Charge translocation by the Na/K pump

Obtaining “ghosts” of red blood cells with a set ion concentration as models for studying Na+, K+ - ATPase

The article considers the production of red blood cell “ghosts” and their application as a model for studying Na+, K+ - ATPase. Various variants of liposomes are used as test objects for studying ion transport, but the best model of biological membranes for studying ion transport and ATPase pumps are the «ghosts» of red blood cells. The activity of blood Na+/K+ - ATPase is determined mainly in the suspension of red blood cell “ghosts”. This is due to the need to remove the interfering compound – hemoglobin - and obtain “ghosts” that are necessary for accurate measurement of the activity of the enzyme. In this scientific work, studies were conducted to determine the concentrations of Na+ and K+ ions in the blood and red blood cells of cows and calves of various ages by atomic absorption and atomic emission spectroscopy. The values of these concentrations of Na+ and K+ ions were used to obtain red blood cell “ghosts”. “Ghosts” of red blood cells with a set concentration of sodium and potassium ions were obtained by the method of J. T. Dodge, in the modification of Zhumadilov Zh. Sh. and Gening T. P. and in our modification. Studies of the concentration of Na+ and K+ ions in the obtained «ghosts» of red blood cells showed their full compliance with the calculated values.

Movement of 'gating charge' is coupled to ligand binding in a G-protein-coupled receptor

Activation by agonist binding of G-protein-coupled receptors (GPCRs) controls most signal transduction processes. Although these receptors span the cell membrane, they are not considered to be voltage sensitive. Recently it was shown that both the activity of GPCRs and their affinity towards agonists are regulated by membrane potential. However, it remains unclear whether GPCRs intrinsically respond to changes in membrane potential. Here we show that two prototypical GPCRs, the m2 and m1 muscarinic receptors (m2R and m1R), display charge-movement-associated currents analogous to 'gating currents' of voltage-gated channels. The gating charge-voltage relationship of m2R correlates well with the voltage dependence of the affinity of the receptor for acetylcholine. The loop that couples m2R and m1R to their G protein has a crucial function in coupling voltage sensing to agonist-binding affinity. Our data strongly indicate that GPCRs serve as sensors for both transmembrane potential and external chemical signals.

Structure-function relationships in the NA+,K+-pump

The Na,K-pump was discovered about 50 years ago. Since then there has been a methodic investigation of its structure and functional characteristics. The development of the Albers-Post model for the transport cycle was a milestone that provided the framework for detailed understanding of the transport process. The pump is composed of 2 subunits that exist in the membrane as an alphabeta heterodimer. All known enzymatic functions of the pump occur through the alpha subunit. Although necessary for activity, the complete role of the beta subunit is not understood fully. Numerous studies have established that the alphabeta protomer is the minimal functional unit needed to perform the Albers-Post reaction cycle. However, higher orders of aggregation [(alphabeta)n] are commonly detected. There is little evidence that oligomerization has functional consequence for ion transport. The Na+,K+-adenosine triphosphatase (ATPase) is a member of the P-type ATPase family of transporters. Proteins within this family have common amino acid sequence motifs that share functional characteristics and structure. Low-resolution 3-dimensional reconstruction of 2-dimensional crystal diffractions provide evidence for the similarity in tertiary structure of the alpha subunit and the Ca2+ATPase (a closely related P-type ATPase). The spatial location of the beta subunit also is obvious in these reconstructions. Recent high-resolution reconstructions from 3-dimensional crystals of the Ca2+ATPase provide structural details at the atomic level. It now is possible to interpret structurally some of the key steps in the Albers-Post reaction. Some of these high-resolution interpretations are translatable to the Na+,K+-ATPase, but a high-resolution structure of the Na,K-pump is needed for the necessary details of those aspects that are unique to this transporter.

Development of models of active ion transport for whole-cell modelling: cardiac sodium-potassium pump as a case study

This study presents a method for the reduction of biophysically-based kinetic models for the active transport of ions. A lumping scheme is presented which exploits the differences in timescales associated with fast and slow transitions between model states, while maintaining the thermodynamic properties of the model. The goal of this approach is to contribute to modelling of the effects of disturbances to metabolism, associated with ischaemic heart disease, on cardiac cell function. The approach is illustrated for the sodium-potassium pump in the myocyte. The lumping scheme is applied to produce a 4-state representation from the detailed 15-state model of Läuger and Apell, Eur. Biophys. J. 13 (1986) 309, for which the principles of free energy transduction are used to link the free energy released from ATP hydrolysis (deltaGATP) to the transition rates between states of the model. An iterative minimisation algorithm is implemented to determine the transition rate parameters based on the model fit to experimental data. Finally, the relationship between deltaGATP and pump cycling direction is investigated and compared with recent experimental findings.

Electrophysiology of the sodium-potassium-ATPase in cardiac cells

Like several other ion transporters, the Na(+)-K(+) pump of animal cells is electrogenic. The pump generates the pump current I(p). Under physiological conditions, I(p) is an outward current. It can be measured by electrophysiological methods. These methods permit the study of characteristics of the Na(+)-K(+) pump in its physiological environment, i.e., in the cell membrane. The cell membrane, across which a potential gradient exists, separates the cytosol and extracellular medium, which have distinctly different ionic compositions. The introduction of the patch-clamp techniques and the enzymatic isolation of cells have facilitated the investigation of I(p) in single cardiac myocytes. This review summarizes and discusses the results obtained from I(p) measurements in isolated cardiac cells. These results offer new exciting insights into the voltage and ionic dependence of the Na(+)-K(+) pump activity, its effect on membrane potential, and its modulation by hormones, transmitters, and drugs. They are fundamental for our current understanding of Na(+)-K(+) pumping in electrically excitable cells.

