Na⁺ transport in the normal and failing heart - remember the balance

In the heart, intracellular Na(+) concentration ([Na(+)]i) is a key modulator of Ca(2+) cycling, contractility and cardiac myocyte metabolism. Several Na(+) transporters are electrogenic, thus they both contribute to shaping the cardiac action potential and at the same time are affected by it. [Na(+)]i is controlled by the balance between Na(+) influx through various pathways, including the Na(+)/Ca(2+) exchanger and Na(+) channels, and Na(+) extrusion via the Na(+)/K(+)-ATPase. [Na(+)]i is elevated in HF due to a combination of increased entry through Na(+) channels and/or Na(+)/H(+) exchanger and reduced activity of the Na(+)/K(+)-ATPase. Here we review the major Na(+) transport pathways in cardiac myocytes and how they participate in regulating [Na(+)]i in normal and failing hearts. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes."

Can the sodium-calcium exchanger initiate or suppress calcium sparks in cardiac myocytes?

Positive feedback of Calcium (Ca)-induced Ca release is the mechanism of Ca spark formation in cardiac myocytes. To initiate this process, a certain amount of Ca in the cleft space is necessary. When the membrane potential becomes higher during excitation-contraction coupling, Ca can enter through both Ca current (I(CaL)) and sodium-calcium exchanger (NCX) and may activate ryanodine receptors to initiate a Ca spark. On the other hand, at the resting membrane potential (V(m) ~-80 mV), NCX removes Ca from the cell (forward mode). If Ca released from the sarcoplasmic reticulum is quickly removed via forward mode NCX before Ca-induced Ca release starts, the Ca release becomes nonspark Ca leak. This would also be influenced by the cleft/noncleft distribution of NCX, which is unknown. Using a physiologically detailed mathematical model of subcellular Ca cycling, we analyze how NCX strength and distribution alter Ca spark formation. During excitation-contraction coupling, most Ca sparks are induced by I(CaL) with very few due to NCX current. At the resting membrane potential if most NCX is localized to the cleft, spontaneous Ca sparks are significantly reduced.

Pharmacological inhibition of soluble epoxide hydrolase provides cardioprotection in hyperglycemic rats

Glycemic regulation improves myocardial function in diabetic patients, but finding optimal therapeutic strategies remains challenging. Recent data have shown that pharmacological inhibition of soluble epoxide hydrolase (sEH), an enzyme that decreases the endogenous levels of protective epoxyeicosatrienoic acids (EETs), improves glucose homeostasis in insulin-resistant mice. Here, we tested whether the administration of sEH inhibitors preserves cardiac myocyte structure and function in hyperglycemic rats. University of California-Davis-type 2 diabetes mellitus (UCD-T2DM) rats with nonfasting blood glucose levels in the range of 150-200 mg/dl were treated with the sEH inhibitor 1-(1-acetypiperidin-4-yl)-3-adamantanylurea (APAU) for 6 wk. Administration of APAU attenuated the progressive increase of blood glucose concentration and preserved mitochondrial structure and myofibril morphology in cardiac myocytes, as revealed by electron microscopy imaging. Fluorescence microscopy with Ca(2+) indicators also showed a 40% improvement of cardiac Ca(2+) transients in treated rats. Sarcoplasmic reticulum Ca(2+) content was decreased in both treated and untreated rats compared with control rats. However, treatment limited this reduction by 30%, suggesting that APAU may protect the intracellular Ca(2+) effector system. Using Western blot analysis on cardiac myocyte lysates, we found less downregulation of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA), the main route of Ca(2+) reuptake in the sarcoplasmic reticulum, and lower expression of hypertrophic markers in treated versus untreated UCD-T2DM rats. In conclusion, APAU enhances the therapeutic effects of EETs, resulting in slower progression of hyperglycemia, efficient protection of myocyte structure, and reduced Ca(2+) dysregulation and SERCA remodeling in hyperglycemic rats. The results suggest that sEH/EETs may be an effective therapeutic target for cardioprotection in insulin resistance and diabetes.

