doi: 10.1161/CIRCRESAHA.107.164459
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© 2007 American Heart Association, Inc.
Editorials |
Local Sodium, Global Reach
Filling the Gap Between Salt and Hypertension
From the Departments of Physiology (M.P.B., W.G.W.) and Medicine (M.P.B.) and the Center for Heart, Hypertension, and Kidney Disease (M.P.B., W.G.W.), University of Maryland School of Medicine, Baltimore, Md.
Correspondence to Mordecai P. Blaustein, MD, Department of Physiology, University of Maryland School of Medicine, 655 W. Baltimore Street, Baltimore, MD 21201. E-mail mblaustein{at}som.umaryland.edu
See related article, pages 1030–1038
Key Words: sodium • subplasma membrane microdomains • TRPC6 • receptor-operated channels • Na+/Ca2+ exchanger
The plasma membrane (PM) Na+/Ca2+ exchanger (NCX) in vascular smooth muscle is an unique link between the trans-PM Na+ electrochemical gradient and intracellular Ca2+ and, therefore, between Na+ ions and Ca2+ signaling, vascular tone and blood pressure.1 The mechanisms by which Na+ normally enters the myocytes and influences the Na+ gradient and NCX activity are, however, incompletely understood. Our view of how Na+ ions help regulate sarco-/endoplasmic reticulum (S/ER) Ca2+ stores and contractility in arteries has now been signally enhanced by Poburko and colleagues.2 Using CoroNa green, a Na+-sensitive fluorochrome, they observed local Na+ concentration transient increases ("LNats") in cultured arterial myocytes. The LNats were generated by Na+ entry through cation-selective TRPC6 channels, a member of the TRP (transient receptor potential) channel family.2 This is direct, dynamic evidence for a predicted sub-PM compartment with greatly restricted Na+ diffusion3,4 in which the local rise in Na+ concentration should drive Ca2+ into the myocytes via NCX.
The present study has broad implications for Ca2+ homeostasis and signaling. Earlier vascular smooth muscle studies indicated that other members of the TRP channel family might also admit Na+ to sub-PM domains.3,5 Indirect evidence,6 as well as an electron microprobe study, indicate that cardiomyocytes, too, can exhibit elevated local sub-PM Na+ concentrations ([Na+]SPM).7 Moreover, comparable diffusion-restricted, sub-PM cytosolic compartments may also be present in other types of cells (e.g., astrocytes8).
To explain how S/ER Ca2+ stores in smooth muscles could refill from the extracellular fluid without inducing contractions,9,10 van Breeman and colleagues postulated a "privileged pathway" (the Ca2+ "buffer barrier"), through which Ca2+ could move directly between the extracellular fluid and the sub-PM ("junctional") S/ER, jS/ER.9 One mechanism purportedly involved in this Ca2+ transfer was the NCX.9
This model was supported by the discovery that NCX in smooth muscles (and neurons and astrocytes) is confined to PM microdomains that overlie closely-apposed jS/ER,11,12 as are Na+ pumps with an 
2 or 3 catalytic subunit.13–15 In contrast, coexpressed Na+ pumps with an 1 subunit, the predominant "housekeepers" that maintain the low bulk cytosolic Na+ concentration ([Na+]CYT), are excluded from these microdomains.13,15 Cation-selective TRPC-containing store- or receptor-operated channels,3,5 which also are located in these PM microdomains,15–17 are, therefore, key Na+ entry pathways. The jS/ER, the PM microdomains, and the tiny volume of cytosol between them (perhaps 10–19 to 10–18 l), form a structural and functional unit, the "PLasmERosome" (Figure).3
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LNats,2 which presumably arise in PLasmERosomes, are surprisingly long-lasting, on the order of 1 minute. Thus, Na+ diffusion between the PLasmERosomes and bulk cytosol must be markedly restricted. The nature of the diffusion barrier is unknown, but intracellular Na+ gradients2 could not be sustained even for 1 second if Na+ diffusivity was comparable to that measured in muscle cytoplasm.18 This helps explain how Na+ pumps with an 2 or 3 subunit can function in cells that also express 4 times as many pumps with an 1 subunit,19,20 which have a much higher affinity for intracellular Na+.21 The implication is that the membrane potential and the balance between Na+ entry through receptor- and store-operated channels, and Na+ extrusion via the 2/3 Na+ pumps, control [Na+]SPM and the local Na+ electrochemical gradient. This gradient drives Ca2+ either into or out of the myocytes via NCX, and thereby controls the local sub-PM Ca2+ concentration, [Ca2+]SPM. Indeed, [Ca2+]SPM transients have been observed in arterial smooth muscle.15,22,23 The [Ca2+]SPM, in turn, influences the transport of Ca2+ into the jS/ER (mediated by SERCA pumps), and thereby helps regulate Ca+ signaling,5,8,17,24 vascular tone and blood pressure.20,24
Mitochondria accumulate Ca2+ when global [Ca2+]CYT rises, and mitochondrial NCX may then help the mitochondria extrude Ca2+. When mitochondrial NCX was inhibited by CGP37157,25 ATP-stimulated global [Na+]CYT rose, as did the frequency of LNats.2 The structural and functional details of the PLasmERosome/SR/mitochondria and bulk cytosol interrelationships are yet to be fully elucidated.
