Characteristics of electrogenic sodium pumping in rat myometrium

Sodium channels and transporters in the myometrium

In excitable cells such as neurons and cardiomyocytes, sodium influx across the plasma membrane contributes to the resting membrane potential, and sodium is the key ion for generating action potentials. In myometrial smooth muscle cells, however, the functions of sodium influx have not been fully elucidated. This review briefly discusses the contribution of Na+ pumps to myometrial excitability but given the brevity of this article, we focus on the evidence that sodium influx through various types of channels may play numerous roles in controlling myometrial excitability.

Reconstruction of Cell Surface Densities of Ion Pumps, Exchangers, and Channels from mRNA Expression, Conductance Kinetics, Whole-Cell Calcium, and Current-Clamp Voltage Recordings, with an Application to Human Uterine Smooth Muscle Cells

Uterine smooth muscle cells remain quiescent throughout most of gestation, only generating spontaneous action potentials immediately prior to, and during, labor. This study presents a method that combines transcriptomics with biophysical recordings to characterise the conductance repertoire of these cells, the ‘conductance repertoire’ being the total complement of ion channels and transporters expressed by an electrically active cell. Transcriptomic analysis provides a set of potential electrogenic entities, of which the conductance repertoire is a subset. Each entity within the conductance repertoire was modeled independently and its gating parameter values were fixed using the available biophysical data. The only remaining free parameters were the surface densities for each entity. We characterise the space of combinations of surface densities (density vectors) consistent with experimentally observed membrane potential and calcium waveforms. This yields insights on the functional redundancy of the system as well as its behavioral versatility. Our approach couples high-throughput transcriptomic data with physiological behaviors in health and disease, and provides a formal method to link genotype to phenotype in excitable systems. We accurately predict current densities and chart functional redundancy. For example, we find that to evoke the observed voltage waveform, the BK channel is functionally redundant whereas hERG is essential. Furthermore, our analysis suggests that activation of calcium-activated chloride conductances by intracellular calcium release is the key factor underlying spontaneous depolarisations.

A well-known problem in electrophysiologal modeling is that the parameters of the gating kinetics of the ion channels cannot be uniquely determined from observed behavior at the cellular level. One solution is to employ simplified “macroscopic” currents that mimic the behavior of aggregates of distinct entities at the protein level. The gating parameters of each channel or pump can be determined by studying it in isolation, leaving the general problem of finding the densities at which the channels occur in the plasma membrane. We propose an approach, which we apply to uterine smooth muscle cells, whereby we constrain the list of possible entities by means of transcriptomics and chart the indeterminacy of the problem in terms of the kernel of the corresponding linear transformation. A graphical representation of this kernel visualises the functional redundancy of the system. We show that the role of certain conductances can be fulfilled, or compensated for, by suitable combinations of other conductances; this is not always the case, and such “non-substitutable” conductances can be regarded as functionally non-redundant. Electrogenic entities belonging to the latter category are suitable putative clinical targets.

A computational model of the ionic currents, Ca2+ dynamics and action potentials underlying contraction of isolated uterine smooth muscle

Uterine contractions during labor are discretely regulated by rhythmic action potentials (AP) of varying duration and form that serve to determine calcium-dependent force production. We have employed a computational biology approach to develop a fuller understanding of the complexity of excitation-contraction (E-C) coupling of uterine smooth muscle cells (USMC). Our overall aim is to establish a mathematical platform of sufficient biophysical detail to quantitatively describe known uterine E-C coupling parameters and thereby inform future empirical investigations of physiological and pathophysiological mechanisms governing normal and dysfunctional labors. From published and unpublished data we construct mathematical models for fourteen ionic currents of USMCs: currents (L- and T-type), current, an hyperpolarization-activated current, three voltage-gated currents, two -activated current, -activated current, non-specific cation current, - exchanger, - pump and background current. The magnitudes and kinetics of each current system in a spindle shaped single cell with a specified surface area∶volume ratio is described by differential equations, in terms of maximal conductances, electrochemical gradient, voltage-dependent activation/inactivation gating variables and temporal changes in intracellular computed from known fluxes. These quantifications are validated by the reconstruction of the individual experimental ionic currents obtained under voltage-clamp. Phasic contraction is modeled in relation to the time constant of changing . This integrated model is validated by its reconstruction of the different USMC AP configurations (spikes, plateau and bursts of spikes), the change from bursting to plateau type AP produced by estradiol and of simultaneous experimental recordings of spontaneous AP, and phasic force. In summary, our advanced mathematical model provides a powerful tool to investigate the physiological ionic mechanisms underlying the genesis of uterine electrical E-C coupling of labor and parturition. This will furnish the evolution of descriptive and predictive quantitative models of myometrial electrogenesis at the whole cell and tissue levels.

Functional effects of caloxin 1c2, a novel engineered selective inhibitor of plasma membrane Ca(2+)-pump isoform 4, on coronary artery

Coronary artery smooth muscle expresses the plasma membrane Ca²⁺ pump (PMCA) isoforms PMCA4 and PMCA1. We previously reported the peptide inhibitor caloxin 1b1 that was obtained by using extracellular domain 1 of PMCA4 as the target (Am J Physiol Cell.290 [2006] C1341). To engineer inhibitors with greater affinity and isoform selectivity, we have now created a phage display library of caloxin 1b1-like peptides. We screened this library by affinity chromatography with PMCA from erythrocyte ghosts that contain mainly PMCA4 to obtain caloxin 1c2. Key properties of caloxin 1c2 are (a) Ki = 2.3 ± 0.3 μM which corresponds to a 20× higher affinity for PMCA4 than that of caloxin 1b1 and (b) it is selective for PMCA4 since it has greater than 10-fold affinity for PMCA4 than for PMCA1, 2 or 3. It had the following functional effects on coronary artery smooth muscle: (a) it increased basal tone of the de-endothelialized arteries; the increase being similar at 10, 20 or 50 μM, and (b) it enhanced the increase in the force of contraction at 0.05 but not at 1.6 mM extracellular Ca²⁺ when Ca²⁺ extrusion via the Na⁺–Ca²⁺ exchanger and the sarco/endoplasmic reticulum Ca²⁺ pump were inhibited. We conclude that PMCA4 is pivotal to Ca²⁺ extrusion in coronary artery smooth muscle. We anticipate caloxin 1c2 to aid in understanding the role of PMCA4 in signal transduction and home-ostasis due to its isoform selectivity and ability to act when added extracellularly.

