Activation Of Electrogenic Na Research Articles

Aging Leads to Over-Expression of Na<sup>+</sup>/K<sup>+</sup> Pump Units in Liver and Na<sup>+</sup>/Ca<sup>2+</sup> Exchangers in Brain Cortex

The metabolic controlling of tissue hydration is the fundamental parameter determining cell functional activity and its dysfunction is the common consequence of any cell pathology, including aging. The aim of the present study is to reveal the differences of age-dependent metabolic controlling of cell hydration of excitable tissue such as brain cortex and non-excitable tissues such as liver and spleen. For this purpose, the age-dependent sensitivity of cell hydration in excitable and non-excitablet issues is studied through depressing metabolic activity by cooling and its activation by supplying animals with distilled water, by inactivation of Na+/K+ pump and activation of Na+/Ca2+ exchange in the reverse mode. The obtained data bring us to the conclusion that the metabolic regulation of cell hydration in excitable tissue is realized by the activation of electrogenic Na+/K+ pump and Na+/Ca2+ exchange in the cell membrane and the formation of endogen water by mitochondrial activity, while the regulation of cell hydration in non-excitable tissue is carried out only by the activity of mitochondrial function. Aging leads to an over-expression of Na+/K+ pump units in liver and Na+/Ca2+ exchanger in brain cortex of rats.

Relative Contribution of Local Ca2+ Releases (LCRS) and AP-Induced Ca2+ Transient Decay to Diastolic Depolarization in Rabbit Sa Node Cells

Background: Intracellular Ca2+ dynamics contribute to cardiac pacemaker cell function via activation of electrogenic Na+/Ca2+ exchanger during diastolic depolarization. Two fundamental mechanisms have been implicated for the diastolic Ca2+ signaling: LCRs and the decaying AP-induced Ca2+ transients. Their relative contributions, however, are unknown.Methods: We evaluated the relative contributions based on simultaneous high-speed camera recordings of Ca2+ signal, alongside membrane potential recordings by perforated patch clamp in single isolated rabbit SA node cells prior to and during beta-adrenergic receptor stimulation with isoproterenol (100 nM). We obtained further insights by employing simulations of our recent numerical model featuring local Ca2+ dynamics in 3D in these cells.Results: All cells tested (n=5) exhibited both LCRs and AP-induced-transient residuals during diastolic depolarization. Individual LCRs have been characterized in the 2D movies using our novel computer algorithm (see our poster on automated analysis of LCRs). The values of LCR signal mass and Ca2+ transient residuals were integrated within the entire cell perimeter in the focal plane of video-recording and compared. Both our analysis of the experimental data and model simulations demonstrated that LCRs provide a major part of the net Ca2+ diastolic signal, both prior to and during isoproterenol exposure. The relative contribution of LCRs, however, substantially increased after isoproterenol application. Specifically, while the contribution of the transient residuals into the net diastolic Ca2+ signal prior to isoproterenol is about 30%, the net signal following isoproterenol becomes almost completely driven by the LCRs, Ca2+ transient decays are accelerated and almost fully complete prior the maximum diastolic depolarization.Conclusions: Both LCRs and local Ca2+ transient residuals contribute to diastolic depolarization. As pacemaker rate increases LCR signal waxes, but the contribution of Ca2+ transient residuals wanes.

Regulation of the Basolateral Na+, K+-ATPase in the Activation of Electrogenic Na+Absorption Induced by Isoproterenol in the Rat Distal Colon

We investigated the effects of cAMP on the basolateral Na(+), K(+)-ATPase and apical amiloride-sensitive Na(+) channel in the rat distal colon. Under mucosal treatment with nystatin, isoproterenol and forskolin increased the basolateral Na(+) pump current. Under serosal treatment with ouabain, both agents slightly increased the amiloride-sensitive Na current, but not to a significant level. We concluded that cAMP activates the basolateral Na(+), K(+)-ATPase in the rat distal colon.

