Temperature adaptation of active sodium-potassium transport and of passive permeability in erythrocytes of ground squirrels

Renal Mitochondrial Response to Low Temperature in Non-Hibernating and Hibernating Species

SIGNIFICANCE

Therapeutic hypothermia is commonly applied to limit ischemic injury in organ transplantation, during cardiac and brain surgery and after cardiopulmonary resuscitation. In these procedures, the kidneys are particularly at risk for ischemia/reperfusion injury (IRI), likely due to their high rate of metabolism. Although hypothermia mitigates ischemic kidney injury, it is not a panacea. Residual mitochondrial failure is believed to be a key event triggering loss of cellular homeostasis, and potentially cell death. Subsequent rewarming generates large amounts of reactive oxygen species that aggravate organ injury. Recent Advances: Hibernators are able to withstand periods of profoundly reduced metabolism and body temperature ("torpor"), interspersed by brief periods of rewarming ("arousal") without signs of organ injury. Specific adaptations allow maintenance of mitochondrial homeostasis, limit oxidative stress, and protect against cell death. These adaptations consist of active suppression of mitochondrial function and upregulation of anti-oxidant enzymes and anti-apoptotic pathways.

CRITICAL ISSUES

Unraveling the precise molecular mechanisms that allow hibernators to cycle through torpor and arousal without precipitating organ injury may translate into novel pharmacological approaches to limit IRI in patients.

FUTURE DIRECTIONS

Although the precise signaling routes involved in natural hibernation are not yet fully understood, torpor-like hypothermic states with increased resistance to ischemia/reperfusion can be induced pharmacologically by 5'-adenosine monophosphate (5'-AMP), adenosine, and hydrogen sulfide (H₂S) in non-hibernators. In this review, we compare the molecular effects of hypothermia in non-hibernators with natural and pharmacologically induced torpor, to delineate how safe and reversible metabolic suppression may provide resistance to renal IRI. Antioxid. Redox Signal. 27, 599-617.

A Novel Stopped-Flow Assay for Quantitating Carbonic-Anhydrase Activity and Assessing Red-Blood-Cell Hemolysis

We report a novel carbonic-anhydrase (CA) assay and its use for quantitating red-blood-cell (RBC) lysis during stopped-flow (SF) experiments. We combine two saline solutions, one containing HEPES/pH 7.03 and the other, ~1% CO₂/44 mM [Formula: see text]/pH 8.41, to generate an out-of-equilibrium CO₂/[Formula: see text] solution containing ~0.5% CO₂/22 [Formula: see text]/pH ~7.25 (10°C) in the SF reaction cell. CA catalyzes relaxation of extracellular pH to ~7.50: [Formula: see text] + H⁺ → CO₂ + H₂O. Proof-of-concept studies (no intact RBCs) show that the pH-relaxation rate constant (k_ΔpH)-measured via pyranine fluorescence-rises linearly with increases in [bovine CAII] or [murine-RBC lysate]. The y-intercept (no CA) was k_ΔpH = 0.0183 s^-1. Combining increasing amounts of murine-RBC lysate with ostensibly intact RBCs (pre-SF hemolysis ≅0.4%)-fixing total [hemoglobin] at 2.5 μM in the reaction cell to simulate hemolysis from ostensibly 0 to 100%-causes k_ΔpH to increase linearly. This y-intercept (0% lysate/100% ostensibly intact RBCs) was k_ΔpH = 0.0820 s^-1, and the maximal k_ΔpH (100% lysate/0% intact RBCs) was 1.304 s^-1. Thus, mean percent hemolysis in the reaction cell was ~4.9%. Phenol-red absorbance assays yield indistinguishable results. The increase from 0.4 to 4.9% presumably reflects mechanical RBC disruption during rapid mixing. In all fluorescence studies, the CA blocker acetazolamide reduces k_ΔpH to near-uncatalyzed values, implying that all CA activity is extracellular. Our lysis assay is simple, sensitive, and precise, and will be valuable for correcting for effects of lysis in physiological SF experiments. The underlying CA assay, applied to blood plasma, tissue-culture media, and organ perfusates could assess lysis in a variety of applications.

