4517
J Physiol 593.20 (2015) pp 4517–4518
JOURNAL CLUB
Upstream stiffness, downstream problems: not all arteries are equal Aaron L. Owen1 and J. Mikhail Kellawan2 School of Medicine and Public Health, University of Wisconsin, Madison, WI, USA 2 Department of Kinesiology, University of Wisconsin, Madison, WI, USA
1
Email: [email protected]
The Journal of Physiology
Introduction
In humans, a healthy aorta is an effective pressure reservoir cushioning the smaller arteries from potentially harmful fluctuations in blood pressure waves emanating from the left ventricle. However, arterial stiffening that occurs from natural ageing and several cardiovascular diseases attenuates the effectiveness of the aorta to protect downstream peripheral vessels from the adverse effects of increased pressure. These effects are evident in cultured vascular endothelial cells where increased circumferential stress related to large increases in intra-luminal pressure lead to dysfunction (Cheng et al. 1998). Just downstream of the aorta, the brain has a relatively high-volume flow and therefore may be especially susceptible to microvascular damage resulting from the increased pulsatility caused by the stiffening of large arteries. Looking beyond the brain, skeletal muscle is an important end organ in blood pressure regulation whose circulation might also be affected by aortic stiffening. Despite many observations of overt macrovascular problems caused by stiffening, the specific mechanisms linking large artery stiffening to downstream peripheral microvascular dysfunction remain largely unknown. Study design and effectiveness of the model
Recently Walker et al. (2015) uncovered a mechanism by which cerebral artery impairment develops secondary to aortic stiffening. The authors hypothesized that large artery stiffness would impair endothelium-dependent dilatation (EDD) in both cerebral (middle cerebral artery, MCA) and skeletal muscle arteries (gastrocnemius feed artery, GFA). Furthermore, the impaired EDD would be due to decreased nitric oxide (NO) function
caused by increased oxidative stress. In their study, aortic stiffening was modelled in young mice with a heterozygote elastin gene deletion (Eln+/− ). The resulting elastin insufficiency significantly increased aortic stiffness as indicated by a 35% higher aortic pulse wave velocity (PWV), and less elastin but similar collagen in Eln+/− mice vs. Eln+/+ mice. In Eln+/− mice, the stiffness of MCA did not differ, but GFA was slightly stiffer. Normal amounts of elastin content and number of elastin laminae were observed in both MCA and GFA from both mouse strains. Indeed, previous studies also suggest that the mechanical consequences of elastin haploinsufficiency remain limited to the large arteries, as a single gene copy seems sufficient to maintain normal elastin content in smaller arteries and other tissues. Effects of large artery stiffness on cerebral vasculature
In support of their hypothesis, acetylcholine (ACh)-induced maximal EDD in MCA was 40% lower in the Eln+/− , indicating stiffening in large arteries can lead to endothelial dysfunction in cerebral microcirculation. To determine the contributions of NO and prostaglandins, the ACh responses were repeated with addition of NOS inhibitor (NG -nitro-L-arginine methyl ester; L-NAME) and COX inhibitor (indomethacin), respectively. L-NAME eliminated the difference in the maximal dilatation to ACh between groups, reflecting that ACh produces dilatation in the MCA via a NO-dependent pathway that was disrupted in the Eln+/− MCA segments. Interestingly, prostaglandins seemingly inhibit ACh-mediated dilatation in MCA of both groups, as addition of indomethacin increased dilatation from L-NAME alone. These data indicate that cyclooxygenase-derived ROS and/or thromboxanes may be restraining ACh-mediated dilatation in the MCA. The authors also observed a large increase in markers for superoxide and protein nitration in MCAs from Eln+/− compared with Eln+/+ mice indicating greater oxidative stress. They then successfully reversed the dysfunction in EDD by incubating the Eln+/− MCA segments in the superoxide scavenger TEMPOL, thus confirming oxidative stress as the
