Effects of pH changes on sodium pump fluxes in squid giant axon

Impacts and mechanisms of CO2 narcosis in bumble bees: narcosis depends on dose, caste and mating status and is not induced by anoxia

Carbon dioxide (CO2) is commonly used to immobilize insects and to induce reproduction in bees. However, despite its wide use and potential off-target impacts, its underlying mechanisms are not fully understood. Here, we used Bombus impatiens to examine whether CO2 impacts are mediated by anoxia and whether these mechanisms differ between female castes or following mating in queens. We examined the behavior, physiology and gene expression of workers, mated queens and virgin queens following exposure to anoxia, hypoxia, full and partial hypercapnia, and controls. Hypercapnia and anoxia caused immobilization, but only hypercapnia resulted in behavioral, physiological and molecular impacts in bees. Recovery from hypercapnia resulted in increased abdominal contractions and took longer in queens. Additionally, hypercapnia activated the ovaries of queens, but inhibited those of workers in a dose-dependent manner and caused a depletion of fat-body lipids in both castes. All responses to hypercapnia were weaker following mating in queens. Analysis of gene expression related to hypoxia and hypercapnia supported the physiological findings in queens, demonstrating that the overall impacts of CO2, excluding virgin queen ovaries, were unique and were not induced by anoxia. This study contributes to our understanding of the impacts and the mechanistic basis of CO2 narcosis in insects and its impacts on bee physiology. This article has an associated ECR Spotlight interview with Anna Cressman.

Ion Channels, Transporters, and Sensors Interact with the Acidic Tumor Microenvironment to Modify Cancer Progression

Solid tumors, including breast carcinomas, are heterogeneous but typically characterized by elevated cellular turnover and metabolism, diffusion limitations based on the complex tumor architecture, and abnormal intra- and extracellular ion compositions particularly as regards acid-base equivalents. Carcinogenesis-related alterations in expression and function of ion channels and transporters, cellular energy levels, and organellar H⁺ sequestration further modify the acid-base composition within tumors and influence cancer cell functions, including cell proliferation, migration, and survival. Cancer cells defend their cytosolic pH and HCO₃^- concentrations better than normal cells when challenged with the marked deviations in extracellular H⁺, HCO₃^-, and lactate concentrations typical of the tumor microenvironment. Ionic gradients determine the driving forces for ion transporters and channels and influence the membrane potential. Cancer and stromal cells also sense abnormal ion concentrations via intra- and extracellular receptors that modify cancer progression and prognosis. With emphasis on breast cancer, the current review first addresses the altered ion composition and the changes in expression and functional activity of ion channels and transporters in solid cancer tissue. It then discusses how ion channels, transporters, and cellular sensors under influence of the acidic tumor microenvironment shape cancer development and progression and affect the potential of cancer therapies.

Effects of Alkalinity and pH on Survival, Growth, and Enzyme Activities in Juveniles of the Razor Clam, Sinonovacula constricta

In order to clarify the possibility of rearing razor clams (Sinonovacula constricta) in inland saline water (ISW) and to facilitate their breeding under these stressful conditions, we performed semi-static acute and chronic toxicity tests to determine the effects of carbonate alkalinity (CA) and pH on the survival and growth rate, and critical metabolic enzyme activity in juvenile of S. constricta (JSC). (1) Acute toxicity test. As the water CA increased from 1.22 to 45.00 mmol L-1, the survival rate decreased significantly, which was exacerbated by the increase in the pH. When the water CA was set at 2.5 mmol L-1, the 48 h lethal concentration 50% (LC50) for JSCs with respect to pH was 9.86. When the water pH was 9.0, 9.5, and 10.0, the 48 h LC50 values for JSCs with respect to CA were 10.38, 8.79, and 3.11 mmol L-1, respectively. (2) Chronic toxicity test. Four experimental groups comprising the control, CAS, pHS, and CA-pHS were designated according to the target ISW data. After 3 months of stress, the JSC survival rate in each group exceeded 85%, but survival was significantly lower in the CA-pHS group than the control group (p < 0.05) in the first month. For the JSCs in various groups, the shell length growth rate (SGR) and weight gain (WG) rate were significantly lower in the CA-pHS group than the other groups (p < 0.05 for SGR; p < 0.001 for WG) in the first month. However, the difference in the growth rate among groups decreased in the next 2 months. For the JSCs in the CA-pHS group, the oxygen consumption, ammonia-N excretion, Na+/K+-ATPase, aspartate aminotransferase, and superoxide dismutase levels were significantly higher than those in the other groups during the first month, but there were no significant differences between the groups subsequently. The acetylcholinesterase and lysozyme levels did not differ significantly among groups during stress for 3 months. The integrated biomarker response index showed that stressors comprising high pH and CA could be tolerated well by JSCs over long periods of stress. These results indicate that water CA and pH together affect the survival, growth, and physiological activity of JSCs. S. constricta is suitable for culture in ISW.

