Heart rate-associated mechanical stress impairs carotid but not cerebral artery compliance in dyslipidemic atherosclerotic mice

Abstract

The cardiac cycle imposes a mechanical stress that dilates elastic carotid arteries, while shear stress largely contributes to the endothelium-dependent dilation of downstream cerebral arteries. In the presence of dyslipidemia, carotid arteries stiffen while the endothelial function declines. We reasoned that stiffening of carotid arteries would be prevented by reducing resting heart rate (HR), while improving the endothelial function would regulate cerebral artery compliance and function. Thus we treated or not 3-mo-old male atherosclerotic mice (ATX; LDLr(-/-):hApoB(+/+)) for 3 mo with the sinoatrial pacemaker current inhibitor ivabradine (IVA), the β-blocker metoprolol (METO), or subjected mice to voluntary physical training (PT). Arterial (carotid and cerebral artery) compliance and endothelium-dependent flow-mediated cerebral dilation were measured in isolated pressurized arteries. IVA and METO similarly reduced (P < 0.05) 24-h HR by ≈15%, while PT had no impact. As expected, carotid artery stiffness increased (P < 0.05) in ATX mice compared with wild-type mice, while cerebral artery stiffness decreased (P < 0.05); this paradoxical increase in cerebrovascular compliance was associated with endothelial dysfunction and an augmented metalloproteinase-9 (MMP-9) activity (P < 0.05), without changing the lipid composition of the wall. Reducing HR (IVA and METO) limited carotid artery stiffening, but plaque progression was prevented by IVA only. In contrast, IVA maintained and PT improved cerebral endothelial nitric oxide synthase-dependent flow-mediated dilation and wall compliance, and both interventions reduced MMP-9 activity (P < 0.05); METO worsened endothelial dysfunction and compliance and did not reduce MMP-9 activity. In conclusion, HR-dependent mechanical stress contributes to carotid artery wall stiffening in severely dyslipidemic mice while cerebrovascular compliance is mostly regulated by the endothelium.

Figures

Fig. 1

Time during which exercise occurred and changes in heart rate induced by training. A: recording of the running activity of 3 different LDLr−/−:hApoB+/+ (ATX) mice housed in a cage with free access to a running wheel. Cumulative duration of the training, which occurred mostly during the night, was 9 ± 1 h. B: example of the simultaneous recording of the speed of running and of the changes in heart rate (HR) monitored by telemetry in an ATX mouse. C: schematization of the heart rate in sedentary, ivabradine (IVA)-, or metoprolol (METO)-treated ATX mice and in mice exposed to exercise. Heart rate is expressed in beats per minute (bpm) or in beats per 24 h. Difference in heart rate (ΔHR) between IVA- or METO-treated ATX mice and trained ATX mice is indicated.

Fig. 2

Effect of aging and atherosclerosis on the compliance of peripheral carotid arteries expressed as strain (%) from 3- and 6-mo wild-type (WT) mice and ATX mice (A) and following 3-mo exposure to voluntary physical training (PT) or treatment with either IVA or METO (B). Results are means ± SE of 7–16 mice. *P < 0.05 vs. 6-mo WT; ϕP < 0.05 IVA vs. ATX; αP < 0.05 METO vs. ATX.

Fig. 3

Age and atherosclerosis increase cerebral compliance and alter endothelial function. Cerebrovascular compliance expressed as circumferential strain (%; A), distensibility expressed as incremental distensibility (ID; %/mmHg; B), and flow-mediated endothelium-dependent dilation (C) of pressurized cerebral arteries from 3- and 6-mo WT mice and ATX mice. Effects of endothelial nitric oxide synthase inhibition by Nω-nitro-L-arginine (L-NNA) on flow-mediated dilation of cerebral arteries isolated from 3-mo WT and ATX mice (D) and 6-mo WT and ATX mice (E). Results are means ± SE of 7 mice. **P < 0.05 vs. 3-mo WT mice; *P < 0.05 vs. 6-mo WT mice; ωP < 0.05 vs. 3-mo ATX mice.

Fig. 4

Effect of a chronic treatment of 6-mo ATX mice with IVA (A) and METO (B) and exposure to voluntary PT (C) on the cerebral compliance expressed as circumferential strain (%), the distensibility expressed as ID (%/mmHg), and flow-mediated endothelium-dependent dilation of pressurized cerebral arteries. Results are means ± SE of 6 to 10 mice. *P < 0.05 vs. 6-mo WT mice; φP < 0.05 vs. 6-mo ATX mice.

Fig. 5

Correlation between endothelial flow-mediated dilation (%) observed at 12.0 ± 0.5 dyn/cm2 and the circumferential strain (%) observed at an intraluminal pressure of 60 mmHg, in cerebral arteries isolated from 3- and 6-mo WT mice and ATX mice, and in 6-mo ATX mice treated for 3 mo with either IVA or METO, or ATX mice exposed for 3 mo to voluntary PT. There is a negative and significant correlation between both parameters (P = 0.0227; r = 0.824; n = 7). Results are means ± SE.

Fig. 6

A: representative example of zymography illustrating metalloproteinase-9 (MMP-9) activities of cerebral vessels from WT and ATX mice. B: effect of a chronic treatment of 6-mo ATX mice with IVA or METO and after 3-mo exposure to voluntary PT on the gelatinase activity of cerebral vessels. Each graph represents the %changes to respective controls. Results are means ± SE of 6 mice. **P < 0.05 vs. 3-mo WT mice; *P < 0.05 vs. 6-mo WT mice; ωP < 0.05 vs. 3-mo ATX mice; ϕP < 0.05 vs. 6-mo ATX mice.

Fig. 7

A: representative example of Western blot illustrating collagen I and III (Col I/III) protein expression of total cerebral vessels from WT and ATX mice. B: effect of a chronic treatment of 6-mo ATX mice with IVA or METO and after 3-mo exposure to voluntary exercise (PT) on Col I/III protein expression of total cerebral vessels. Each graph represents the protein abundancy normalized to its respective internal standard after adjustment for loading with Ponceau red. Results are means ΔSE of 4 mice. C: representative example of Western blot illustrating elastin expression of total cerebral vessels from 6-mo WT and ATX mice. D: bar graph of the elastin abundancy normalized to α-actin. E: representative example of cerebral arteries stained with Verhoeff-Van Gieson to reveal elastic fibers in black and collagen in red (×100). No differences were detected between cerebral arteries isolated from 6-mo WT and ATX mice.

Fig. 8

Effect of 3-mo chronic treatement of ATX mice with IVA or METO and effect of 3-mo exposure to voluntary PT on the the progression of atheroclerotic lesions in the aortic arch and thoracic aorta. Aortic lesions quantification (A) and representative picture of Oil Red O-stained aorta (B). Results are means ± SE of 9 –20 mice. *P < 0.05 vs. 6-mo WT mice; ωP < 0.05 vs. 3-mo ATX mice; ϕP < 0.05 vs. 6-mo ATX mice.