C 2015 The Authors. The Journal of Physiology C 2015 The Physiological Society
mechanism for impaired EDD observed in Eln+/− MCA. Furthermore, when L-NAME was added to TEMPOL the improvement of EDD was abolished, confirming that ROS interaction with NO contributes to the attenuated EDD observed in Eln+/− MCA. The current study focuses exclusively on mechanical factors as the mechanism for increased ROS, but there are additional signalling pathways that could also be contributing. Specifically, Eln+/− mice exhibit greater plasma renin activity which would lead to increased renin–angiotensin– aldosterone system (RAAS)-mediated activation of NADPH oxidase ROS production (Landmesser et al. 2003). Moreover, Eln+/− mice are born with essential hypertension. Hypertension is associated with chronic, low-grade inflammation, which may negatively impact vascular function via RAAS supporting RAAS systemic influence on vascular ROS production. With these concepts in mind, future studies might focus on systemic signals like RAAS and/or inflammation, alone and in combination with mechanical factors to offer deeper insight into this complex pathophysiology. Effects of large artery stiffness on skeletal muscle vasculature
In contrast to their hypothesis and the MCA findings, EDD of GFA was largely intact, leading the authors to conclude that skeletal muscle feed arteries are only marginally affected by large artery stiffness. It is noteworthy that the GFA of Eln+/− mice exhibited a slightly lower sensitivity to ACh and that prostaglandins no longer seem to contribute to EDD in the GFA of the Eln+/− group. As the authors did not report data for markers of oxidative stress in GFA, we do not know if there is less oxidative stress than in MCA or if GFA is protected by some adaptive mechanism. Consistent with this preservation, arteries from the fast-twitch gastrocnemius in rat model of ageing demonstrated intact EDD; however, feed arteries from slow-twitch soleus muscle from the aged animal demonstrated reduced NO-mediated EDD (Muller-Delp et al. 2002). Thus, a worthwhile follow-up study in the Eln+/− mice would be to address the effect (or lack thereof) of large artery stiffness on skeletal muscle arteries across both oxidative and glycolytic muscles. DOI: 10.1113/JP271236
4518 Not all arteries are equal
While the authors provide data to support their hypotheses in regard to the MCA, the unexpected lack of effects in the GFA are unresolved. The authors suggest two distinct possibilities for the differences observed between cerebral and skeletal muscle vasculature: (1) MCA experiences greater pulsatility compared to the GFA due to close proximity to the heart, relatively high-volume flow to the brain, and/or differences in elasticity in between MCA and resistance arteries; (2) MCA may be more dependent on NO to elicit dilatation. While these suggestions are plausible, there has been a lack of research on the direct effect of increased pulsatile pressure on EDD, and the authors offer no data specific to the direct effect of increased pulsatility within each artery. In future studies, direct measurement of cerebral and skeletal muscle resistance artery pulsatility in vivo in Eln+/− mouse models is warranted to identify differences between the two circulations. Despite the unexplained EDD difference between cerebral and skeletal muscle resistance arteries, this study provides compelling evidence of the causality of large artery stiffening on arterial dysfunction of the MCA. The authors classified the mechanism of this dysfunction as the reduction in NO bioavailability resulting from the oxidative stress occurring from increased pulsatility. The authors make a logical connection between their observations and data from cultured endothelial cells, where cyclic strain induces a sustained increase in intracellular ROS production (Cheng et al. 1998). Taken together, the data from Walker et al. and previous literature outline a pathophysiological mechanism connecting increased large artery stiffness to cerebral endothelial dysfunction (Fig. 1).