Distinct pH dependencies of Na+/K+ selectivity at the two faces of Na,K-ATPase

The sodium pump (Na,K-ATPase) in animal cells is vital for actively maintaining ATP hydrolysis-powered Na⁺ and K⁺ electrochemical gradients across the cell membrane. These ion gradients drive co- and countertransport and are critical for establishing the membrane potential. It has been an enigma how Na,K-ATPase discriminates between Na⁺ and K⁺, despite the pumped ion on each side being at a lower concentration than the other ion. Recent crystal structures of analogs of the intermediate conformations E2·Pi·2K⁺ and Na⁺-bound E1∼P·ADP suggest that the dimensions of the respective binding sites in Na,K-ATPase are crucial in determining its selectivity. Here, we found that the selectivity at each membrane face is pH-dependent and that this dependence is unique for each face. Most notable was a strong increase in the specific affinity for K⁺ at the extracellular face (i.e. E2 conformation) as the pH is lowered from 7.5 to 5. We also observed a smaller increase in affinity for K⁺ on the cytoplasmic side (E1 conformation), which reduced the selectivity for Na⁺ Theoretical analysis of the pK_a values of ion-coordinating acidic amino acid residues suggested that the face-specific pH dependences and Na⁺/K⁺ selectivities may arise from the protonation or ionization of key residues. The increase in K⁺ selectivity at low pH on the cytoplasmic face, for instance, appeared to be associated with Asp⁸⁰⁸ protonation. We conclude that changes in the ionization state of coordinating residues in Na,K-ATPase could contribute to altering face-specific ion selectivity.

Biochemical adaptation to ocean acidification

The change in oceanic carbonate chemistry due to increased atmospheric PCO2 has caused pH to decline in marine surface waters, a phenomenon known as ocean acidification (OA). The effects of OA on organisms have been shown to be widespread among diverse taxa from a wide range of habitats. The majority of studies of organismal response to OA are in short-term exposures to future levels of PCO2. From such studies, much information has been gathered on plastic responses organisms may make in the future that are beneficial or harmful to fitness. Relatively few studies have examined whether organisms can adapt to negative-fitness consequences of plastic responses to OA. We outline major approaches that have been used to study the adaptive potential for organisms to OA, which include comparative studies and experimental evolution. Organisms that inhabit a range of pH environments (e.g. pH gradients at volcanic CO2 seeps or in upwelling zones) have great potential for studies that identify adaptive shifts that have occurred through evolution. Comparative studies have advanced our understanding of adaptation to OA by linking whole-organism responses with cellular mechanisms. Such optimization of function provides a link between genetic variation and adaptive evolution in tuning optimal function of rate-limiting cellular processes in different pH conditions. For example, in experimental evolution studies of organisms with short generation times (e.g. phytoplankton), hundreds of generations of growth under future conditions has resulted in fixed differences in gene expression related to acid–base regulation. However, biochemical mechanisms for adaptive responses to OA have yet to be fully characterized, and are likely to be more complex than simply changes in gene expression or protein modification. Finally, we present a hypothesis regarding an unexplored area for biochemical adaptation to ocean acidification. In this hypothesis, proteins and membranes exposed to the external environment, such as epithelial tissues, may be susceptible to changes in external pH. Such biochemical systems could be adapted to a reduced pH environment by adjustment of weak bonds in an analogous fashion to biochemical adaptation to temperature. Whether such biochemical adaptation to OA exists remains to be discovered.

Disturbed acid-base transport: an emerging cause of hypertension

Genome-wide association studies and physiological investigations have linked alterations in acid-base transporters to hypertension. Accordingly, Na⁺-coupled HCO⁻₃-transporters, Na⁺/H⁺-exchangers, and anion-exchangers have emerged as putative mechanistic components in blood pressure disturbances. Even though hypertension has been studied extensively over the last several decades, the cause of the high blood pressure has in most cases not been identified. Renal, cardiovascular, and neuronal dysfunctions all seem to play a role in hypertension development but their relative importance and mutual interdependency are still being debated. Multiple functional and structural alterations have been described in patients and animals with hypertension but it is typically unclear whether they are causes or consequences of hypertension or represent mechanistically unrelated associations. Perturbed blood pressure regulation has been demonstrated in several animal models with disrupted expression of acid-base transporters; and reciprocally, disturbed acid-base transport function has been described in hypertensive individuals. In addition to regulating intracellular and extracellular pH, Na⁺-coupled HCO⁻₃-transport, Na⁺/H⁺-exchange, and anion-exchange also contribute to water and electrolyte balance in cells and systemically. Since acid-base transporters are widely expressed, alterations in transport activities likely affect multiple cell and organ functions, and it is a significant challenge to determine the mechanisms linking perturbed acid-base transport function to hypertension. It is the purpose of this review to evaluate the current evidence for involvement of acid-base transporters in hypertension development and discuss the cellular and integrative mechanisms, which may link changes in acid-base transport to blood pressure disturbances.

Molecular Mechanisms of K+ Selectivity in Na/K Pump

The sodium–potassium (Na/K) pump plays an essential role in maintaining cell volume and secondary active transport of other solutes by establishing the Na+ and K+ concentration gradients across the plasma membrane of animal cells. The recently determined crystal structures of the Na/K pump to atomic resolution provide a new impetus to investigate molecular determinants governing the binding of Na+ and K+ ions and conformational transitions during the functional cycle. The pump cycle is generally described by the alternating access mechanism, in which the pump toggles between different conformational states, where ions can bind from either the intracellular or the extracellular side. However, important issues concerning the selectivity of the Na/K pump remain to be addressed. In particular, two out of the three binding sites are shared between Na+ and K+ and it is not clear how the protein is able to select K+ over Na+ when it is in the outwardly facing phosphorylated conformation (E2P), and Na+ over K+ when it is in the inwardly facing conformation (E1). In this review article, we will first briefly review the recent advancement in understanding the microscopic mechanism of K+ selectivity in the Na/K pump at the E2·Pi state and then outline the remaining challenges to be addressed about ion selectivity.