Fifty-eight years in science--a commentary

After 58 years in science, mostly in pharmacology, one gains perspective. Mine is that there have been important changes over this time, some good and some questionable. In this commentary, I try to reveal how I got to this stage, partially explaining my biases, and possibly helping others learn from my experiences including mistakes. Changing from seeking an M.D. to cellular biology and then to pharmacology early in my career were the best moves I made. The next best move was migration to Canada, away from the McCarthy-McCarran hysteria. Arriving at a time after the end of World War II when science in Canada was expanding was very good luck. I had an excellent opportunity to enjoy both the administration (as Chair of the first independent Department of Pharmacology at the University of Alberta) and the practice of pharmacology (as a practitioner of research on smooth muscle in health and disease). For me, the practice of research has always won over administration when a choice had to be made. Early on, I began to ask questions about educational practices and tried to evaluate them. This led me to initiate changes in laboratories and to seek nondidactic educational approaches such as problem-based learning. I also developed questions about the practice of anonymous peer review. After moving to McMaster in 1975, I was compelled to find a solution for a failed "Pharmacology Program" and eventually developed the first "Smooth Muscle Research Program". Although that was a good solution for the research component, it did not solve the educational needs. This led to the development of "therapeutic problems", which were used to help McMaster medical students educate themselves about applied pharmacology. Now these problems are being used to educate pharmacology honours and graduate students at the University of Alberta. The best part of all these activities is the colleagues and friends that I have interacted with and learned from over the years, and the realization that many of them have collaborated with me again in this volume.

Ouabain-induced excitation of colonic smooth muscle due to block of K+ conductance by intracellular Na+ ions

The mechanism by which ouabain causes excitation of canine colonic circular smooth muscle was investigated. Ouabain-induced depolarization and increase in contractility were related to the concentration of extracellular sodium and prevented by complete substitution of sodium ions with N-methyl-D-glucamine or lithium ions. Absence of external sodium ions did not prevent the depolarization and increase in contractility induced by tetraethylammonium. Exposure of the muscle strips to sodium-free solutions produced a transient hyperpolarization and decrease in the input membrane resistance consistent with the hypothesis that intracellular sodium blocks potassium conductance. The relationship between the membrane potential and the extracellular potassium concentration indicated that the resting membrane potential is mainly determined by the membrane potassium conductance. Our data suggest the following mechanism of action for ouabain: (a) ouabain blocks Na+/K+ pump thereby increasing the intracellular sodium concentration; (b) increase in intracellular sodium inhibits membrane potassium conductance, which depolarizes the membrane and prolongs the slow wave plateau, resulting in an increase of the force of contraction. The direct contribution of the sodium pump to the resting membrane potential, if any, can only be minor (< 6 mV).

The nature of fuel provision for the Na+,K(+)-ATPase in porcine vascular smooth muscle

1. The specific contributions of aerobic glycolysis and oxidative metabolism to Na+ pump activity were quantitated in porcine carotid arteries under aerobic conditions. 2. Active reaccumulation of potassium by potassium-depleted tissues could be supported by oxidative metabolism alone, anaerobic metabolism in the presence of glucose, or a combination of oxidative metabolism and aerobic glycolysis, but not under anaerobic conditions in the absence of glucose. 3. Increasing levels of potassium added to potassium-depleted arteries under aerobic conditions resulted first in stimulation of aerobic lactate release which saturated at 0.028-0.036 mumol min-1 g-1, which was then followed by a stimulation of oxidative metabolism. This behaviour is opposite to the classic Pasteur effect. 4. The dependence of potassium uptake and lactate release on the concentration of potassium added to potassium-depleted arteries ('potassium re-entry concentration') under aerobic conditions were qualitatively similar. The K0.5 (concentration at which the velocity is half-maximally activated) and Vmax (the maximum velocity) for lactate release were 1.2 +/- 0.3 mM and 0.037 mumol min-1 g-1, respectively; those for K+ uptake were 4.3 +/- 0.4 mM and 0.399 mumol min-1 g-1. 5. The stoichiometric ratio between potassium uptake and ATP as calculated from lactate release approximated theoretical values of 2:1 (assuming 1 ATP per lactate) when potassium re-entry concentrations were less than 2 mM; higher concentrations of potassium produced ratios up to 9:1. 6. Physiological pump rates, as determined by potassium efflux studies, corresponded to potassium re-entry concentrations of less than or equal to 2 mM, the same potassium re-entry concentrations where the stoichiometry between potassium transport and aerobic glycolysis approximated the theoretical ratio of 2:1. Increases in oxidative metabolism were not detected in this range, but were detected at potassium re-entry concentrations of greater than or equal to 4 mM. 7. It was concluded that at physiological Na+ pump rates, aerobic glycolytic metabolism supported the N+,K(+)-ATPase; at higher pump rates, oxidative metabolism was required for pump support as well.

GABAergic synaptic current in dissociated nucleus basalis of Meynert neurons of the rat

gamma-Aminobutyric acid (GABA)-mediated spontaneous inhibitory postsynaptic currents (IPSCs) were recorded from dissociated rat nucleus basalis of Meynert neurons which still had their synaptic boutons attached. The membrane currents were recorded by the whole-cell patch-clamp technique. Elevated extracellular K+ concentration and the addition of the calcium ionophore, A23187, enhanced the amplitude and frequency of spontaneous IPSCs. Ryanodine and Ca(2+)-free external solution containing EGTA or BAPTA markedly decreased the spontaneous IPSC activities. Spontaneous IPSC activities were reversibly reduced by baclofen and increased by phaclofen, indicating that the GABAB receptor regulates the release of GABA from nerve terminals and acts as a negative autoreceptor.