Na(+)/K)+)-ATPase α2-isoform preferentially modulates Ca2(+) transients and sarcoplasmic reticulum Ca2(+) release in cardiac myocytes

AIMS

Na(+)/K(+)-ATPase (NKA) is essential in regulating [Na(+)](i), and thus cardiac myocyte Ca(2+) and contractility via Na(+)/Ca(2+) exchange. Different NKA-α subunit isoforms are present in the heart and may differ functionally, depending on specific membrane localization. In smooth muscle and astrocytes, NKA-α2 is located at the junctions with the endo(sarco)plasmic reticulum, where they could regulate local [Na(+)], and indirectly junctional cleft [Ca(2+)]. Whether this model holds for cardiac myocytes is unclear.

METHODS AND RESULTS

The ouabain-resistant NKA-α1 cannot be selectively blocked to assess its effect. To overcome this, we used mice in which NKA-α1 is ouabain sensitive and NKA-α2 is ouabain resistant (SWAP mice). We measured the effect of ouabain at low concentration on [Na(+)](i), Ca(2+) transients, and the fractional sarcoplasmic reticulum (SR) Ca(2+) release in cardiac myocytes from wild-type (WT; NKA-α2 inhibition) and SWAP mice (selective NKA-α1 block). At baseline, Na(+) and Ca(2+) regulations are similar in WT and SWAP mice. For equal levels of total NKA inhibition (~25%), ouabain significantly increased Ca(2+) transients (from ΔF/F(0)= 1.5 ± 0.1 to 1.8 ± 0.1), and fractional SR Ca(2+) release (from 24 ± 3 to 29 ± 3%) in WT (NKA-α2 block) but not in SWAP myocytes (NKA-α1 block). This occurred despite a similar and modest increase in [Na(+)](i) (~2 mM) in both groups. The effect in WT mice was mediated specifically by NKA-α2 inhibition because at a similar concentration ouabain had no effect in transgenic mice where both NKA-α1 and NKA-α2 are ouabain resistant.

CONCLUSION

NKA-α2 has a more prominent role (vs. NKA-α1) in modulating cardiac myocyte SR Ca(2+) release.

Hyperamylinemia contributes to cardiac dysfunction in obesity and diabetes: a study in humans and rats

RATIONALE

Hyperamylinemia is common in patients with obesity and insulin resistance, coincides with hyperinsulinemia, and results in amyloid deposition. Amylin amyloids are generally considered a pancreatic disorder in type 2 diabetes. However, elevated circulating levels of amylin may also lead to amylin accumulation and proteotoxicity in peripheral organs, including the heart.

OBJECTIVE

To test whether amylin accumulates in the heart of obese and type 2 diabetic patients and to uncover the effects of amylin accumulation on cardiac morphology and function.

METHODS AND RESULTS

We compared amylin deposition in failing and nonfailing hearts from lean, obese, and type 2 diabetic humans using immunohistochemistry and Western blots. We found significant accumulation of large amylin oligomers, fibrils, and plaques in failing hearts from obese and diabetic patients but not in normal hearts and failing hearts from lean, nondiabetic humans. Small amylin oligomers were even elevated in nonfailing hearts from overweight/obese patients, suggesting an early state of accumulation. Using a rat model of hyperamylinemia transgenic for human amylin, we observed that amylin oligomers attach to the sarcolemma, leading to myocyte Ca(2+) dysregulation, pathological myocyte remodeling, and diastolic dysfunction, starting from prediabetes. In contrast, prediabetic rats expressing the same level of wild-type rat amylin, a nonamyloidogenic isoform, exhibited normal heart structure and function.

CONCLUSIONS

Hyperamylinemia promotes amylin deposition in the heart, causing alterations of cardiac myocyte structure and function. We propose that detection and disruption of cardiac amylin buildup may be both a predictor of heart dysfunction and a novel therapeutic strategy in diabetic cardiomyopathy.