Effects of pH on potassium: new explanations for old observations

Maintenance of extracellular K(+) concentration within a narrow range is vital for numerous cell functions, particularly electrical excitability of heart and muscle. Potassium homeostasis during intermittent ingestion of K(+) involves rapid redistribution of K(+) into the intracellular space to minimize increases in extracellular K(+) concentration, and ultimate elimination of the K(+) load by renal excretion. Recent years have seen great progress in identifying the transporters and channels involved in renal and extrarenal K(+) homeostasis. Here we apply these advances in molecular physiology to understand how acid-base disturbances affect serum potassium.

Effects of pH on potassium: new explanations for old observations

Maintenance of extracellular K(+) concentration within a narrow range is vital for numerous cell functions, particularly electrical excitability of heart and muscle. Potassium homeostasis during intermittent ingestion of K(+) involves rapid redistribution of K(+) into the intracellular space to minimize increases in extracellular K(+) concentration, and ultimate elimination of the K(+) load by renal excretion. Recent years have seen great progress in identifying the transporters and channels involved in renal and extrarenal K(+) homeostasis. Here we apply these advances in molecular physiology to understand how acid-base disturbances affect serum potassium.

Protonation of key acidic residues is critical for the K⁺-selectivity of the Na/K pump

The sodium-potassium (Na/K) pump is a P-type ATPase that generates Na⁺ and K⁺ concentration gradients across the cell membrane. For each ATP molecule, the pump extrudes three Na⁺ and imports two K⁺ by alternating between outward- and inward-facing conformations that preferentially bind K⁺ or Na⁺, respectively. Remarkably, the selective K⁺ and Na⁺ binding sites share several residues, and how the pump is able to achieve the selectivity required for the functional cycle is unclear. Here, free energy perturbation molecular dynamics (FEP/MD) simulations based on the crystal structures of the Na/K pump in a K⁺-loaded state (E2·P_i) reveal that protonation of the high-field acidic side-chains involved in the binding sites is critical to achieve the proper K⁺ selectivity. This prediction is tested with electrophysiological experiments showing that the selectivity of the E2P state for K⁺ over Na⁺ is affected by extracellular pH.

Resting membrane potential regulates Na(+)-Ca2+ exchange-mediated Ca2+ overload during hypoxia-reoxygenation in rat ventricular myocytes

In the heart, reperfusion following an ischaemic episode can result in a marked increase in [Ca2+]i and cause myocyte dysfunction and death. Although the Na(+)-Ca2+ exchanger has been implicated in this response, the ionic mechanisms that are responsible have not been identified. In this study, the hypothesis that the diastolic membrane potential can influence Na(+)-Ca2+ exchange and Ca2+ homeostasis during chemically induced hypoxia-reoxygenation has been tested using right ventricular myocytes isolated from adult rat hearts. Superfusion with selected [K+]o of 0.5, 2.5, 5, 7, 10 and 15 mM yielded the following resting membrane potentials: -27.6+/-1.63 mV, -102.2+/-1.89, -86.5+/-1.03, -80.1+/-1.25, -73.6+/-1.02 and -66.4+/-1.03, respectively. In a second set of experiments myocytes were subjected to chemically induced hypoxia-reoxygenation at these different [K+]o, while [Ca2+]i was monitored using fura-2. These results demonstrated that after chemically induced hypoxia-reoxygenation had caused a marked increase in [Ca2+]i, hyperpolarization of myocytes with 2.5 mM [K+]o significantly reduced [Ca2+]i (7.5+/-0.32 vs. 16.9+/-0.55%); while depolarization (with either 0.5 or 15 mM [K+]o) significantly increased [Ca2+]i (31.8+/-3.21 and 20.8+/-0.36 vs. 16.9+/-0.55%, respectively). As expected, at depolarized membrane potentials myocyte hypercontracture and death increased in parallel with Ca2+ overload. The involvement of the Na(+)-Ca2+ exchanger in Ca2+ homeostasis was evaluated using the Na(+)-Ca2+ exchanger inhibitor KB-R7943. During reoxygenation KB-R7943 (5 microM) almost completely prevented the increase in [Ca2+]i both in control conditions (in 5 mM [K+]o: 2.2+/-0.40 vs. 10.8+/-0.14%) and in depolarized myocytes (in 15 mM [K+]o: -2.1+/-0.51 vs. 11.3+/-0.05%). These findings demonstrate that the resting membrane potential of ventricular myocytes is a critical determinant of [Ca2+]i during hypoxia-reoxygenation. This appears to be due mainly to an effect of diastolic membrane potential on the Na(+)-Ca2+ exchanger, since at depolarized potentials this exchanger mechanism operates in the reverse mode, causing a significant Ca2+ influx.