Biological processing of dinuclear ruthenium complexes in eukaryotic cells

The biological processing - mechanism of cellular uptake, effects on the cytoplasmic and mitochondrial membranes, intracellular sites of localisation and induction of reactive oxygen species - of two dinuclear polypyridylruthenium(ii) complexes has been examined in three eukaryotic cells lines. Flow cytometry was used to determine the uptake of [{Ru(phen)2}2{μ-bb12}](4+) (Rubb12) and [Ru(phen)2(μ-bb7)Ru(tpy)Cl](3+) {Rubb7-Cl, where phen = 1,10-phenanthroline, tpy = 2,2':6',2''-terpyridine and bbn = bis[4(4'-methyl-2,2'-bipyridyl)]-1,n-alkane} in baby hamster kidney (BHK), human embryonic kidney (HEK-293) and liver carcinoma (HepG2) cell lines. The results demonstrated that the major uptake mechanism for Rubb12 and Rubb7-Cl was active transport, although with a significant contribution from carrier-assisted diffusion for Rubb12 and passive diffusion for Rubb7-Cl. Flow cytometry coupled with Annexin V/TO-PRO-3 double-staining was used to compare cell death by membrane damage or apoptosis. Rubb12 induced significant direct membrane damage, particularly with HepG2 cells, while Rubb7-Cl caused considerably less membrane damage but induced greater levels of apoptosis. Confocal microscopy, coupled with JC-1 assays, demonstrated that Rubb12 depolarises the mitochondrial membrane, whereas Rubb7-Cl had a much smaller affect. Cellular localisation experiments indicated that Rubb12 did not accumulate in the mitochondria, whereas significant mitochondrial accumulation was observed for Rubb7-Cl. The effect of Rubb12 and Rubb7-Cl on intracellular superoxide dismutase activity showed that the ruthenium complexes could induce cell death via a reactive oxygen species-mediated pathway. The results of this study demonstrate that Rubb12 predominantly kills eukaryotic cells by damaging the cytoplasmic membrane. As this dinuclear ruthenium complex has been previously shown to exhibit greater toxicity towards bacteria than eukaryotic cells, the results of the present study suggest that metal-based cationic oligomers can achieve selective toxicity against bacteria, despite exhibiting a non-specific membrane damage mechanism of action.

Substrate-specific changes in mitochondrial respiration in skeletal and cardiac muscle of hibernating thirteen-lined ground squirrels

During torpor, the metabolic rate (MR) of thirteen-lined ground squirrels (Ictidomys tridecemlineatus) is considerably lower relative to euthermia, resulting in part from temperature-independent mitochondrial metabolic suppression in liver and skeletal muscle, which together account for ~40% of basal MR. Although heart accounts for very little (<0.5%) of basal MR, in the present study, we showed that respiration rates were decreased up to 60% during torpor in both subsarcolemmal (SS) and intermyofibrillar (IM) mitochondria from cardiac muscle. We further demonstrated pronounced seasonal (summer vs. winter [i.e., interbout] euthermia) changes in respiration rates in both mitochondrial subpopulations in this tissue, consistent with a shift in fuel use away from carbohydrates and proteins and towards fatty acids and ketones. By contrast, these seasonal changes in respiration rates were not observed in either SS or IM mitochondria isolated from hind limb skeletal muscle. Both populations of skeletal muscle mitochondria, however, did exhibit metabolic suppression during torpor, and this suppression was 2- to 3-fold greater in IM mitochondria, which provide ATP for Ca(2+)- and myosin ATPases, the activities of which are likely quite low in skeletal muscle during torpor because animals are immobile. Finally, these changes in mitochondrial respiration rates were still evident when standardized to citrate synthase activity rather than to total mitochondrial protein.

Mammalian cell injury induced by hypothermia- the emerging role for reactive oxygen species

Hypothermia is a well-known strategem to protect biological material against injurious or degradative processes and is widely used in experimental and especially in clinical applications. However, hypothermia has also proved to be strongly injurious to a variety of cell types. Hypothermic injury to mammalian cells has long been attributed predominantly to disturbances of cellular ion homeostasis, especially of sodium homeostasis. For many years, reactive oxygen species have hardly been considered in the pathogenesis of hypothermic injury to mammalian cells. In recent years, however, increasing evidence for a role of reactive oxygen species in hypothermic injury to these cells has accumulated. Today there seems to be little doubt that reactive oxygen species decisively contribute to hypothermic injury in diverse mammalian cells. In some cell types, such as liver and kidney cells, they even appear to play the central role in hypothermic injury, outruling by far a contribution of the cellular ion homeostasis. In these cells, the cellular chelatable, redox-active iron pool appears to be decisively involved in the pathogenesis of hypothermic injury and of cold-induced apoptosis that occurs upon rewarming of the cells after a (sublethal) period of cold incubation.