Voltage dependence of the apparent affinity for external Na(+) of the backward-running sodium pump

The steady-state voltage and [Na(+)](o) dependence of the electrogenic sodium pump was investigated in voltage-clamped internally dialyzed giant axons of the squid, Loligo pealei, under conditions that promote the backward-running mode (K(+)-free seawater; ATP- and Na(+)-free internal solution containing ADP and orthophosphate). The ratio of pump-mediated (42)K(+) efflux to reverse pump current, I(pump) (both defined by sensitivity to dihydrodigitoxigenin, H(2)DTG), scaled by Faraday's constant, was -1.5 +/- 0.4 (n = 5; expected ratio for 2 K(+)/3 Na(+) stoichiometry is -2.0). Steady-state reverse pump current-voltage (I(pump)-V) relationships were obtained either from the shifts in holding current after repeated exposures of an axon clamped at various V(m) to H(2)DTG or from the difference between membrane I-V relationships obtained by imposing V(m) staircases in the presence or absence of H(2)DTG. With the second method, we also investigated the influence of [Na(+)](o) (up to 800 mM, for which hypertonic solutions were used) on the steady-state reverse I(pump)-V relationship. The reverse I(pump)-V relationship is sigmoid, I(pump) saturating at large negative V(m), and each doubling of [Na(+)](o) causes a fixed (29 mV) rightward parallel shift along the voltage axis of this Boltzmann partition function (apparent valence z = 0.80). These characteristics mirror those of steady-state (22)Na(+) efflux during electroneutral Na(+)/Na(+) exchange, and follow without additional postulates from the same simple high field access channel model (Gadsby, D.C., R.F. Rakowski, and P. De Weer, 1993. Science. 260:100-103). This model predicts valence z = nlambda, where n (1.33 +/- 0.05) is the Hill coefficient of Na binding, and lambda (0.61 +/- 0.03) is the fraction of the membrane electric field traversed by Na ions reaching their binding site. More elaborate alternative models can accommodate all the steady-state features of the reverse pumping and electroneutral Na(+)/Na(+) exchange modes only with additional assumptions that render them less likely.

Binding of the third Na+ ion to the cytoplasmic side of the Na,K-ATPase is electrogenic

A new experimental setup was constructed to allow parallel measurements of total internal reflection fluorescence and of capacitance changes in Na,K-ATPase-containing membranes. Effects correlated with cytoplasmic sodium binding to Na,K-ATPase were investigated. Ion binding-induced fluorescence changes of the electrochromic dye RH421 in membrane fragments adsorbed on a transparent capacitative electrode corresponded perfectly to capacitance increases detected by a lock-in technique. From these electric measurements it was possible to estimate a dielectric coefficient of about 0.25 for the electrogenic binding of the third Na+ ion. Binding of K+ to cytoplasmic sites was electroneutral.

Depolarization increases the apparent affinity of the Na+-K+ pump to cytoplasmic Na+ in isolated guinea-pig ventricular myocytes

1. In order to investigate the possible effect of membrane potential on cytoplasmic Na+ binding to the Na+-K+ pump, we studied Na+-K+ pump current-voltage relationships in single guinea-pig ventricular myocytes whole-cell voltage clamped with pipette solutions containing various concentrations of Na+ ([Na+]pip) and either tetraethylammonium (TEA+) or N-methyl-D-glucamine (NMDG+) as the main cation. The experiments were conducted at 30 C under conditions designed to abolish the known voltage dependence of other steps in the pump cycle, i.e. in Na+-free external media containing 20 mM Cs+. 2. Na+-K+ pump current (Ip) was absent in cells dialysed with Na+-free pipette solutions and was almost voltage independent at 50 mM Na+pip (potential range: -100 to +40 mV). By contrast, the activation of Ip by 0.5-5 mM Na+pip was clearly voltage sensitive and increased with depolarization, independently of the main intracellular cation species. 3. The apparent affinity of the Na+-K+ pump for cytoplasmic Na+ increased monotonically with depolarization. The [Na+]pip required for half-maximal Ip activation (K0.5 value) amounted to 5.6 mM at -100 mV and to 2.2 mM at +40 mV. 4. The results suggest that cytoplasmic Na+ binding and/or a subsequent partial reaction in the pump cycle prior to Na+ release is voltage dependent. From the voltage dependence of the K0.5 values the dielectric coefficient for intracellular Na+ binding/translocation was calculated to be approximately 0.08. The voltage-dependent mechanism might add to the activation of the cardiac Na+-K+ pump during cardiac excitation.

Role of the intracellular domain of the beta subunit in Na,K pump function

The catalytic alpha subunit of the (Na,K)- and (H,K)-ATPases needs to be coexpressed with a beta subunit in order to produce cation transport activity. Although the isoform of the beta subunit is known to influence the functional characteristics of the Na,K pump, the role of the different domains of the beta subunit is not fully understood. We have studied the function of a Na,K pump resulting from the expression of a wild-type alpha subunit with a N-terminally truncated mutant of the beta subunit using the two-electrode voltage clamp and the cut-open oocyte techniques. While the maximal activity, measured as the K+-activated outward current, was not significantly altered, the beta N-terminal truncation induced an ouabain-sensitive conductance in the absence of extracellular K+. The voltage dependence of the ouabain-sensitive charge distribution indicated that in the Na/Na exchange conditions, the E1-E2 conformation equilibrium was shifted towards the E2 conformation, a change resulting from alteration of both the forward and the backward reaction rate. Removal of the intracellular domain of the beta subunit modifies several aspects of the whole enzyme function by a mechanism that must imply the state of the extracellular and/or transmembrane parts of the alpha/beta subunit complex.