Extracellular protons regulate the extracellular cation selectivity of the sodium pump

The effects of 0.3-10 nM extracellular protons (pH 9.5-8.0) on ouabain-sensitive rubidium influx were determined in 4,4'-diisocyanostilbene-2, 2'-disulfonate (DIDS)-treated human and rat erythrocytes. This treatment clamps the intracellular H. We found that rubidium binds much better to the protonated pump than the unprotonated pump; 13-fold better in rat and 34-fold better in human erythrocytes. This clearly shows that protons are not competing with rubidium in this proton concentration range. Bretylium and tetrapropylammonium also bind much better to the protonated pump than the unprotonated pump in human erythrocytes and in this sense they are potassium-like ions. In contrast, guanidinium and sodium bind about equally well to protonated and unprotonated pump in human red cells. In rat red cells, protons actually make sodium bind less well (about sevenfold). Thus, protons have substantially different effects on the binding of rubidium and sodium. The effect of protons on ouabain binding in rat red cells was intermediate between the effects of protons on rubidium binding and on sodium binding. Remarkably, all four cationic inhibitors (bretylium, guanidinium, sodium, and tetrapropylammonium) had similar apparent inhibitory constants for the unprotonated pump ( approximately 5-10 mM). The K(d) for proton binding to the human pump, with the empty transport site facing extracellularly is 13 nM, whereas the extracellular transport site loaded with sodium is 9.5 nM, and with rubidium is 0.38 nM. In rat red cells there is also a substantial difference in the K(d) for proton binding to the sodium-loaded pump (14.5 nM) and the rubidium-loaded pump (0.158 nM). These data suggest that important rearrangements occur at the extracellular pump surface as the pump moves between conformations in which the outward facing transport site has sodium bound, is empty, or has rubidium bound and that guanidinium is sodium-like and bretylium and tetrapropylammonium are rubidium-like.

The K-Cl cotransporter in the lobster stretch receptor neurone--a kinetic analysis

Experiments were performed to define quantitatively the substrate (K(+) and Cl(-)) dependence of the transport function (production of equally large and oppositely directed K(+)and Cl(-) flows/currents) of an earlier (Theander et al., 1999) identified electroneutral K-Cl cotransporter in the slowly adapting stretch receptor neurone of the European lobster. The experiments were based on microelectrode techniques. This allowed us to perform steady-state measurements of the so-called "instantaneous" current-voltage relationships (around a holding voltage of -65 mV after a blockage of the cell's action potential and hyperpolarization-activated currents) and intracellular ion concentrations at various settings of the extracellular K(+) and Cl(-) concentrations. From the results, we could then define steady-state values of all of the cell's non-KCl cotransporter K(+) and Cl(-) currents. Finally, the negative sums of the inferred non-KCl cotransporter K(+) and Cl(-) currents could be taken as equivalents of the K-Cl cotransporter's K(+) and Cl(-) currents for the reason that, in steady state, all membrane currents add up to zero. For the cotransporter currents, thus inferred for a range from 2.5/410.5 to 40.0/448.0 mM external K(+)/Cl(-), we found that their absolute values increased in a nonlinear fashion from about 5 nA cell(-1) at the lowest, to about 20 nA cell(-1) at the highest external K(+)/Cl(-) concentrations. Formally, this relationship could be reproduced by a Hill function-based enzyme kinetic expression simulating inward and outward transmembrane electroneutral ion transports. Following insertion of this expression into a comprehensive model of electrical membrane functions and intracellular solute and solvent control in the lobster stretch receptor neurone, the model predictions suggested that the K-Cl cotransporter does play an important role in (a) keeping intracellular Cl(-) low for a proper function of the cell's inhibitory system, and (b) enabling rapid transmembrane K(+) shifts that provide for a stabilization of the cell's membrane voltage and membrane excitability in cases of varying extracellular K(+) concentrations. The model predictions gave, however, no clear evidence that the K-Cl cotransporter is critically involved in the cell's volume regulation in conditions of varying extracellular osmolalities.

Postischemic Na(+)-K(+)-ATPase reactivation is delayed in the absence of glycolytic ATP in isolated rat hearts

Normalization of intracellular sodium (Na) after postischemic reperfusion depends on reactivation of the sarcolemmal Na(+)-K(+)-ATPase. To evaluate the requirement of glycolytic ATP for Na(+)-K(+)-ATPase function during postischemic reperfusion, 5-s time-resolution 23Na NMR was performed in isolated perfused rat hearts. During 20 min of ischemia, Na increased approximately twofold. In glucose-reperfused hearts with or without prior preischemic glycogen depletion, Na decreased immediately upon postischemic reperfusion. In glycogen-depleted pyruvate-reperfused hearts, however, the decrease of Na was delayed by approximately 25 s, and application of the pyruvate dehydrogenase (PDH) activator dichloroacetate (DA) did not shorten this delay. After 30 min of reperfusion, Na had almost normalized in all groups and contractile recovery was highest in the DA-treated hearts. In conclusion, some degree of functional coupling of glycolytic ATP and Na(+)-K(+)-ATPase activity exists, but glycolysis is not essential for recovery of Na homeostasis and contractility after prolonged reperfusion. Furthermore, the delayed Na(+)-K(+)-ATPase reactivation observed in pyruvate-reperfused hearts is not due to inhibition of PDH.

Extracellular pH modulates kinetics of the Na(+),K(+)-ATPase

To investigate effects of pH on the Na(+),K(+)-ATPase, we used the Xenopus oocytes to measure transient charge movements in the absence of extracellular K(+), and steady-state currents mediated by the pump as well as ATPase activity. The activity of purified Na(+), K(+)-ATPase strongly depends on pH, which has been attributed to protonation of intracellular sites. The steady-state current reflects pump activity, the transient charge movement voltage-dependent interaction of external Na(+) ions with the pump molecule and/or conformational changes during Na(+)/Na(+) exchange. The steady-state current exhibits a characteristic voltage dependence with maximum at about 0 mV at low external K(+) (< or =2 mM) and with 50 Na(+). This dependency is not significantly affected by changes in external pH in the range from pH 9 to pH 6. Only below pH 6, the voltage dependence of pump current becomes less steep, and may be attributed to a pH-dependent inhibition of the forward pump cycle by external Na(+). External stimulation of the pump by K(+) in the absence of Na(+) can be described by a voltage-dependent K(m) value with an apparent valency z(K). At higher external pH the z(K) value is reduced. The transient current signal in the absence of external K(+) can be described by the sum of three exponentials with voltage-dependent time constants of about 50 ms, 700 micros and less than 100 micros during pulses to 0 mV. The charge distribution was calculated by integration of the transient current signals. The slowest component and the associated charge distributions do not significantly depend on external pH changes. The intermediate component of the transients is represented by a voltage-dependent rate constant which shows a minimum at about -120 mV and increases with decreasing pH. Nevertheless, the contribution to the charge movement is not altered by pH changes due to a simultaneous increase of the amplitude of this component. We conclude that reduction of external pH counteracts external K(+) and Na(+) binding.