Cell volume and ionic transport systems after cold preservation of coronary endothelial cells

BACKGROUND

Hypothermia-induced changes in cell volume and ionic transport systems of coronary endothelial cells may play a role in the development of coronary artery disease in cardiac transplant recipients.

METHODS

Coronary endothelial cells were incubated in University of Wisconsin solution or culture control medium for up to 48 hours at 4 degrees C. Parallel control cultures were incubated at 37 degrees C. Na/K-ATPase and Na/K/Cl cotransport activities were determined as ouabain- and furosemide-sensitive 86Rb+ uptake, respectively. Cell volume changes and cell death were analyzed by a FACScan flow cytometer and the release of lactate dehydrogenase, respectively.

RESULTS

Coronary endothelial cells stored in University of Wisconsin solution up to 6 hours showed an increased Na/K-ATPase activity compared to control cells, whereas no changes were observed in Na/K/Cl cotransport activity or cell volume. Long-term preservation (24 and 48 hours) was associated with a partial loss of cell viability, as demonstrated by lactate dehydrogenase release, and dramatic alterations in ionic transport system activities.

CONCLUSIONS

University of Wisconsin solution seems to prevent coronary endothelial cells Na/K/Cl cotransport activity changes during cold preservation, which could alter cell volume regulation and cause cell injury.

A role for sodium in hypoxic but not in hypothermic injury to hepatocytes and LLC-PK1 cells

BACKGROUND

Hypothermia is considered to be responsible for sodium influx during cold hypoxic incubation. However, we have previously shown that hypothermia alone leads to a pronounced decrease in cellular sodium content when liver endothelial cells or hepatocytes are incubated under such conditions. In the research described here, we therefore studied the effects of hypothermia and hypoxia, alone or combined, on cellular sodium homeostasis and assessed the role sodium plays in the pathogenesis of hypoxic and hypothermic injury to cultured liver and kidney cells.

METHODS

Isolated hepatocytes and LLC-PK1 cells were incubated in Krebs-Henseleit buffer or a sodium-free modification thereof under normoxic and hypoxic conditions at 4 degrees C as well as at 37 degrees C. Cytosolic sodium concentration was determined in isolated hepatocytes under both warm and cold conditions using digital fluorescence microscopy and the Na+-sensitive dye sodium-binding benzofuran isophthalate.

RESULTS

When hepatocytes were incubated under cold normoxic conditions the cellular sodium concentration decreased. However, it increased strongly under hypoxic conditions at 4 degrees C and at 37 degrees C. When either hepatocytes or LLC-PK1 cells were incubated under hypoxic conditions at 4 degrees C or 37 degrees C, sodium-free medium provided protection. In contrast, sodium-free medium did not alleviate the hypothermic injury observed when cells were incubated under cold normoxia.

CONCLUSIONS

The sodium influx observed during cold hypoxia is triggered by hypoxia and not by hypothermia. Sodium plays a prominent role in hypoxic injury to cultured liver and kidney cells, although hypothermic injury of these cells is independent of sodium homeostasis.

Temperature effects on ion transport across the erythrocyte membrane of the frog Rana temporaria

Unidirectional K+ and Na+ influxes in the frog erythrocytes incubated in Cl- or NO(3)- media with 2.7 mM K+ were measured using 86Rb and 22Na as tracers. K+ influx was inhibited by 35-55% in the presence of 0.2-1.0 mM furosemide but it was unaffected by 0.1-0.2 mM bumetanide. Furosemide at a concentration of 0.5 mM had no effect on K+ loss from the frog red cells incubated in a nominally K(+)-free medium. Together with our previous studies the data support the existence of K-Cl cotransport and the absence of Na-K-2Cl cotransport in the frog erythrocyte membrane. Cell cooling from 20 to 5 degrees C caused a decrease in K+ influx and K+ efflux via the K-Cl cotransporter (3.2- and 3.7-fold, respectively) giving an apparent energy of activation (EA) of about 60 kJ/mol and Q10 value of 2.5. Only small decline (approximately 30%) in the ouabain-sensitive K+ influx was found as temperature was changed from 20 to 5-10 degrees C. Low values of Q10 (approximately 1.5) and EA (27.3 kJ/mol) were obtained for passive K+ influx in the frog erythrocytes (ouabain-insensitive in NO(3)- medium) at temperature within 5-20 degrees C. However, the temperature coefficients were greater for passive Na+ influx and passive K+ efflux (Q10 approximately 2.4-2.5 and EA approximately 56-58 kJ/mol). The temperature dependence of all ion transport components displayed discontinuities showing no changes at temperature between 5 and 10 degrees C. Thus, cooling of the frog red cells is associated with a greater decrease of Na+ influx and K+ efflux than passive and active K+ influx. These data indicate that the preservation of a relative high activity of the Na,K-pump during cell cooling and also the temperature-induced changes in the K-Cl cotransport activity and ion passive diffusion contribute to maintenance of ion concentration gradients in the frog erythrocytes at decreased temperature.

