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

Effect of radiologic contrast media on cell volume regulatory mechanisms in human red blood cells

RATIONALE AND OBJECTIVES

The authors performed this study to evaluate cell volume regulation in human red blood cells (RBCs) after incubation in solutions of three contrast media: iohexol (830 mOsm), ioxaglate (520 mOsm), and iodixanol (300 mOsm).

MATERIALS AND METHODS

Whole blood sampled from six healthy subjects was exposed to Ringer solutions containing 25% or 5% vol/vol iohexol (final osmolality, 440 or 340 mOsm, respectively), ioxaglate (final osmolality, 395 or 335 mOsm, respectively), iodixanol (final osmolality, 330 or 315 mOsm, respectively), or NaCl (control solutions with the same osmolality as that of the contrast media). In some experiments, control RBCs were subjected to a hyposmotic solution (100 mOsm). RBC volumes were obtained with a Coulter counter.

RESULTS

The RBCs showed normal regulatory cell shrinkage after hyposmotically induced swelling. All 25% vol/vol contrast material solutions and their control solutions induced RBC shrinkage (range, 6% +/- 1 [standard error] to 22% +/- 3). The same was true for cells exposed to 5% vol/vol contrast material (range, 4% +/- 1 to 7% +/- 1). The shrinkage phase was followed by cell swelling (10% +/- 2 to 20% +/- 2 for 25% contrast material and their control solutions and 8% +/- 1 to 15% +/- 2 for 5% contrast material and their control solutions). No contrast material-exposed RBCs increased their volumes to the level reached with their control solutions.

CONCLUSION

RBCs exposed to hyperosmotic iohexol, ioxaglate, or iodixanol solutions shrank and then swelled. The degree of shrinkage and subsequent swelling could not be explained simply with the osmolality of the test solutions. Physicochemical properties of the contrast media must be involved, putatively affecting electrolyte fluxes over the RBC membrane. Possible targets of these effects are the K+/Cl- symporter, K+ channels, and the Na+/K+/Cl- symporter.

Metabolic depression and Na+/K+ gradients in the aestivating Australian goldfields frog, Neobatrachus wilsmorei

During aestivation the metabolic rate of the Australian goldfields frog Neobatrachus wilsmorei was reduced by 80% from its standard metabolic rate. The in vitro rate of oxygen consumption of isolated muscle and skin from aestivating frogs was up to 50% lower than that of the non-aestivating frogs. This in vitro rate of oxygen consumption was maintained for 6-12 h, indicating an intrinsic metabolic depression of tissues during aestivation. Frogs became dehydrated during aestivation. Muscle, skin and liver also became dehydrated during aestivation, but brain and kidney did not. Na+ and K+ contents and extracellular space measurement for muscle indicated that ion gradients were maintained across the muscle cell membrane during aestivation. Increases in plasma concentrations of Na+ and K+ were matched with similar increases in muscle intracellular ion concentrations. Extracellular space measurements were unsuccessful in the other tissues, but K+ content in all tissues (per dry weight) was maintained during aestivation, and the concentration of plasma K+ did not increase above that which can be accounted for by dehydration, indicating that K+ gradients were maintained.

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.

Cold activation of Na influx through the Na-H exchange pathway in guinea pig red cells

Previous work showed that amiloride partially inhibits the net gain of Na in cold-stored red cells of guinea pig and that the proportion of unidirectional Na influx sensitive to amiloride increases dramatically with cooling. This study shows that at 37 degrees C amiloride-sensitive (AS) Na influx in guinea pig red blood cells is activated by cytoplasmic H+, hypertonic incubation, phorbol ester in the presence of extracellular Ca2+ and is correlated with cation-dependent H+ loss from acidified cells. Cytoplasmic acidification increases AS Na efflux into Na-free medium. These properties are consistent with the presence of a Na-H exchanger with a H+ regulatory site. Elevation of cytoplasmic free Mg2+ above 3 mM greatly increases AS Na influx: this correlates with a Na-dependent loss of Mg2+, indicating the presence of a Na-Mg exchanger. At 20 degrees C activators of Na-H exchange have little or no further stimulatory effect on the already elevated AS Na influx. AS Na influx is much larger than either Na-dependent H+ loss or AS Na efflux at 20 degrees C. The affinity of the AS Na influx for cytoplasmic H+ is greater at 20 degrees C than at 37 degrees C. Depletion of cytoplasmic Mg2+ does not abolish the high AS Na influx at 20 degrees C. Thus, elevation of AS Na influx with cooling appears to be due to increased activity of a Na-H exchanger (operating in a "slippage" mode) caused by greater sensitivity to H+ at a regulatory site.

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.

Seasonal changes in cation transport in red blood cells of grey squirrel (Sciurus carolinensis) in relation to thermogenesis and cellular adaptation to cold

1. Unidirectional influx of 42K was measured in red cells of grey squirrels at seasonal intervals over two years. 2. Na/K pump-related (i.e. ouabain-sensitive) K influx at 37 degrees C was maximal in cells collected in January and was more than three times greater than cells collected in summer. Na/K pump activity, maximized by loading the cells with Na, exhibited a similar difference. 3. At 5 degrees C in fresh cells, ouabain-sensitive K influx, expressed as per cent of that at 37 degrees C, was highest in March. In Na-loaded cells it was lowest in summer. 4. Passive "leak" K influx (i.e., the residual influx remaining in presence of ouabain and bumetanide) was highest in October, and declined progressively to the summer months, when it was only 27% of that in October. 5. Cotransport (i.e., bumetanide-sensitive K influx) exhibited the same seasonal pattern as Na/K pump activity in fresh cells. 6. Net gain of Na in cells stored at 5 degrees C for three days in March was less than half of that in January or summer. 7. High transport activity in January may correlate with a requirement for increased non-shivering thermogenesis. However, red cells of grey squirrels exhibit maximum resistance to low temperature in March and at this time resemble the red cells of hibernating mammals.