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(Circulation Research. 2003;93:586.)
© 2003 American Heart Association, Inc.

Editorials

Brain in the Brawn

The Neuronal Nitric Oxide Synthase as a Regulator of Myogenic Tone

Ingrid Fleming

From the Institut für Kardiovaskuläre Physiologie, Klinikum der J.W. Goethe-Universität, Frankfurt am Main, Germany.

Correspondence to Dr Ingrid Fleming, Institut für Kardiovaskuläre Physiologie, Klinikum der J.W. Goethe-Universität, Theodor-Stern-Kai 7, D-60596 Frankfurt/Main, Germany. E-mail fleming{at}em.uni-frankfurt.de

Key Words: neuronal nitric oxide synthase • 20-hydroxyeicosatetraenoic acid • plasma membrane calcium ATPase • myogenic contraction

Nitric oxide (NO) plays a central role in the regulation of cardiovascular homeostasis and is involved in the regulation of vascular tone and cardiac contractility as well as gene expression and cell proliferation. Furthermore, NO modulates renin secretion and salt and fluid reabsorption in the kidney.¹ Three isoforms of NO synthase (NOS) have been identified, the neuronal NOS (nNOS or NOS I), the inducible NOS (iNOS or NOS II), and the endothelial NOS (eNOS or NOS III). While all of these enzymes potentially affect blood pressure, only eNOS-deficient mice are hypertensive.^2,3

Although nNOS is expressed in cardiac myocytes,^4,5 as well as in vascular smooth muscle cells,^6–8 relatively little is known about the role played by nNOS-derived NO in cardiovascular homeostasis. Both pro- and antihypertensive actions have been attributed to nNOS, and selective inhibitors of this isoform have been reported to normalize blood pressure,⁹ as well as to attenuate flow-induced vasodilatation in eNOS^-/- mice.⁸

The regulation of nNOS activity, like that of eNOS, is determined by phosphorylation of the enzyme as well as by its association with a number of regulatory proteins.¹⁰ One protein that associates with nNOS in human embryonic kidney (HEK293) cells and is reported to play a major role in regulating the activity of the Ca²⁺-dependent enzyme, is the plasma membrane Ca²⁺/calmodulin-dependent Ca²⁺-ATPase (PMCA).¹¹ The ATPase binds to nNOS via an interaction between its carboxyl terminus and the PDZ domain of nNOS. Increasing expression of the PMCA4b isoform markedly attenuates NO synthesis by nNOS, an effect not observed in cells overexpressing a mutant PMCA that was devoid of Ca²⁺-transporting activity.¹¹ Thus, it appears that the PCMA4b, by decreasing the concentration of Ca²⁺ in the local vicinity of nNOS, can reduce NO and subsequently cGMP production.

In this issue of Circulation Research, Gros et al¹² investigated the relationship between PMCA4b and nNOS by generating doxycycline-responsive transgenic mice that selectively overexpress the human PMCA4b (hPMCA4b) in arterial smooth muscle cells. The authors found that a 2-fold increase in hPMCA4b expression had a subtle effect on thapsigargin-insensitive Ca²⁺-dependent ATPase activity but no significant effect on basal [Ca²⁺]_i or Ca²⁺ sensitivity. The authors’ most impressive finding was that hPMCA4b expression was accompanied by a markedly enhanced myogenic response in isolated mesenteric arteries as well as by increased contractile responses to phenylephrine and prostaglandin F2 but not to KCl. Global NOS inhibition using N-nitro-L-arginine and selective inhibition of nNOS with N-propyl-L-arginine did not affect the myogenic response in mesenteric arteries from hPMCA4b-expressing mice but significantly attenuated responses in non–hPMCA4b-expressing littermates. Since overexpression of the Ca²⁺-ATPase and nNOS inhibition elicited similar effects, the authors suggest that the small increase in Ca²⁺-dependent ATPase activity in the hPMCA4b-overexpressing mice was sufficient to deplete Ca²⁺ from nNOS and to inhibit its activity, thus alleviating the intrinsic functional inhibition of nNOS on contractile responses. There is at least circumstantial evidence to support this conclusion since cGMP levels were lower in aortic smooth muscle cells from hPMCA4b-overexpressing mice. As mentioned above, this is not the first report suggesting that nNOS could play a significant role in the regulation of vascular tone, but it provides compelling evidence that NO generated by nNOS can affect the myogenic response.

The myogenic response is a main determinant of vascular tone in situ and consistent with the effect observed on contraction, Gros et al¹² report that hPMCA4b-expressing mice have higher systolic and diastolic blood pressures than their hPMCA4b-deficient littermates. Blood pressure was normalized by treatment with doxycycline, which prevented hPMCA4b expression. However, if the enhanced expression of hPMCA4b increases blood pressure via its inhibitory effect on nNOS activity, why do nNOS knockout animals not demonstrate a manifest hypertension? The most likely explanation is that additional mechanisms are activated to compensate for the global lack of nNOS in these animals.