Effect of norepinephrine on intracellular pH in kidney proximal tubule: role of Na+-(HCO-3)n cotransport

We examined the effect of norepinephrine (NE) on intracellular pH (pHi) and activity of Na+ (aNai) in the isolated perfused kidney proximal tubule of Ambystoma, using single-barreled voltage and ion-selective microelectrodes. In control HCO-3 Ringer, addition of 10(-6) M NE to the bath reversibly depolarized the basolateral membrane potential (V1), the luminal membrane potential (V2), and the transepithelial potential difference (V3) and increased pHi by 0. 14 +/- 0.02. These effects were mimicked by isoproterenol but were abolished after pretreatment with SITS or in the absence of CO2/HCO-3. Removal of bath Na+ depolarized V1 and V2, hyperpolarized V3, and decreased pHi. These effects are largely mediated by the electrogenic Na+-(HCO-3)n cotransporter. In the presence of NE, the effects of Na+ removal on membrane potential differences and the rate of change of pHi were significantly smaller. Reducing bath HCO-3 concentration from 10 to 2 mM at constant CO2 (pH 6.8) depolarized V1 and V2, decreased pHi, and lowered aNai. These changes are also due to Na+-(HCO-3)n. In the presence of NE, reducing bath [HCO-3] caused a smaller depolarizations of V1 and V2, and the rate of pHi decrease was significantly reduced. Our results indicate: 1) NE causes an increase in pHi; 2) the NE-induced alkalinization is mediated by a SITS-sensitive and HCO-3-dependent transporter on the basolateral membrane; and 3) in the presence of NE, the reduced effects caused by basolateral HCO-3 changes or Na+ removal are indicative of an inhibitory effect of NE on Na+-(HCO-3)n cotransport.

Stopped-flow kinetic investigations of conformational changes of pig kidney Na+,K+-ATPase

The kinetics of Na+-dependent partial reactions of the Na+,K+-ATPase were investigated via the stopped-flow technique using the fluorescent labels RH421 and BIPM. After the enzyme is mixed with MgATP, both labels give almost identical kinetic responses. Under the chosen experimental conditions two exponential time functions are necessary to fit the data. The dominant fast phase, 1/tau1 approximately 180 s-1 (saturating [ATP] and [Na+], pH 7.4 and 24 degrees C), is attributed to phosphorylation of the enzyme and a subsequent conformational change (E1ATP(Na+)3 --> E2P(Na+)3 + ADP). The rate of the phosphorylation reaction measured by the acid quenched-flow technique was 190 s-1 at 100 microM ATP, suggesting that phosphorylation controls the kinetics of the RH421 signal and that the conformational change is very fast (>/=600 s-1). The rate of the RH421 signal was optimal at pH 7.5. The Na+ concentration dependence of 1/tau1 showed half-saturation at a Na+ concentration of 8-10 mM with positive cooperativity involved in the occupation of the Na+ binding sites. The apparent dissociation constant of the high affinity ATP binding site determined from the ATP concentration dependence of 1/tau1 was 7.0 (+/-0.6) microM, while the apparent Kd for the low affinity site and the rate constant for the E2 to E1 conformational change evaluated in the absence of Mg2+ were 143 (+/-17) microM and </= 28 s-1. At RH421 concentrations in the micromolar range, a decrease in the value of 1/tau1 is observed. On the basis of rapid quenched-flow measurements, this inhibition can be attributed to a reaction step subsequent to phosphorylation. This accounts for previously observed kinetic discrepancies between RH421 and BIPM.

Na+,K(+)-ATPase pump currents in giant excised patches activated by an ATP concentration jump

The giant-patch technique was used to study the Na+,K(+)-ATPase in excised patches from rat or guinea pig ventricular myocytes. Na+,K(+)-pump currents showed a saturable ATP dependence with aK(m) of approximately 150 microM at 24 degrees C. The pump current can be completely abolished by ortho-vanadate. Dissociation of vanadate from the enzyme in the absence of extracellular Na+ was slow, with a Koff of 3.10(-4) S-1 (K1 approximately 0.5 microM, at 24 degrees C). Stationary currents were markedly dependent on intracellular pH, with a maximum at pH 7.9. Temperature-dependence measurements of the stationary pump current yielded an activation energy of approximately 100 kJ mol-1. Partial reactions in the transport cycle were investigated by generating ATP concentration jumps through photolytic release of ATP from caged ATP at pH 7.4 and 6.3. Transient outward currents were obtained at pH 6.3 with a fast rising phase followed by a slower decay to a stationary current. It was concluded that the fast rate constant of approximately 200 s-1 at 24 degrees C (pH 6.3) reflects a step rate-limiting the electrogenic Na+ release. Simulating the data with a simple three-state model enabled us to estimate the turnover rate under saturating substrate concentrations, yielding rates (at pH 7.4) of approximately 60 s-1 and 200 s-1 at 24 degrees C and 36 degrees C, respectively.

