Effect of transverse aortic constriction on cardiac structure, function and gene expression in pregnant rats

Abstract

Background: There is an increased risk of heart failure and pulmonary edema in pregnancies complicated by hypertensive disorders. However, in a previous study we found that pregnancy protects against fibrosis and preserves angiogenesis in a rat model of angiotensin II induced cardiac hypertrophy. In this study we test the hypothesis that pregnancy protects against negative effects of increased afterload.

Methods: Pregnant (gestational day 5.5-8.5) and non-pregnant Wistar rats were randomized to transverse aortic constriction (TAC) or sham surgery. After 14.2 ± 0.14 days echocardiography was performed. Aortic blood pressure and left ventricular (LV) pressure-volume loops were obtained using a conductance catheter. LV collagen content and cardiomyocyte circumference were measured. Myocardial gene expression was assessed by real-time polymerase chain reaction.

Results: Heart weight was increased by TAC (p<0.001) but not by pregnancy. Cardiac myocyte circumference was larger in pregnant compared to non-pregnant rats independent of TAC (p = 0.01), however TAC per se did not affect this parameter. Collagen content in LV myocardium was not affected by pregnancy or TAC. TAC increased stroke work more in pregnant rats (34.1 ± 2.4 vs 17.5 ± 2.4 mmHg/mL, p<0.001) than in non-pregnant (28.2 ± 1.7 vs 20.9 ± 1.5 mmHg/mL, p = 0.06). However, it did not lead to overt heart failure in any group. In pregnant rats, α-MHC gene expression was reduced by TAC. Increased in the expression of β-MHC gene was higher in pregnant (5-fold) compared to non-pregnant rats (2-fold) after TAC (p = 0.001). Nine out of the 19 genes related to cardiac remodeling were affected by pregnancy independent of TAC.

Conclusions: This study did not support the hypothesis that pregnancy is cardioprotective against the negative effects of increased afterload. Some differences in cardiac structure, function and gene expression between pregnant and non-pregnant rats following TAC indicated that afterload increase is less tolerated in pregnancy.

Figure 1. Light microscopy of heart tissue.

Histological section of heart tissue (200x) stained with toluidine blue to enhance the contour of the cardiomyocytes for measuring their circumference and immunohistochemistry using non-muscular β-actin to stain capillaries (brown). One cardiomyocyte cut in short axis and containing a nucleus (blue) is outlined and magnified (box) to show how cardiomyocyte circumference was measured (in arbitrary units).

Figure 2. Blood pressure, heart weight and heart weight/tibia length ratio.

Mean±SEM for diastolic (A) and systolic (B) blood pressure in ascending aorta, heart weight (C) and heart weight/tibia length ratio (D). Comparisons between groups were made using one-way ANOVA and Holm-Sidak post hoc test. NP, non-pregnant, TAC, transverse aorta constriction. p<0.05 compared to non-pregnant sham (*), non-pregnant TAC (†), or pregnant sham rats (‡).

Figure 3. Myocyte transverse circumference and collagen content in myocardial tissue.

Myocyte transverse circumference (A) expressed in arbitrary units (a.u.) and collagen content in myocardial tissue (B) expressed as % tissue area stained by Sirius Red presented as mean±SEM. Comparisons between groups were made using one-way ANOVA and Holm-Sidak post hoc test. NP, non-pregnant, TAC, transverse aorta constriction. p<0.05 compared to non-pregnant sham (*).

Figure 4. Pressure-volume loops, cardiac output, end-diastolic pressure and stroke work.

Typical pressure-volume loops obtained by conductance catheter from a sham operated non-pregnant rat (full line) and a pregnant rat with transverse aortic constriction (dotted line) (A). Mean±SEM for calculated cardiac output (B), end-diastolic pressure (C) and calculated stroke work (D). Comparisons between groups were made using one-way ANOVA and Holm-Sidak post hoc test. NP, non-pregnant, TAC, transverse aorta constriction. p<0.05 compared to non-pregnant sham (*), non-pregnant TAC (†), or pregnant sham rats (‡).