Potassium transport and nucleoside metabolism in human red cells

Study of Hemolysis During Storage of Blood in the Blood Bank of a Tertiary Health Care Centre

The aim of RBC storage system in a blood bank is to counteract damage to the metabolic machinery and the membrane, to improve post-transfusion viability. In recent years, the need for strict control over the quality of blood has been emphasised. Such quality indicator includes extend of hemolysis and morphological changes of RBC during storage. This study was design to see extend of hemolysis and level of plasma lactate dehydrogenase (LDH) and plasma potassium, during processing and storage at different intervals under blood bank condition. Forty-six donors were selected and blood units were collected and stored under blood bank conditions. Mean plasma haemoglobin of stored blood was estimated by tetra methyl benzidine method (TMB) and percentage hemolysis was calculated on day 0, 1, 7, 21, 28, 35 and 42 days. Similarly plasma LDH and plasma potassium level was also assessed during storage. It was noted that free haemoglobin level and percentage hemolysis progressively increased with storage along with the level of LDH and potassium. However, extend of hemolysis did not exceed the permissible limit of 0.8% up to 42 days of storage. 15 blood bags which showed visual hemolysis on day 28 did not exceeded the threshold of 0.8% hemolysis, when interpreted by TMB method. It was concluded that TMB method is better than visual method for determination of hemolysis. The reduced hemolysis at this centre may be accounted for the use of additive solution SAGM (Saline, Adenine, Glucose, Mannitol) and DEHP (di-2-ethyl hexyl phthalate) as plasticizer in blood bags for storage.

Net influx and efflux of 22Na in erythrocytes from normotensive offspring of patients with essential hypertension

Fresh erythrocytes from 35 young normotensive individuals (20 males), the offspring of patients with essential hypertension (EH), and from 24 normotensive controls (14 males) were incubated at 37 degrees C in an isotonic buffer with 22Na. After 80 min, the net influx (net accumulation) of 22Na was not significantly different in either group, but after 140 min it was significantly higher (8.2%, p less than 0.01) in erythrocytes from male offspring as opposed to male controls. After both 80 and 140 min of incubation, the net influx of 22Na was significantly higher in males than in females in the respective groups. During a 60-minute period the net efflux of 22Na from preincubated (180 min) erythrocytes in 14 males in the offspring group was not significantly different from that found in 10 male controls. These findings could indicate that little differences exists in vitro between offspring and controls in the undirectional fluxes across the membrane. The difference between the male groups in the net influx after 140 min could indicate a higher Na content in the intracellular space in the offspring group, as assumption subsequently verified in a follow-up. The net influx of 22Na after 140 min was significantly correlated (R=0.58, p less than 0.05) to total plasma aldosterone male controls, but not in the offspring. No differences in the mean concentrations of total aldosterone, plasma and urinary electrolytes or in serum gamma glutamyl transferase were noted between offspring and controls. Abnormal cellular handling of sodium at least in the male offspring of patients with EH for at least two generations may be an important predisposing factor for EH.

Physiology of Nucleoside Transporters: Back to the Future

Nucleoside transporters (NTs) are integral membrane proteins responsible for mediating and facilitating the flux of nucleosides and nucleobases across cellular membranes. NTs are also responsible for the uptake of nucleoside analog drugs used in the treatment of cancer and viral infections, and they are the target of certain compounds used in the treatment of some types of cardiovascular disease. The important role of NTs as drug transporters and therapeutic targets has necessarily led to intense interest into their structure and function and the relationship between these proteins and drug efficacy. In contrast, we still know relatively little about the fundamental physiology of NTs. In this review, we discuss various aspects of the physiology of NTs in mammalian systems, particularly noting tissues and cells where there has been little recent research. Our central thesis is reference back to some of the older literature, combined with current findings, will provide direction for future research into NT physiology that will lead to a fuller understanding of the role of these intriguing proteins in the everyday lives of cells, tissues, organs, and whole animals.