Glucose availability alters ischaemia-induced changes in intracellular pH and calcium of isolated rat spinal roots

Peripheral nerves in diabetic patients show an enhanced liability to ischaemic lesions. Using an in vitro model, we have now analysed the possible role of intracellular proton (pHi) and calcium concentrations ([Ca2+]i) for the pathophysiology of this phenomenon. Isolated rat spinal roots were preincubated for 3 to 6 h in either 5 or 25 mM of D-glucose before transient exposure to gaseous hypoxia or cyanide. Intracellular pH and Ca2+ concentrations were measured photometrically by means of the fluorescent dyes carboxy-SNARF-1 and a combination of Calcium Green-1 and Fura Red, respectively. The following observations were made. (a) The presence of 25 mM D-glucose resulted in stronger intracellular acidification and much slower post-hypoxic recovery of pHi as compared to 5 mM D-glucose. (b) Intracellular calcium increased during hypoxia and recovered quickly on reoxygenation. There were no statistically significant differences between the Ca2+ signals in either high or normal concentrations of D-glucose, although on average less rise was seen in high glucose. (c) Inhibition of glycolysis with iodoacetate reduced the acidification but amplified the rise in [Ca2+]i seen during transient hypoxia. These data suggest that hypoxia-induced nerve acidification rather than a rise in [Ca2+]i might contribute to ischaemic lesions found in diabetic neuropathy.

Microenvironment of respiratory neurons in the in vitro brainstem-spinal cord of neonatal rats

1. O2-, K(+)- and pH-sensitive microelectrodes were used to measure extracellular oxygen pressure (PO2), K+ activity (aKo) and pH (pHo) in ventral regions of the medulla oblongata containing respiratory neurons in the in vitro brainstem-spinal cord preparation from 0 to 4-day-old rats. 2. The location of respiratory neurons was mapped by extracellular recordings with conventional microelectrodes, or with the reference barrel of ion-sensitive microelectrodes. The major populations of respiratory neurons were distributed in the ventrolateral reticular formation near the nucleus ambiguus at depths of 300-600 microns. In this area, aKo baseline increased from 3.2 to 3.8 mM whereas steady-state values of PO2 and pHo fell from 120 to 7 mmHg and from 6.9 to 6.7, respectively. 3. During rhythmic inspiratory discharges recorded with suction electrodes from ventral roots of spinal (C3-C5) and cranial (IX, X, XII) nerves, aKo transiently increased by up to 100 microM, and PO2 fell maximally by 0.4 mmHg. During episodes of non-rhythmic neuronal discharge, aKo increased by as much as 0.4 mM and PO2 decreased by about 10 mmHg. In contrast, no variations in pHo could be detected during such activities. 4. Activation of medullary neurons by tetanic electrical stimulation of axonal tracts in the ventrolateral column of the spinal cord at the level of the phrenic motoneuron pool produced aKo elevations of up to 5 mM, decreases of PO2 by up to 50 mmHg, and pHo increases by a maximum of 0.07 pH units. These aKo and PO2 transients were reduced by more than 80% during blockade of synaptic transmission with 5 mM manganese (Mn2+) and completely blocked by 1 microM tetrodotoxin (TTX). 5. The tissue PO2 gradient as well as activity-related decreases of PO2 were completely abolished after block of oxidative cellular metabolism by addition of 2-10 mM cyanide (CN-) to the bathing solution. 6. Inhibition of the Na(+)-K+ pump by addition of 3-50 microM ouabain (3-10 min) caused a reversible increase of aKo by 0.8-3 mM, a delayed recovery of stimulus-induced aKo elevations, and produced a disturbance of the respiratory rhythm. 7. The sensitivity of the respiratory network to oxygen depletion was tested by superfusing the neuraxis with hypoxic solutions gassed with N2 instead of O2 (5-20 min).(ABSTRACT TRUNCATED AT 400 WORDS)

Consequences of CO2 acidosis for transmembrane Na+ transport and membrane current in rabbit cardiac Purkinje fibres

