Acute and chronic influence of temperature on red blood cell anion exchange

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.

The permeability of red blood cells to chloride, urea and water

This study extends permeability (P) data on chloride, urea and water in red blood cells (RBC), and concludes that the urea transporter (UT-B) does not transport water. P of chick, duck, Amphiuma means, dog and human RBC to (36)Cl(-), (14)C-urea and (3)H2O was determined under self-exchange conditions. At 25°C and pH 7.2-7.5, PCl is 0.94 × 10(-4)-2.15 × 10(-4) cm s(-1) for all RBC species at [Cl]=127-150 mmol l(-1). In chick and duck RBC, P(urea) is 0.84 × 10(-6) and 1.65 × 10(-6) cm s(-1), respectively, at [urea]=1-500 mmol l(-1). In Amphiuma, dog and human RBC, P(urea) is concentration dependent (1-1000 mmol l(-1), Michaelis-Menten-like kinetics; K1/2;=127, 173 and 345 mmol l(-1)), and values at [urea]=1 mmol l(-1) are 29.5 × 10(-6), 467 × 10(-6) and 260 × 10(-6) cm s(-1), respectively. Diffusional water permeability, Pd, was 0.84 × 10(-3) (chick), 5.95 × 10(-3) (duck), 0.39 × 10(-3) (Amphiuma), 3.13 × 10(-3) (dog) and 2.35 × 10(-3) cm s(-1) (human). DIDS, DNDS and phloretin inhibit PCl by >99% in all RBC species. PCMBS, PCMB and phloretin inhibit P(urea) by >99% in Amphiuma, dog and human RBC, but not in chick and duck RBC. PCMBS and PCMB inhibit Pd in duck, dog and human RBC, but not in chick and Amphiuma RBC. Temperature dependence, as measured by apparent activation energy, EA, of PCl is 117.8 (duck), 74.9 (Amphiuma) and 89.6 kJ mol(-1) (dog). The EA of P(urea) is 69.6 (duck) and 53.3 kJ mol(-1) (Amphiuma), and that of Pd is 34.9 (duck) and 32.1 kJ mol(-1) (Amphiuma). The present and previous RBC studies indicate that anion (AE1), urea (UT-B) and water (AQP1) transporters only transport chloride (all species), water (duck, dog, human) and urea (Amphiuma, dog, human), respectively. Water does not share UT-B with urea, and the solute transport is not coupled under physiological conditions.

Sulfate influx on band 3 protein of equine erythrocyte membrane (Equus caballus) using different experimental temperatures and buffer solutions

The aim of this study was to assess the anion transport in equine erythrocytes through the measurement of the sulfate uptake operating from band 3 using different experimental temperatures and buffer solutions. Blood samples of six clinically healthy horses were collected via jugular vein puncture, and an emochrome-citometric examination was performed. The blood was divided into four aliquots and by centrifugation and aspiration the plasma and buffy coat were carefully discarded. The red blood cells were washed with an isosmotic medium and centrifuged. The obtained cell suspensions were incubated with two different experimental buffer solutions (buffer A: 115 mM Na2SO4, 10 mM NaCl, 20 mM ethylenediaminetetraacetic acid, 30 mM glucose; and buffer B: 115 mM Na2SO4, 10 mM NaCl, 20 mM ethylenediaminetetraacetic acid, 30 mM MgCl2) in a water bath for 1 h at 25 °C and 37 °C. Normal erythrocytes, suspended at 3% hematocrit, were used to measure the SO4= influx by absorption spectrophotometry at 425 nm wavelength. Unpaired Student's t-test showed a statistically significant decrease (P < 0.01) of rate constants in equine erythrocytes at 25 °C versus 37 °C using both experimental buffer solutions. Comparing the buffer A with buffer B unpaired Student's t-test showed statistically lower values (P < 0.0001) for A solution versus B solution both at 25 °C and at 37 °C. The greater inhibition of SO4 (=) influx measured in equine erythrocytes indicates the increased formation of the sulfydryl bonds in band 3 and the modulation of the sulfydryl groups, culminating in the conformational changes in band 3.

Respiratory properties of blood in the harbor porpoise, Phocoena phocoena

Harbor porpoises are active divers that exchange O2 and CO2 with the environment during a fast single breath upon surfacing. We investigated blood O2-transporting properties, buffer characteristics, Cl– transport via the erythrocyte anion exchanger (AE1), circulating nitric oxide metabolites and hemoglobin nitrite reduction in harbor porpoises with the aim to evaluate traits that are adaptive for diving behavior. Blood O2 affinity was higher in harbor porpoises than in similar sized terrestrial mammals, as supported by our parallel recordings of O2 equilibria in sheep and pig blood. Further, O2 affinity tended to increase with increasing body mass. A high O2 affinity favors O2 extraction from the lungs, but a normal Bohr effect (ΔlogP50/ΔpH=–0.46) gradually lowers O2 affinity during dives (where CO2 accumulates) to assist O2 off-loading to perfused tissues. The true plasma non-bicarbonate buffer value was moderately higher than in terrestrial mammals and increased upon deoxygenation. Plasma bicarbonate was also relatively high, contributing to increase the overall buffer capacity. The apparent Cl– permeability of harbor porpoise erythrocytes was similar to the human value at 37°C, showing absence of a comparative increase in the velocity of erythrocyte HCO –3/Cl– exchange to aid CO2 excretion. The Q10 for AE1-mediated Cl– transport in harbor porpoises was lower than in humans and seemed to match the Q10 for metabolism (Q10≈2). Plasma nitrite, plasma nitrate and hemoglobin-mediated nitrite reduction were elevated compared with mammalian standards, suggesting that increased nitric oxide bioavailability and nitrite-derived nitric oxide could play important roles in diving physiology.

