Role of fetal nutrient restriction and postnatal catch-up growth on structural and mechanical alterations of rat aorta

Abstract

Key points: Intrauterine growth restriction (IUGR), induced by maternal undernutrition, leads to impaired aortic development. This is followed by hypertrophic remodelling associated with accelerated growth during lactation. Fetal nutrient restriction is associated with increased aortic compliance at birth and at weaning, but not in adult animals. This mechanical alteration may be related to a decreased perinatal collagen deposition. Aortic elastin scaffolds purified from young male and female IUGR animals also exhibit increased compliance, only maintained in adult IUGR females. These mechanical alterations may be related to differences in elastin deposition and remodelling. Fetal undernutrition induces similar aortic structural and mechanical alterations in young male and female rats. Our data argue against an early mechanical cause for the sex differences in hypertension development induced by maternal undernutrition. However, the larger compliance of elastin in adult IUGR females may contribute to the maintenance of a normal blood pressure level.

Abstract: Fetal undernutrition programmes hypertension development, males being more susceptible. Deficient fetal elastogenesis and vascular growth is a possible mechanism. We investigated the role of aortic mechanical alterations in a rat model of hypertension programming, evaluating changes at birth, weaning and adulthood. Dams were fed ad libitum (Control) or 50% of control intake during the second half of gestation (maternal undernutrition, MUN). Offspring aged 3 days, 21 days and 6 months were studied. Blood pressure was evaluated in vivo. In the thoracic aorta we assessed gross structure, mechanical properties (intact and purified elastin), collagen and elastin content and internal elastic lamina (IEL) organization. Only adult MUN males developed hypertension (systolic blood pressure: MUN_males = 176.6 ± 5.6 mmHg; Control_males = 136.1 ± 4.9 mmHg). At birth MUN rats were lighter, with smaller aortic cross-sectional area (MUN_males = (1.51 ± 0.08) × 10⁵ μm² , Control_males = (2.8 ± 0.04) × 10⁵ μm² ); during lactation MUN males and females exhibited catch-up growth and aortic hypertrophy (MUN_males = (14.5 ± 0.5) × 10⁵ μm² , Control_males = (10.4 ± 0.9) × 10⁵ μm² ), maintained until adulthood. MUN aortas were more compliant until weaning (functional stiffness: MUN_males = 1.0 ± 0.04; Control_males = 1.3 ± 0.03), containing less collagen with larger IEL fenestrae, returning to normal in adulthood. Purified elastin from young MUN offspring was more compliant in both sexes; only MUN adult females maintained larger elastin compliance (slope: MUN_females = 24.1 ± 1.9; Control_females = 33.3 ± 2.8). Fetal undernutrition induces deficient aortic development followed by hypertrophic remodelling and larger aortic compliance in the perinatal period, with similar alterations in collagen and elastin in both sexes. The observed alterations argue against an initial mechanical cause for sex differences in hypertension development. However, the maintenance of high elastin compliance in adult females might protect them against blood pressure rise.

Keywords: IUGR; collagen; elastin; fetal programming; hypertension; sexual dimorphism; vascular mechanics.

Figure 1. Body weight in male and female rats aged 24 h, 3 days, 21 days and 6 months

MUN, maternal undernutrition. The number of dams is shown in parentheses (from each dam 2 pups of each sex were averaged, with the exception of 24 h data which includes all pups from each dam). Bar graphs represent the mean, and error bars are SEM. Mean differences between Control and MUN groups are shown below each graph together with confidence intervals (^* P < 0.05 when compared to sex‐ and age‐matched Control rats).

Figure 2. Systolic and diastolic blood pressure (SBP, DBP) and heart rate (HR) measured in anaesthetized male and female rats aged 21 days and 6 months

MUN, maternal undernutrition. The number of dams is shown in parentheses (from each dam 2 pups of each sex were averaged). Bar graphs represent the mean, and error bars are SEM. Mean differences between Control and MUN groups are shown below each graph together with confidence intervals (^* P < 0.05 when compared to sex‐ and age‐matched Control rats).

Figure 3. Thoracic aorta structure

Internal diameter (A) and medial cross‐sectional area (CSA, B) from thoracic aorta rings without tension. MUN, maternal undernutrition. The number of dams is shown in parentheses (from each dam 2 pups of each sex were averaged). Bar graphs represent the mean, and error bars are SEM. Mean differences between Control and MUN groups are shown below each graph together with confidence intervals (^* P < 0.05 when compared to sex‐ and age‐matched Control rats).

Figure 4. Mechanical properties of rat aortic segments

A, mean stretch–pressure relationship in 3‐day‐old rats. B, mean stretch–tension relationship in 21‐day‐old rats. C, mean stretch–tension relationship in 6‐month‐old rats. Inset shows the mean B values or slope (derived for the exponential fit) from each experimental group. MUN, maternal undernutrition. The number of dams is shown in parentheses (from each dam 2 pups of each sex were averaged). Bar graphs represent the mean, and error bars are SEM. Mean differences between Control and MUN groups are shown below each graph together with confidence intervals (^* P < 0.05 when compared to sex‐ and age‐matched Control rats).

Figure 5. Effect of different incubation times with NaOH on aortic collagen and elastin degradation

Representative images of adventitial collagen and internal elastic lamina from a 21‐day‐old Control rat aorta incubated with 0.1 N NaOH at 0, 15, 30 or 45 min. Images were obtained with a laser scanning confocal microscope at Ex488 nm/Em500–560 nm to visualize elastin and Ex647/Em665 to visualize collagen, using a ×20 objective with zoom 4. Scale bar, 25 μm. [Color figure can be viewed at http://wileyonlinelibrary.com]

Figure 6. Mechanical tests on purified aortic elastin

A, mean stretch–tension relationship in 21‐day‐old rat aorta. B, mean stretch–tension relationship in 6‐month‐old rat aorta. Inset shows the mean slope values. MUN, maternal undernutrition. The number of dams is shown in parentheses (2 pups from each dam were used). Bar graphs represent the mean, and error bars are SEM. Mean differences between Control and MUN groups are shown below each graph together with confidence intervals (^* P < 0.05 when compared to sex‐ and age‐matched Control rats).

Figure 7. Relative aortic scleroprotein content

Elastin (A) and collagen (B). MUN, maternal undernutrition. W, weight. The number of dams is shown in parentheses (from each dam 1–2 pups of each sex were averaged). Bar graphs represent the mean, and error bars are SEM. Mean differences between Control and MUN groups are shown below each graph together with confidence intervals (^* P < 0.05 when compared to sex‐ and age‐matched Control rats).

Figure 8. Internal elastic lamina organization in rat aorta

Relative area of fenestrae and representative confocal images of the internal elastic lamina of 21‐day‐old (A) and 6‐month‐old animals (B). MUN, maternal undernutrition. Images were obtained with a ×20 objective, zoom 8 (scale bar, 100 μm). The relative area of fenestra (area of fenestra relative to the entire image) was measured from binary images. The number of dams is shown in parentheses (from each dam 1–2 pups of each sex were averaged). Bar graphs represent the mean, and error bars are SEM. Mean differences between Control and MUN groups are shown below each graph together with confidence intervals (^* P < 0.05 when compared to sex‐matched Control rats). [Color figure can be viewed at http://wileyonlinelibrary.com]