The concentration of ouabain binding sites in biopsies of uterine muscle

The concentration of Na, K-ATPase in biopsies of uterine muscle was determined by measurement of [3H]ouabain binding in the presence of vanadate. For this purpose a method previously described for skeletal muscle (Nørgaard et al. 1983) was modified. Biopsies were obtained from uterine muscle from pregnant women (during caesarian section), non-pregnant women (during hysterectomy) and from adult, non-pregnant guinea-pigs and rats. The ouabain binding site concentration in uterine muscle of the pregnant women averaged 72 +/- 2 pmol g-1 wet wt (n = 8), with an apparent dissociation constant (KD) for ouabain of 3 x 10(-9) mol l-1. The ouabain-binding capacity in uterine muscle of the non-pregnant women amounted to 83 +/- 9 pmol g-1 wet wt (n = 8). In uterine muscle of the guinea-pig, two populations of ouabain binding sites were observed: one with a maximum binding capacity of 230 pmol g-1 wet wt and an apparent KD of 1.6 x 10(-6) mol l-1, and one with a maximum capacity of 62 pmol g-1 wet wt and an apparent KD of 5 x 10(-8) mol l-1. Immediate freezing of the biopsies in liquid N2 and storage at -60 degrees C for up to 6 weeks caused no change in ouabain-binding capacity. The dry weight/wet weight ratio of the samples from different subjects showed values of around 20%. It is concluded that the concentration of Na-K pumps in human uterine muscle can be quantified by [3H]ouabain binding using samples weighing 5-10 mg.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of the reactivity of the guinea-pig isolated trachealis by respiratory epithelium: effects of cooling

1. Examination has been made of the effects of epithelium removal on the reactivity of guinea-pig trachealis to methacholine at 37 degrees C and 22 degrees C, and on responses to activation of the Na+/K(+)-pump by abrupt temperature increase from 22 degrees C to 37 degrees C. 2. At 37 degrees C, epithelium removal increased the sensitivity of isolated tracheal strips to methacholine without affecting the maximum isometric contractile response. Epithelium removal resulted at 22 degrees C in a decrease in sensitivity to methacholine, i.e. an effect opposite to that seen at 37 degrees C. While the maximum response of intact strips to methacholine was enhanced at 22 degrees C, the maximum response of denuded preparations was decreased. 3. The increase in sensitivity to methacholine at 37 degrees C after epithelium removal was mimicked in intact preparations by indomethacin (1 microM). Indomethacin did not mimic the decrease in methacholine sensitivity and maximum response caused by epithelium removal at 22 degrees C. 4. Following incubation at 22 degrees C, abrupt increase in temperature to 37 degrees C elicited relaxation in both epithelium-containing and epithelium-denuded tracheal strips. In epithelium-containing preparations the relaxation was more pronounced and followed by contraction. Ouabain (1 microM) converted the relaxation of denuded preparations to contraction, but was ineffective in intact strips. The relaxation of intact strips was, however, inhibited by a greater ouabain concentration (10 microM). 5. These findings indicate that the modulatory effect of the epithelium is temperature-dependent. In cooled preparations, the epithelium enhances reactivity. At 37 degrees C, an epithelium-derived factor reduces reactivity, and this may partially be due to activation of the electrogenic Na+/K+-pump.

14 beta-Hydroxyprogesterone binds to the digitalis receptor, inhibits the sodium pump and enhances cardiac contractility

1. Certain derivatives of progesterone are potent inhibitors of high affinity, specific binding of 3H-cardiac glycosides. The steroids interact at the cardiac glycoside site on Na,K-ATPase and inhibit the enzyme (the sodium pump) in cardiac and other tissues. However, the active congeners identified previously have been, unlike the cardiac glycosides, predominantly cardiodepressant. 2. Because a 14 beta-hydroxy substituent is an important determinant of activity of the cardiotonic cardiac glycosides, we synthesized 14 beta-hydroxyprogesterone. This derivative has about one-tenth the potency of the aglycone, ouabagenin, in a [3H]-ouabain binding assay. 3. Like ouabagenin, but in contrast to the cardiodepressant congeners of progesterone, 14 beta-hydroxyprogesterone consistently elicited positive inotropy in isolated cardiac muscle and enhanced both the magnitude and frequency of fluctuations in scattered light (an index of oscillatory intracellular release of calcium). 4. Thus, at least one hydroxylated derivative (and putative endogenous metabolite) of progesterone, mimics the cardiac effects of cardiac glycosides including enhanced contractility.

Dependence of the electrogenic pump current of Xenopus oocytes on external potassium

The membrane potential of Xenopus oocytes showed a variable response to an increase of the K+ concentration in the bathing solution, [K+]e, from 2.5 mM to 20 mM. In 54% of the cases (n = 52) the cells hyperpolarized (by max. 70 mV). In the presence of 10(-5) M ouabain, all cells depolarized suggesting that the hyperpolarization was caused by an electrogenic Na+/K+ pump. In cells stored overnight in a Na+-free solution the transition from 2.5 to 20 mM [K+]e always caused depolarization indicating that the stimulation of the pump requires high internal sodium, [Na+]i. Cells stored overnight in a Na+-rich solution had a [Na+]i of 30.7 +/- 7 mM, i.e. the Na+/K+ pump was saturated with sodium (Lafaire and Schwarz 1986). With 9 such cells we determined the K+ activation of the Na+/K+ pump. The activation follows Hill kinetics with Imax = 90.5 nA, Ks = 2.3 mM, and n = 1.68.

Forskolin effects on longitudinal myometrial strips from the pregnant rat: relationship with membrane potential and cyclic AMP

The effects of forskolin on tension, membrane potential and cyclic AMP accumulation were studied in longitudinal myometrial strips from pregnant rats. 0.1 microM forskolin reduced the amplitude of spontaneous contractions by decreasing the frequency of action potential discharge without a change in resting potential or cyclic AMP accumulation. Forskolin, 1.0 microM, abolished contractions and action potentials, hyperpolarized the membrane and increased cyclic AMP accumulation. Ouabain, 1 mM, depolarized the muscle and increased resting tension. Ouabain reduced potential change produced by forskolin but did not prevent the relaxation or cAMP accumulation. Therefore changes in membrane potential are not prerequisite for the inhibitory actions of forskolin. The cyclic AMP-related relaxation may result primarily from intracellular events that remove calcium from the contractile elements.