Journal Club
J Physiol 593.20
Eln+/− MCA segments were treated with a ROS scavenger (TEMPOL). While these data make a strong argument, data directly linking increased mechanical pulsatility in the MCA to a mechanism for increased oxidative stress in vivo would strengthen the conclusions. Furthermore, other mechanisms related to the hypertension (i.e. RAAS or inflammation) may also drive some of these vascular changes. Additionally, these mechanisms require evaluation in females, since evidence strongly suggests women with large artery stiffness demonstrate far worse health outcomes (Coutinho et al. 2013). Therefore, determining mechanistic differences between men and women may be important to improving human health. While large artery stiffness would impact pressure transduction to all vessels, the data from the current study indicate skeletal muscle arteries are more resistant than the cerebrovasculature to upstream increases in pulsatility. Cerebral blood flow is typically well regulated both to meet the metabolic demands of the brain and to shield it from potentially harmful spikes in pressure. However, despite the complex local mechanisms contributing to the responsiveness of the cerebrovasculature, the current study highlights that large artery structure and function may impact cerebral blood flow regulation. Therefore, future investigations into the pathophysiology of the cerebrovasculature should be put into the context of the cardiovascular system as a whole. ROS
2. Pulsatility
References Cheng JJ, Wung BS, Chao YJ & Wang DL (1998). Cyclic strain-induced reactive oxygen species involved in ICAM-1 gene induction in endothelial cells. Hypertension 31, 125–130. Coutinho T, Borlaug BA, Pellikka PA, Turner ST & Kullo IJ (2013). Sex differences in arterial stiffness and ventricular-arterial interactions. J Am Coll Cardiol 61, 96–103. Landmesser U, Dikalov S, Price SR, McCann L, Fukai T, Holland SM, Mitch WE & Harrison DG (2003). Oxidation of tetrahydrobiopterin leads to uncoupling of endothelial cell nitric oxide synthase in hypertension. J Clin Invest 111, 1201–1209. Muller-Delp JM, Spier SA, Ramsey MW & Delp MD (2002). Aging impairs endothelium-dependent vasodilation in rat skeletal muscle arterioles. Am J Physiol Heart Circ Physiol 283, H1662–H1672. Walker AE, Henson GD, Reihl KD, Morgan RG, Dobson PS, Nielson EI, Ling J, Mecham RP, Li DY, Lesniewski LA & Donato AJ (2015). Greater impairments in cerebral artery compared with skeletal muscle feed artery endothelial function in a mouse model of increased large artery stiffness. J Physiol 593, 1931–1943.
Additional information Competing interests
None declared.
Acknowledgements
The authors would like to sincerely thank Dr William G. Schrage for his suport and critical evaluation of this manuscript. ROS
3. ROS
ROS ROS ROS
Endothelial Cell ROS
4. NO ROS
ROS
Summary, perspectives, and conclusions
Walker et al. executed a well-designed study that provides evidence that large artery stiffness increases ROS production in cerebral arteries, which negatively impacts NO bioavailability, thus disrupting vascular endothelial function. These conclusions are made from the following observations: (1) blunted EDD in MCA segments taken from mouse model of arterial stiffness (Eln+/− ); (2) similar EDD between Eln+/− and wild-type when NO formation was pharmacologically inhibited (with L-NAME); (3) EDD was restored when
1. LAS 5. MCA EDD
Figure 1. Schematic representation of pathway from large artery stiffness (LAS) to dysfunctional cerebrovascular endothelial-dependent dilatation (EDD) (1) Increased large artery stiffness can be a part of normal ageing or the result of several cardiovascular diseases. (2) A stiff aorta loses its effectiveness at dampening pulse pressure, leading to increased aortic pulse wave velocity and increased pulsatility in the cerebral resistance vessels. (3) The resulting pulsatility causes increased circumferential stress, which generates reactive oxygen species (ROS) in endothelial cells. (4) ROS (superoxide) reacts with NO and decreases its bioavailability as an endothelium-derived dilating factor. (5) Decreased NO bioavailability causes dysfunctional endothelium-dependent dilatation in MCA. C 2015 The Authors. The Journal of Physiology C 2015 The Physiological Society
