Changes in the structure-function relationship of elastin and its impact on the proximal pulmonary arterial mechanics of hypertensive calves

Abstract

Extracellular matrix remodeling has been proposed as one mechanism by which proximal pulmonary arteries stiffen during pulmonary arterial hypertension (PAH). Although some attention has been paid to the role of collagen and metallomatrix proteins in affecting vascular stiffness, much less work has been performed on changes in elastin structure-function relationships in PAH. Such work is warranted, given the importance of elastin as the structural protein primarily responsible for the passive elastic behavior of these conduit arteries. Here, we study structure-function relationships of fresh arterial tissue and purified arterial elastin from the main, left, and right pulmonary artery branches of normotensive and hypoxia-induced pulmonary hypertensive neonatal calves. PAH resulted in an average 81 and 72% increase in stiffness of fresh and digested tissue, respectively. Increase in stiffness appears most attributable to elevated elastic modulus, which increased 46 and 65%, respectively, for fresh and digested tissue. Comparison between fresh and digested tissues shows that, at 35% strain, a minimum of 48% of the arterial load is carried by elastin, and a minimum of 43% of the change in stiffness of arterial tissue is due to the change in elastin stiffness. Analysis of the stress-strain behavior revealed that PAH causes an increase in the strains associated with the physiological pressure range but had no effect on the strain of transition from elastin-dominant to collagen-dominant behavior. These results indicate that mechanobiological adaptations of the continuum and geometric properties of elastin, in response to PAH, significantly elevate the circumferential stiffness of proximal pulmonary arterial tissue.

Fig. 1.

Thickness measurements, tissue dissection, and biopsy locations (typical). ID, internal diameter; MC, biopsy taken in circumferential direction; ML, biopsy taken in longitudinal direction.

Fig. 2.

Detail of material testing system (MTS) used to generate stress (σ)-strain (ɛ) data for arterial tissue samples.

Fig. 3.

Typical loading/unloading curves for fresh and elastin tissue tests. F_n, width-normalized force.

Fig. 4.

Top: typical σ-ɛ behavior of fresh (σ_fresh) and elastin (σ_elast) tissue. ɛ_trans is the ɛ of transition from the elastin-dominant region (A) to the transitional region (B), associated with increasing collagen engagement. Bottom: typical curvature (κ) plot of fresh tissue. H, height; κ_PSS, average prestrain-stiffening κ; κ_trans, κ of the stress-strain curve at the transition strain; κ_max, maximum κ calculated for a given tissue sample; ɛ_H, ɛ associated with maximum curvature minus 20% ɛ; ɛ_L, 20% ɛ; ɛ_PSS, prestrain-stiffening ɛ.

Fig. 5.

Comparison of average thickness values and elastin area fractions between control and hypoxic populations. Values are means ± SD. RPA, LPA, MPA: right, left, and main pulmonary artery, respectively. *P < 0.05.

Fig. 6.

Comparison of mean values for artery stiffness (Φ) and modulus (E) of the RPA, LPA, and MPA for control and hypoxic populations. Values are means ± SD. *P < 0.05.

Fig. 7.

Average control and hypertensive tissue stiffness at systolic and diastolic pressures. Values are means ± SD. Comparative values are presented in Table 1.

Fig. 8.

Comparison of mean values for elastin stiffness and modulus for the RPA, LPA, and MPA for control and hypoxic populations. Values are means ± SD. *P < 0.05.

Fig. 9.

Mean pressure-ɛ data for control and hypertensive tissues. Shaded regions indicate the predicted physiological conditions for control (A) and hypertensive (B) tissues. ɛ_cd, Collagen-dominant ɛ.