Rapid decrease in cellular sodium and chloride content during cold incubation of cultured liver endothelial cells and hepatocytes

Hypothermia, as used for organ preservation in transplantation medicine, is generally supposed to lead to an intracellular accumulation of sodium, and subsequently of chloride, via inhibition of the Na+/K+-ATPase. However, on studying the cellular sodium concentration of cultured liver endothelial cells using fluorescence microscopy, we found a 55% decrease in the cellular sodium concentration after 30 min of cold incubation in University of Wisconsin (UW) solution. To confirm this surprising result, we set up a capillary electrophoresis method that allowed us to determine the cellular contents of inorganic cations and of inorganic anions. Using this method we measured a decrease in the cellular sodium content from 104+/-11 to 55+/-4 nmol/mg of protein, accompanied by a decrease in the chloride content from 71+/-9 to 25+/-5 nmol/mg of protein, after 30 min of cold incubation in UW solution. When the endothelial cells were incubated in cold Krebs-Henseleit buffer or in cold cell culture medium instead of UW solution, similar early decreases in cellular sodium and chloride contents were observed, thus excluding the possibility of the decreases being dependent on the preservation solution used. Furthermore, experiments with cultured rat hepatocytes yielded a similar decrease in sodium content during initiation of cold incubation in UW solution, so the decrease does not appear to be cell-specific either. These results suggest that, contrary to current opinion, sodium efflux predominates over sodium influx during the early phase of cold incubation of cells.

Elevating intracellular free Mg2+ preserves sensitivity of Na(+)-K+ pump to ATP at reduced temperatures in guinea pig red blood cells

Red cells of hibernating species have a higher relative rate of Na(+)-K+ pump activity at low temperature than the red cells of a mammal with a typical sensitivity to cold. The kinetics of ATP stimulation of the Na(+)-K+ pump were determined in guinea pig and ground squirrel red cells at different temperatures between 5 and 37 degrees C by measuring ouabain-sensitive K+ influx at different levels of ATP. In guinea pig cells, elevation of intracellular free Mg2+ to 2 mmol.1-1 by use of the divalent cation ionophore A23187 caused the apparent affinity of the pump for ATP to increase with cooling to 20 degrees C, rather than to decrease, as occurs in cells not loaded with Mg2+. In ground squirrel cells raising intracellular free Mg2+ had little effect on apparent affinity of the pump for ATP at 20 degrees C. ATP affinity rose slightly with cooling both in Mg(2+)-enriched and in control ground squirrel cells. Increased intracellular free Mg2+ in guinea pig cells stimulated Na(+)-K+ pump activity so that at 20 degrees C the pump rate was the same in the Mg(2+)-enriched guinea pig and control ground squirrel cells. Pump activity in Mg(2+)-enriched guinea pig cells at 5 degrees C was significantly improved but still lower than pump activity in control cells from ground squirrel. Thus, loss of affinity of the Na(+)-K+ pump for ATP that occurs with cooling in cold-sensitive guinea pig red cells can be, at least partially, prevented by elevating cytoplasmic free Mg2+. Conversely, in ground squirrel red cells natural rise of free Mg2+ may in part account for the preservation of the ATP affinity of their Na(+)-K+ pump with cooling.