Chronic Actions of Nicotine on the Electrogenic Activity of Na+,K+-ATPase and the Contractile Characteristics of Rat Diaphragm

Rats received chronic nicotine via subcutaneous injections at a dose of 6 mg/kg/day for 2–3 weeks. Resting membrane potential and action potentials were recorded from muscle fibers at the end of this period, along with single and tetanic (20 and 50 impulses/sec) isometric contractions in isolated rat diaphragm. The electrogenic contribution of the α2 isoform of Na+,K+,-ATPase was assessed using ouabain (1 μM). In rats receiving chronic nicotine, the resting membrane potential was decreased by 2.2 ± 0.6 mV (p < 0.01); the electrogenic contribution of the α2 isoform of Na+,K+,-ATPase was about two times less than in the control group of animals (3.7 ± 0.6 and 6.4 ± 0.6 mV respectively, p < 0.01). Chronic nicotine had no effect on the force of single or tetanic muscle contractions in response to direct and indirect stimulation. There was a decrease in the durations of single contractions, along with a decrease in the rise time of tetanic contractions. There were no changes in the dynamics of tiring. It is suggested that chronic nicotine administration leads to decreases in the electrogenic activity of the α2 isoform of Na+,K+,-ATPase, with a resultant impairment to the electrogenesis of the diaphragm in rats.

Spike integration and cellular memory in a rhythmic network from Na+/K+ pump current dynamics

The output of a neural circuit results from an interaction between the intrinsic properties of neurons within the circuit and the features of the synaptic connections between them. The plasticity of intrinsic properties has been primarily attributed to modification of ion channel function and/or number. In this study, we demonstrate a mechanism for intrinsic plasticity in rhythmically active Drosophila neurons that is not conductance-based. Larval motor neurons show a long lasting sodium-dependent afterhyperpolarization (AHP) following bursts of action potentials that is mediated by the electrogenic activity of Na+/K+ ATPase. This AHP persists for multiple seconds following volleys of action potentials and is able to function as a pattern-insensitive integrator of spike number that is independent of external calcium. This current also interacts with endogenous Shal K+ conductances to modulate spike timing for multiple seconds following rhythmic activity, providing a cellular memory of network activity on a behaviorally relevant time scale.

Indirect effects of acetylcholine on the electrogenic sodium pump in bull‐frog atrial muscle fibres.

1. Effects of acetylcholine (ACh) on the activity of electrogenic Na+ pump in bullfrog atrial muscle fibres were examined using the single sucrose-gap voltage clamp technique. 2. In the K+-free solution, 10 microM-ACh induced a large outward current (ACh-induced current) with an increase in the membrane conductance. 3. The amplitude of the ACh-induced current decreased to 15% of the control 10 min after application of 1 microM-ouabain, suggesting the contribution of electrogenic Na+ pump to the ACh-induced current. The remaining ACh-induced current was not affected even if the concentration of ouabain was increased ten times. 4. The K+-activated current induced by an activation of the electrogenic Na+ pump was suppressed or reversed its direction during the course of the ACh-induced current. 5. The ACh-induced current was completely inhibited by applications of either atropine or barium ions while the K+-activated current was not affected. 6. Both ouabain-sensitive and -insensitive ACh-induced currents were decreased when the membrane was hyperpolarized and eliminated around -95 mV. 7. The ouabain-sensitive component was decreased by increasing the external K+ concentration [K+]o; the proportions of this current to ACh-induced current in 0.5, 0.75, 1 and 2 mM [K+]o were 54, 42, 34 and 14%, respectively. 8. The current-voltage (i-v) relation obtained in 2 or 4 mM [K+]o, where the currents carried by Na+ and Ca2+ were blocked by application of 1 microM-TTX and 1 mM-Cd2+, exhibits marked inward-going rectification but does not show a clear N-shaped feature. Ba2+ (1 mM) induced an inward current at the holding potential (-80 mV) and eliminated the inward-going rectification of the membrane. 9. These results suggest that the increase in the K+ permeability by ACh increases the concentration of K+ immediately outside of the membrane, which in turn stimulates the electrogenic Na+ pump mechanism. The physiological significance of the action of ACh on the electrogenic Na+ pump in bull-frog atrium is discussed in relation to the background K+ current (IK,1).