Demarcation of Ca2+ transport processes in guinea pig stomach smooth muscle

Microsomal fractions were isolated from gastric antrum and fundus smooth muscle of guinea pigs. Ca2+ uptake into and Ca2+ release from the membrane vesicles were studied by a rapid filtration method, and Ca2+ transport properties of the different regions of the stomach were compared. ATP-dependent Ca2+ uptake was similar in microsomes isolated from both regions. This uptake was increased by oxalate and was not affected by NaN3. Oxalate affected Ca2+ permeability of both antrum and fundus microsome vesicles similarly. Fundus microsome vesicles preincubated in 100 mM NaCl and then diluted to 1/20 concentration with Na+-free medium had significantly higher ATP-independent Ca2+ uptake than vesicles preincubated in 100 mM KCl and treated the same way. This was not true for antrum vesicles. Monensin abolished Na+-dependent Ca2+ uptake, and NaCl enhanced Ca2+ efflux from fundus microsome vesicles. The halflife values of Ca2+ loss from fundus vesicles in the presence of NaCl were significantly smaller than those in the presence of KCl. The release of Ca2+ from the vesicles within the first 3 min was accelerated by NaCl to three times that by KCl. However, NaCl had no effect on Ca2+ release from antrum microsome vesicles. Results suggest two distinct mechanisms of stomach membrane Ca2+ transport: (1) ATP-dependent Ca2+ uptake and (2) Na+-Ca2+ exchange; the latter in the fundus only.

The facilitating effect of gangliosides on the electrogenic (Na+/K+) pump and on the resistance of the membrane potential to hypoxia in neuromuscular preparation

The effects have been investigated of a mixture of gangliosides from beef brain cortex (GM1, GD1a, GD1b and GT1) either added to the bathing medium or injected intraperitoneally on muscle fibres and nerve terminals in mouse diaphragm. The electrogenic (Na+/K+) pump activity of muscle fibres enriched with sodium was increased by 38% after 2-h pretreatment with gangliosides (5 X 10(-8) mol X 1(-1]. Muscles from animals treated with gangliosides did not show the substantial depolarization of the resting membrane potential (RMP) in K+-free solution (6 h) shown by control muscles. Further, treatment with gangliosides slowed the changes in muscle fibre RMP and frequency of the miniature end-plate potentials in oxygen deprived muscles.

Contractions of rat uterine smooth muscle induced by acetylcholine and angiotensin II in Ca2+-free medium

The effects of acetylcholine (ACh, 10(-4)M) and angiotensin II (Ang II, 10(-6) M) have been studied on the mechanical and electrical activities of rat myometrial strips perfused in Ca2+-free EGTA-containing solutions. Both ACh and Ang II produced transient contractions, the amplitude of which can be taken as a measurement of the amount of Ca2+ present in a drug-sensitive Ca2+ store. The degree of filling of this store depended on the external Ca2+ concentration, and on the presence of contractile responses during the Ca2+ loading period. The existence of two pathways (either direct or transcytoplasmic) is suggested for Ca2+ uptake into the internal Ca2+ store. The rate of filling of the Ca2+ store in 2.1 mM-Ca2+-containing solution was faster (time to half-maximal response, t 1/2 = 29 +/- 2.2 s, n = 4) than the rate of depletion in Ca2+-free solution (t 1/2 = 3 +/- 0.3 min, n = 3). The gradual depletion of this store was much slower at 18 degrees C than at 35 degrees C, and in the presence of vanadate which is known to inhibit Ca2+-ATPases. Methoxyverapamil (D600, 10(-6)-10(-5) M) had no appreciable effect on the direct Ca2+ uptake or on the release of Ca2+ from the store by ACh and Ang II. Mn2+ (10(-3) M) completely inhibited the direct pathway to the internal Ca2+ store and also reduced the release of Ca2+. ACh and Ang II induced repetitive depolarizations close to zero potential which did not parallel the transient contractions as a function of the time of perfusion in Ca2+-free solution. Applications of 2 mM EGTA, 135 mM K+ or Ca2+ antagonists which suppressed or reduced the drug-induced depolarizations did not affect appreciably the drug-induced contractions. These results suggest that myometrial cells have an intracellular Ca2+ store sensitive to different stimulus substances. This store is not affected by depolarization of the plasma membrane and is certainly different from that described in voltage-clamp experiments.

Thallium uptake by canine iliac arteries

Most of the thallium uptake by canine iliac arteries (70%) was inhibited by ouabain. The component sensitive to ouabain was significantly increased by incubating tissues in K+-free solution and reduced by low temperatures, metabolic inhibitors, and loading with lithium. The apparent Km for thallium for the ouabain-sensitive uptake was 0.47 +/- 0.046 mM. External K+ inhibited thallium uptake and the apparent Ki was estimated to be 6.5 mM. Thallium thus appears to have a greater affinity for the ouabain-sensitive components. The residual component of thallium uptake was unaffected by maneuvers that affected the ouabain-sensitive uptake, which thus appears to be a suitable indicator of Na+ pump activity in vascular smooth muscle.

