by potassium loading or the I.V. administration of a 5% sodium bicarbonate solution or in which potassium secretion was suppressed by dietary deprivation of potassium or sodium.2. (42)K was used to measure unidirectional fluxes across the luminal and peritubular cell membranes and to assess the magnitude of cellular potassium partaking in the transport process. This was accomplished by the simultaneous perfusion of the peritubular capillary network with (42)K-Ringer and of the distal tubular lumen with initially tracer-free solution. From the steady-state flux and the time course of tracer washout into the lumen after discontinuing the peritubular perfusion, unidirectional fluxes, rate coefficients of ion transfer and cellular transport pools could be measured.3. Transepithelial movement of potassium involves mixing with a variable cellular potassium transport pool. The latter is significantly elevated in conditions of enhanced distal tubular potassium secretion; cellular potassium labelling is reduced in conditions in which potassium secretion has been suppressed by potassium deprivation.4. Evidence is presented that changes in the peritubular transport pattern are primarily responsible for modifications of potassium translocation. Thus, stimulation of potassium secretion is associated with increased peritubular potassium uptake; a reduced potassium uptake across the peritubular cell membrane accounts for the fall in potassium secretion in potassium-depleted animals. Whereas passive entry of potassium across the peritubular membrane is augmented in potassium-loaded animals, the induction of metabolic alkalosis by the administration of 5% sodium bicarbonate stimulates active potassium uptake across the peritubular cell membrane. Sodium deprivation stimulates active reabsorptive transfer of potassium from the tubular lumen.

Effects of various secretagogues upon 42K and 22NA uptake during in vitro hormone release from the rat adenohypophysis

1. The in vitro uptake of (22)Na and (42)K was measured simultaneously in rat adenohypophyses during hormone release produced by several secretagogues and during inhibition of hormone release in Ca-free media.2. Intracellular adenohypophysial [Na(+)] and [K(+)] changed only slightly when the uptake changed. This would indicate that relative permeability changes were the primary effect of the treatments.3. The uptake of (42)K was increased by elevated external [K(+)], but was unaffected by the presence or absence of Ca(2+). Acid extracts of hypothalamus-stalk-median eminence or cerebellum also increased the (42)K uptake.4. The uptake of (22)Na or (24)Na was decreased by elevated [K(+)]. Uptake was increased in Ca-free Krebs-Ringer bicarbonate; but was unaltered when [K(+)] was concurrently increased.5. Neither purified growth hormone releasing hormone, synthetic lysine-vasopressin, dibutyryl cyclic AMP nor theophylline had an effect on the uptake of either K(+) or Na(+).6. The rapid uptake of (22)Na and its smaller volume of distribution compared to absolute measurements of intracellular [Na(+)] suggest that the plasma membrane of adenohypophysial cells is relatively impermeable to Na(+).7. We conclude that changes in the uptake of Na(+) and K(+) associated with hormone release are incidental to the release process.8. Hormone release produced by elevated external [K(+)] is most likely due to a non-specific increase in permeability of the cell membranes, facilitating Ca(2+) entry into the cytoplasm.9. The results suggest that the low resting transmembrane potentials of adenohypophysial cells may be due to their conjoint relatively high permeability to both K(+) and Ca(2+), rather than K(+) and Na(+).

Sodium and potassium flux rates in normal human leucocytes in an artificial extracellular fluid

1. Sodium and potassium efflux and influx rates were studied in normal human leucocytes in an artificial extracellular fluid using radioactive isotopes.

2. The rate constant for sodium efflux was 4.2 h−1 corresponding to a sodium efflux rate of 487 mmol kg cell dry weight−1 h−1. Approximately three-quarters of this flux was ouabain sensitive.

3. Potassium influx was 346 mmol kg cell dry weight−1 h−1. Approximately two-thirds of the potassium influx was ouabain insensitive.

