The effects of beta-stimulation on the Na(+)-K+ pump current-voltage relationship in guinea-pig ventricular myocytes

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

Profound regulation of Na/K pump activity by transient elevations of cytoplasmic calcium in murine cardiac myocytes

Small changes of Na/K pump activity regulate internal Ca release in cardiac myocytes via Na/Ca exchange. We now show conversely that transient elevations of cytoplasmic Ca strongly regulate cardiac Na/K pumps. When cytoplasmic Na is submaximal, Na/K pump currents decay rapidly during extracellular K application and multiple results suggest that an inactivation mechanism is involved. Brief activation of Ca influx by reverse Na/Ca exchange enhances pump currents and attenuates current decay, while repeated Ca elevations suppress pump currents. Pump current enhancement reverses over 3 min, and results are similar in myocytes lacking the regulatory protein, phospholemman. Classical signaling mechanisms, including Ca-activated protein kinases and reactive oxygen, are evidently not involved. Electrogenic signals mediated by intramembrane movement of hydrophobic ions, such as hexyltriphenylphosphonium (C6TPP), increase and decrease in parallel with pump currents. Thus, transient Ca elevation and Na/K pump inactivation cause opposing sarcolemma changes that may affect diverse membrane processes.

Protein kinase-dependent oxidative regulation of the cardiac Na+-K+ pump: evidence from in vivo and in vitro modulation of cell signalling

The widely reported stimulation of the cardiac Na(+)-K(+) pump by protein kinase A (PKA) should oppose other effects of PKA to increase contractility of the normal heart. It should also reduce harmful raised myocyte Na(+) levels in heart failure, yet blockade of the β1 adrenergic receptor (AR), coupled to PKA signalling, is beneficial. We treated rabbits with the β1 AR antagonist metoprolol to modulate PKA activity and studied cardiac myocytes ex vivo. Metoprolol increased electrogenic pump current (Ip) in voltage clamped myocytes and reduced glutathionylation of the β1 pump subunit, an oxidative modification causally related to pump inhibition. Activation of adenylyl cyclase with forskolin to enhance cAMP synthesis or inclusion of the catalytic subunit of PKA in patch pipette solutions abolished the increase in Ip in voltage clamped myocytes induced by treatment with metoprolol, supporting cAMP/PKA-mediated pump inhibition. Metoprolol reduced myocardial PKA and protein kinase C (PKC) activities, reduced coimmunoprecipitation of cytosolic p47(phox) and membranous p22(phox) NADPH oxidase subunits and reduced myocardial O2(•-)-sensitive dihydroethidium fluorescence. Treatment also enhanced coimmunoprecipitation of the β1 pump subunit with glutaredoxin 1 that catalyses de-glutathionylation. Since angiotensin II induces PKC-dependent activation of NADPH oxidase, we examined the effects of angiotensin-converting enzyme inhibition with captopril. This treatment had no effect on PKA activity but reduced the activity of PKC, reduced β1 subunit glutathionylation and increased Ip. The PKA-induced Na(+)-K(+) pump inhibition we report should act with other mechanisms that enhance contractility of the normal heart but accentuate the harmful effects of raised cytosolic Na(+) in the failing heart. This scheme is consistent with the efficacy of β1 AR blockade in the treatment of heart failure.

Regulation of the cardiac sodium pump

In cardiac muscle, the sarcolemmal sodium/potassium ATPase is the principal quantitative means of active transport at the myocyte cell surface, and its activity is essential for maintaining the trans-sarcolemmal sodium gradient that drives ion exchange and transport processes that are critical for cardiac function. The 72-residue phosphoprotein phospholemman regulates the sodium pump in the heart: unphosphorylated phospholemman inhibits the pump, and phospholemman phosphorylation increases pump activity. Phospholemman is subject to a remarkable plethora of post-translational modifications for such a small protein: the combination of three phosphorylation sites, two palmitoylation sites, and one glutathionylation site means that phospholemman integrates multiple signaling events to control the cardiac sodium pump. Since misregulation of cytosolic sodium contributes to contractile and metabolic dysfunction during cardiac failure, a complete understanding of the mechanisms that control the cardiac sodium pump is vital. This review explores our current understanding of these mechanisms.