A comparison of effect of temperature on phosphorus metabolites, pH and Mg2+ in human and ground squirrel red cells

1. 31P NMR spectra were obtained at temperatures ranging from 2 to 30 degrees C from freshly drawn human (cold-sensitive) and ground squirrel (cold-tolerant) red cells. The concentration of ATP was also determined by luciferin-luciferase assay over the same temperature range. 2. The concentration of ATP as determined by NMR or by the luciferin-luciferase assay did not change with temperature in either species. The absolute concentration of ATP in human cells determined by NMR was not significantly different from the total ATP determined enzymatically. 3. The concentration of 2,3-diphosphoglycerate was higher and that of pyridine nucleotides lower in human than in ground squirrel red cells. This species difference was independent of temperature. 4. Intracellular pH, as determined from the positions of the NMR peaks of 2- and 3-phosphates of diphosphoglycerate, became more alkaline as the temperature was lowered. 5. Free intracellular magnesium, determined from the difference in the positions of the peaks for alpha- and beta-phosphorus of ATP, increased in the ground squirrel red cells and decreased in the human red cells with cooling from 30 to 2 degrees C. Total magnesium, as determined by atomic emission spectroscopy, did not change with temperature in red cells of either species. 6. The intensities of all phosphorus metabolite signals from the ground squirrel cells increased with decreasing temperature, while those from the human cells were unaffected. Since chemical shift anisotropy in the presence of magnesium is a powerful spin-lattice relaxation mechanism for phosphates, this is additional evidence for the temperature dependence of free magnesium concentration in the ground squirrel cells. 7. We conclude that there is no difference in phosphorus metabolites or intracellular pH which could account for the differential cold sensitivity in human and ground squirrel red cells. We suggest that, in the cold-tolerant red cells from the ground squirrel, magnesium is released from binding sites as the temperature is lowered. The change in free intracellular Mg2+ may account at least in part for the unusually low temperature sensitivity of the Na(+)-K+ pump in the red cells of this species.

The effects of short term cold storage upon ATP and 2,3-BPG levels in the blood of euthermic and hibernating thirteen-lined ground squirrels Spermophilus tridecemlineatus

1. Whole blood was collected from hibernating and euthermic ground squirrels. 2. Blood from hibernating animals had approximately 50% less ATP and 2,3-biphosphoglycerate (2,3-BPG) than did that from euthermic animals. 3. In euthermic blood subjected to in vitro cold storage, levels of ATP decreased to those observed in hibernators, but 2,3-BPG did not decline significantly. 4. During cold storage of hibernator blood, no significant changes occurred in levels of these organophosphates.

ATP dependence of Na(+)-K+ pump of cold-sensitive and cold-tolerant mammalian red blood cells

1. The ATP concentration of intact, cold-tolerant (ground squirrel) red cells and cold-sensitive (guinea-pig and human) red cells was monitored by use of the firefly tail, luciferin-luciferase assay. ATP kinetics of the pump in intact red blood cells was investigated by altering cell [ATP] by progressive depletion of ATP in the presence of 2-deoxy-D-glucose and then by measurement of ouabain-sensitive K+ influx at each level of [ATP] at various temperatures between 37 and 5 degrees C. Na(+)-K(+)-ATPase activity of broken membranes was also determined in parallel experiments using ouabain-sensitive release of 32P from [gamma-32P]ATP as a measure of activity. 2. Without depletion, there is no immediate decrease in [ATP] of intact cold-sensitive cells at low temperature (5 degrees C) at times when there are marked differences in the activities of the Na(+)-K+ pump of cold-tolerant and cold-sensitive cells. 3. At 37 degrees C Na(+)-K(+)-ATPase of all three species exhibited two components of ATP dependence at 37 degrees C, one with high velocity, low affinity, the other with low velocity, high affinity. Affinities of both components rose with cooling. 4. A similar, two component pattern was observed in intact guinea-pig and human red cells at 37 degrees C, except that the segment corresponding to the high affinity component had an apparent Km (Michaelis-Menten constant) 3- to 4-fold higher than that of the broken membrane preparation. 5. Cooling intact guinea-pig and human red cells decreased the apparent affinity of the high velocity, low affinity component for ATP, so that at 20 degrees C the value of Km approached or exceeded the levels of physiological ATP concentration. Below 20 degrees C only one component with values corresponding to that of the low velocity, high affinity component could be observed. 6. In intact ground squirrel cells only the low affinity, high velocity component was apparent between 37 and 5 degrees C. Its affinity for ATP rose with cooling between 37 and 5 degrees C.