Effect of cell ageing on Ca2+ influx into human red cells

The effect of cell ageing on Ca2+ entry was studied in this work, using sub-populations of young and old human red cells, separated by stringent percoll density gradients. Additionally, the influence of an osmotic gradient was investigated as a model for shear stress. Ca2+ entry was assessed at 37 degrees C, under conditions where the Ca2+ pump was either inhibited by NaVO3 (0.5-10 mM) or inactivated by ATP depletion. The entry was linear with time up to 1 h. No differences in Ca2+ influx between the two sub-populations were detected in isotonic Na(+)-medium. In contrast, after incubation in anisosmotic media, Ca2+ entry into old cells was significantly higher than into younger cells. In hypotonic Na(+)-medium, the entry into old cells was not affected by La3+ (10 microM) whilst it was partially blocked by Gd3+ at a similar level (half-maximal effect attained with about 1 microM Gd3+). The entry into young cells was only slightly stimulated by these lanthanides at low concentrations (10 microM), regardless of the tonicity of incubation medium. Further increasing Gd3+ levels above 10 microM markedly enhanced Ca2+ entry into both cell types. The selective blockade of Ca2+ influx by low Gd3+ concentrations suggests presence of mechano-sensitive channels, that become preferentially activated in old cells. Activation of these channels during in-vivo microcirculation may help to explain the increased Ca2+ content of senescent cells.

Differences in Ca2+ pumping activity between sub-populations of human red cells

It has been shown recently that the free Ca2+ content of human red cells rises during ageing in vivo. With the aim of determining the mechanisms involved in such a change, we have investigated some aspects of Ca2+ homeostasis. Both the initial rate of Ca2+ influx and some kinetic parameters of the Ca2+ pump of human red cells were studied in light and dense sub-populations obtained through stringent, self-formed Percoll gradients. At 37 degrees C and pH 7.4, no differences in Ca2+ entry were found. By contrast, either at pH 7.0 or 7.4, the maximal Ca2+ extrusion rate of the approximately 10% heaviest cells was one-half of the corresponding lighter ones. The results demonstrate that the elevated free Ca2+ concentration distinctive of senescent cells, arises from a reduction in Ca2+ extrusion capacity during ageing. The possible physiological significance of this finding is discussed.

Quantitative requirement for ATP for active transport in isolated renal cells

We investigated the quantitative relationship between cellular ATP concentration and Na+-K+-ATPase activity as measured by ouabain-sensitive 86Rb influx in rabbit proximal renal cells. Cellular ATP was reduced in a stepwise manner by rotenone (10(-7) to 10(-5) M) and was increased by 10 mM adenosine. During these maneuvers, ouabain-sensitive 86Rb influx was linearly related to cellular ATP and did not saturate up to 9.9 mM ATP. In contrast, Na+-K+-ATPase activity in membranes prepared from these cells saturated at 2.0 mM ATP at various sodium (10-100 mM) and potassium (4-100 mM) concentrations. Sodium-dependent phosphate uptake and alpha-methylglucoside (alpha-MG) uptake were both inhibited to a similar degree when cellular ATP was reduced. We conclude that 1) the ATP requirement for saturation of Na+-K+-ATPase is higher in intact renal cells than in the membranes, and 2) the uptake of phosphate and alpha-MG are similarly influenced by reduction in ATP. This effect of ATP on phosphate and AMG uptake is most likely an indirect one and is secondary to changes in the sodium gradient across the cell.

Calcium-induced transient potassium efflux in human red blood cells

Human red blood cells pretreated with low-ionic-strength solutions and resuspended in saline respond biphasically to extracellular Ca. At first, addition of Ca causes a large transient K efflux of as much as 600 mM . liter cell H2O-1 . h-1; this is followed by a decrease in K flux below control levels. The first phase (phase I) resembles the Gardos effect in several respects. It is inhibited by oligomycin, by external K, and by increased exposure time to Ca. Further, the K permeability of phase I is similar to that of the Gardos effect (5 X 10(-8)-9 X 10(-8) cm/s), and the cells hyperpolarize in a low-K medium when Ca2+ is added. However, phase I is not identical to the Gardos phenomenon. For example, La, which prevents the Gardos response, is ineffective on phase I. Moreover, external Ba prevents the development of phase I but not the Gardos response, whereas internal Ba prevents the Gardos response. Attempts to demonstrate a Ca leak or pump failure during phase I have failed; passive Ca movements of both treated and normal cells are similar. The results suggest that low-ionic-strength solution exposes Ca-sensitive sites to the external medium; these sites are maintained when the cells are returned to saline.