Anger, emotion, and arrhythmias: from brain to heart

Strong emotion and mental stress are now recognized as playing a significant role in severe and fatal ventricular arrhythmias. The mechanisms, although incompletely understood, include central processing at the cortical and brain stem level, the autonomic nerves and the electrophysiology of the myocardium. Each of these is usually studied separately by investigators from different disciplines. However, many are regulatory processes which incorporate interactive feedforward and feedback mechanisms. In this review we consider the whole as an integrated interactive brain-heart system.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Effects of nanomolar concentration dihydroouabain on calcium current and intracellular calcium in guinea pig ventricular myocytes

The effects of nanomolar concentration of dihydroouabain (DHO) on L-type calcium current (ICa-L), TTX-sensitive calcium current (ICa(TTX)), and intracellular calcium concentration ([Ca2+]i) were investigated in guinea pig ventricular myocytes. The whole-cell patch-clamp technique was used to record ICa-L and ICa(TTX); [Ca2+]i was detected and recorded with the confocal microscopy. The nanomolar concentration of DHO increased the ICa-L, ICa(TTX), and [Ca2+]i, which could be partially inhibited by nisoldipine or TTX, but still appeared in the absence of extracellular K+ and Na+. These data suggest that DHO could increase [Ca2+]i in non-beating myocytes via stimulating the ICa-L and ICa(TTX), or perhaps triggering directly a release of intracellular calcium.

Assessment of the cytoprotective role of adenosine in an in vitro cellular model of myocardial ischemia

This work aimed to detect functional adenosine receptors in isolated rat cardiomyocytes and to study the influence of stimulation of these receptors in an in vitro model of ischemia. Cultures of cardiomyocytes were prepared from newborn rat ventricles. The contractions were photometrically monitored. In this preparation, adenosine induced a positive chronotropic response. This effect was reproduced by CGS 21680 (2-(4-[2-carboxyethyl]-phen-ethyl-amino) adenosine-5'N-ethylunosamide), a specific adenosine A(2) receptor agonist, and antagonized by DMPX (3,7-dimethyl-1-propargylxanthine), an adenosine A(2) receptor antagonist. However, R-PIA (R-N(6)-(2-phenylisopropyl)-adenosine; a specific adenosine A(1) receptor agonist) induced a negative chronotropic effect that was abolished by its corresponding adenosine A(1) antagonist DPCPX (1,3-dipropyl-8-cyclo-pentyl-adenosine). Substrate-free hypoxia, as simulation of ischemia, induced a progressive decrease and then arrest of spontaneous cell contractions. The spontaneous rhythmic contractile activity was restored during reoxygenation following simulated ischemia. Adenosine A(1) receptor stimulation with R-PIA induced a decrease of hypoxia-induced damage. This effect was antagonized by DPCPX, an adenosine A(1) receptor antagonist. Conversely, the cells treated with CGS 21680 did not display complete recovery after reoxygenation. In addition, this effect was abolished by DMPX, since the cells recovered normal function after reoxygenation. To conclude, it appeared that cardiomyocytes possess both functional adenosine A(1) and A(2) receptors and that only the activation of adenosine A(1) receptor had a cytoprotective effect against simulated ischemia-induced cardiac cell injury.

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.

Rapid inhibition of the Na+-K+ pump affects Na+-Ca2+ exchanger-mediated relaxation in rabbit ventricular myocytes