Diversities of transport of sodium in rodent red cells

1. Na-K pumps of rodent red cells reveal variations among species in terms of kinetic properties such as ouabain sensitivity, Na/K coupling and temperature sensitivity and variations within an individual organism related to such physiological challenges as K deficiency, calorie deficiency and seasonal changes in temperature. 2. Passive Na entry among rodents collectively occurs through the same routes as in red cells of other mammals, but red cells of hamsters, rats and thirteen-lined ground squirrels lack or are deficient in an amiloride-sensitive, shrinkage-activated Na-H exchange. 3. In guinea-pig this pathway appears to be both activated and uncoupled by cooling from 37 to 20 degrees C. 4. Red cells of rodents in general and hamsters in particular are rich in a Na-Mg exchange pathway. In hamsters, this appears to be the only amiloride-sensitive pathway in simple media. 5. In hamster cells, Na entry through the amiloride-sensitive Mg-activated pathway exhibits the same kinetics as previously shown for Na activation of Mg extrusion.

Maintenance of cation gradients in cold-stored erythrocytes of guinea pig and ground squirrel: improvement by amiloride

Red blood cells of ground squirrel, a hibernator, gain Na at one-third the rate of guinea pig red blood cells when stored in saline medium at 5 degrees C for several days. This result correlates with the known slower loss of K during storage in ground squirrel cells. In ground squirrel cells Na gain is balanced by K loss, so that there is no net gain of solute; in guinea pig cells the total cation content rises progressively. Amiloride, a drug which inhibits Na entry, retards Na uptake in cells of both species. Surprisingly, amiloride also slowed K loss and, in guinea pig red cells, the decline of ATP content. In guinea pig cells amiloride reduced the gain of total cation by half. The results substantiate the difference in cold sensitivity of ion regulation of red blood cells of these two species and demonstrate the possible usefulness of amiloride-type drugs in nonfreezing preservation of red blood cells.

Membrane function in mammalian hibernation

For homeotherms the maintenance of a high, uniform body temperature requires a constant energy supply and food intake. For many small mammals, the loss of heat in winter exceeds energy supply, particularly when food is scarce. To survive, some animals have developed a capacity for adaptive hypothermia in which they lower their body temperature to a new regulatory set-point, usually a few degrees above the ambient. This process, generally known as hibernation, reduces the temperature differential, metabolic activity, as well as the energy demand, and thus facilitates survival during winter. Successful hibernation in mammals requires that the enzymatic processes are regulated in such a manner that metabolic balance is maintained at both the high body temperature of the summer-active animal (37 degrees C) and the low body temperature of the winter-torpid animal (approx. 5 degrees C). This means that the cellular membranes have thermal properties capable of maintaining a balanced metabolism at these extreme physiological temperatures. The available evidence indicates that, for some tissues, preparation for hibernation involves an alteration in the lipid composition and thermal properties of cellular membranes. Marked differences in the thermal response of cellular membranes have been observed on a seasonal basis and, in some membranes, differences in lipid composition have been associated with the torpid state. However, to date, no consistent changes in lipid composition which would account for, or explain, the changes in membrane thermal response, have been detected. An important point to emphasize is that the process of 'homeoviscous adaptation', which occurs in procaryotes and some poikilotherms during acclimation to low temperatures, is not a characteristic feature of most membranes of mammalian hibernators.

Differential effects of temperature on three components of passive permeability to potassium in rodent red cells

The effect of temperature on ouabain-insensitive fluxes of K+ was characterized in red cells from a non-hibernator (guinea-pig) and a hibernator (thirteen-lined ground squirrel). The residual K+ influx which remains in the presence of ouabain and bumetanide, and which is linearly dependent on [K+]o was the same in the erythrocytes of the two species at low temperature (5 degrees C). At 5 degrees C co-transport of K+ was abolished in guinea-pig red cells but was still present in ground squirrel red cells. In guinea-pig cells, ouabain-and-bumetanide-insensitive K+ flux was increased by Ca2+ at low temperatures. This flux was inhibited by quinine and selective for K+ over Na+, indicating activation of the Ca2+-sensitive K+ pathway (Gárdos channel). Ouabain-and-bumetanide-insensitive K+ permeability in red cells from the ground squirrel was insensitive to Ca2+ added to the medium at low temperature. When ground squirrel red cells were depleted of ATP or treated with A23187, Ca2+ induced a flux which was inhibitable by quinine. Hence, ground squirrel red cells possess Gárdos channels. The temperature sensitivity of the K+ channels was assessed using A23187-mediated K+ influx as a measure of Gárdos channel activation. The influence of temperature on the Ca2+-stimulated K+ fluxes under these conditions was indistinguishable between the two species. It is concluded that K+ loss through the Ca2+-sensitive K+ channel is minimal in hibernators' erythrocytes because of more efficient regulation of cytoplasmic Ca2+ during cold storage.

