Maladaptive aortic remodeling in hypertension associates with dysfunctional smooth muscle contractility

Abstract

Intramural cells are responsible for establishing, maintaining, and restoring the functional capability and structural integrity of the aortic wall. In response to hypertensive loading, these cells tend to increase wall content via extracellular matrix turnover in an attempt to return wall stress and/or material stiffness toward homeostatic values despite the elevated pressure. Using a common rodent model of induced hypertension, we found marked mouse-to-mouse differences in thoracic aortic remodeling over 2-4 wk of pressure elevation, with mechanoadaptation in some but gross maladaptation in most mice despite the same experimental conditions and overall genetic background. Consistent with our hypothesis, we also found a strong correlation between maladaptive aortic remodeling and a dysfunctional ability of the vessel to vasoconstrict, with maladaptation often evidenced by marked adventitial fibrosis. Remarkably, mouse-to-mouse variability did not correlate with the degree or duration of pressure elevation over the 2- to 4-wk study period. These findings suggest both a need to study together the structure, mechanical properties, and function across layers of the wall when assessing aortic health and a need for caution in using common statistical comparisons across small seemingly well-defined groups that may mask important underlying individual responses, an area of investigation that demands increasing attention as we move toward an era of precision diagnosis and patient care. NEW & NOTEWORTHY There are three primary findings. Marked mouse-to-mouse differences exist in large vessel hypertensive remodeling in an otherwise equivalent cohort of animals. The degree of maladaptation correlates strongly with decreases in smooth muscle contractile capacity. Finally, short-term maladaptive remodeling is independent of the precise degree or duration of the pressure elevation provided that thresholds are exceeded. Therapeutic targets should thus be personalized and focus on both layer-to-layer interactions and early interventions.

Keywords: aortic remodeling; central artery stiffness; precision medicine; smooth muscle tone; wall stress.

Fig. 1.

Mean passive mechanical testing data for the proximal descending thoracic aorta (DTA) from male wild-type mice. A–D: data are from noninfused (control, black circles, n = 5) and ANG II-infused (duration of 13–28 days, white circles, n = 23) mice. E–H: shown, too, are the same mean noninfused (black circles, n = 5) data compared against data for 3 ANG II-infused samples (n = 1 each), which are representative of vessels that had a near normal (norm, dark gray circles), compromised (comp, light gray squares), or impaired (imp, white inverted triangles) ability to vasoconstrict in response to 80 mM KCl stimulation. Structural responses were revealed by pressure-diameter data at group-specific in vivo axial stretches (A and E) and axial force-axial stretch data (B and F) at 100 mmHg. Material responses were revealed by mean circumferential (Circ.) Cauchy stress-stretch data at in vivo axial stretches (C and G) and axial Cauchy stress-stretch data (D and H) at 100 mmHg. Bars (means ± SE) are barely evident in most of curves in A–D because of either low variability (controls) or a large sample size (infused).

Fig. 2.

Values of passive geometric and mechanical metrics for the descending thoracic aorta (DTA) from all n = 23 ANG II-infused mice computed at a common pressure of 100 mmHg but individual values of in vivo axial stretch and then plotted as a function of the duration of ANG II infusion (A–D) or mouse-specific systolic pressure (E–H). Data were further distinguished based on their ability to vasoconstrict in response to 80 mM KCl (cf. Fig. 4) as follows: near normal (dark gray circles, n = 3), compromised (light gray squares, n = 16), and impaired (white inverted triangles, n = 4). Neither the duration of infusion nor systolic pressure correlated with DTA deformed thickness, circumferential (Circ.) stress or stiffness, or stored energy; although not shown, the same lack of correlation held for deformed outer diameter, axial stretch, and axial stress and stiffness (i.e., P > 0.05 and Pearson’s linear correlation coefficient |ρ| < 0.5).

Fig. 3.