The direct influence of Na+-K+ pump activity on the ability of the Na+-Ca2+ exchanger to remove Ca2+ was investigated in isolated adult rabbit ventricular myocytes. Cell shortening was measured using an edge-detection system. Cytoplasmic [Ca2+] was monitored using the fluorescent indicator indo-1. Electrophysiological parameters were recorded using high-resistance microelectrodes. The Na+-K+ pump was rapidly inhibited by removal of extracellular K+ and measurements were taken almost immediately to minimise effects on other cellular compartments. Activity of the Na+-Ca2+ exchanger was monitored during release of Ca2+ from the sarcoplasmic reticulum (SR) elicited by rapid application of 15 mM caffeine. When Na+-K+ pump activity was affected by K+ removal, cell relaxation and indo-1 fluorescence decline were slowed by approximately 40 %. The charge calculated by integrating the caffeine-induced transient inward current was unchanged, suggesting that there was no difference in the SR Ca2+ content in the two conditions. However Ca2+ flux via the Na+-Ca2+ exchanger was slower when the Na+-K+ pump was inhibited. Similar experiments were performed by inhibiting the Na+-K+ pump using 0.5 mM strophanthidin. In this condition similar results to the ones observed by K+ removal were obtained, suggesting a specific role of the Na+-K+ pump in the phenomenon observed. This study suggests that the activity of the Na+-K+ pump influences Na+-Ca2+ exchanger function in the absence of changes in SR Ca2+ content. This can be explained by a slower removal of Na+ from the subsarcolemmal space. The source of the increase in subsarcolemmal [Na+] requires further investigation. However, calculations derived from modelling suggest that the Na+-Ca2+ exchanger itself could be involved.

Cyclic AMP increases cell surface expression of functional Na,K-ATPase units in mammalian cortical collecting duct principal cells

Cyclic AMP (cAMP) stimulates the transport of Na(+) and Na,K-ATPase activity in the renal cortical collecting duct (CCD). The aim of this study was to investigate the mechanism whereby cAMP stimulates the Na,K-ATPase activity in microdissected rat CCDs and cultured mouse mpkCCD(c14) collecting duct cells. db-cAMP (10(-3) M) stimulated by 2-fold the activity of Na,K-ATPase from rat CCDs as well as the ouabain-sensitive component of (86)Rb(+) uptake by rat CCDs (1.7-fold) and cultured mouse CCD cells (1.5-fold). Pretreatment of rat CCDs with saponin increased the total Na,K-ATPase activity without further stimulation by db-cAMP. Western blotting performed after a biotinylation procedure revealed that db-cAMP increased the amount of Na,K-ATPase at the cell surface in both intact rat CCDs (1.7-fold) and cultured cells (1.3-fold), and that this increase was not related to changes in Na,K-ATPase internalization. Brefeldin A and low temperature (20 degrees C) prevented both the db-cAMP-dependent increase in cell surface expression and activity of Na,K-ATPase in both intact rat CCDs and cultured cells. Pretreatment with the intracellular Ca(2+) chelator bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid also blunted the increment in cell surface expression and activity of Na,K-ATPase caused by db-cAMP. In conclusion, these results strongly suggest that the cAMP-dependent stimulation of Na,K-ATPase activity in CCD results from the translocation of active pump units from an intracellular compartment to the plasma membrane.

Dynamics and consequences of potassium shifts in skeletal muscle and heart during exercise

Since it became clear that K(+) shifts with exercise are extensive and can cause more than a doubling of the extracellular [K(+)] ([K(+)](s)) as reviewed here, it has been suggested that these shifts may cause fatigue through the effect on muscle excitability and action potentials (AP). The cause of the K(+) shifts is a transient or long-lasting mismatch between outward repolarizing K(+) currents and K(+) influx carried by the Na(+)-K(+) pump. Several factors modify the effect of raised [K(+)](s) during exercise on membrane potential (E(m)) and force production. 1) Membrane conductance to K(+) is variable and controlled by various K(+) channels. Low relative K(+) conductance will reduce the contribution of [K(+)](s) to the E(m). In addition, high Cl(-) conductance may stabilize the E(m) during brief periods of large K(+) shifts. 2) The Na(+)-K(+) pump contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in the heart. This will contribute considerably to the lowering of intracellular [K(+)] ([K(+)](c)) and will attenuate the exercise-induced rise of intracellular [Na(+)] ([Na(+)](c)). 4) The rise of [Na(+)](c) is sufficient to activate the Na(+)-K(+) pump to completely compensate increased K(+) release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for control of pump activity not only through hormones, but through a hitherto unidentified mechanism. 5) Ionic shifts within the skeletal muscle t tubules and in the heart in extracellular clefts may markedly affect excitation-contraction coupling. 6) Age and state of training together with nutritional state modify muscle K(+) content and the abundance of Na(+)-K(+) pumps. We conclude that despite modifying factors coming into play during muscle activity, the K(+) shifts with high-intensity exercise may contribute substantially to fatigue in skeletal muscle, whereas in the heart, except during ischemia, the K(+) balance is controlled much more effectively.