The effect of intracellular calcium on the sodium pump of human red cells

The inhibitory effect of cytoplasmic Ca on Na-pump-mediated Na-K exchange was investigated in intact red cells under conditions of constant cell volume, membrane potential and inorganic ion composition. The ionized cytoplasmic Ca concentration ( [Ca2+]i) was controlled using the ionophore A23187. In normal cells, ouabain-sensitive 24Na efflux was inhibited with an apparent affinity for [Ca2+]i which depended on the concentration of A23187; 50% inhibition required 20-40 microM and 160-300 microM-cytoplasmic Ca2+ with 10 microM and 0.63 microM-A23187 respectively. Cytoplasmic Ca also affected cell ATP content which fell rapidly on addition of A23187 and subsequently increased, steadied or continued to fall more slowly depending on the Ca and A23187 concentrations. Half-maximal fall required 5-15 microM and 110-170 microM-cytoplasmic Ca2+ at 10 microM and 0.63 microM-A23187 respectively. Removal of Ca from the cells failed to reverse either the Na pump inhibition or the fall in cell ATP. In ATP-enriched cells cytoplasmic Ca caused inhibition of ouabain-sensitive 24Na efflux in an A23187-dependent manner with apparent affinities for [Ca2+]i similar to those observed in the normal cells. Inhibition was complete at high [Ca2+]i. As in the normal cells, the ATP content of the cells fell in the presence of cytoplasmic Ca, but always remained above 1.2 m-mole/l. cells. This was higher than the ATP content of Ca-free normal intact cells. A23187 had no effect on the inhibition by Ca of ouabain-sensitive ATPase activity in isolated red cell membrane preparations. Both under conditions near optimal for Na-K-ATPase activity and under conditions resembling those in the cytoplasm, inhibition was half-maximal at about 25 microM-Ca2+ and in the latter case complete at below 400 microM-Ca2+. The apparent ATP-dependence of ouabain-sensitive Na efflux in the presence of cytoplasmic Ca was distinctly different in the normal and ATP-enriched cells but in both groups of cells it was similar for data obtained with high and low concentrations of A23187. The data for Na pump inhibition by cytoplasmic Ca in the intact cells were well fitted by several kinetic models involving either [Ca2+]i or CaATP as the inhibitory species and a low affinity dependence of pump activity on MgATP or total ATP. However, for any model, the apparent affinities for CaATP or for Ca2+ required to fit the ATPase data were 2.5-10 times higher than those required to fit the data for Na efflux.(ABSTRACT TRUNCATED AT 400 WORDS)

Does tissue ATP content limit active sodium transport across intestinal epithelia in vitro?

The ATP content of isolated epithelia of rabbit descending colon, incubated in oxygenated Ringer solution containing glucose, is increased by addition of 1 mM adenosine from 10.9 +/- 1.4 to 22.2 +/- 2.3 pmoles/mg, but the transport rate of the Na pump is not altered. It is therefore concluded that epithelial ATP synthesis is not rate-limiting for Na transport in this tissue under conditions of in vitro aerobic incubation.

Regulation of the Ca2+-pump by calmodulin in intact cells

ATP-enriched human red cells display high rates of Ca2+-dependent ATP hydrolysis (16 mmol . litre cells-1 . h-1) with a high Ca2+ affinity (K0.5 approximately 0.2 microM). The finding suggests a mechanism for regulation of cell Ca2+ levels, involving highly-cooperative stimulation of active Ca2+ extrusion following binding of calmodulin to the (Ca2+ +Mg2+)-ATPase.

Effects of adenosine and its analogues on the perfused hind limb artery and vein of anaesthetized dogs

1. The effects of infusion of adenosine and its analogues on arterial and venous resistance have been studied in the vascularly and sympathetically isolated hind limb of chloralose-anaesthetized dogs. Resistance changes have been assessed by monitoring changes in perfusion pressures at constant flow through the femoral artery and metatarsal vein.2. With sympathetic stimulation (10 V, 2 msec, 0.5-2 Hz applied to the cut peripheral end of the lumbar sympathetic trunk), continuous infusion of adenosine, to produce a concentration of approximately 2 x 10(-5)m, resulted in a near maximal sustained decrease in arterial perfusion pressure of 35.3+/-3.6%, and a decrease of about half this in venous perfusion pressure.3. Bolus injections of adenosine into the artery, to produce a concentration of about 7 x 10(-5)m, caused a transient decrease in resistance similar to that observed with continuous infusion. However, the venous response was smaller than that induced by continuous infusion.4. Withdrawal of sympathetic stimulation to the limb had little effect on adenosine-induced vasodilatation in the artery, but abolished the small response of the vein.5. Less than 5% vasodilatation was produced in the artery and vein by 2-deoxyadenosine, inosine, guanosine, xanthosine, cytidine or uridine when infused in amounts up to ten times, or by sodium phosphate (pH 7.4) in amounts one hundred times the maximal amounts of adenosine given.6. These results suggest that adenosine caused vasodilatation, at least in arterioles, largely by acting directly on vascular muscle rather than via presynaptic inhibition of noradrenaline release.