Role of phospholemman phosphorylation sites in mediating kinase-dependent regulation of the Na+-K+-ATPase

Phospholemman (PLM) is a major target for phosphorylation mediated by both PKA (at Ser68) and PKC (at both Ser63 and Ser68) in the heart. In intact cardiac myocytes, PLM associates with and inhibits Na(+)-K(+)-ATPase (NKA), mainly by reducing its affinity for internal Na(+). The inhibition is relieved upon PLM phosphorylation by PKA or PKC. The aim here was to distinguish the role of the Ser63 and Ser68 PLM phosphorylation sites in mediating kinase-induced modulation of NKA function. We expressed wild-type (WT) PLM and S63A, S68A, and AA (Ser63 and Ser68 to alanine double mutant) PLM mutants in HeLa cells that stably express rat NKA-α(1) and we measured the effect of PKA and PKC activation on NKA-mediated intracellular Na(+) concentration decline. PLM expression (WT or mutant) significantly decreased the apparent NKA affinity for internal Na(+) and had no significant effect on the maximum pump rate (V(max)). PKA activation with forskolin (20 μM) restored NKA Na(+) affinity in cells expressing WT but not AA PLM and did not affect V(max) in either case. Similarly, PKC activation with 300 nM phorbol 12,13-dibutyrate increased NKA Na(+) affinity in cells expressing WT, S63A, and S68A PLM and had no effect in cells expressing AA PLM. Neither forskolin nor phorbol 12,13-dibutyrate affected NKA function in the absence of PLM. We conclude that PLM phosphorylation at either Ser63 or Ser68 is both necessary and sufficient for completely relieving the PLM-induced NKA inhibition.

Na+ transport in cardiac myocytes; Implications for excitation-contraction coupling

Intracellular Na(+) concentration ([Na(+)](i)) is very important in modulating the contractile and electrical activity of the heart. Upon electrical excitation of the myocardium, voltage-dependent Na(+) channels open, triggering the upstroke of the action potential (AP). During the AP, Ca(2+) enters the myocytes via L-type Ca(2+) channels. This triggers Ca(2+) release from the sarcoplasmic reticulum (SR) and thus activates contraction. Relaxation occurs when cytosolic Ca(2+) declines, mainly due to re-uptake into the SR via SR Ca(2+)-ATPase and extrusion from the cell via the Na(+)/Ca(2+) exchanger (NCX). NCX extrudes one Ca(2+) ion in exchange for three Na(+) ions and its activity is critically regulated by [Na(+)](i). Thus, via NCX, [Na(+)](i) is centrally involved in the regulation of intracellular [Ca(2+)] and contractility. Na(+) brought in by Na(+) channels, NCX and other Na(+) entry pathways is extruded by the Na(+)/K(+) pump (NKA) to keep [Na(+)](i) low. NKA is regulated by phospholemman, a small sarcolemmal protein that associates with NKA. Unphosphorylated phospholemman inhibits NKA by decreasing the pump affinity for internal Na(+) and this inhibition is relieved upon phosphorylation. Here we discuss the main characteristics of the Na(+) transport pathways in cardiac myocytes and their physiological and pathophysiological relevance.

Isoform specificity of the Na/K-ATPase association and regulation by phospholemman