Evidence for K+ and Cl- binding inside muscle from diffusion studies

Intrafiber diffusion of 42K+, 36Cl−, and 14C-sorbitol was measured along the longitudinal axis of a single muscle fiber which had been placed inside the lumen of a glass capillary at least 24 h beforehand. The mean diffusion coefficients (× 10−5 cm2/s) in myoplasm at 10 °C and at pH 7.5 were 0.728 ± 0.008, 0.683 ± 0.006, and 0.216 ± 0.005 for K+, Cl−, and sorbitol, respectively. The K+ coefficient decreased, the Cl− coefficient increased, and the sorbitol coefficient remained unchanged as the pH of the muscle-capillary preparation was increased. By applying Wang's theory to explain diffusion in polyelectrolyte solutions (1954), we have estimated the diffusible volume (1 − ϕ) and the binding fractions (ƒK and ƒCl) of K+ and Cl− in myoplasm. From pH 5.2 to 10, ƒK varied from 0 to 0.13 and ƒCl varied from 0.13 to 0. Analysis of this K+ diffusion data along with the Na+ diffusion data from an earlier study (Can. J. Physiol. Pharmacol. 50, 228–237, 1972) leads to the prediction that myoplasm at physiological pH has a minimum binding capacity for Na+ and K+ of about 70 mmol/kg dry weight and a selectivity of 3.3 for Na+ over K+. Estimations of the diffusible volume ranged from 0.7 to 0.8, indicating that probably all the intrafiber water (74–78% by weight) is being utilized in the diffusion process.

Ionic transport and membrane potential of rat liver cells in normal and low-chloride solutions

1. The ouabain-sensitive component of Na efflux and K influx amount to 58 and 72% respectively. Taking into account also (a) the diffusional passive fluxes of Na (5%) and K (13%), as estimated by Ussing's equation and (b) the ouabain-insensitive Na-Na exchange (28%), 85% (K) and 90% (Na) of the measured total fluxes can be accounted for.2. Na efflux is diminished when K is partially or totally removed from the medium. This effect is reversible, indicating probably activation of the Na pump by external K.3. The coupling ratio of Na and K ouabain-sensitive fluxes is equal to 1.58, suggesting that three Na ions are removed from and two K ions are carried into the cell in one cycle of the pump. Hence, in liver cell membranes, the Na pump must be electrogenic.4. A tenfold decrease in [Cl](o) by substitution with an impermeant anion results in a membrane hyperpolarization and a decrease in [Cl](i). Cl loss from the liver is compensated by an equivalent loss of intracellular K to preserve electroneutrality.5. The measurement of passive fluxes indicates that Cl removal from the perfusing solutions increases P(K) but does not alter P(Na).6. Addition of ouabain brings about a depolarization which is three times greater in low-Cl solutions (21.9 mV) than in normal-Cl solutions (6.8 mV).7. It is concluded that hyperpolarization which develops when Cl ions are removed can be accounted for entirely by (a) the increase in P(K), (b) the increase of the contribution of the electrogenic pump to membrane potential.

Nitroglycerin and heterogeneity of myocardial blood flow. Reduced subendocardial blood flow and ventricular contractile force

The effects of both intracoronary and intravenous administration of nitroglycerin on transmural distribution of blood flow in the left ventricle after partial coronary artery occlusion was investigated using two independent methods. In 16 open chest, anesthetized dogs, tubing supplying the cannulated left coronary artery was partially occluded. Strain gauges sutured paralled to superficial and deep fibers of the myocardium separately recorded the contractile force of each layer. With occlusion set so that depression of the deep contractile force was imminent. 12 mug intracoronary nitroglycerin in seven dogs depressed only the deep contractile force without changing systemic hemodynamics. Intravenous administration of 180 mug nitroglycerin in nine dogs resulted in a decrease of deep contractile force and aortic pressure often associated with an increase in superficial contractile force. Distribution of myocardial blood flow during peak coronary flow after intracoronary administration of nitroglycerin or during a decrease in aortic pressure after intravenous nitroglycerin administration was determined by the tissue uptake of an intracoronary bolus of rubidium-(80). This was compared with the uptake of potassium-(42) injected before nitroglycerin. Intravenous or intracoronary administration of nitroglycerin caused a significant reduction in subendocardial blood flow with a decrease in the subendocardial/subepicardial ratio of isotope. These experiments suggest that under conditions of acute partial coronary occlusion, the autoregulatory response results in more fully dilated subendocardial vessels causing them to be less responsive to nitroglycerin. Nitroglycerin may then reduce the vascular resistance in the subepicardial more than the subendocardial vessels, resulting in a "steal" of blood flow from deep to superficial myocardium.