Temperature dependence of H+ transport across erythrocyte membrane of Rana temporaria grass frog in media containing Cl- and SO4(2-)

H+ transport across the erythrocyte membrane was studied in Rana temporaria grass frog. The temperature coefficients and activation energy of H+ transport were calculated in media containing Cl- and SO4(2-). Our results show that kinetic characteristics of H+ transport depend on function of band 3 protein in the erythrocyte membrane.

Evolution of vertebrate haemoglobins: Histidine side chains, specific buffer value and Bohr effect

This review highlights the use of analytical tools, recently developed in the comparative method of evolutionary biology, for the study of haemoglobin (Hb) adaptation. It focuses on the functional consequences of a previously largely ignored structural feature of Hb, namely the degree and positional specificity of histidine (His) substitution in Hb chains. The importance of His side chains for hydrogen ion buffering, blood CO(2) transport capacity and the molecular mechanism of the Bohr effect in vertebrate Hbs is discussed. Using phylogenetically independent contrasts, a significant correlation between the specific buffer value of Hb and the number of predicted physiological buffer groups from Hb sequence data is shown. In a new result, the evolution of the number of physiological buffer groups in 77 vertebrate species is reconstructed on a phylogenetic tree. The analysis predicts that teleost fishes, passeriform birds and some snakes have independently evolved a much-reduced specific buffer value of Hb, possibly for enhancing the efficiency of an acid load to change oxygen affinity via the Bohr effect. This analysis demonstrates how in comparative physiology analysis of genetic databases in an evolutionary framework can identify candidate species for further experimental in vitro and whole animal studies.

Red blood cell pH, the Bohr effect, and other oxygenation-linked phenomena in blood O2 and CO2 transport

The discovery of the S-shaped O2 equilibrium curve and the Bohr effect in 1904 stimulated a fertile and continued research into respiratory functions of blood and allosteric mechanisms in haemoglobin (Hb). The Bohr effect (influence of pH/CO2 on Hb O2 affinity) and the reciprocal Haldane effect (influence of HbO2 saturation on H+/CO2 binding) originate in the Hb oxy-deoxy conformational change and allosteric interactions between O2 and H+/CO2 binding sites. In steady state, H+ is passively distributed across the vertebrate red blood cell (RBC) membrane, and intracellular pH (pHi) changes are related to changes in extracellular pH, Hb-O2 saturation and RBC organic phosphate content. As the Hb molecule shifts between the oxy and deoxy conformation in arterial-venous gas transport, it delivers O2 and takes up CO2 and H+ in tissue capillaries (elegantly aided by the Bohr effect). Concomitantly, the RBC may sense local O2 demand via the degree of Hb deoxygenation and release vasodilatory agents to match local blood flow with requirements. Three recent hypotheses suggest (1) release of NO from S-nitroso-Hb upon deoxygenation, (2) reduction of nitrite to vasoactive NO by deoxy haems, and (3) release of ATP. Inside RBCs, carbonic anhydrase (CA) provides fast hydration of metabolic CO2 and ensures that the Bohr shift occurs during capillary transit. The formed H+ is bound to Hb (Haldane effect) while HCO3- is shifted to plasma via the anion exchanger (AE1). The magnitude of the oxylabile H+ binding shows characteristic differences among vertebrates. Alternative strategies for CO2 transport include direct HCO3- binding to deoxyHb in crocodilians, and high intracellular free [HCO3-] (due to high pHi) in lampreys. At the RBC membrane, CA, AE1 and other proteins may associate into what appears to be an integrated gas exchange metabolon. Oxygenation-linked binding of Hb to the membrane may regulate glycolysis in mammals and perhaps also oxygen-sensitive ion transport involved in RBC volume and pHi regulation. Blood O2 transport shows several adaptive changes during exposure to environmental hypoxia. The Bohr effect is involved via the respiratory alkalosis induced by hyperventilation, and also via the pHi change that results from modulation of RBC organic phosphate content. In teleost fish, beta-adrenergic activation of Na+/H+ exchange rapidly elevates pHi and O2 affinity, particularly under low O2 conditions.

Apparent diffusion limitations on branchial CO2 transfer are revealed by severe experimental anaemia in brown bullhead (Ameiurus nebulosus)

In this study, the rapid (within 2 h) effects of acute anaemia on blood gas and acid-base status, as well as cardiorespiratory variables, were examined in brown bullhead (Ameiurus nebulosus). Anaemia was induced by blood withdrawal coupled to volume replacement with saline. Lowering haematocrit from the control value of 23.5+/-1.0% (mean+/-S.E.M.; N=37) to 5.9+/-0.3% (N=37) resulted in a significant increase (by 2.63+/-0.51 torr; N=7) in arterial CO(2) tension (PaCO(2)) over the subsequent 2-h period in the absence of a change in arterial O(2) tension (PaO(2)). Treatment with bovine carbonic anhydrase (CA) reduced the extent of the PaCO(2) increase to the point where it was not statistically significant. In both control and CA-treated fish, arterial pH decreased during acute anaemia; the acidosis was of mixed respiratory and metabolic origin in control fish and primarily metabolic in CA-treated fish. Inducing anaemia caused increases in both cardiac output (V*b) and heart rate that were similar in control and CA-treated fish. Experimental elevation of V*b equivalent to that observed during anaemia, but in the absence of lowered haematocrit, increased PaCO(2) significantly by 1.49+/-0.74 to 1.64+/-0.78 torr (N=5) without affecting PaO(2). These findings suggest that CO(2) excretion in bullhead, as in rainbow trout, is effectively diffusion-limited, and that approximately half of the increase in PaCO(2) measured during the initial 2 h of anaemia results from the impact of increased blood flow (hence decreased gill transit time) in a diffusion-limited system.