The kinetics of ouabain-sensitive ionic transport in the rabbit carotid artery

1. Ouabain (0.1 mM)-sensitive 42K influx and 24Na efflux have been measured in rabbit carotid arteries under conditions of high cellular potassium, [K]i, as well as sodium, [Na]i. About 50% of the total fluxes are ouabain-sensitive (active) under conditions of high [K]i. 2. The extracellular space, determined by 60Co-EDTA, was relatively large in comparison to cellular water. The ionic concentrations in normal solution, estimated from isotope flux components, are: [Na]i = 24; [K]i = 169; [Cl]i = 68 mmol/l cell water. 3. The ouabain=sensitive 42K influx and 24Na efflux in high-K tissues were measured at varying external concentrations of potassium, [K]o, and normal concentrations of external sodium, [Na]o. Sigmoidal kinetics were observed and fitted to a co-operative interaction model. The maximal efflux of 24Na, 0.245 muequiv/g wet weight per minute, was about 1.4 times that for 42K influx. Half-maximal stimulation was achieved at [K]0.5o of 2.4 mM for Na, and 3.4 mM for K transport. The flux ratio of Na to K approximated 1.5. 4. Increased 42K efflux was found in the presence of ouabain and the passive influx of 42K was corrected for this effect. In the absence of this correction the ouabain-sensitive 42K influx would be reduced, and the Na/K flux ratio raised to about 2. 5. The [K]o-dependence of ouabain-sensitive fluxes was measured on Na-loaded tissues. 24Na efflux exhibited saturation kinetics with a maximum of 1.18 muequiv/g wet weight per minute and [K]0.5o = 3.1 mM. The 42K influx was two thirds the active Na efflux for [K]o less than or equal to 5 mM. At high [K]o, however, the influx greatly exceeded the predicted levels. Evidence is presented for a ouabain-sensitive membrane hyperpolarization being responsible for an additional influx of 42K. 6. The ouabain-sensitive 24Na efflux showed a sigmoidal dependence on [Na]i in the presence of [K]o = 10 mM and normal [Na]o. The maximal efflux was 0.88 muequiv/g weight per minute and [Na]0.5i = 49 mmol/l cell water, which is about twice the physiological operating point. 7. It is concluded that active Na and K transport in rabbit carotid artery follow sigmoidal kinetics and the flux ratio is about 1.5. Changes in [K]o and [Na]i over the physiological range can markedly affect transport, and may regulate vascular contraction by their action on electrogenic transport.

Purification of human granulocytes by centrifugal elutriation and measurement of transmembrane potential

Viable cell samples containing 93% pure granulocytes were obtained from human blood using the techniques of dextran sedimentation followed by centrifugal elutriation. The resting transmembrane potential (Em) of human granulocytes was estimated using the fluorescent lipophilic cation, Di-S-C3(5), from the null point for potassium--i.e., the external K concentration at which there is no change in Em in response to valinomycin (a K ionophore). The Em of human granulocytes, as calculated from the Nernst potential for K at the null point, is approximately -100 mV. Data indicate that this large transmembrane potential is due in part to the presence of an electrogenic Na-K pump in human granulocytes which is stimulated by external potassium and inhibited by ouabain.

Electrophysiological study of the action of angiotensin II on the rat myometrium

Angiotensin II (A II) produces a contraction of visceral and vascular smooth muscles of different species. The accompanying electrophysiological changes were measured on strips of rat myometrium at 35 degrees C using the double sucrose-gap technique. A II at concentrations from 5 x 10(-10) to 10(-6)M produces a depolarization and an increase in membrane conductance. This increase in membrane conductance is not membrane potential dependent since it is observed even when the membrane potential is maintained at the resting level. When all Na + in the test solution is replaced by either Li + or Mg (2+), the depolarizing effect of 10(-6)M A II is either markedly reduced or abolished. Under these conditions, A II produces a small initial hyperpolarization, which is modified by external potassium concentration changes and abolished by tetraethylammonium chloride. When all Cl- is replaced by either NO (-3) or cyclohexanesulfamate, A II (10(-6) M) still produces a 20-m V depolarization. The removal of extracellular Ca (2+) or K+ does not have any effect on the depolarizing action of A II, which also is not changed by 10(-3) M ouabain. In conclusion, A II produces a depolarization of the uterine smooth muscle membrane through an increase in the membrane conductance to Na+. The membrane conductance to potassium is increased simultaneously. The contraction induced by A II shows two components: a phasic component triggered by the Ca (2+) entry associated with spike production and a tonic component due to the release of Ca(2+) from intracellular stores.

Electrophysiological recordings from spontaneously contracting reaggregates of cultured smooth muscle cells from guinea pig vas deferens

Smooth muscle cells were enzymatically dispersed from vasa deferentia of adult male guinea pigs (250-400 g). These cells reassociated in vitro to form monolayers and small spherical reaggregates (0.05-0.3 mm in Diam). Within 48 h of being placed in culture, cells in both types of preparation began to contract spontaneously. The contractions were rhythmic and slow. Cells in the monolayers stopped contracting after approximately 1 wk in vitro, but the reaggregates continued to contract spontaneously for at least 3 wk. Electron microscopy of the reaggregates revealed the presence of thick and thin myofilaments. Overshooting action potentials were recorded in many of the cells penetrated (primarily in reaggregates), and were accompanied by visible contractions of the aggregate or monolayer. Quiescent cells could often be excited by intracellularly applied depolarizing and hyperpolarizing (anodal-break) current pulses. The resting potentials had a mean value of -58 +/- 2 mV. The action potentials were usually preceded by a spontaneous depolarization. The action potentials had slow rates of rise (1--4 V/s) which were unaffected by tetrodotoxin (TTX, 1 microgram/ml), a known blocker of fast Na+ -channels. Verapamil (1 microgram/ml) blocked the action potentials. The mean value of input resistance was 6.9 +/- 0.5 M omega (n = 12). These electrophysiological properties are similar to those of intact adult vas deferens smooth muscle cells. Thus, the cultured adult vas deferens smooth muscle cells retain their functional properties in vitro even after long periods.

Changes in total and quantal release of acetylcholine in the mouse diaphragm during activation and inhibition of membrane ATPase

1. Acetylcholine (ACh) released from mouse diaphragm was gel filtrated and estimated by bio-assay and compared with electrophysiologically measured quantal release, expressed either as frequency of miniature end-plate potentials or quantum content of end-plate potentials. 2. Activation of Na+-K+-dependent membrane ATPase (membrane ATPase) in Na+-loaded muscles lowered the total amount of ACh released at rest to one tenth of the control value, but quantal release remained unchanged. 3. Inhibition of membrane ATPase by 2 X 10(-5) M-ouabain or by K-free solution led to an increase in total release and to a delayed progressive increase in quantal release. When Ca2+ was removed only the total release was enhanced. 4. Depolarization of the diaphragm by 8, 11 and 14 mM-K increased both total and quantal release only in the presence of Ca2+ in the perfusion medium. When Ca2+ was removed, no significant increase in release was observed. 5. The total and quantal release in response to 2 Hz stimulation of the preparation was increased 1.4 and 45 times, respectively. It is concluded that the total amount of ACh released at rest consists of two fractions, quantal and non-quantal, the former representing about 1% of the total release.