The present work advances the concept that local [Na+]SPM controls vascular tone by directly demonstrating local [Na+]SPM, and by identifying a key cation channel that may be involved, TRPC6. Nevertheless, the mechanisms of activation of LNats in arteries may differ from those in cultured cells; different GPCRs (G protein–coupled receptors) and different receptor- operated channels/TRPCs may be involved. It seems unlikely that LNats will be activated by ATP in intact arteries. In the cultured smooth muscle cells used by Poburko,2 ATP (1 mmol/L) activated metabotropic purinergic receptors. But in isolated mouse mesenteric arteries, the effects of bath-applied ATP (0.1 mmol/L) are entirely dependent on a different (ionotropic) purinergic receptor, P2X1. Both the vasoconstrictor effect and an endothelium dependent vasodilator effect of ATP are completely absent in mesenteric arteries of P2X1 receptor–deficient mice.26 It seems much more likely that TRPC6-dependent LNats would be activated physiologically in arteries after norepinephrine binding to well-known GPCRs (viz. 1-adrenoceptors, or 1-ARs). In freshly dispersed rabbit mesenteric artery myocytes, the vasoconstrictor, angiotensin II, acting on AT1 GPCRs, triggers a cation conductance that likely is mediated by TRPC6.27 In intact arteries, however, the role of Na+ or Ca2+ entry through TRPC6 has proven difficult to evaluate; aortas of mice deficient in TRPC6 display enhanced, not reduced, contractile responses to 1-AR activation.28 In the myocytes from these TRPC6–/– animals, the enhanced cation influx associated with the potentiated contraction seems to be attributable to enhanced constitutive activity of a closely related channel, TRPC3. Expression of TRPC6 and GPCR-stimulated currents are clearly enhanced in the mesenteric arteries of DOCA-salt hypertensive rats, however,29 implicating TRPC6 in the altered agonist responsiveness of these arteries. TRPC6 is also implicated in the production of myogenic tone.30 Nevertheless, caution should be used in extrapolating results from cultured myocytes2 to intact arterial smooth muscle. In cultured cells, TRPC6 and NCX appear to have dominant roles in controlling intracellular Na+ and Ca2+. In many arteries however, voltage-gated Ca2+ channels play major roles in myogenic tone and agonist-induced Ca2+ entry.
Now that LNats can be observed experimentally, with a molecular identity reasonably well established, we should be able to obtain more mechanistic information. The details of activation are still uncertain, although Ca2+ and calmodulin are likely involved, and either Ca2+-calmodulin dependent kinase II or myosin light chain kinase.31 TRPC6 channels heterologously expressed in HEK293 cells are activated by diacylglycerol and Ca2+-calmodulin dependent kinase II, but are subsequently inactivated by protein kinase C (Figure).32 Interestingly, most LNats occur early during the response to ATP, at a time when release of S/ER Ca2+ causes a large increase in cytosolic [Ca2+]. Perhaps this Ca2+ activates the TRPC6 through Ca2+-calmodulin dependent kinase II (Figure). These unresolved details notwithstanding, the LNats2 shed new light on the key roles of TRP channels and NCX in regulating [Na+]SPM and global Ca2+ signals in vascular smooth muscle. This opens significant opportunity for investigating the links between salt and vascular contractility and hypertension.
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Acknowledgments |
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Sources of Funding
The authors were supported by research grants from the National Heart Lung and Blood Institute and the National Institute of Neurological Diseases and Stroke.
Disclosures
None.
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Footnotes |
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The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
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References |
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Related Article:
Circ. Res. 2007 101: 1030-1038.
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