Isoform-Specific Regulation of the Na(+)-K(+) Pump in Heart

Guinea pig ventricular myocytes coexpress two isoforms of the Na(+)-K(+) pump. These two isoforms respond differently to the physical environment and are coupled to autonomic input through different signal transduction cascades. The expression of different isoforms provides each cell type with a mechanism of programming specific responses to environmental changes.

Activation of the cAMP-protein kinase A pathway facilitates Na+ translocation by the Na+-K+ pump in guinea-pig ventricular myocytes

1. The effects of the adenylyl cyclase activator forskolin on steady-state and transient currents generated by the Na+-K+ pump were studied in guinea-pig ventricular myocytes by means of whole-cell voltage clamp at 30 C. 2. In external solution containing 144 mM Na+ (Na+o) and 10 mM K+ (K+o), steady-state Na+-K+ pump current (Ip) activated by 5 mM pipette Na+ (Na+pip) at -20 mV was reversibly augmented by forskolin (4 microM) to 133 +/- 4 % of the control current (n = 15). The forskolin analogue 1, 9-dideoxyforskolin (10 microM), which does not activate adenylyl cyclases, did not increase Ip (n = 2). Application of the protein kinase A (PKA) inhibitor H-89 (10 microM) in the continued presence of forskolin reversed the forskolin-induced elevation of Ip (n = 3). 3. The forskolin effect on Ip persisted in the presence of 50 mM Na+pip which ensured that the internal Na+-binding sites of the Na+-K+ pump were nearly saturated. Under these conditions, the drug increased Ip to 142 +/- 3 % of the control Ip when the pipette free Ca2+ concentration ([Ca2+]pip) was 0.013 nM (n = 5) and to 138 +/- 4 % of the control Ip when free [Ca2+]pip was 15 nM (n = 9). 4. In Na+-free external solution, Ip activated by 50 mM Na+pip and 1.5 mM K+o was likewise increased by forskolin but to a lesser extent than in Na+-containing medium (116 +/- 3 % of control, n = 10). 5. In order to investigate exclusively partial reactions in the Na+ limb of the pump cycle, transient pump currents under conditions of electroneutral Na+-Na+ exchange were studied. Transient pump currents elicited by voltage jumps displayed an initial peak and then decayed monoexponentially. Moved charge (Q) and the rate constant of current decay varied with membrane potential (V). The Q-V relationship followed a Boltzmann distribution characterized by the midpoint voltage (V0.5) and the maximum amount of movable charge (DeltaQmax). Forskolin (2-10 microM) shifted V0.5 to more negative values while DeltaQmax was not affected (n = 11). The effects of forskolin on transient pump currents were mimicked by 8-bromo-cAMP (500 microM; n = 2) and abolished by a peptide inhibitor of PKA (PKI, 10 microM; n = 5). 6. We conclude that activation of the cAMP-PKA pathway in guinea-pig ventricular myocytes increases Na+-K+ pump current at least in part by modulating partial reactions in the Na+ limb of the pump cycle. Under physiological conditions, the observed stimulation of the cardiac Na+-K+ pump may serve to shorten the action potential duration and to counteract the increased passive sarcolemmal Na+ and K+ fluxes during sympathetic stimulation of the heart.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

Isoform-specific regulation of the sodium pump by alpha- and beta-adrenergic agonists in the guinea-pig ventricle