Active calcium transport in red cell ghosts resealed in dextran solutions

1. Human erythrocytes when lysed and resealed to Ca in the presence of dextran can be readily separated from the suspending medium by low-speed centrifugation. 2. Ghosts trapped Ca and EGTA at the same ratio as present in the haemolytic medium and remained tight to Ca after washing and subsequent incubation for up to 90 min at 37 degrees C. 3. Ca extrusion could be promoted by substrates other than ATP only from ghosts that had been loaded with low free Ca concentrations (1--22 microM). The order of activation by the various substrates employed was ATP greater than adenine + inosine greater than inosine. 4. The kinetics of extrusion depended markedly on internal free Ca. The system showed a high affinity state (KCa about 3 microM; V = 0.34 mumol Ca/ml ghosts per min) at low concentrations (1--22 microM) and a low affinity state (KCa about 250 microM; V = 0.17 mumol Ca/ml ghosts per min) at high concentrations (0.2--4.0 mM). 5. Both at low and at high free Ca, La-sensitive ATP hydrolysis was closely correlated with La-dependent Ca efflux, in keeping with an stoichiometry of 1.6. The rate of extrusion was maximal in the presence of 160 mM KCl and decreased to various extents when K was fully replaced by different cations, following the order K greater than Na = choline greater than Mg. 7. The efflux rate of high-K ghosts, resealed to alkaline cations, was stimulated by external Na, whilst Mg and choline was practically without effect. 8. The results indicate that human red cells possess a powerful Ca extrusion mechanism, the activity of which can be modulated by alkaline cations.

The magnesium dependence of sodium-pump-mediated sodium-potassium and sodium-sodium exchange in intact human red cells

1. The magnesium content of human red blood cells was controlled by varying the magnesium concentration in the medium in the presence of the ionophore A23187. The new magnesium levels attained were very stable, which allowed the magnesium dependence of the sodium pump to be investigated.2. The effects of magnesium were shown to occur at the inner surface of the red cell membrane for the range of magnesium concentrations tested (10(-7) to 6 x 10(-3)m).3. At intracellular ionized magnesium concentrations below 0.8 mm the activation of ouabain-sensitive sodium-potassium exchange by internal ionized magnesium could be resolved into two or three components: (a) a small component, about 5% of the maximum flux, which is apparently independent of the ionized magnesium concentration below 2 mum, (b) a saturating component with a K((1/2)) of between 30 and 45 mum, and possibly (c) a component which increases linearly with ionized magnesium concentration and which only becomes apparent at concentrations above 0.1 mm.4. At intracellular ionized magnesium concentrations below 0.8 mm, activation of ouabain-sensitive sodium-sodium exchange by internal ionized magnesium could be resolved into two components: (a) a small component, about 6% of the maximal flux, which is apparently independent of the ionized magnesium concentration below 2 mum, and (b) a saturating component with a K((1/2)) of about 9 mum. At ionized magnesium concentrations between about 0.2 and 0.8 mm the rate of sodium-sodium exchange remained constant at the maximal level.5. The intracellular concentration of ATP decreased and the ADP concentration increased as the magnesium content of the cells was reduced from the normal level. A small increase in ATP and a small decrease in ADP was seen when the magnesium content was increased above the normal level. The variation in the ATP: ADP ratio from 2.5 at very low magnesium levels to about 6 at normal magnesium levels can account, at least in part, for the different K((1/2)) values of sodium-potassium and sodium-sodium exchange.6. When the concentration of ionized magnesium was increased above about 0.8 mm both sodium-potassium and sodium-sodium exchange were inhibited. Sodium-sodium exchange was more strongly inhibited than sodium-potassium exchange.7. The possible sites of action of magnesium in the sodium pump cycle are discussed.

Is the Ca2+-sensitive K+ channel under metabolic control in human red cells?