Phospholemman (PLM) phosphorylation mediates enhanced Na/K-ATPase (NKA) function during adrenergic stimulation of the heart. Multiple NKA isoforms exist, and their function/regulation may differ. We combined fluorescence resonance energy transfer (FRET) and functional measurements to investigate isoform specificity of the NKA-PLM interaction. FRET was measured as the increase in the donor fluorescence (CFP-NKA-alpha1 or CFP-NKA-alpha2) during progressive acceptor (PLM-YFP) photobleach in HEK-293 cells. Both pairs exhibited robust FRET (maximum of 23.6 +/- 3.4% for NKA-alpha1 and 27.5 +/- 2.5% for NKA-alpha2). Donor fluorescence depended linearly on acceptor fluorescence, indicating a 1:1 PLM:NKA stoichiometry for both isoforms. PLM phosphorylation induced by cAMP-dependent protein kinase and protein kinase C activation drastically reduced the FRET with both NKA isoforms. However, submaximal cAMP-dependent protein kinase activation had less effect on PLM-NKA-alpha2 versus PLM-NKA-alpha1. Surprisingly, ouabain virtually abolished NKA-PLM FRET but only partially reduced co-immunoprecipitation. PLM-CFP also showed FRET to PLM-YFP, but the relationship during progressive photobleach was highly nonlinear, indicating oligomers involving >or=3 monomers. Using cardiac myocytes from wild-type mice and mice where NKA-alpha1 is ouabain-sensitive and NKA-alpha2 is ouabain-resistant, we assessed the effects of PLM phosphorylation on NKA-alpha1 and NKA-alpha2 function. Isoproterenol enhanced internal Na(+) affinity of both isoforms (K((1/2)) decreased from 18.1 +/- 2.0 to 11.5 +/- 1.9 mm for NKA-alpha1 and from 16.4 +/- 2.5 to 10.4 +/- 1.5 mm for NKA-alpha2) without altering maximum transport rate (V(max)). Protein kinase C activation also decreased K((1/2)) for both NKA-alpha1 and NKA-alpha2 (to 9.4 +/- 1.0 and 9.1 +/- 1.1 mm, respectively) but increased V(max) only for NKA-alpha2 (1.9 +/- 0.4 versus 1.2 +/- 0.5 mm/min). In conclusion, PLM associates with and modulates both NKA-alpha1 and NKA-alpha2 in a comparable but not identical manner.

Extracellular potassium dependence of the Na+-K+-ATPase in cardiac myocytes: isoform specificity and effect of phospholemman

Cardiac Na(+)-K(+)-ATPase (NKA) regulates intracellular Na(+), which in turn affects intracellular Ca(2+) and contractility via the Na(+)/Ca(2+) exchanger. Extracellular K(+) concentration ([K(+)]) is a central regulator of NKA activity. Phospholemman (PLM) has recently been recognized as a critical regulator of NKA in the heart. PLM reduces the intracellular Na(+) affinity of NKA, an effect relieved by PLM phosphorylation. Here we tested whether the NKA alpha(1)- vs. alpha(2)- isoforms have different external K(+) sensitivity and whether PLM and PKA activation affects the NKA affinity for K(+) in mouse cardiac myocytes. We measured the external [K(+)] dependence of the pump current generated by the ouabain-resistant NKA isoform in myocytes from wild-type (WT) mice (i.e., current due to NKA-alpha(1)) and mice in which the NKA isoforms have swapped ouabain affinities (alpha(1) is ouabain sensitive and alpha(2) is ouabain resistant) to assess current due to NKA-alpha(2). We found that NKA-alpha(1) has a higher affinity for external K(+) than NKA-alpha(2) [half-maximal pump activation (K(0.5)) = 1.5 +/- 0.1 vs. 2.9 +/- 0.3 mM]. The apparent external K(+) affinity of NKA was significantly lower in myocytes from WT vs. PLM-knockout mice (K(0.5) = 2.0 +/- 0.2 vs. 1.05 +/- 0.08 mM). However, PKA activation by isoproterenol (1 microM) did not alter the K(0.5) of NKA for external K(+) in WT myocytes. We conclude that 1) NKA-alpha(1) has higher affinity for K(+) than NKA-alpha(2) in cardiac myocytes, 2) PLM decreases the apparent external K(+) affinity of NKA, and 3) phosphorylation of PLM at the cytosolic domain does not alter apparent extracellular K(+) affinity of NKA.

Phospholemman-mediated activation of Na/K-ATPase limits [Na]i and inotropic state during beta-adrenergic stimulation in mouse ventricular myocytes

BACKGROUND

Cardiac Na/K-ATPase (NKA) regulates intracellular Na ([Na](i)), which in turn affects intracellular Ca and thus contractility via Na/Ca exchange. Recent evidence shows that phosphorylation of the NKA-associated small transmembrane protein phospholemman (PLM) mediates beta-adrenergic-induced NKA stimulation.

