The current produced by the E779A mutant rat Na(+)/K(+) pump alpha1-subunit expressed in HEK 293 cells

A mathematical model of plasma membrane electrophysiology and calcium dynamics in vascular endothelial cells

Vascular endothelial cells (ECs) modulate smooth muscle cell (SMC) contractility, assisting in vascular tone regulation. Cytosolic Ca2+ concentration ([Ca2+]i) and membrane potential ( Vm) play important roles in this process by controlling EC-dependent vasoactive signals and intercellular communication. The present mathematical model integrates plasmalemma electrophysiology and Ca2+ dynamics to investigate EC responses to different stimuli and the controversial relationship between [Ca2+]i and Vm. The model contains descriptions for the intracellular balance of major ionic species and the release of Ca2+ from intracellular stores. It also expands previous formulations by including more detailed transmembrane current descriptions. The model reproduces Vm responses to volume-regulated anion channel (VRAC) blockers and extracellular K+ concentration ([K+]o) challenges, predicting 1) that Vm changes upon VRAC blockade are [K+]o dependent and 2) a biphasic response of Vm to increasing [K+]o. Simulations of agonist-induced Ca2+ mobilization replicate experiments under control and Vm hyperpolarization blockade conditions. They show that peak [Ca2+]i is governed by store Ca2+ release while Ca2+ influx (and consequently Vm) impacts more the resting and plateau [Ca2+]i. The Vm sensitivity of rest and plateau [Ca2+]i is dictated by a [Ca2+]i “buffering” system capable of masking the Vm-dependent transmembrane Ca2+ influx. The model predicts plasma membrane Ca2+-ATPase and Ca2+ permeability as main players in this process. The heterogeneous Vm impact on [Ca2+]i may elucidate conflicting reports on how Vm influences EC Ca2+. The present study forms the basis for the development of multicellular EC-SMC models that can assist in understanding vascular autoregulation in health and disease.

Na,K-ATPase mutations in familial hemiplegic migraine lead to functional inactivation

The Na,K-ATPase is an ion-translocating transmembrane protein that actively maintains the electrochemical gradients for Na+ and K+ across the plasma membrane. The functional protein is a heterodimer comprising a catalytic alpha-subunit (four isoforms) and an ancillary beta-subunit (three isoforms). Mutations in the alpha2-subunit have recently been implicated in familial hemiplegic migraine type 2, but almost no thorough studies of the functional consequences of these mutations have been provided. We investigated the functional properties of the mutations L764P and W887R in the human Na,K-ATPase alpha2-subunit upon heterologous expression in Xenopus oocytes. No Na,K-ATPase-specific pump currents could be detected in cells expressing these mutants. The binding of radiolabelled [3H]ouabain to intact cells suggested that this could be due to a lack of plasma membrane expression. However, plasma membrane isolation showed that the mutated pumps are well expressed at the plasma membrane. 86Rb+-flux and ATPase activity measurements demonstrated that the mutants are inactive. Therefore, the primary disease-causing mechanism is loss-of-function of the Na,K-ATPase alpha2-isoform.

Electrophysiological analysis of the mutated Na,K-ATPase cation binding pocket

Na,K-ATPase mediates net electrogenic transport by extruding three Na+ ions and importing two K+ ions across the plasma membrane during each reaction cycle. We mutated putative cation coordinating amino acids in transmembrane hairpin M5-M6 of rat Na,K-ATPase: Asp776 (Gln, Asp, Ala), Glu779 (Asp, Gln, Ala), Asp804 (Glu, Asn, Ala), and Asp808 (Glu, Asn, Ala). Electrogenic cation transport properties of these 12 mutants were analyzed in two-electrode voltage-clamp experiments on Xenopus laevis oocytes by measuring the voltage dependence of K+-stimulated stationary currents and pre-steady-state currents under electrogenic Na+/Na+ exchange conditions. Whereas mutants D804N, D804A, and D808A hardly showed any Na+/K+ pump currents, the other constructs could be classified according to the [K+] and voltage dependence of their stationary currents; mutants N776A and E779Q behaved similarly to the wild-type enzyme. Mutants E779D, E779A, D808E, and D808N had in common a decreased apparent affinity for extracellular K+. Mutants N776Q, N776D, and D804E showed large deviations from the wild-type behavior; the currents generated by mutant N776D showed weaker voltage dependence, and the current-voltage curves of mutants N776Q and D804E exhibited a negative slope. The apparent rate constants determined from transient Na+/Na+ exchange currents are rather voltage-independent and at potentials above -60 mV faster than the wild type. Thus, the characteristic voltage-dependent increase of the rate constants at hyperpolarizing potentials is almost absent in these mutants. Accordingly, dislocating the carboxamide or carboxyl group of Asn776 and Asp804, respectively, decreases the extracellular Na+ affinity.

Electrogenicity of Na,K- and H,K-ATPase activity and presence of a positively charged amino acid in the fifth transmembrane segment

The transport activity of the Na,K-ATPase (a 3 Na+ for 2 K+ ion exchange) is electrogenic, whereas the closely related gastric and non-gastric H,K-ATPases perform electroneutral cation exchange. We have studied the role of a highly conserved serine residue in the fifth transmembrane segment of the Na,K-ATPase, which is replaced with a lysine in all known H,K-ATPases. Ouabain-sensitive 86Rb uptake and K+-activated currents were measured in Xenopus oocytes expressing the Bufo bladder H,K-ATPase or the Bufo Na,K-ATPase in which these residues, Lys800 and Ser782, respectively, were mutated. Mutants K800A and K800E of the H,K-ATPase showed K+-stimulated and ouabain-sensitive electrogenic transport. In contrast, when the positive charge was conserved (K800R), no K+-induced outward current could be measured, even though rubidium transport activity was present. Conversely, the S782R mutant of the Na,K-ATPase had non-electrogenic transport activity, whereas the S782A mutant was electrogenic. The K800S mutant of the H,K-ATPase had a more complex behavior, with electrogenic transport only in the absence of extracellular Na+. Thus, a single positively charged residue in the fifth transmembrane segment of the alpha-subunit can determine the electrogenicity and therefore the stoichiometry of cation transport by these ATPases.