It is widely known that a rise in internal Ca2+ leads to an increased K+ permeability of human red blood cells [1,2,3]. Binding of Ca2+ to some membrane receptors is required for the opening of the K+ channel [4]. This requirement, however, seems to alter after "ageing" red cells in vitro in acid-citrate-dextrose solutions. Thus, the free Ca2+ concentration producing half-maximal effect on K+ permeability ([Ca2+]K+-50) of 4-weeks stored cells is approx. 2.10(-4) M (calculated from ref. 3 using 50% free Ca2+ according to Schatzmann [5]); nearly ten times lower than that reported for fresh cells [6]. This observation suggests the possibility that the K+ channel may become more sensitive to Ca2+ on cold storage. The experiments described below support this idea.

The alteration by ouabain of calcium movements in human red cell ghosts

1. The influence of ouabain on net Ca movements was studied in human erythrocyte ghosts by atomic absorption spectrophotometry. 2. Ouabain (10(-5) M) showed a dual effect, altering both entry and exit of Ca from K-rich ghosts incubated in a high-Na medium in the presence of 10 mM-Ca. 3. Stimulation of Ca entry was observed in the first 15 min at 37 degrees C, whereas during the subsequent 15 min incubation ouabain elicited Ca extrusion. This latter effect was eliminated when the ouabain concentration was raised to 1 mM. 4. Ouabain-dependent Ca movements were abolished by replacing both internal K and external Na with choline. They were also absent from ghosts prepared at a high lytic ratio (1 : 100) or obtained from ATP-depleted cells. 5. A moderate increase in cell ATP enhanced the effect of ouabain on Ca efflux whilst it was eliminated at higher ATP levels. 6. The actions of ouabain markedly depended on the initial ADP/ATP ratio in ghosts, being optimal at about 2-5. 7. The results suggest that the effects of ouabain on Ca movements are mediated through the Na pump. Reversal of this pump in Na-rich K-free medium may provide the energy for active Ca transport.

Erythrocyte ghosts (Na+ + K+) ATPase activity in Duchenne's dystrophy and myotonia

In Duchenne muscular dystrophy the activity of (Na+ + K+)ATPase in erythrocyte ghosts is reduced and its reaction to ouabain is paradoxical both in low sodium and high sodium systems. No such changes were seen in a case of Becker dystrophy, in limb-girdle dystrophy, and in neurogenic atrophy of muscles. In myotonic dystrophy and congenital myotonia the activity of ATPase and its inhibition by ouabain were depressed.

Inosine-stimulated insulin release and metabolism of inosine in isolated mouse pancreatic islets

Inosine is a potent primary stimulus of insulin secretion from isolated mouse islets. The inosine-induced insulin secretion was totally depressed during starvation, but was completely restored by the addition of 5 mM-caffeine to the medium and partially restored by the addition of 5 mM-glucose. Mannoheptulose (3 mg/ml) potentiated the effect of 10 mM-inosine in islets from fed mice. The mechanism of the stimulatory effect of inosine was further investigated, and it was demonstrated that pancreatic islets contain a nucleoside phosphorylase capable of converting inosine into hypoxanthine and ribose 1-phosphate. Inosine at 10 mM concentration increased the lactate production and the content of ATP, glucose 6-phosphate (fructose 1,6-diphosphate + triose phosphates) and cyclic AMP in islets from fed mice. In islets from starved mice inosine-induced lactate production was decreased and no change in the concentration of cyclic AMP could be demonstrated, whereas the concentration of ATP and glucose 6-phosphate rose. Inosine (10 mM) induced a higher concentration of (fructose 1,6-diphosphate + triose phosphates) in islets from starved mice than in islets from fed mice suggesting that in starvation the activities of glyceraldehyde 3-phosphate dehydrogenase or other enzymes below this step in glycolysis are decreased. Formation of glucose from inosine was negligible. Inosine had no direct effect on adenylate cyclase activity in islet homogenates. The observed changes in insulin secretion and islet metabolism mimic what is seen when glucose and glyceraldehyde stimulate insulin secretion, and as neither ribose nor hypoxanthine-stimulated insulin release, the results are interpreted as supporting the substrate-site hypothesis for glucose-induced insulin secretion according to which glucose has to be metabolized in the beta-cells before secretion is initiated.

