Biomechanical and structural responses of the aorta to intermittent hypobaric hypoxia in a rat model

Abstract

High altitude hypoxia is a condition experienced by diverse populations worldwide. In addition, several jobs require working shifts where workers are exposed to repetitive cycles of hypobaric hypoxia and normobaric normoxia. Currently, few is known about the biomechanical cardiovascular responses of this condition. In the present study, we investigate the cycle-dependent biomechanical effects of intermittent hypobaric hypoxia (IHH) on the thoracic aorta artery, in terms of both structure and function. To determine the vascular effects of IHH, functional, mechanical and histological approaches were carried out in the thoracic aorta artery, using uniaxial, pre-stretch, ring opening, myography, and histological tests. Three groups of rats were established: control (normobaric normoxia, NN), 4-cycles of intermittent hypoxia (short-term intermittent hypobaric hypoxia, STH), and 10-cycles of intermittent hypoxia (long-term intermittent hypobaric hypoxia, LTH). The pre-stretch and ring opening tests, aimed at quantifying residual strains of the tissues in longitudinal and circumferential directions, showed that the hypoxia condition leads to an increase in the longitudinal stretch and a marked decrease of the circumferential residual strain. The uniaxial mechanical tests were used to determine the elastic properties of the tissues, showing that a general stiffening process occurs during the early stages of the IH (STH group), specially leading to a significative increase in the high strain elastic modulus ([Formula: see text]) and an increasing trend of low strain elastic modulus ([Formula: see text]). In contrast, the LTH group showed a more control-like mechanical behavior. Myography test, used to assess the vasoactive function, revealed that IH induces a high sensitivity to vasoconstrictor agents as a function of hypoxic cycles. In addition, the aorta showed an increased muscle-dependent vasorelaxation on the LTH group. Histological tests, used to quantify the elastic fiber, nuclei, and geometrical properties, showed that the STH group presents a state of vascular fibrosis, with a significant increase in elastin content, and a tendency towards an increase in collagen fibers. In addition, advanced stages of IH (LTH), showed a vascular remodeling effect with a significant increase of internal and external diameters. Considering all the multidimensional vascular effects, we propose the existence of a long-term passive adaptation mechanism and vascular dysfunction as cycle-dependent effects of intermittent exposures to hypobaric hypoxia.

Figure 1

Experimental exposure to hypoxic cycles (a) and aorta topological overview (b).

Figure 2

Residual strain tests representation. (a) Axial pre-stretch test scheme. (b) Ring opening test scheme.

Figure 3

Tensile test representative curve and experimental setup. (a) stress-stretch curve of aortic tissue and mechanical parameters. (b) experimental setup: A: Longitudinal specimen, B: Steel clamps.

Figure 4

Counting protocol. (A) Histology image sample and ROI selection within media layer. (B) Gaussian blur and grayscale filter. (C) thresholding and watershed separation. (D) Particle analyzer result.

Figure 5

Axial pre-stretch mapping: Each segment represents one region of the thoracic aorta artery, going from the beginning of the artery (proximal) to the thoracic aorta distal end. Values are expressed as mean ± SEM (n = 6 per group).

Figure 6

Ring opening test. (a) Opening angle measurements in thoracic region of aorta artery. (b) image samples of aorta rings (1: NN, 2: STH, 3: LTH). Values are expressed as mean ± SEM (n = 6 per group). Significant difference (p ≤ 0.05): *vs NN.

Figure 7

Stress-stretch curves. Stress-stretch mean curves of longitudinal tests on aorta arteries of NN (blue), STH (yellow) and LTH (orange) groups until the first sample rupture. The filled areas represent the error envelope. All values are presented as mean ± S.E.M (n = 6 per group).

Figure 8

Vasoactive function of aorta artery. Vascular response to potassium (A, K+), phenylephrine (B, PE), methacholine (C, Metch) and sodium nitroprusside (D, SNP). Maximal responses (Emax, Kmax or Rmax) and sensitivity (EC50 or pD2) were calculated (inserted histograms). Groups are NN (light blue squares/bars), STH (yellow squares/bars) and LTH (red squares /bars). Values are means ± SEM (n = 6 per group). Significant difference (p ≤ 0.05): *versus NN; ^†versus STH.