Metabolic component in the epithelial intracellular potential of rabbit cornea

Effects of the change of external ionic composition and the addition of metabolic inhibitors on rabbit cornea were studied by recording the epithelial intracellular potential. High K and Li Ringer's solutions, applied to the corneal endothelial side, caused a marked depolarization of the epithelial cells, but no potential change was seen when applied to the epithelial side. Ouabain, MIAA and NaCN applied to the endothelial side reduced the epithelial potential, while those applied to the epithelial side did not change the potential. DNP and FDNB also had no effect when applied to the epithelial side only. The thermal dependence of the epithelial intracellular potentials of whole eye (0.85 mV/degrees C) and excised cornea (2.01 mV/degrees C) preparations were greater than about 0.2 mV/degrees C predicted by the Nernst equation. It is concluded that the epithelial cell layer of rabbit cornea act as a tight barrier against diffusion of K ion and metabolic inhibitors from the tear side to the epithelial basal cell. A high thermal dependence of the epithelial intracellular potential may depend greatly on the pump ingibition.

Ouabain-sensitive ion fluxes in the smooth muscle of the guinea-pig's taenia coli

1. Tissues with raised intracellular Na levels, produced by incubation in K-free media, were used throughout. The uptake of 42K by these Na-loaded tissues was followed for 10 min in the presence and absence of 1-37 X 10(-4) M ouabain, this being sufficient to inhibit Na pumping maximally. Subtraction of the uptake seen in the presence from that seen in the absence of ouabain gave estimates of the pumped ouabain-sensitive K uptake. 2. In Na-free (MgCl2) medium this depended on the [K]0 in a sigmoidal fashion with a half maximal [K]0 for activation of some 4mM. The maximal uptake of K was 3 m-mole/kg.min corresponding to a transmembrane flux of some 12-5 p-mole. cm-2.sec-1. 3. In the presence of Na the K activation curve became more obviously sigmoid and higher concentrations of K were needed to achieve a given active K influx. The results were well fitted by assuming that Na and K competed for two identical, non-interacting sites on the external pump face. 4. Addition of K during the efflux of 24Na into a Na-free (MgCl2) medium led to an increased rate of tracer loss. The magnitude of this increase depended on the [K] used in a hyperbolic fashion and it was abolished by addition of ouabain. The [K] causing half-maximal activation of ouabain-sensitive Na efflux was in the order of 1-2 mM. 5. When the [K] in the uptake media was 1-5 mM; Na, Li, Rb and Cs all inhibited ouabain-sensitive K uptake, the order of effectiveness being Rb greater than Cs greater than Na greater than Li. With a E1TKA10 OF 0-15 MM low concentrations of Cs and Rb were shown to stimulate K uptake. Such an effect is predicted by assuming two ion binding sites on the pump's outer face, and that the pump can translocate mixtures of K and either Rb or Cs...

Inhibitory mechanisms of isoprenaline in the guinea-pig taenia coli

The beta-receptor stimulating effect of isoprenaline on the spontaneous activities of the guinea-pig taenia coli was investigated in the presence of an alpha-receptor blocking agent, phentolamine (2 X 10(-6) M). Substitution of the external Naion with Li, choline, tris-hydroxymethyl aminomethane or Kion greatly reduced the inhibitory effect of isoprenaline. A treatment with ouabain (4 X 10(-6) M) also had effects similar to those of Na removal. The spontaneous activity was transiently suppressed when 5.9 mM K ion was re-admitted into K-free solution, and this suppressing effect was blocked by ouabain and also by reducing Na concentration to 5.9mM. However, isoprenaline retained its effects in K-free solution containing normal Na concentration or 5.9 mM Na. The relaxation by isoprenaline in these solutions was hardly reduced by ouabain (4 X 10(-6) M). These observations suggest that the beta-receptor stimulating effects of isoprenaline are partly mediated through an activation of the Na-pump and partly through some other mechanism which is inactivated when the preparation is deprived of Na or when the membrane is depolarized.

Intracellular ion concentration and electrical activity in potassium-depleted mammalian soleus muscle fibers

The relationship between the intracellular cation concentration and the membrane potential has been studied in "Na-rich" soleus muscle fibers of rats which had been fed a K-free diet for 10-50 days. The resting potentials of "Na-rich" muscle fibers closely agreed with the theoretical potential expected from ionic theory when a quantitative dissociation of active cation transport with Na ions extrusion exceeding K ions uptake was eliminated due to the recovery of "Na-rich" fibers in Krebs solution with 10 mM K for 2 h at 37 degrees C. The hyperpolarized membrane potentials during cellular Na ions extrusion were accounted for by the sum of the potentials produced by the electrogenic Na-pump and by the ionic diffusion potential. On the other hand, the amplitude of overshoot of action potentials decreased linearly with the logarithmic increase of the intracellular Na concentration ([Na]i). The maximum rate of rise of action potentials also changed as a fraction of [Na]i, though, at the early period of K-deficiency the inhibitory effect of the increased [Na]i on the maximum rate of rise was transiently masked by the hyperpolarization produced by the electrogenic Na-pump which secondarily led to a progressive reduction of Na inactivation, while the maximum rate of fall was a linear function of [K]i.

Inhibition of spontaneous contractility of the human and the rabbit fallopian tube by potassium

Spontaneous contractions of circular and longitudal muscle of the ampulla and isthmus of human and rabbit Fallopian tubes were recorded isometrically. Addition of K+ to tissue in K+-free medium always caused immediate inhibition of spontaneous contractility. K+-induced inhibition was prevented by ouabain and reversed by removal of K+. It is suggested that K+ caused inhibition of contractility by activating electrogenic Na+ pumping with resultant hyperpolarization of the muscle cells.