Relationship between tubular net sodium reabsorption and peritubular potassium uptake in the perfused Necturus kidney

1. K influx from peritubular space into renal tubular cells, varphi(i) (K), was measured in doubly perfused Necturus kidneys by studying tissue uptake of (42)K added exclusively to the portal circulation. Concomitantly, net tubular Na reabsorption, varphi(n) (Na), was measured by clearance techniques. varphi(n) (Na) and varphi(i) (K) were varied widely by replacing solutions of physiological composition (controls) with solutions containing high K, low K, low Na, cyclamate instead of Cl, ouabain (10(-7)-10(-4)M) or ethacrynic acid (10(-5)-10(-4)M).2. The ratio of varphi(n) (Na) to varphi(i) (K) was found to vary with the experimental conditions, the control value of about 2 was maintained over a threefold variation in absolute Na reabsorption. This ratio increased with low K or ouabain to values near 4. With high K, ethacrynic acid, low Na or cyclamate the relationship was one or lower. Thus, net Na reabsorption can be uncoupled from peritubular K influx.3. These results can be best explained if there are two Na pumps working in parallel: pump A transporting Na (with Cl) and pump B, a Na-for-K-exchange pump. The ratio of Na efflux to K influx could approach infinity if only pump A works (if B is inhibited) and could approach one if only B works. It should vary between these limits in controls when both pumps are active, or when neither of the two pumps is completely inhibited.4. Alternatively, the experimental findings could be explained by a Na pump with a coupling ratio that varies within two extreme values, from high Na-K ratios (with Na reabsorption at, or near, control values but with very low K influx values) to low ratios (with normal K influx values but with low Na reabsorption values).

Ion distribution and the role of calcium in cellular function. Ion distribution and ion movements in smooth muscle

The steady-state fluxes of K + , Cl - and Na + from smooth muscles in normal Krebs solution are described, and some of the problems encountered in the interpretation of such results are discussed. Sodium fluxes are particularly difficult to analyse, and the type of model used to estimate sodium permeability from flux curves, determines the value calculated to a large extent. In order to simplify the ionic distribution in the tissue, in the hope of obtaining more information about the handling of Na and K by the guinea-pig taenia coli, potassium-free (high Na) tissues and sodium-free (high K) tissues were prepared by soaking for 4 h in the relevant solution. The tissues reach a steady state, and the effluxes of 42 K from high K tissues and 24 Na from high Na tissues were measured and compared. In these two conditions the electrochemical gradients for Na + and for K + are probably identical. Nevertheless, the fluxes are markedly different. At 37 °C the Na flux is much faster than the K flux, and a part of the Na exchange is very temperature sensitive. There is also some evidence for a sodium calcium exchange mechanism. It is hoped that further experiments will produce results that will help to understand the processes involved in these exchanges.

The effect of amiloride on sodium and potassium fluxes in red cells

1. The effect of amiloride on the influx and efflux of (24)Na and (42)K in red cells was studied. The drug was added to the bathing Ringer or else incorporated in resealed ghosts.2. Amiloride does not inhibit the active or the passive (ouabain insensitive) extrusion of (24)Na.3. Amiloride inhibits the influx of (24)Na into red cells by 70%.4. Whether added to the inside or to the outside of the cells amiloride has no effect on the efflux of (42)K.5. Amiloride does not modify the uptake of (42)K from control Ringer. This uptake is strongly inhibited by the removal of Na. Amiloride has no effect on the extent of this inhibition.6. It is concluded that amiloride specifically inhibits the passive penetration of Na, and has no effect on the Na-K-pumping mechanism. However, at the concentration which inhibits 70% of the influx, amiloride fails to produce an observable effect on the ouabain-insensitive Na-efflux.7. On the basis that the information obtained could be extrapolated to other membranes, the effect of amiloride on epithelial membranes is discussed.

Identification of ischemic area of left ventricle by visualization of 43 K myocardial deposition

A new method to determine myocardial blood distribution was used to study a patient with a recent large transmural anterior myocardial infarct. The method consisted of injecting 43 potassium into a peripheral arm vein and recording the radioactivity over the chest with a scintillation (Anger) camera. Elimination of noncardiac radioactivity in the analysis of data was accomplished by recording the rapid passage of injected 99m technetium through the heart. The cardiac deposition of the potassium was visualized by identifying the contour of the heart from the technetium scan. In this patient deposition of 43 potassium was markedly reduced in the septal region of the myocardium. This correlated well with the ischemic area suggested by coronary angiography, which demonstrated proximal occlusion of the left anterior descending coronary artery and absence of obstructive disease in the other coronary arteries. Intracoronary artery injection of 43 K gave confirmation of decreased deposition in the same area. This method should prove helpful in the study of certain clinical problems, such as the evaluation of the patency of coronary artery bypass grafts.