Temperature dependence of active K+ transport in cation dimorphic sheep erythrocytes

Arrhenius diagrams of K+ pump fluxes measured between 15 degrees C and 41 degrees C were discontinuous in high K+ but not in low K+ sheep red cells. Exposure of low K+ cells to anti-L caused a bimodal temperature response of K+ pump flux with a transition temperature, Tc, similar to that found in high K+ cells but with comparatively higher activation energies above Tc.

Potassium transport and lipid composition in mammalian red blood cell membranes

Potassium influxes in red cells from eight species have been found to follow exponential relationship with membrane phosphatidylcholine and sphingomyelin content. This relationship with membrane phosphatidylcholine and sphingomyelin content. This relationship with membrane phospholipid patterns was found to exist with both ouabain sensitive and insensitive fraction of potassium transport. When published values of chloride and phosphate permeabilities were compared with potassium permeabilities, correlations were found in seven out of nine of the species studied. On the basis of these findings it appears that potassium, phosphate, and chloride permeabilities in red blood cells of most species are related to the membrane phosphatidylcholine and sphingomyelin content; that is, membrane permeabilities increase with increasing amounts of phosphatidylcholine and decrease with increasing amounts of sphingomyelin. These results indicate that the membrane lipid is an important factor in transport processes in mammalian red blood cells.

Cultured cells from renal cortex of hibernators and nonhibernators. Regulation of cell K+ at low temperature

Cells were grown as primary monolayer cultures from kidney cortex of guinea pigs (nonhibernators), hamsters and ground squirrels (both hibernating species). When plates of cells were placed at 5 degrees C, cells of guinea pigs lost 37% of their K+ in 2 h and those of the hibernator lost about 10%. Uptake of 42K into the cells exhibited a simple, single exponential time course at both temperatures. Unidirectional efflux of K+ was equal to K+ influx in all cultures at 37 degrees C and, within limits of error, in hibernator cells at 5 degrees C. Efflux was 3-to 5-fold greater than influx in guinea pig cells at 5 degrees C. After 2 h in the cold the ouabain sensitive K+ influx remaining (7-15% of that at 37 degrees C) was about the same in the cells of the 3 species. Cells from active hamsters and from hibernating ground squirrels, however, exhibited significantly greater pump activity after 45 min in the cold (19 and 14%, respectively). The stimulation of K+ influx by increasing [K+] did not show an increase in Km+ at 5 degrees C in cells of guinea pigs and ground squirrels. Lowering [K+]c and/or raising [Na+]c by treatment in low- and high-K+ media caused only slight stimulation of K+ influx, except in cells of ground squirrels at 5 degrees C in which the stimulation was at least 11-times greater than at 37 degrees C or in cells of guinea pigs at either temperature. This altered kinetic response of K+ transport to cytoplasmic ion stimulation with cooling accounted for about one-third of the improved regulation of K+ at 5 degrees C in ground squirrel cells; the other two-thirds was attributable to a greater decrease in K+ leak with cooling. The inhibition of active transport by cold in all 3 species was much less severe than that previously seen in any (Na++K+)-ATPase of mammalian cells.

Protein-liposome interactions and their relevance to the structure and function of cell membranes

Recent studies on the interactions of soluble proteins, membrane proteins and enzymes with phospholipid model membranes are reviewed. Similarities between the properties of such systems and the behavior of biomembranes, such as alterations in the redox potential of cytochrome c after binding to membranes and effects of phospholipid fluidity on (Na+K) ATPase activity, are emphasized. The degree of correspondence between the behavior of model systems and natural membranes encourages the continuing use of model membranes in studies on protein-lipid interactions. However, some of the data on the increase of surface pressure of phospholipid monolayers by proteins and increases in the permeability of liposomes indicate that many soluble proteins also have a capability to interact hydrophobically with phospholipids. Thus a sharp distinction between both peripheral and integral membrane proteins and non-membrane proteins are not seen by these techniques. Cautious use of such studies, however, should lead to greater understanding of the molecular basis of cell membrane structure and function in normal and pathological states. Studies implicating protein-lipid interactions and (Na+K) ATPase activity in membrane alterations in disease states are also briefly discussed.