Overexpression of one constituent of the Ca²⁺ homeostatic machinery would be expected to have distinct consequences on [Ca²⁺]_i. However, in cultured aortic smooth muscle cells isolated from hPMCA4b-deficient and hPMCA4b-expressing mice, Gros et al¹² found no difference in basal [Ca²⁺]_i. Notably, in these cells, the expression of hPMCA4b was associated with a decrease in the expression of the murine PMCA1 and PMCA4 as well as an increase in SERCA2a, SERCA2b, and the inositol 1,4,5-trisphosphate-activated Ca²⁺ channel (IP3R) mRNA. Thus, it seems that the systems that control [Ca²⁺]_i are closely regulated and that [Ca²⁺]_i is held constant by adaptive changes in the expression and/or activity of other Ca²⁺ pumps/channels. Although the effects described by Gros et al¹² in cultured smooth muscle cells were more marked than those observed in freshly isolated aortae, similar alterations in SERCA and IP3R expression have previously been reported in rat endothelial cells overexpressing PMCA1.¹³

While the increased myogenic response was the most pronounced effect of hPMCA4b overexpression, the molecular mechanisms underlying this response remain to be investigated in detail. Given the reported compensatory increase in SERCA and IP3R mRNA expression, it is possible that the enhanced release of intracellular Ca²⁺ by increases in transmural pressure as well as by phenylephrine could account for the effects observed.¹² However, data gathered over the last 5 to 8 years have convincingly shown a link between vascular 20-hydroxyeicosatetraenoic acid (20-HETE) generation and myogenic responses in renal, cerebral, and mesenteric arteries.^14–16 20-HETE is endogenously produced by smooth muscle cells after an increase in [Ca²⁺]_i, and, once formed, increases smooth muscle tone (and enhances sensitivity to phenylephrine) by inhibiting large conductance Ca²⁺-dependent K⁺ channels inducing depolarization and contraction (see recent reviews^17–19). This effect is related to the activation of L-type Ca²⁺ channels,²⁰ as well as the activation of the Rho kinase and the phosphorylation of myosin light chain.²¹ Endothelium-derived factors are able to modulate myogenic contraction and at least part of their action can be attributed to interference with the formation and actions of 20-HETE. For example, NO may modulate the formation of 20-HETE by binding to and inactivating the cytochrome P450 enzyme that generates this eicosanoid. Indeed, the NO-mediated inhibition of 20-HETE formation has been proposed to account for the natriuretic and diuretic actions of NO,²² as well as the cGMP-independent relaxant effects of NO in renal and cerebral arteries.^23,24

While it has been generally assumed that the NO that modulates 20-HETE generation is derived from endothelial cells, it is just as likely that NO derived from nNOS in vascular smooth muscle cells can influence the same cellular processes. It will therefore be interesting to determine whether the overexpression of hPMCA4b is linked to changes in 20-HETE levels, Rho kinase activity, and myosin light chain phosphorylation, and whether the reported increase in 20-HETE production in mesenteric arteries from spontaneously hypertensive rats²⁵ is directly related to an increase in PMCA expression²⁶ and a decrease in nNOS-derived NO production (Figure). For such a signaling cascade to be functional, it is essential that Ca²⁺ levels are tightly regulated in specific intracellular microdomains and that the extrusion of Ca²⁺ by the PMCA, which are known to be concentrated with nNOS in caveolae, does not affect Ca²⁺ signaling process required to initiate 20-HETE production and contraction.

View larger version (30K): [in this window] [in a new window]   Putative scheme by which NO derived from nNOS could modulate the myogenic response. An increase in transmural pressure would be expected to stimulate stretch-activated Ca2+ channels (SAC), thereby enhancing smooth muscle [Ca2+]i and the synthesis of 20-HETE from arachidonic acid (AA). The latter elicits contraction by further increasing [Ca2+]i and enhancing myosin light chain phosphorylation. Since NO inhibits the generation of 20-HETE by cytochrome P450 4A (CYP 4A) isoforms, this autacoid should attenuate the myogenic response. Inhibition of nNOS using N-propyl-L-arginine or as a consequence of an increase in the expression of the plasma membrane Ca2+/calmodulin-dependent Ca2+-ATPase (PMCA) would be expected to alleviate this intrinsic inhibition and thus to enhance the myogenic tone.

Acknowledgments

The author’s work is supported by the Deutsche Forschungsgemeinschaft (SFB553/B1 and B5).

Footnotes

The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.

References

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