Sensitization-induced sodium influx in airway smooth muscle cells of guinea pigs

Airway smooth muscle (ASM) preparations isolated from Camm-Hartley male guinea pigs were passively in vitro sensitized using Austen's technique. During this sensitization, cells were frequently penetrated with glass microelectrodes (a resistance of 80–90 MΩ) for a period of 120 min. Simultaneously, changes in the isometric force of ASM preparations were also monitored. It was found that: (1) administration of serum (1:10 dilution) from sensitized animals caused a depolarization of ASM cells (−52.3 ± 0.7 mV), achieving a peak at 4 min, followed by slow sustained hyperpolarization reaching a steady state after 15 min; (Resulting resting membrane potential of ASM cells was −60.0 ± 0.7 mV); (2) in the presence of ouabain (10 −5 M), the development of hyperpolarization of ASM cells was completely inhibited; (3) replacement of Na + with choline chloride or lithium chloride partially inhibited the development of depolarization of ASM cells; (4) in the presence of specific sodium entry blocker, amiloride the sensitization-caused depolarization was abolished; (5) the presence of choline chloride, lithium chloride or amiloride in the experimental medium completely inhibited the contractile response of sensitized tissues to a specific antigen; (6) response of ASM preparations to a specific antigen was absent during depolarization of ASM cells, but was always present during their hyperpolarization. It is concluded that in vitro passive sensitization leads to the interaction of serum specific antibodies with the membrane of airway smooth muscle cells so that permeability of membrane for Na + is increased (transient depolarization) followed by the activation of electrogenic Na +-pump (sustained hyperpolarization). These events seem to determine the contractile response of sensitized ASM cells to a specific antigen.

Vasopressin induced rhythmic activity in rat basilar artery.

The effects of vasopressin on membrane potential and tension were studied in isolated segments of basilar arteries from the University of Iowa colonies of normotensive inbred Kyoto-Wistar rats (WKY) and stroke-prone spontaneously hypertensive rats (SP-SHR). In the presence of vasopressin (0.01-0.3 IU/ml), basilar arteries from WKY, but not from SP-SHR, developed rhythmic contractions. These contractions were recorded in 13 of 14 WKY basilar arteries, were unaffected by pretreatment with 6-hydroxydopamine, and were characterized by 20-100 dyne oscillations in tension, occurring 1-3 cycles/min, and superimposed on the vasopressin-induced contraction (averaging 60 dynes at 0.01 IU/ml or 160 dynes at 0.3 IU/ml). However, resting membrane potentials were not different in SP-SHR vs. WKY at 37 degrees C, and both strains showed about the same (11 mV) depolarization by 0.1 IU/ml of vasopressin. The rhythmic contractions were enhanced by K+-free solution, and abolished in the presence of high K+ solution (30 mM), suggesting that active Na+-K+ transport may be involved in modulating the rhythmic activity. These findings are consistent with the hypothesis that the vasopressin-induced rhythmic contractions in WKY basilar arteries are at least partly dependent on a reduced activity of electrogenic Na+-K+ active transport in WKY as compared to SP-SHR.

Acute resetting of carotid sinus baroreceptors. II. Possible involvement of electrogenic Na+ pump.

In the accompanying manuscript [Am. J. Physiol. 247 (Heart Circ. Physiol. 16): H824-H832, 1984] we demonstrated that a mechanical mechanism alone cannot account for acute resetting of baroreceptors. To determine if changes in the activity of a baroreceptor electrogenic Na+ pump contributed to resetting, single-unit baroreceptor discharge was recorded from the carotid sinus nerve while resetting protocols were performed before and after exposing the vascularly isolated carotid sinus to treatments known to block the Na+ pump [ouabain and low extracellular K+ concentration ([K+]o)]. Ouabain (0.1-0.5 microgram/ml) blocked the increase in baroreceptor threshold pressure that occurred when intrasinus pressure was increased by 30 mmHg for 15 min [delta threshold = 16 +/- 3 (SE) mmHg before and 1.2 +/- 2.3 mmHg after ouabain]. In 12 experiments carotid sinus pressure was increased from 70 to 160 mmHg for 5 min and then returned to 70 mmHg for 10 min in the presence of both normal [K+]o (5.8 mM) and low [K+]o. Exposure to the low K+ solution resulted in a significantly smaller increase in threshold pressure when intrasinus pressure was increased from 70 to 160 mmHg (9 +/- 2.7 vs. 18 +/- 2.1 mmHg). When K+ was replaced, threshold pressure again increased by 18 +/- 2.3 mmHg, the increase in threshold was reversed. Thus, since ouabain blocked and low [K+]o attenuated acute resetting of the baroreceptors, we propose that changes in the activity of an electrogenic Na+ pump contribute to acute resetting.

