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Research Papers

Fundamental Roles of Axial Stretch in Isometric and Isobaric Evaluations of Vascular Contractility

[+] Author and Article Information
Alexander W. Caulk

Department of Biomedical Engineering,
Yale University,
New Haven, CT 06520

Jay D. Humphrey

Fellow ASME
Department of Biomedical Engineering,
Yale University,
New Haven, CT 06520;
Vascular Biology and Therapeutics Program,
Yale University,
New Haven, CT 06520

Sae-Il Murtada

Department of Biomedical Engineering,
Yale University,
55 Prospect Street,
New Haven, CT 06520
e-mail: sae-il.murtada@yale.edu

1Corresponding author.

Manuscript received April 10, 2018; final manuscript received November 13, 2018; published online January 25, 2019. Assoc. Editor: Jonathan Vande Geest.

J Biomech Eng 141(3), 031008 (Jan 25, 2019) (10 pages) Paper No: BIO-18-1174; doi: 10.1115/1.4042171 History: Received April 10, 2018; Revised November 13, 2018

Vascular smooth muscle cells (VSMCs) can regulate arterial mechanics via contractile activity in response to changing mechanical and chemical signals. Contractility is traditionally evaluated via uniaxial isometric testing of isolated rings despite the in vivo environment being very different. Most blood vessels maintain a locally preferred value of in vivo axial stretch while subjected to changes in distending pressure, but both of these phenomena are obscured in uniaxial isometric testing. Few studies have rigorously analyzed the role of in vivo loading conditions in smooth muscle function. Thus, we evaluated effects of uniaxial versus biaxial deformations on smooth muscle contractility by stimulating two regions of the mouse aorta with different vasoconstrictors using one of three testing protocols: (i) uniaxial isometric testing, (ii) biaxial isometric testing, and (iii) axially isometric plus isobaric testing. Comparison of methods (i) and (ii) revealed increased sensitivity and contractile capacity to potassium chloride and phenylephrine (PE) with biaxial isometric testing, and comparison of methods (ii) and (iii) revealed a further increase in contractile capacity with isometric plus isobaric testing. Importantly, regional differences in estimated in vivo axial stretch suggest locally distinct optimal biaxial configurations for achieving maximal smooth muscle contraction, which can only be revealed with biaxial testing. Such differences highlight the importance of considering in vivo loading and geometric configurations when evaluating smooth muscle function. Given the physiologic relevance of axial extension and luminal pressurization, we submit that, when possible, axially isometric plus isobaric testing should be employed to evaluate vascular smooth muscle contractile function.

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Figures

Grahic Jump Location
Fig. 1

Dose–response data collected for DTA (black) and IAA (gray) using three different classes of mechanical testing protocols (left-to-right) to assess responses to KCl ((a)–(c)) and PE ((d)–(f)). Direct quantitative comparison between methods is difficult since each experimental framework naturally yields different quantities: circumferential force (uniaxial isometric testing; (a), (d)), transmural pressure (biaxial isometric testing; (b), (e)), and outer diameter (biaxial isometric–isobaric testing; (c), (f)). Data are presented as mean ± SEM.

Grahic Jump Location
Fig. 2

Active circumferential stress responses to KCl ((a), (b)) or PE ((c), (d)) in the DTA ((a), (c)) and IAA ((b), (d)) normalized to the maximum change in circumferential stress for uniaxial isometric (uniax isom), biaxial isometric (biax isom), and biaxial isometric–isobaric (biax isom-isob) testing. Note that calculation of Cauchy stress facilitates direct comparison across methods. These results suggest a markedly different sensitivity of potassium channels due to axial loading (uniaxial versus biaxial testing) and ability to change configuration (isometric or fixed diameter versus isobaric or fixed pressure testing) while α-adrenergic receptors were less sensitive to the type of mechanical loading. Data are presented as mean ± SEM.

Grahic Jump Location
Fig. 3

Variation in [EC50] ((a), (c)) and maximum change in circumferential stress ((b), (d)) in response to KCl ((a), (b)) and PE ((c), (d)) as evaluated by uniaxial isometric (uniax isom), biaxial isometric (biax isom), and biaxial isometric–isobaric (biax isom-isob) testing of murine DTA and IAA. Maximum change in circumferential stress is largest in biaxial isometric–isobaric testing and best reflects the true in vivo capability of VSMCs to regulate the mechanical environment. Data are presented as mean ± SEM. (*) p <0.05, (**) p <0.01, (***) p <0.001 denote differences between testing protocols. (#) p <0.05, (##) p <0.01, (###) p <0.001 denote differences between aortic regions.

Grahic Jump Location
Fig. 4

Optimal biaxial configuration of aortic segments prior to KCl stimulation. Circumferential (a) and axial stretches (b) were higher in biaxial testing protocols (biax isom, biax isom-isob) compared to uniaxial testing (uniax isom). Axial stretches in biaxial testing were larger in IAA segments compared to DTA segments, suggesting regionally distinct optimal biaxial configurations for achieving maximal smooth muscle contractility. Data are presented as mean ± SEM. (*) p <0.05, (**) p <0.01, (***) p <0.001 denote differences between testing protocols. (#) p <0.05, (##) p <0.01, (###) p <0.001 denote differences between aortic regions.

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