METHODS AND RESULTS

Here, we tested whether PLM phosphorylation during beta-adrenergic activation limits the rise in [Na](i), Ca transient amplitude, and triggered arrhythmias in mouse ventricular myocytes. In myocytes from wild-type (WT) mice, [Na](i) increased on field stimulation at 2 Hz from 11.1+/-1.8 mmol/L to a plateau of 15.2+/-1.5 mmol/L. Isoproterenol induced a decrease in [Na](i) to 12.0+/-1.2 mmol/L. In PLM knockout (PLM-KO) mice in which beta-adrenergic stimulation does not activate NKA, [Na](i) also increased at 2 Hz (from 10.4+/-1.2 to 17.0+/-1.5 mmol/L) but was unaltered by isoproterenol. The PLM-mediated decrease in [Na](i) in WT mice could limit the isoproterenol-induced inotropic state. Indeed, the isoproterenol-induced increase in the amplitude of Ca transients was significantly smaller in the WT mice (5.2+/-0.4- versus 7.1+/-0.5-fold in PLM-KO mice). This also was the case for the sarcoplasmic reticulum Ca content, which increased by 1.27+/-0.09-fold in WT mice versus 1.53+/-0.09-fold in PLM-KO mice. The higher sarcoplasmic reticulum Ca content in PLM-KO versus WT mice was associated with an increased propensity for spontaneous Ca transients and contractions in PLM-KO mice.

CONCLUSIONS

These data suggest that PLM phosphorylation and NKA stimulation are an integral part of the sympathetic fight-or-flight response, tempering the rise in [Na](i) and cellular Ca loading and perhaps limiting Ca overload-induced arrhythmias.

Functional analysis of Na+/K+-ATPase isoform distribution in rat ventricular myocytes

The Na+/K+-ATPase (NKA) is the main route for Na+ extrusion from cardiac myocytes. Different NKA α-subunit isoforms are present in the heart. NKA-α1 is predominant, although there is a variable amount of NKA-α2 in adult ventricular myocytes of most species. It has been proposed that NKA-α2 is localized mainly in T-tubules (TT), where it could regulate local Na+/Ca2+ exchange and thus cardiac myocyte Ca2+. However, there is controversy as to where NKA-α1 vs. NKA-α2 are localized in ventricular myocytes. Here, we assess the TT vs. external sarcolemma (ESL) distribution functionally using formamide-induced detubulation of rat ventricular myocytes, NKA current (IPump) measurements and the different ouabain sensitivity of NKA-α1 (low) and NKA-α2 (high) in rat heart. Ouabain-dependent IPump inhibition in control myocytes indicates a high-affinity NKA isoform (NKA-α2, K1/2 = 0.38 ± 0.16 μM) that accounts for 29.5 ± 1.3% of IPump and a low-affinity isoform (NKA-α1, K1/2 = 141 ± 17 μM) that accounts for 70.5% of IPump. Detubulation decreased cell capacitance from 164 ± 6 to 120 ± 8 pF and reduced IPump density from 1.24 ± 0.05 to 1.02 ± 0.05 pA/pF, indicating that the functional density of NKA is significantly higher in TT vs. ESL. In detubulated myocytes, NKA-α2 accounted for only 18.2 ± 1.1% of IPump. Thus, ∼63% of IPump generated by NKA-α2 is from the TT (although TT are only 27% of the total sarcolemma), and the NKA-α2/NKA-α1 ratio in TT is significantly higher than in the ESL. The functional density of NKA-α2 is ∼4.5 times higher in the T-tubules vs. ESL, whereas NKA-α1 is almost uniformly distributed between the TT and ESL.