J Physiol 593.20 (2015) pp 4517–4518
JOURNAL CLUB
Upstream stiffness, downstream problems: not all arteries are equal Aaron L. Owen1 and J. Mikhail Kellawan2 School of Medicine and Public Health, University of Wisconsin, Madison, WI, USA 2 Department of Kinesiology, University of Wisconsin, Madison, WI, USA
1
Email: [email protected]
The Journal of Physiology
Introduction
In humans, a healthy aorta is an effective pressure reservoir cushioning the smaller arteries from potentially harmful fluctuations in blood pressure waves emanating from the left ventricle. However, arterial stiffening that occurs from natural ageing and several cardiovascular diseases attenuates the effectiveness of the aorta to protect downstream peripheral vessels from the adverse effects of increased pressure. These effects are evident in cultured vascular endothelial cells where increased circumferential stress related to large increases in intra-luminal pressure lead to dysfunction (Cheng et al. 1998). Just downstream of the aorta, the brain has a relatively high-volume flow and therefore may be especially susceptible to microvascular damage resulting from the increased pulsatility caused by the stiffening of large arteries. Looking beyond the brain, skeletal muscle is an important end organ in blood pressure regulation whose circulation might also be affected by aortic stiffening. Despite many observations of overt macrovascular problems caused by stiffening, the specific mechanisms linking large artery stiffening to downstream peripheral microvascular dysfunction remain largely unknown. Study design and effectiveness of the model
Recently Walker et al. (2015) uncovered a mechanism by which cerebral artery impairment develops secondary to aortic stiffening. The authors hypothesized that large artery stiffness would impair endothelium-dependent dilatation (EDD) in both cerebral (middle cerebral artery, MCA) and skeletal muscle arteries (gastrocnemius feed artery, GFA). Furthermore, the impaired EDD would be due to decreased nitric oxide (NO) function
caused by increased oxidative stress. In their study, aortic stiffening was modelled in young mice with a heterozygote elastin gene deletion (Eln+/− ). The resulting elastin insufficiency significantly increased aortic stiffness as indicated by a 35% higher aortic pulse wave velocity (PWV), and less elastin but similar collagen in Eln+/− mice vs. Eln+/+ mice. In Eln+/− mice, the stiffness of MCA did not differ, but GFA was slightly stiffer. Normal amounts of elastin content and number of elastin laminae were observed in both MCA and GFA from both mouse strains. Indeed, previous studies also suggest that the mechanical consequences of elastin haploinsufficiency remain limited to the large arteries, as a single gene copy seems sufficient to maintain normal elastin content in smaller arteries and other tissues. Effects of large artery stiffness on cerebral vasculature
In support of their hypothesis, acetylcholine (ACh)-induced maximal EDD in MCA was 40% lower in the Eln+/− , indicating stiffening in large arteries can lead to endothelial dysfunction in cerebral microcirculation. To determine the contributions of NO and prostaglandins, the ACh responses were repeated with addition of NOS inhibitor (NG -nitro-L-arginine methyl ester; L-NAME) and COX inhibitor (indomethacin), respectively. L-NAME eliminated the difference in the maximal dilatation to ACh between groups, reflecting that ACh produces dilatation in the MCA via a NO-dependent pathway that was disrupted in the Eln+/− MCA segments. Interestingly, prostaglandins seemingly inhibit ACh-mediated dilatation in MCA of both groups, as addition of indomethacin increased dilatation from L-NAME alone. These data indicate that cyclooxygenase-derived ROS and/or thromboxanes may be restraining ACh-mediated dilatation in the MCA. The authors also observed a large increase in markers for superoxide and protein nitration in MCAs from Eln+/− compared with Eln+/+ mice indicating greater oxidative stress. They then successfully reversed the dysfunction in EDD by incubating the Eln+/− MCA segments in the superoxide scavenger TEMPOL, thus confirming oxidative stress as the