1. Guinea-pig ventricle was used in the RNase protection assays to determine which alpha-isoforms of the Na+-K+ pumps are present, and ventricular myocytes were used in whole cell patch clamp studies to investigate the actions of alpha- and beta-adrenergic agonists on Na+-K+ pump current. 2. RNase protection assays showed that two isoforms of the alpha-subunit of the Na+-K+-ATPase are present in guinea-pig ventricle. The mRNA for the alpha1-isoform comprises 82 % of the total pump message, the rest being the alpha2-isoform. 3. We have previously shown that beta-adrenergic agonists affect Na+-K+ pump current (Ip) through a protein kinase A (PKA)-dependent pathway. We now show that these beta-effects are targeted to the alpha1-isoform of the Na+-K+ pumps. 4. We have also previously shown that alpha-adrenergic agonists increase Ip through a protein kinase C (PKC)-dependent pathway. We now show that these alpha-isoform effects are targeted to the alpha2-isoform of the Na+-K+ pumps. 5. These results suggest the effects of adrenergic activation on Na+-K+ pump activity in the heart can be regionally specific, depending on which alpha-isoform of the Na+-K+ pump is expressed.

Antiarrhythmic drugs and cardiac ion channels: mechanisms of action

In this review a description and an analysis are given of the interaction of antiarrhythmic drugs with their molecular target, i.e. ion channels and receptors. Our approach is based on the concept of vulnerable parameter, i.e. the electrophysiological property which plays a crucial role in the genesis of arrhythmias. To prevent or stop the arrhythmia a drug should modify the vulnerable parameter by its action on channel or receptor targets. In the first part, general aspects of the interaction between drugs channel molecules are considered. Drug binding depends on the state of the channel: rested, activated pre-open, activated open, or inactivated state. The change in channel behaviour with state is presented in the framework of the modulated-receptor hypothesis. Not only inhibition but also stimulation can be the result of drug binding. In the second part a detailed and systematic description and an analysis are given of the interaction of drugs with specific channels (Na+, Ca2+, K+, "pacemaker") and non-channel receptors. Emphasis is given to the type of state-dependent block involved (rested, activated and inactivated state block) and the change in channel kinetics. These properties vary and determine the voltage- and frequency-dependence of the change in ionic current. Finally, the question is asked as to whether the available drugs by their action on channels and receptors modify the vulnerable parameter in the desired way to stop or prevent arrhythmias.

alpha-Adrenergic effects on Na+-K+ pump current in guinea-pig ventricular myocytes

1. The whole-cell patch clamp was employed to study Na+-K+ pump current (Ip) in acutely isolated myocytes. alpha-Adrenergic receptors were activated with noradrenaline (NA) after blocking beta-adrenergic receptors with propranolol. Ip was measured as the current blocked by strophanthidin (Str). 2. Activation of alpha-receptors by NA increased Ip in a concentration-dependent manner. The K0.5 depended on intracellular calcium ([Ca2+]i), however maximal stimulation did not. At 15 nM [Ca2+]i the K0.5 was 219 nM NA whereas at 1.4 microM [Ca2+]i it was 3 nM. 3. The voltage dependence of Ip was not shifted by NA at either high or low [Ca2+]i. At each voltage, maximal stimulation of Ip was 14-15 %. 4. Staurosporine (St), an inhibitor of protein kinase C (PKC), eliminated the alpha-receptor-mediated stimulation of Ip at either high or low[Ca2+]i. 5. The stimulation of Ip was independent of changes in intracellular sodium or external potassium concentrations, and did not reflect a change in affinity for Str. 6. Phenylephrine, methoxamine and metaraminol, three selective alpha1-adrenergic agonists, stimulate Ip in a similar manner to NA. Stimulation of Ip by NA was eliminated by prazosin, an alpha1-antagonist, but was unaffected by yohimbine, an alpha2-antagonist. 7. We conclude noradrenaline activates ventricular alpha1-receptors, which are specifically coupled via PKC to increase Na+-K+ pump current. The sensitivity of the coupling is [Ca2+]i dependent, however the maximal increase in pump current is [Ca2+]i and voltage independent.