Light activated electrogenic Na+-Ca2+-exchange in fly photoreceptors: Modulation by Na+/K+-pump activity

We found recently that illumination of white-eyed Musca photoreceptors following application of ruthenium red (RR), (a known inhibitor of Ca2+-uptake into intracellular organelles) induced a transient post-illumination-afterdepolarization (TA). We found that the TA depended strongly on Na+- and Ca2+-gradients across the cell membrane suggesting that it arose from activation of electrogenic Na+-Ca2+-exchange. Since the magnitude of the TA was dependent on the Na+-gradient, in this work we studied the effects of the Na+/K+-pump, which controls the Na+-gradient, on the TA. Increasing extracellular K+-concentration which largely increased pump activity also increased the TA. Application of 100 ΜM ouabain, for a short time, depressed the TA considerably without causing any noticeable effect on the resting or the receptor potential. Only after longer exposure to ouabain, when the TA was totally depressed, were the resting and receptor potentials abolished by only a few strong light flashes. Trains of strong light pulses either facilitated or inhibited the TA depending on the degree of increase in internal Ca2+-ions by light and on the activity of the Na+/K+-pump.

Activation of electrogenic Na+/K+ exchange by extracellular K+ in canine cardiac Purkinje fibers.

Transient increments in sodium pump current were elicited in small voltage-clamped Purkinje fibers suspended in a fast flow system by briefly exposing them to K+-free fluid, to temporarily inhibit the pump, and then suddenly returning them to K+-containing fluid. The exponential time course of decay of the current increment provides a measure of the pump rate constant for Na+ extrusion. The dependence of that rate constant, and of the peak amplitude of the increment in pump current, on the extracellular K+ concentration was determined. The results indicate: that in cardiac Purkinje cells, as in many other cells, the pump is half-maximally activated by about 1 mM K+; that the coupling ratio for Na+/K+ exchange is independent of either intracellular Na+ concentration or external K+ concentration; and that a simple model in which intracellular Na+ concentration is determined by a passive "leak," and an active extrusion of Na+, seems sufficient to account for moderate changes in cellular Na+ concentration.

Effect of transmembrane ion gradients on Raman spectra of sealed, hemoglobin-free erythrocyte membrane vesicles.

Sealed hemoglobin-free erythrocyte vesicles have been isolated. Imposition of transmembrane cation gradients increases the intensity of Raman scattering in the CH3-stretching region as observed with unsealed ghosts at temperatures greater than 38 degrees C and pH less than 7.0 [Verma, S. P. & Wallach, D. F. H. (1976) Proc. Natl. Acad. Sci. USA 73, 3358--3561]. Modifications in the amide I and amide III frequencies consistent with increased helicity of membrane proteins are observed upon imposition of a cation gradient. Spectrin-free vesicles also demonstrate cation gradient-sensitive intensity changes in the CH3-stretching region. However, no evidence for cation gradient-related protein conformation changes is found with these vesicles. The transmembrane potential of these vesicles has been altered by variations in anion composition and the electrogenic activity of Na+,K+-ATPase. The membrane potential was monitored by cyanine dye fluorescence. Imposition of a membrane potential (negative inside) also increased the intensity of Raman scattering in the CH3-stretching region. These results suggest that a transmembrane potential (negative inside) and/or cation gradient can energize membranes by compression of the apolar region and transfer of protein methyl residues into polar regions.

Activation of Electrogenic Na&lt;sup&gt;+&lt;/sup&gt; Pump by Epinephrine in Bullfrog Atrium

The effects of epinephrine (1 X 10(-6)--1 X 10(-5) M) on the resting membrane potential and the K+-activated hyperpolarization of bullfrog atrial heart muscles were studied by the single sucrose-gap method or by intracellular microelectrode filled with 3 M KC1. Epinephrine-induced hyperpolarization (Ep-hyperpolarization) was completely eliminated by the effect of ouabain or by removal of extracellular K+. The amplitude of K+-activated hyperpolarization, which was produced when the extracellular K+ concentration was raised from zero to 2 mM, was markedly increased in the presence of epinephrine. The membrane depolarizations, due to high extracellular K+ concentration in the presence of ouabain (5 X 10(-6) M) which completely and reversibly eliminated the K+-activated hyperpolarization, were not altered under the effect of epinephrine. These results suggested that Ep-hyperpolarization of bullfrog atrial heart muscles was due to the acceleration of electrogenic Na+ pump which produced the K+-activated hyperpolarization.