Comparison of geometric and material metrics between noninfused (left, n = 5) and ANG II-infused (right, n = 23) specimens. Values were calculated at a common pressure of 100 mmHg but individual values of in vivo axial stretch. Statistical significance was assessed using a t-test with Welch’s correction for unequal variances. Circ., circumferential. *P < 0.05, statistical significance.

Fig. 4.

A and B: maximum change (Δ) in outer diameter (OD) of the descending thoracic aorta (DTA) upon stimulation with 80 mM KCl at specimen-specific optimal combinations of pressure and axial stretch. All ANG II-infused (n = 23) specimens were compared with noninfused control specimens (n = 5). A: the percent change in OD was calculated as 100 × (OD prestimulus − OD poststimulus)/(OD prestimulus). Statistical significance was assessed by a t-test with Welch’s correction for unequal variances (*P < 0.001). B: histogram of the maximum change in diameter calculated as follows: (OD prestimulus – OD poststimulus). A normal distribution fit well the observed variations exhibited by the ANG II-infused group (dashed line); the associated fit for specimens in the control group (blue shaded curve) are plotted for comparison. The mean (μ) of both groups is annotated, and the means ± standard deviation (μ ± σ) of the ANG II-infused group are annotated to delineate specimens with impaired (white, < μ − σ), compromised (light gray, μ ± σ), or near normal (dark gray, > μ + σ) abilities to vasoconstrict. C and D: representative tracings of the online measurement of OD and axial force upon stimulation with 80 mM KCl shortly after 0 min for one ANG II-infused specimen with impaired contractility (in gray); shown for comparison are representative data from the control group (in black). Note the biaxial consequences of KCl-stimulated smooth muscle contractility in the control vessels.

Fig. 5.

A: computed area fractions of cross sections of the aortic wall that were medial (white) or adventitial (gray). B–E: shown, too, are cross sections for one representative vessel per subgroup [top to bottom: Verhoeff Van Gieson stain (VVG), Masson’s trichrome stain (MTC), and picrosirius red stain (PSR)] and merged immunofluorescent stains against smooth muscle myosin heavy chain or smooth muscle α-actin in red, with DAPI in blue. The edge of the adventitia is outlined with a white dashed line in rows 4 and 5. Values were compared between noninfused (control; B) and ANG II-infused (C–E) specimens grouped based on their ability to vasoconstrict in response to 80 mM KCl (recall Fig. 4) as follows: near normal (C), compromised (D), and impaired (E). F: list of computed cross-sectional area fractions in the adventitia for the red-, orange-, yellow-, and green-stained collagen fibers from polarized light imaging of PSR-stained sections (mean of 2 representative samples/group). Overall, note the slight decrease in medial smooth muscle myosin heavy chain and α-actin with decreased contractile capacity as well as the marked increase in adventitial collagen and myofibroblasts (white arrows, bottom right) in the impaired contractility specimen. Scale bar = 100 μm.

Fig. 6.

Individual values of passive geometric (deformed wall thickness and outer diameter and axial stretch) and mechanical (biaxial wall stress, material stiffness, and energy storage) metrics plotted versus the individual maximum percent change (Δ) in outer diameter in response to 80 mM KCl. Symbols correspond to ANG II-infused (duration of 13–28 days) specimens delineated based on their ability to vasoconstrict (see Fig. 4) as follows: nearly normal (dark gray circles, n = 3), compromised (light gray squares, n = 16), and impaired (white inverted triangles, n = 4). Best-fit linear regression lines are plotted with solid lines to indicate a strong correlation (Pearson’s linear correlation coefficient |ρ| > 0.5 and P < 0.05) or a dashed line (axial stiffness only) to indicate a moderate correlation (ρ ≈ 0.5 and P < 0.1), all based on ANG II-infused samples only. See Table A1 in the appendix for a complete listing of correlations. Noninfused (control, black circles, n = 5) data are overlaid for comparison. All values were calculated at a common pressure of 100 mmHg but individual values of in vivo axial stretch.