Clinical and experimental studies on adenine, various nucleosides and their analogs in hematology

In red blood cells as well as in platelets there appears to be a decrease in adenine nucleotides during storage under blood bank conditions. This can be decreased by use of anticoagulant preservatives with higher phosphate content than the standard ACD solution, through the addition of adenine and inosine. Maintenance of higher ATP levels appears to be related to longer circulating life span after transfusion into patients in the case of red blood cells but not platelets. Inosine and more alkaline preservative medium also contribute to the maintenance of 2,3-DPG levels in red blood cells, and with it to the maintenance of normal hemoglobin dissociation curves and thus oxygen-carrying capacity. Certain nucleoside analogs may contribute to the preservation of platelets and of whole blood by their platelet-aggregation inhibitory activity. Platelet-aggregation inhibitors may also be useful in preventing thromboembolic episodes with potentially greater safety than anticoagulants.

The influence of the chloride gradient across red cell membranes on sodium and potassium movements

1. A study has been made to see whether active and passive movements of sodium and potassium in human red blood cells are influenced by changing the chloride gradient and hence the potential difference across the cell membrane.2. Chloride distribution was measured between red cells and isotonic solutions with a range of concentrations of chloride and non-penetrating anions (EDTA, citrate, gluconate). The cell chloride concentration was greater than that outside with low external chloride, suggesting that the sign of the membrane potential was reversed. The chloride ratio (internal/external) was approximately equal to the inverse of the hydrogen ion ratio at normal and low external chloride, and inversely proportional to external pH. These results show that chloride is passively distributed, making it valid to calculate the membrane potential from the chloride ratio.3. Ouabain-sensitive (pump) potassium influx and sodium efflux were decreased by not more than 20 and 40% respectively on reversing the chloride gradient, corresponding to a change in membrane potential from -9 to +30 mV. In contrast, passive (ouabain-insensitive) movements were reversibly altered - potassium influx was decreased about 60% and potassium efflux was increased some tenfold. Sodium influx was unaffected by the nature of the anion and depended only on the external sodium concentration, whereas ouabain-insensitive sodium efflux was increased about threefold. When external sodium was replaced by potassium there was a decrease in ouabain-insensitive sodium efflux with normal chloride, but an increase in low-chloride medium.4. Net movements of sodium and potassium were roughly in accord with the unidirectional fluxes.5. The results suggest that reversing the chloride gradient and, therefore, the sign of the membrane potential, had little effect on the sodium pump, but caused a marked increase in passive outward movements of both sodium and potassium ions.

The control by internal calcium of membrane permeability to sodium and potassium

1. A study has been made of the relationship between the concentration of internal calcium and the permeability of human red cell membranes to sodium and potassium.2. Fresh red cells contain very little calcium, but after being depleted of ATP by ageing they took up calcium from Ringer solution. The entry was unaffected by external sodium and potassium but was markedly pH dependent. When supplied with energy, calcium-loaded cells actively extruded calcium by a saturable process which was also unaffected by the distribution of sodium and potassium across the membranes. The activity of the calcium pump was sufficient to maintain the low internal concentration found under physiological conditions.3. Raising internal calcium in metabolically poor cells caused a loss of cell potassium which was greater than the concomitant sodium gain. These changes were reversed when ATP was supplied. External calcium had no effect. The increase in permeability to sodium and potassium was enhanced by the simultaneous addition of fluoride, and, even more so, of iodoacetate. These inhibitors had no effect on membrane permeability unless calcium was also present. Inosine potentiated the action of fluoride and iodoacetate in causing potassium loss, by allowing more calcium to enter the cells.4. The results suggest that the permeability of human red cell membranes to sodium and potassium is regulated by internal calcium, which in turn is controlled by a calcium pump that utilizes ATP.

Potassium fluxes in dialyzed squid axons

Measurements have been made of K influx in squid giant axons under internal solute control by dialysis. With [ATP](i) = 1 microM, [Na](i) = 0, K influx was 6 +/- 0.6 pmole/cm(2) sec; an increase to [ATP](i) = 4 mM gave an influx of 8 +/- 0.5 pmole/cm(2) sec, while [ATP](i) 4, [Na](i) 80 gave a K influx of 19 +/- 0.7 pmole/cm(2) sec (all measurements at approximately 16 degrees C). Strophanthidin (10 microM) in seawater quantitatively abolished the ATP-dependent increase in K influx. The concentration dependence of ATP-dependent K influx on [ATP](i), [Na](i), and [K](o) was measured; an [ATP](i) of 30 microM gave a K influx about half that at physiological concentrations (2-3 mM). About 7 mM [Na](i) yielded half the K influx found at 80 mM [Na](i). The ATP-dependent K influx responded linearly to [K](o) from 1-20 mM and was independent of whether Na, Li, or choline was the principal cation of seawater. Substances tested as possible energy sources for the K pump were acetyl phosphate, phosphoarginine, PEP, and d-ATP. None was effective except d-ATP and this substance gave 70% of the maximal flux only when phosphoarginine or PEP was also present.