Regulation of Ca2+ and Na+ in normal and failing cardiac myocytes

Ca(2+) in cardiac myocytes regulates contractility and relaxation, and Ca(2+) and Na (+)regulation are linked via Na(+)/Ca(2+) exchange (NCX). Heart failure (HF) is accompanied by contractile dysfunction and arrhythmias, both of which may be due to altered cellular Ca(2+) handling. Smaller Ca(2+) transient and sarcoplasmic reticulum (SR) Ca(2+) content cause systolic dysfunction in HF. The reduced SR Ca(2+) content is due to: (a) reduced SR Ca(2+)-ATPase function (which also contributes to diastolic dysfunction), (b) increased expression and function of NCX (which competes with SR Ca(2+)-ATPase during relaxation, but preserves diastolic function), and (c) enhanced diastolic SR Ca(2+) leak. Relative contributions of these may vary with HF etiology and stage. Triggered arrhythmias (e.g., delayed afterdepolarizations [DADs]) are prominent in HF. DADs are due to spontaneous SR Ca(2+) release and consequent activation of transient inward NCX current, which in HF allows DADs to more readily trigger arrhythmogenic action potentials. Thus NCX and Na(+) are critical in systolic and diastolic function and arrhythmias. [Na(+)](i) is elevated in HF, which may limit SR unloading and provide some Ca(2+) influx during the HF action potential, thus limiting the depression of systolic function. High [Na(+)](i) in HF is due to enhanced Na(+) influx. Cellular Na(+)/K(+)-ATPase (NKA) function appears unaltered, despite reduced NKA expression. This dichotomy led us to test NKA regulation by phospholemman (PLM). We find that PLM regulates NKA in a manner analogous to phospholamban regulation of SR Ca(2+)-ATPase (i.e., inhibition that is relieved by PLM phosphorylation). We measured intermolecular FRET between PLM and NKA, which is reduced upon PLM phosphorylation. The lower expression level of more phosphorylated PLM in HF may explain the above dichotomy. Thus, altered Ca(2+) and Na(+) handling contributes to altered contractile function and arrhythmogenesis in HF.

Differential distribution and regulation of mouse cardiac Na+/K+-ATPase alpha1 and alpha2 subunits in T-tubule and surface sarcolemmal membranes

OBJECTIVES

Two Na+/K+-ATPase (NKA) alpha-subunit isoforms, alpha1 and alpha2, are expressed in the adult mouse heart. The subcellular distribution of these isoforms in T-tubule and surface sarcolemmal (SSL) membranes and their regulation by cAMP-dependent protein kinase (PKA) is unclear.

METHODS

We used formamide-induced detubulation of mouse ventricular myocytes to investigate differential functional distribution and regulation by PKA of alpha1 and alpha2 in T-tubule versus SSL membranes by measuring NKA current (I(pump)) and NKA-mediated Na+ efflux (-d[Na](i)/dt).

RESULTS

I(pump) is composed of 88% alpha(1)-mediated I(pump) (Ialpha1) and 12% alpha2-mediated I(pump) (Ialpha2). alpha1 and alpha2 subunits demonstrate distinct ouabain affinities (105+/-6 and 0.3+/-0.1 micromol/L respectively) but similar affinity for intracellular Na+ (K(1/2)Na+ of 16.6+/-0.8 and 16.7+/-2.6 mmol/L respectively). Detubulation reduced (i) I(pump) density (1.42+/-0.1 to 1.20+/-0.04 pA/pF), (ii) cell capacitance (181+/-12 to 127+/-17 pF), and (iii) Ialpha2 contribution (12 to 6%). Total I(pump) density was approximately 60% higher in T-tubule (1.94 pA/pF, derived) vs. SSL membranes. Although T-tubule membranes represent only 30% of total surface area, they generate approximately 70% of Ialpha2 and approximately 37% of Ialpha1. Ialpha1 density was substantially higher than Ialpha2 in SSL (Ialpha1:Ialpha2 = 16:1) but this was markedly reduced in T-tubules (4:1). In addition to differential localisation, isoprenaline (ISO, 1 micromol/L) significantly increased alpha1-mediated NKA Na+ affinity (from 16.6+/-0.8 to 13.3+/-1.4 mmol/L) and caused a small increase in maximal NKA Na+ efflux rate. ISO had no effect on alpha2-mediated NKA activity.

CONCLUSION

These data suggest that NKA alpha1 and alpha2 subunits are differentially localised and regulated by PKA in T-tubule and SSL membranes and may have distinct regulatory roles in cardiac excitation-contraction coupling.