C 2015 The Authors. The Journal of Physiology C 2015 The Physiological Society
mechanism for impaired EDD observed in Eln+/− MCA. Furthermore, when L-NAME was added to TEMPOL the improvement of EDD was abolished, confirming that ROS interaction with NO contributes to the attenuated EDD observed in Eln+/− MCA. The current study focuses exclusively on mechanical factors as the mechanism for increased ROS, but there are additional signalling pathways that could also be contributing. Specifically, Eln+/− mice exhibit greater plasma renin activity which would lead to increased renin–angiotensin– aldosterone system (RAAS)-mediated activation of NADPH oxidase ROS production (Landmesser et al. 2003). Moreover, Eln+/− mice are born with essential hypertension. Hypertension is associated with chronic, low-grade inflammation, which may negatively impact vascular function via RAAS supporting RAAS systemic influence on vascular ROS production. With these concepts in mind, future studies might focus on systemic signals like RAAS and/or inflammation, alone and in combination with mechanical factors to offer deeper insight into this complex pathophysiology. Effects of large artery stiffness on skeletal muscle vasculature
In contrast to their hypothesis and the MCA findings, EDD of GFA was largely intact, leading the authors to conclude that skeletal muscle feed arteries are only marginally affected by large artery stiffness. It is noteworthy that the GFA of Eln+/− mice exhibited a slightly lower sensitivity to ACh and that prostaglandins no longer seem to contribute to EDD in the GFA of the Eln+/− group. As the authors did not report data for markers of oxidative stress in GFA, we do not know if there is less oxidative stress than in MCA or if GFA is protected by some adaptive mechanism. Consistent with this preservation, arteries from the fast-twitch gastrocnemius in rat model of ageing demonstrated intact EDD; however, feed arteries from slow-twitch soleus muscle from the aged animal demonstrated reduced NO-mediated EDD (Muller-Delp et al. 2002). Thus, a worthwhile follow-up study in the Eln+/− mice would be to address the effect (or lack thereof) of large artery stiffness on skeletal muscle arteries across both oxidative and glycolytic muscles. DOI: 10.1113/JP271236
4518 Not all arteries are equal
While the authors provide data to support their hypotheses in regard to the MCA, the unexpected lack of effects in the GFA are unresolved. The authors suggest two distinct possibilities for the differences observed between cerebral and skeletal muscle vasculature: (1) MCA experiences greater pulsatility compared to the GFA due to close proximity to the heart, relatively high-volume flow to the brain, and/or differences in elasticity in between MCA and resistance arteries; (2) MCA may be more dependent on NO to elicit dilatation. While these suggestions are plausible, there has been a lack of research on the direct effect of increased pulsatile pressure on EDD, and the authors offer no data specific to the direct effect of increased pulsatility within each artery. In future studies, direct measurement of cerebral and skeletal muscle resistance artery pulsatility in vivo in Eln+/− mouse models is warranted to identify differences between the two circulations. Despite the unexplained EDD difference between cerebral and skeletal muscle resistance arteries, this study provides compelling evidence of the causality of large artery stiffening on arterial dysfunction of the MCA. The authors classified the mechanism of this dysfunction as the reduction in NO bioavailability resulting from the oxidative stress occurring from increased pulsatility. The authors make a logical connection between their observations and data from cultured endothelial cells, where cyclic strain induces a sustained increase in intracellular ROS production (Cheng et al. 1998). Taken together, the data from Walker et al. and previous literature outline a pathophysiological mechanism connecting increased large artery stiffness to cerebral endothelial dysfunction (Fig. 1).