Some aspects of adenosine triphosphate synthesis from adenine and adenosine in human red blood cells

1. The synthesis of ATP has been studied in human erythrocytes. Fresh cells showed no net synthesis of ATP when incubated with adenine or adenosine, although labelled adenine was incorporated into ATP in small amounts.2. Cold-stored cells (3-6 weeks old) became progressively depleted of adenine nucleotides but incubation with adenosine or adenine plus inosine restored the ATP concentration to normal within 4 hr. Incorporation of labelled adenine or adenosine into the ATP of incubated stored cells corresponded to net ATP synthesis by these cells.3. Synthesis of ATP from adenosine plus adenine together was 75% derived from adenine and only 25% from adenosine, indicating that nucleotide synthesis from adenine inhibits the simultaneous synthesis of nucleotide from adenosine.

Sodium fluxes in internally dialyzed squid axons

The effects which alterations in the concentrations of internal sodium and high energy phosphate compounds had on the sodium influx and efflux of internally dialyzed squid axons were examined. Nine naturally occurring high energy phosphate compounds were ineffective in supporting significant sodium extrusion. These compounds were: AcP, PEP, G-3-P, ADP, AMP, GTP, CTP, PA, and UTP.(1) the compound d-ATP supported 25-50% of the normal sodium extrusion, while ATP supported 80-100%. The relation between internal ATP and sodium efflux was nonlinear, rising most steeply in the range 1 to 10 microM and more gradually in the range 10 to 10,000 microM. There was no evidence of saturation of efflux even at internal ATP concentrations of 10,000 microM. The relation between internal sodium and sodium efflux was linear in the range 2 to 240 mM. The presence of external strophanthidin (10 microM) changed the sodium efflux to about 8-12 pmoles/cm(2) sec regardless of the initial level of efflux; this changed level was not altered by subsequent dialysis with large concentrations of ATP. Sodium influx was reduced about 50 % by removal of either ATP or Na and about 70 % by removing both ATP and Na from inside the axon.

Facftors affecting the relative magnitudes of the sodium:potassium and sodium:sodium exchanges catalysed by the sodium pump

1. The effects of external potassium on sodium: potassium exchange and sodium: sodium exchange in human red cells have been estimated from measurements of ouabain-sensitive potassium influx and ouabain-sensitive sodium influx in media containing different concentrations of potassium.2. As the external potassium concentration is increased from zero to 5 mM, sodium:sodium exchange-as judged by ouabain-sensitive sodium influx-is progressively suppressed, and sodium:potassium exchange-as judged by ouabain-sensitive potassium influx-is progressively increased. Both exchanges are half-maximal between 1 and 2 mM-K, and at 5 mM-K sodium: sodium exchange becomes very small as sodium: potassium exchange approaches a maximum.3. Experiments have been carried out, mainly on resealed ghosts, to determine what factors affect the magnitude of the sodium:sodium exchange in potassium-free solutions.4. Sodium:sodium exchange does not occur in the absence of adenosine triphosphate (ATP).5. Ghosts containing high concentrations of sodium, no potassium and high concentrations of ATP show no ouabain-sensitive loss of sodium into potassium-free solutions. The ability to carry out sodium:sodium exchange can be restored by replacing most of the internal sodium with potassium or by preparing the cells so that they contain much more orthophosphate (P(i)) than ATP.6. Ghosts containing sodium in low concentration, potassium in high concentration and with a low [ATP]/([ADP].[P(i)]) ratio show a greater ouabain-sensitive loss of sodium into potassium-free media than into media containing potassium; i.e. external potassium reduces ouabain-sensitive sodium efflux.7. The effect of P(i) is not the result of competitive inhibition of the transport ATPase since P(i) at the concentrations used does not inhibit ATPase activity in fragmented ghosts.