Contractile Smooth Muscle and Active Stress Generation in Porcine Common Carotids

Abstract

The mechanical response of intact blood vessels to applied loads can be delineated into passive and active components using an isometric decomposition approach. Whereas the passive response is due predominantly to the extracellular matrix (ECM) proteins and amorphous ground substance, the active response depends on the presence of smooth muscle cells (SMCs) and the contractile machinery activated within those cells. To better understand determinants of active stress generation within the vascular wall, we subjected porcine common carotid arteries (CCAs) to biaxial inflation-extension testing under maximally contracted or passive SMC conditions and semiquantitatively measured two known markers of the contractile SMC phenotype: smoothelin and smooth muscle-myosin heavy chain (SM-MHC). Using isometric decomposition and established constitutive models, an intuitive but novel correlation between the magnitude of active stress generation and the relative abundance of smoothelin and SM-MHC emerged. Our results reiterate the importance of stretch-dependent active stress generation to the total mechanical response. Overall these findings can be used to decouple the mechanical contribution of SMCs from the ECM and is therefore a powerful tool in the analysis of disease states and potential therapies where both constituent are altered.

Fig. 1

Pressure-deformed outer diameter (left) and axial force-pressure (right) relationships for a representative porcine common carotid artery (sample 3). The mechanical response was recorded under conditions of maximally contracted (•) and fully relaxed (○) SMC states, and at three axial stretch ratios (λ_z) that span the in situ value. Error bars represent the standard deviation of three repeat measurements on the same vessel.

Fig. 2

Representative (top) total stress after isometric contraction (middle), passive stress and (bottom) active circumferential stress–stretch relationships for a porcine common carotid artery at three levels of axial stretch (sample 3). Error bars represent the standard deviation of three repeat measurements on the same vessel. Data points indicate experimentally recorded values, while solid/dashed lines indicate theoretical predictions.

Fig. 3

Histological images (sample 2) of the porcine CCAs at 100× magnification: (a) Verhoeff's elastic fiber counterstained with methyl blue, (b) Hematoxylin and Eosin, (c) smoothelin DAB immunostaining with thresholded inset, and (d) smooth muscle-myosin heavy chain DAB immunostaining with thresholded inset

Fig. 4

Second-order polynomial fit between the mean active circumferential stress (λ_θ = 1.42, λ_z = 1.6) and smoothelin (•) and smooth muscle myosin heavy chain SM-MHC (×) density (area positively stained/area all tissue) content in the porcine CCAs. Error bars ± STD mean.