Journal Club
J Physiol 593.20
Eln+/− MCA segments were treated with a ROS scavenger (TEMPOL). While these data make a strong argument, data directly linking increased mechanical pulsatility in the MCA to a mechanism for increased oxidative stress in vivo would strengthen the conclusions. Furthermore, other mechanisms related to the hypertension (i.e. RAAS or inflammation) may also drive some of these vascular changes. Additionally, these mechanisms require evaluation in females, since evidence strongly suggests women with large artery stiffness demonstrate far worse health outcomes (Coutinho et al. 2013). Therefore, determining mechanistic differences between men and women may be important to improving human health. While large artery stiffness would impact pressure transduction to all vessels, the data from the current study indicate skeletal muscle arteries are more resistant than the cerebrovasculature to upstream increases in pulsatility. Cerebral blood flow is typically well regulated both to meet the metabolic demands of the brain and to shield it from potentially harmful spikes in pressure. However, despite the complex local mechanisms contributing to the responsiveness of the cerebrovasculature, the current study highlights that large artery structure and function may impact cerebral blood flow regulation. Therefore, future investigations into the pathophysiology of the cerebrovasculature should be put into the context of the cardiovascular system as a whole. ROS
2. Pulsatility
References Cheng JJ, Wung BS, Chao YJ & Wang DL (1998). Cyclic strain-induced reactive oxygen species involved in ICAM-1 gene induction in endothelial cells. Hypertension 31, 125–130. Coutinho T, Borlaug BA, Pellikka PA, Turner ST & Kullo IJ (2013). Sex differences in arterial stiffness and ventricular-arterial interactions. J Am Coll Cardiol 61, 96–103. Landmesser U, Dikalov S, Price SR, McCann L, Fukai T, Holland SM, Mitch WE & Harrison DG (2003). Oxidation of tetrahydrobiopterin leads to uncoupling of endothelial cell nitric oxide synthase in hypertension. J Clin Invest 111, 1201–1209. Muller-Delp JM, Spier SA, Ramsey MW & Delp MD (2002). Aging impairs endothelium-dependent vasodilation in rat skeletal muscle arterioles. Am J Physiol Heart Circ Physiol 283, H1662–H1672. Walker AE, Henson GD, Reihl KD, Morgan RG, Dobson PS, Nielson EI, Ling J, Mecham RP, Li DY, Lesniewski LA & Donato AJ (2015). Greater impairments in cerebral artery compared with skeletal muscle feed artery endothelial function in a mouse model of increased large artery stiffness. J Physiol 593, 1931–1943.
Additional information Competing interests
None declared.
Acknowledgements
The authors would like to sincerely thank Dr William G. Schrage for his suport and critical evaluation of this manuscript. ROS
3. ROS
ROS ROS ROS
Endothelial Cell ROS
4. NO ROS
ROS
Summary, perspectives, and conclusions
Walker et al. executed a well-designed study that provides evidence that large artery stiffness increases ROS production in cerebral arteries, which negatively impacts NO bioavailability, thus disrupting vascular endothelial function. These conclusions are made from the following observations: (1) blunted EDD in MCA segments taken from mouse model of arterial stiffness (Eln+/− ); (2) similar EDD between Eln+/− and wild-type when NO formation was pharmacologically inhibited (with L-NAME); (3) EDD was restored when
1. LAS 5. MCA EDD
Figure 1. Schematic representation of pathway from large artery stiffness (LAS) to dysfunctional cerebrovascular endothelial-dependent dilatation (EDD) (1) Increased large artery stiffness can be a part of normal ageing or the result of several cardiovascular diseases. (2) A stiff aorta loses its effectiveness at dampening pulse pressure, leading to increased aortic pulse wave velocity and increased pulsatility in the cerebral resistance vessels. (3) The resulting pulsatility causes increased circumferential stress, which generates reactive oxygen species (ROS) in endothelial cells. (4) ROS (superoxide) reacts with NO and decreases its bioavailability as an endothelium-derived dilating factor. (5) Decreased NO bioavailability causes dysfunctional endothelium-dependent dilatation in MCA. C 2015 The Authors. The Journal of Physiology C 2015 The Physiological Society