1. The influence of sarcolemmal Na(+)-H+ exchange on intracellular Na+ activity (aiNa), intracellular pH (pHi) and membrane holding current (Ih) was investigated in rabbit cardiac Purkinje fibres. pHi and aiNa were measured with liquid sensor ion-selective microelectrodes. A two-microelectrode voltage clamp was used while recording pHi or aiNa.pHi was varied by alternating a nominally CO2-free HEPES buffer and a CO2-HCO3-buffer. 2. The intrinsic buffer capacity was calculated from the decrease in pHi after addition of CO2. The most accurate estimate was obtained when transmembrane pHi regulation was blocked and equalled 18.4 +/- 1.0 mequiv H+/pH unit (mean +/- S.E.M.). 3. aiNa started to rise when pHi fell below 7.0. A hyperpolarization paralleled the increase in aiNa. The magnitude of the rise in aiNa and the hyperpolarization were steeply dependent on pHi. 4. Inhibition of the Na(+)-K+ pump by K(+)-free superfusion increased aiNa. The rate of rise in aiNa was highly dependent on pHi. The rates of rise in 5% (pHi = 7.00 +/- 0.03), 7% (pHi = 6.89 +/- 0.04) and 15% CO2 (pHi = 6.74 +/- 0.02) at constant external pH (pHo) relative to the rate in HEPES solution (pHi = 7.24 +/- 0.02) were: 1.5 +/- 0.2, 2.4 +/- 0.1 and 3.1 +/- 0.2. The acid-induced rise in aiNa was abolished by 2 mM-amiloride. 5. Extracellular acidosis slowed down the recovery of pHi and depressed the rate of rise in aiNa upon intracellular acidification. When both pHi and pHo were decreased to 6.7 the acid-dependent rate increase fell to about 10% of the value found at pHo 7.4. 6. The tetrodotoxin (TTX)-sensitive Na+ current was not influenced by the change in pHi in the range 6.7-7.2. 7. Intracellular acidosis was associated with an early aiNa-independent depolarization and an inward shift in Ih. Current-voltage plots revealed that the initial inward current shift reversed at -81.5 mV on average, showed inward rectification and was largely depressed in the presence of 1 mM-Ba2+. These observations indicate a decrease in K+ conductance when pHi falls. 8. The increase in aiNa elicited a Na(+)-K+ pump-dependent outward current which could override the initial aiNa-independent current shift. At a pHo of 6.7 the initial fall in Ih remained, while the secondary outward current was largely depressed. 9. The rate of active Na+ extrusion and Na(+)-K+ pump current were suppressed by about 30% at pHi 6.7 compared to pHi 7.2.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of diuretics on captopril-induced urinary zinc excretion

The urinary zinc/creatinine ratio has been measured in five groups of patients with essential hypertension and in a group of healthy controls. The first four groups of patients consisted of subjects being treated for at least three months with captopril alone, hydrochlorothiazide alone, captopril plus hydrochlorothiazide, or captopril and furosemide. The fifth group comprised hypertensive patients not on any medication. The first four patient groups exhibited significantly increased urinary zinc/creatinine ratios when compared to the control and untreated hypertensive groups, but in the two combination regimens there was little zincuria. It is suggested that both diuretics inactivate the zincuric effect of captopril by binding to its sulphydryl group within the tubular lumen.

Influence of isolation media on synaptosomal properties: intracellular pH, pCa, and Ca2+ uptake

Preparations of synaptosomes isolated in sucrose or in Na(+)-rich media were compared with respect to internal pH (pHi), internal Ca2+ concentration ([Ca2+]i), membrane potential and 45Ca2+ uptake due to K+ depolarization and Na+/Ca2+ exchange. We found that synaptosomes isolated in sucrose media have a pHi of 6.77 +/- 0.04 and a [Ca2+]i of about 260 nM, whereas synaptosomes isolated in Na(+)-rich ionic media have a pHi of 6.96 +/- 0.07 and a [Ca2+]i of 463 nM, but both types of preparations have similar membrane potentials of about -50 mV when placed in choline media. The sucrose preparation takes up Ca2+ only by voltage sensitive calcium channels (VSCC'S) when K(+)-depolarized, while the Na(+)-rich synaptosomes take up 45Ca2+ both by VSCC'S and by Na+/Ca2+ exchange. The amiloride derivative 2',4'-dimethylbenzamil (DMB), at 30 microM, inhibits both mechanisms of Ca2+ influx, but 5-(N-4-chlorobenzyl)-2',4' dimethylbenzamil (CBZ-DMB), at 30 microM, inhibits the Ca2+ uptake by VSCC'S, but not by Na+/Ca2+ exchange. Thus, DMB and CBZ-DMB permit distinguishing between Ca2+ flux through channels and through Na+/Ca2+ exchange. We point out that the different properties of the two types of synaptosomes studied account for some of the discrepancies in results reported in the literature for studies of Ca2+ fluxes and neurotransmitter release by different types of preparations of synaptosomes.

Fluorescence measurements of cytosolic free Na concentration, influx and efflux in gastric cells

Regulation of cytosolic free Na (Nai) was measured in isolated rabbit gastric glands with the use of a recently developed fluorescent indicator for sodium, SBFI. Intracellular loading of the indicator was achieved by incubation with an acetoxymethyl ester of the dye. Digital imaging of fluorescence was used to monitor Nai in both acid-secreting parietal cells and enzyme-secreting chief cells within intact glands. In situ calibration of Nai with ionophores indicated that SBFI fluorescence (345/385 nm excitation ratio) could resolve 2 mM changes in Nai and was relatively insensitive to changes in K or pH. Measurements on intact glands showed that basal Nai was 8.5 +/- 2.2 mM in parietal cells and 9.2 +/- 3 mM in chief cells. Estimates of Na influx and efflux were made by measuring rates of Nai change after inactivation or reactivation of the Na/K ATPase in a rapid perfusion system. Na/K ATPase inhibition resulting from the removal of extracellular K (Ko) caused Nai to increase at 3.2 +/- 1.5 mM/min and 3.5 +/- 2.7 mM/min in parietal and chief cells, respectively. Na buffering was found to be negligible. Addition of 5 mM Ko and removal of extracellular Na (Nao) caused Nai to decrease rapidly toward 0 mM Na. By subtracting passive Na efflux under these conditions (the rate at which Nai decreased in Na-free solution containing ouabain), an activation curve (dNai/Nai) for the Na/K ATPase was calculated. The pump demonstrated the greatest sensitivity between 5 and 20 mM Nai. At 37 degrees C the pump rate was less than 3 mM/min at 5 mM Nai and 26 mM/min at 25 mM Nai, indicating that the pump has a great ability to respond to changes in Nai in this range. Carbachol, which stimulates secretion from both cell types, was found to stimulate Na influx in both cell types, but did not have detectable effects on Na efflux. dbcAMP+IBMX, potent stimulants of acid secretion, had no effect on Na metabolism.