Na(+)-K+ pump cycle during beta-adrenergic stimulation of adult rat cardiac myocytes

1. The mechanisms underlying the increase in Na(+)-K+ pump current (Ip) caused by adrenergic stimulation were investigated in cultured adult rat cardiac myocytes using the whole-cell patch-clamp technique at 31-33 degrees C. 2. In myocytes perfused internally with 50 mM Na+ (0 K+i, 20 nM Ca2+, caesium aspartate solution) and externally with 5.4 mM K+o, noradrenaline (NA) and isoprenaline (Iso) (1-50 microM) stimulated Ip by 40-45%. 3. Na(+)-dependent transient Ip measurements with 0 mM K+i and 0 mM K+o revealed no change in the total charge transferred by the Na(+)-K+ pump during the conformational change, suggesting that the pump site density was not changed by adrenergic stimulation (2630 +/- 370 pumps micron-2 in control and 2540 +/- 190 pumps micron-2 in the presence of 10 microM NA). 4. With saturating Na+i or K+o (150 and 15-20 mM, respectively), Ip was still stimulated by NA and Iso. Thus, there was no indication that adrenergic activation of the Na(+)-K+ pump was mediated by accumulation of Na+i and K+o or changes in the Na(+)-K+ pump affinity for Na+i and K+o. 5. Both Ip and its increase under adrenergic stimulation were found to depend on [K+]i. While steady-state Ip decreased from 2.2 +/- 0.1 to 1.2 +/- 0.1 pA pF-1 (P < 0.05), the stimulation of Ip by 10 microM Iso increased from 0.38 +/- 0.04 to 0.67 +/- 0.06 pA pF-1 (P < 0.05) with an increase in [K+]i from 0 to 100 mM. 6. Under conditions that cause the Ip-Vm (membrane potential) relationship to express a positive slope ([Na+]o, 150 mM; [K+]o, 5.4 mM) or a negative slope ([Na+]o, 0; [K+]o, 0.3 mM) Iso stimulated Ip with no change in the shape of Ip-Vm curves. Thus, adrenergic stimulation of the Na(+)-K+ pump was not due to an alteration of voltage-dependent steps of the pump cycle. 7. Simulation of these data with a six-step model of the Na(+)-K+ pump cycle suggested that in rat ventricular myocytes a signal from adrenergic receptors increased the Na(+)-K+ pump rate by modulating the rate of K+ de-occlusion and release by the pump.

Effects of acetylcholine on the Na(+)-K+ pump current in guinea-pig ventricular myocytes

1. The whole-cell patch clamp technique was used to study the effects of acetylcholine (ACh) on Na(+)-K+ pump current (Ip) in acutely isolated guinea-pig ventricular myocytes. Studies were performed in the absence and presence of the beta-agonist isoprenaline (Iso). 2. ACh had no effect on Ip at low or high [Ca2+]i at any voltage in the absence of Iso. Iso alone inhibited Ip at low [Ca2+]i and shifted the Ip-V relationship at high [Ca2+]i in a negative direction. Addition of 1 microM ACh reversed these effects of Iso. K0.5 for the effects of ACh was about 16 nM, regardless of [Ca2+]i. 3. The actions of ACh on the heart are usually mediated via muscarinic receptors. Atropine, a muscarinic antagonist, blocked the effects of ACh on Ip in the presence of Iso, suggesting that these effects are also mediated by muscarinic receptors. 4. Muscarinic receptors are usually coupled to a Gi protein, leading to inhibition of adenylyl cyclase and a reduction of cAMP levels. We have shown previously that basal levels of cAMP are very low in guinea-pig ventricular myocytes, and that a membrane-permeant cAMP analogue, chlorophenylthio-cAMP (CPTcAMP), mimics the effects of Iso. ACh did not reverse the effects of CPTcAMP, supporting the hypothesis that the effects of ACh on Ip are also mediated via inhibition of adenylyl cyclase. 5. The present results suggest that a high level of parasympathetic tone alone does not affect the activity of ventricular Na(+)-K+ pumps. However, if sympathetic tone is high, then muscarinic stimulation can reciprocally modulate Na(+)-K+ pump activity.