Contribution of an electrogenic sodium pump to the membrane potential in rabbit sinoatrial node cells

A study has been made of the transient hyperpolarization (K+-induced hyperpolarization) which developed following readmission of potassium after having pre-treated the rabbit sinoatrial node tissue with K+-depleted Tyrode solution for 4--5 min at 35 degrees C. Evidence is presented indicating that the K+-induced hyperpolarization results from the activity of an electrogenic sodium pump: The K+-induced hyperpolarization was inhibited by substituting Li+ for Na+ and by cooling the tissue. The amplitude of the K+-induced hyperpolarization was increased either by increasing K+ concentration in the recovery solution or by decreasing K+ concentration in the pre-treatment K+-depleted solution. By removing Cl- from the perfusates, the amplitude of the K+-induced hyperpolarization increased. In a Cl--depleted solution, the sinoatrial node cell membrane hyperpolarized by approximately 15 mV without a transient depolarization.

Activation of electrogenic sodium pump in mammalian skeletal muscle by external cations

The effect of change of the external ionic composition on "Na-loaded" and "K-depleted" soleus muscle fibres of K-deficient rats was investigated by recording resting membrane potentials. The addition of K, Rb, Cs and NH4 ions to K-free Krebs solution bathing "Na-rich" muscles resulted in a rapid hyperpolarization. The hyperpolarization was abolished by removing the above cations, cooling to ca. 4 degrees C, and adding 0.1 mM ouabain. The effectiveness of cations for activating the electrogenic Na pump was Rb greater than or equal to K greater than NH4 greater than Cs, and NH4 ions seemed to be unique in their stimulating action. The resting cell membrane of "Na-rich" muscles is permeable to cations in the order of Rb = K greater than Cs greater than NH4. Reducing Na ions in Krebs solution had no effect on the rate of Na-pumping in "Na-rich" muscle fibres at a given K concentration. It is concluded that the external K ions could be replaced by Rb, Cs and NH4 ions in activating the electrogenic Na pump in "Na-rich" soleus muscle fibres, but that the electrogenic Na pump in this tissue does not require the external Na ions.

Contribution of an electrogenic sodium pump to membrane potential in mammalian skeletal muscle fibres

1. Relationship between the resting membrane potential and the changes in the intraceullar Na and K concentrations ([Na]i and [K]i) was studied in 'Na-loaded' and K-depleted' soleus (SOL) muscles of rats which had fed a K-free diet for 40 and more days. 2. The extracellular space of the muscles was not significantly different between normal and K-deficient rats. The inulin space in both the 'fresh' and Na-rich' muscles can be determined by the same function relating the space to the muscle weight. 3. Presence of 2-5-15 mM-K in the recovery solution hyperpolarized the 'Na-rich' muscul fibres at the beginning of recovery. The hyperpolarized membrane potential exceeded, beyond the measured potential of 'fresh' muscle fibres, the theoretical potential derived from the ionic theory, or even beyond Ek. Then, the measured membrane potential declined progressively during the immersion in a recovery solution and returned to the steady-state value When a considerable Na extrusion and K uptake took place, the measured membrane potential became equal to Ek. 4.he maximal hyperpolarization occurring immediately after immersion in the recovery solution became smaller and had a shorter duration when increasing the external K concentration ([K]o) from 2-5 to 15mM. 5. The K-sensitive hyperpolarization was completely abolished on exposure to 0mM [K]o, on cooling to ca. 4 degrees C, and in the presence of oubain (10(-4) M). The inhibitory effects were reversed on returning to the control conditions. The membrane potential obtained after inhibition of the electrogenic Na-pump with cooling or ouabain agrees well with that predicted by the 'constant-field' equation. 7. The external Cl ions had a short-circuiting effect on the electrogenic Na-pumping activated on adding K ions. 8. The replacement of Na ions in a recovery solution with Li ions resulted in a faster rate of depolarization from the maximal hyperpolarizationp. It is concluded that the resting membrane potential of 'Na-loaded' and 'K-depleted' SOL muscle fibres is the sum of an ionic diffusion potential predicted by either the Nernst equation or the constant-field equation and of the potential produced by an electrogenic Na-pump.

A study of pace-maker activity in intestinal smooth muscle

1. Electrical activity of longitudinal muscle from cat intestine was recorded in the double sucrose gap.2. Approximately 20% of the preparations demonstrated slow, spontaneous fluctuations of membrane voltage, slow waves. This activity, although quite uniform in a given preparation, showed considerable inter-preparation variation with respect to amplitude, frequency and wave form.3. Application of steady hyperpolarizing current decreased slow-wave frequency and increased slow-wave amplitude while depolarizing currents increased frequency and decreased amplitude.4. Some preparations with no spontaneous slow-wave activity developed slow waves when the membrane was hyperpolarized into a given range which, depending on the preparation, varied in size from 10 to 40 mV. Step or ramp depolarization of the membrane from hyperpolarized levels triggered slow waves in some preparations.5. When the membrane potential of a slow-wave generating preparation was clamped at the resting potential, spontaneous inward-directed current transients were observed.6. No changes in membrane conductance were observed during the course of a slow wave.7. The slow-wave pattern was simulated for individual preparations by applying the membrane current measured under voltage clamp to the passive membrane resistance and capacitance measured independently under current clamp.8. In addition to the defined slow-wave activity, voltage-dependent oscillations in membrane potential were sometimes observed.9. Application of 10(-5)M ouabain irreversibly blocked slow waves and produced a membrane depolarization equal to or slightly greater than the slow wave crest. Repolarization of the membrane to the resting potential, or hyperpolarization, failed to restore slow-wave activity.10. Removal of external potassium produced a reversible sequence of events almost identical to those following ouabain application.11. Replacement of 50% of the external sodium chloride with sucrose produced no changes in slow-wave activity with respect to rates of rise or fall, maximum amplitude or frequency. Sucrose replacement of all external sodium chloride eliminated slow waves after 5 min; however, activity could be restored by a slight hyperpolarization. Longer exposures to the modified bath abolished activity.12. Following a conditioning exposure to potassium-free Krebs solution, readmission of potassium at normal concentration produced a mean hyperpolarization of 20.5 mV and in spontaneous preparations an arrest of activity.13. Pump current in sodium-loaded, non-spontaneously active preparations was measured by voltage clamp and was observed to be voltage-dependent.14. The results of this study indicate that an electrogenic pump is present in longitudinal muscle of cat duodenum, and that oscillations in the level of pump current produce slow waves.