Apical membrane Na+/H+ exchange in Necturus gallbladder epithelium. Its dependence on extracellular and intracellular pH and on external Na+ concentration

Intracellular microelectrode techniques and extracellular pH measurements were used to study the dependence of apical Na+/H+ exchange on mucosal and intracellular pH and on mucosal solution Na+ concentration ([Na+]o). When mucosal solution pH (pHo) was decreased in gallbladders bathed in Na(+)-containing solutions, aNai fell. The effect of pHo is consistent with titration of a single site with an apparent pK of 6.29. In Na(+)-depleted tissues, increasing [Na+]o from 0 to values ranging from 2.5 to 110 mM increased aNai; the relationship was well described by Michaelis-Menten kinetics. The apparent Km was 15 mM at pHo 7.5 and increased to 134 mM at pHo 6.5, without change in Vmax. In Na(+)-depleted gallbladders, elevating [Na+]o from 0 to 25 mM increased aNai and pHi and caused acidification of a poorly buffered mucosal solution upon stopping the superfusion; lowering pHo inhibited both apical Na+ entry and mucosal solution acidification. Both effects can be ascribed to titration of a single site; the apparent pK's were 7.2 and 7.4, respectively. Diethylpyrocarbonate (DEPC), a histidine-specific reagent, reduced mucosal acidification by 58 +/- 4 or 39 +/- 6% when exposure to the drug was at pHo 7.5 or 6.5, respectively. Amiloride (1 mM) did not protect against the DEPC inhibition, but reduced both apical Na+ entry and mucosal acidification by 63 +/- 5 and 65 +/- 9%, respectively. In the Na(+)-depleted tissues mean pHi was 6.7. Cells were alkalinized by exposure to mucosal solutions containing high concentrations of nicotine or methylamine. Estimates of apical Na+ entry at varying pHi, upon increasing [Na+]o from 0 to 25 mM, indicate that Na+/H+ exchange is active at pHi 7.4. Intracellular H+ stimulated apical Na+ entry by titration of more than one site (apparent pK 7.1, Hill coefficient 1.7). The results suggest that external Na+ and H+ interact with one site of the Na+/H+ exchanger and that cytoplasmic H+ acts on at least two sites. The external titratable group seems to be an imidazolium, which is apparently different from the amiloride-binding site. The dependence of Na+ entry on pHi supports the notion that the Na+/H+ exchanger is operational under normal transport conditions.

Cytomegalovirus: sodium entry and development of cytomegaly in human fibroblasts

A possible relationship between net Na+ entry and the development of CMV-induced cytomegaly (cell enlargement) was investigated in human fibroblasts derived from skin-muscle and thyroid tissue. We found that inhibiting cellular Na+ uptake, either by pharmacological means (amiloride, an inhibitor of Na+/H+ exchange) or by replacement of extracellular Na+ (by N-methyl-D-glucamine or choline), inhibited the development of cytomegaly. Furthermore, we noted a temporal parallelism between the development of cytomegaly and enhancement of ouabain-sensitive (O-S) 86Rb+ uptake. O-S 86Rb+ uptake is a monitor for the activity of the sodium pump resident in the plasmalemma of the fibroblasts. The enhanced O-S 86Rb+ uptake reflects either an increased intracellular Na+ concentration or an increased number of sodium pump complexes per fibroblast. Amiloride inhibited the enhancement of O-S 86Rb+ uptake, as well as cytomegaly development. Addition of amiloride at selected times after infection suggested that the same phase of virus replication was sensitive to the inhibitory effect of this drug on the enhancement of O-S 86Rb+ uptake and on the development of cytomegaly. There was also a similar pattern of inhibition of O-S 86Rb+ uptake and cytomegaly with increasing concentrations of amiloride. Thus, there may be a relationship between CMV-induced Na+ entry through activation of the Na+/H+ exchanger and development of cytomegaly.

The interaction of intracellular Mg2+ and pH on Cl- fluxes associated with intracellular pH regulation in barnacle muscle fibers

The intracellular dialysis technique was used to measure unidirectional Cl- fluxes and net acid extrusion by single muscle fibers from the giant barnacle. Decreasing pHi below normal levels of 7.35 stimulated both Cl- efflux and influx. These increases of Cl- fluxes were blocked by disulfonic acid stilbene derivatives such as SITS and DIDS. The SITS-sensitive Cl- efflux was sharply dependent upon pHi, increasing approximately 20-fold as pHi was decreased from 7.35 to 6.7. Under conditions of normal intracellular Mg2+ concentration, the apparent pKa for the activation of Cl- efflux was 7.0. We found that raising [Mg2+]i, but not [Mg2+]o, had a pronounced inhibitory effect on both SITS-sensitive unidirectional Cl- fluxes as well as on SITS-sensitive net acid extrusion. Increasing [Mg2+]i shifted the apparent pKa of Cl- efflux to a more acid value without affecting the maximal flux that could be attained. This relation between pHi and [Mg2+]i on SITS-sensitive Cl- efflux is consistent with a competition between H ions and Mg ions. We conclude that the SITS-inhibitable Cl- fluxes are mediated by the pHi-regulatory transport mechanism and that changes of intracellular Mg2+ levels can modify the activity of the pHi regulator/anion transporter.