Phospholemman phosphorylation mediates the protein kinase C-dependent effects on Na+/K+ pump function in cardiac myocytes

Because phospholemman (PLM) regulates the Na(+)/K(+) pump (NKA) and is a major cardiac phosphorylation target for both protein kinase A (at Ser68) and protein kinase C (PKC) (at both Ser63 and Ser68), we evaluated whether PLM mediates the PKC-dependent regulation of NKA function and protein kinase A/PKC crosstalk in ventricular myocytes. PKC was activated by PDBu (300 nmol/L), and we measured NKA-mediated [Na(+)](i) decline (fluorescence measurements) and current (I(pump)) (voltage clamp). In wild-type mouse myocytes, PDBu increased PLM phosphorylation at Ser63 and Ser68, I(pump) (both at 10 and 100 mmol/L Na(+) in the pipette solution) and maximal NKA-mediated Na(+) extrusion rate (V(max)) from 7.9+/-1.1 to 12.7+/-1.9 mmol.L(-1) per minute without altering NKA affinity for internal Na(+) (K(0.5)). In PLM knockout mice, PDBu had no effect on either V(max) or K(0.5). After pretreatment with isoproterenol (ISO) (1 mumol/L), PDBu still increased the NKA V(max) and PLM phosphorylation at Ser63 and Ser68. Conversely, after pretreatment with PDBu, ISO further increased the Na(+) affinity of NKA and phosphorylation at Ser68, as it did alone without PDBu. The final NKA activity was independent of the application sequence. The NKA activity in PLM knockout myocytes, after normalizing the protein level, was similar to that after PDBu and ISO treatment. We conclude that (1) PLM mediates the PKC-dependent activation of NKA function in cardiac myocytes, (2) PDBu and ISO effects are additive in the mouse (affecting mainly V(max) and K(0.5), respectively), and (3) PDBu and ISO combine to activate NKA in wild-type to the level found in the PLM knockout mouse.

Phospholemman phosphorylation alters its fluorescence resonance energy transfer with the Na/K-ATPase pump

Phospholemman (PLM) or FXYD1 is a major cardiac myocyte phosphorylation target upon adrenergic stimulation. Prior immunoprecipitation and functional studies suggest that phospholemman associates with the Na/K-pump (NKA) and mediates adrenergic Na/K-pump regulation. Here, we tested whether the NKA-PLM interaction is close enough to allow fluorescence resonance energy transfer (FRET) between cyan and yellow fluorescent (CFP/YFP) fusion proteins of Na/K pump and phospholemman and whether phospholemman phosphorylation alters such FRET. Co-expressed NKA-CFP and PLM-YFP in HEK293 cells co-localized in the plasma membrane and exhibited robust FRET. Selective acceptor photobleach increased donor fluorescence (F(CFP)) by 21.5 +/- 4.1% (n = 13), an effect nearly abolished when co-expressing excess phospholemman lacking YFP. Activation of protein kinase C or A progressively and reversibly decreased FRET assessed by either the fluorescence ratio (F(YFP)/F(CFP)) or the enhancement of donor fluorescence after acceptor bleach. After protein kinase C activation, forskolin did not further reduce FRET, but after forskolin pretreatment, protein kinase C could still reduce FRET. This agreed with phospholemman phosphorylation measurements: by protein kinase C at both Ser-63 and Ser-68, but by protein kinase A only at Ser-68. Expression of PLM-YFP and PLM-CFP resulted in even stronger FRET than for NKA-PLM (F(CFP) increased by 37 +/- 1% upon YFP photobleach), and this FRET was enhanced by phospholemman phosphorylation, consistent with phospholemman multimerization. Co-expressed PLM-CFP and Na/Ca exchange-YFP were highly membrane co-localized, but FRET was undetectable. We conclude that phospholemman and Na/K-pump are in very close proximity (FRET occurs) and that phospholemman phosphorylation alters the interaction of Na/K-pump and phospholemman.