The role of sodium pump in the inhibition of smooth muscle responsiveness to agonists during potassium restoration

1. Isometric contractions of cat splenic capsular smooth muscle in response to noradrenaline and histamine were recorded.2. Removal of potassium from the bathing medium did not change the resting tension or the responsiveness to noradrenaline. Restoration of potassium inhibited responses to noradrenaline or histamine only if the muscles were stimulated with an agonist while in the K-free medium.3. This inhibition of responses to the agonists due to potassium was reversed rapidly by removing the ion or reversed slowly by prolonged exposure to the ion. The inhibition was also blocked by procedures or agents which block the sodium pump (ouabain, substitution of NaCl by LiCl), inhibit active processes (low ambient temperature) or prevent intracellular accumulation of sodium (substitution of choline for sodium).4. It is proposed that under special circumstances such as when there is an increase in internal sodium concentration, the sodium pump is probably electrogenic and causes relaxation when activated by external potassium. In the normal muscle the pump is probably electrically neutral.

Metabolism and the electrical activity of anoxic ventricular muscle

1. The action potential duration of anoxic guinea-pig ventricular muscle was related to ATP generated by glycolysis. In 50 mM glucose medium the action potential duration was maintained; in 5 mM glucose medium the action potential duration shortened, the glycolytic rate declined and the ATP content was reduced.2. The action potential amplitude was related to the metabolic state of the muscle but not to the intracellular sodium concentration.3. It is suggested that changes in the action potential duration and overshoot in anoxic muscle may be due to an influence of metabolism on the slow inward current.4. Anoxic muscle incubated for 8 hr in 5 mM glucose medium had an E(m) of -77.1 mV compared to -81.1 mV in fresh muscle. The calculated E(k) of anoxic muscle was -47.4 mV.5. The resting potential of anoxic muscle was separated into two components, one dependent on potassium distribution and the other on the activity of an electrogenic sodium pump.6. The electrogenic pump component was stimulated upon raising the glucose concentration of the medium or upon raising the external potassium concentration.7. The electrogenic pump component was inhibited by ouabain or by reduction of the temperature from 35 to 8 degrees C.

Effects of electrogenic sodium pumping on the membrane potential of longitudinal smooth muscle from terminal ileum of guinea-pig

1. The membrane potential of the separated longitudinal muscle of the guinea-pig terminal ileum was recorded intracellularly with glass micro-electrodes.2. In tissues kept at room temperature and then brought to 35 degrees C for 15-30 min or about 1 hr, the fall in membrane potential upon changing to potassium-free solution was 21.4 +/- 3.5 mV and 13.4 +/- 1.8 mV respectively. Ouabain (1.7 x 10(-6)M) produced a fall in membrane potential of 8.1 +/- 1.1 mV. Returning potassium to potassium-free solution, or changing from ouabain-containing to ouabain-free solution, resulted in an increase in membrane potential which was greater than the initial fall.3. Readmitting potassium to potassium-free solution produced an increase in membrane potential which began within 10 sec and reached a maximum within 15-30 sec. This response was reduced, abolished, or converted to a depolarization by ouabain. In chloride-deficient (13 mM) solution in which membrane resistance was increased, the response to readmitting potassium was increased 2(1/2)-fold so that the membrane potential sometimes exceeded -100 mV, which was probably more negative than E(K). On the basis of these results it was assumed that the response to readmitting potassium was due to the electrogenic activity of the sodium pump.4. The response to briefly readmitting a fixed concentration of potassium increased during the first 30 min in potassium-free solution. This increase was not due to an increase in membrane resistance as this fell with time in potassium-free solution. It was suggested that the increase in the response resulted from the progressive rise in internal sodium concentration which is known to occur in smooth muscle in potassium-free solution.5. Increasing the concentration of potassium over the range approximately 0.1-20 mM, increased the size of the electrogenic potential observed upon readmitting potassium to potassium-free solution. There was a fall in membrane resistance upon readmitting potassium (0.6, 5.9, or 20 mM) which was greater the larger the concentration of potassium. When allowance was made for the fall in membrane resistance, the dependency of the electrogenic response upon the concentration of potassium over the range 0.6-20 mM was much increased.6. The results indicate that the rate of electrogenic sodium pumping in this tissue is increased by increasing the external potassium concentration, and probably by increasing the internal sodium concentration. It was suggested that a rise in the latter could sensitize the pump to an increase in the former.

Ion levels and membrane potential in chick heart tissue and cultured cells

Intracellular concentrations of sodium and potassium as well as resting potentials and overshoots have been determined in heart tissue from chick embryos aged 2-18 days. Intracellular potassium declined from 167 mM at day 2 to 117-119 mM at days 14-18. Intracellular sodium remained nearly constant at 30-35 mM during the same period. The mean resting potential increased from -61.8 mV at day 3 to about -80 mV at days 14-18. The mean overshoot during the same period increased from 12 to 30 mV. P(Na)/P(K) calculated from the ion data and resting potentials declined from 0.08 at day 3 to 0.01 at days 14-18. Thus, the development of embryonic chick heart during days 2-14 is characterized by a declining intracellular potassium concentration and an increasing resting potential and overshoot. Heart cells from 7- to 8-day embryos, cultured either in monolayer or reassociated into aggregates, were compared with intact tissue of the same age. The intracellular concentrations of sodium and potassium were similar in the three preparations and cultured cells responded to incubation in low potassium medium or treatment with ouabain in a manner similar to that of intact tissue. Resting potentials and overshoots were also similar in the three preparations.