Intrauterine growth restriction : effects on cardiac function and structure in adulthood

Abstract

Numerous epidemiological studies world-wide have linked intrauterine growth restriction (IUGR) with an increased incidence of cardiovascular disease later in life, especially when there is ‘catch-up’ in postnatal growth. Over recent decades there has been extensive research in this area and a number of animal models of IUGR have been developed, in order to get a better understanding of how impaired growth in early life programs for vulnerability to cardiovascular disease in adulthood. In this thesis, I have explored how early life growth restriction affects the function and structure of the heart in adulthood and how the adult IUGR heart responds to stress and to the induction of hypertension (which challenges the adaptive capabilities of the myocardium). It was hypothesised that IUGR, induced through maternal protein restriction, acts as an initial insult to the early development of the heart resulting in structural alterations and altered biochemical composition in the myocardium in adulthood. This in turn, leads to adult cardiac dysfunction and renders the heart vulnerable to secondary insults such as the induction of hypertension. Hypertension, leads to left ventricular hypertrophy and it was therefore proposed that when the adaptive capabilities of the IUGR heart are challenged, as a result of hypertension, that the pathological changes in the heart that ensue, will be exacerbated. To address these hypotheses, comprehensive studies of functional and structural changes in the hearts of adult growth-restricted rat offspring compared to offspring that were appropriately grown in utero, were undertaken. The overall aims of this thesis were to examine the effects of early life growth-restriction, on cardiac function, structure and morphology in early adulthood and when combined with a secondary insult, the induction of hypertension. To address the aims, a well-established rat model of maternal protein restriction was used. Rat dams were administered either a low protein diet (LPD; 8.7% casein) or a control normal protein diet (NPD; 20% casein) two weeks prior to mating (to get the dams familiar to the diet), throughout pregnancy and for two weeks after birth (because cardiomyocytes in rat offspring continue to proliferate in the first 2 weeks after birth). The hearts of LPD and NPD offspring were examined in early adulthood at 14 weeks (Chapter 2) and at 18 weeks of age (Chapters 4 and 5); timepoint that coincided with the final endpoint in Chapter 6. In the final series of experiments, a cohort of 14 week adult male LPD and NPD offspring were subcutaneously infused with the vasopressor peptide, angiotensin II (Ang II), for 4 weeks and cardiac structure and function assessed at 18 weeks of age (Chapter 6). Offspring of mothers fed a LPD were born of lower birth weight when compared to offspring of mothers fed a NPD and the LPD offspring remained smaller throughout the postnatal experimental period. At 14 weeks of age, it was demonstrated that early life growth-restriction did not affect ventricular contractility as assessed by a Millar pressure-volume (P-V) conductance catheter system, whereby left ventricular P-V loops and arterial blood pressure were recorded in situ, in a closed chest model, under steady state conditions. However, in the hearts of the LPD offspring, there was increased afterload and an impaired ability to increase stroke volume and cardiac output in response to β-adrenergic stimulation (Chapter 2). Subsequent to these studies, the biochemical composition of the left ventricle and adjoining septum was analysed using Fourier transform infrared (FTIR) micro-spectroscopy (Chapter 4). This chemical method of analysis in rat tissues was developed and optimised in Chapter 3. Importantly, there were marked differences detected in the biochemical composition of the myocardium in the LPD offspring. In particular, there were significant differences in lipid, proteoglycan and carbohydrate content and this could be a likely mechanism for the increased vulnerability to cardiac disease in adulthood in individuals born IUGR. As follow on to these findings, in Chapter 5 cardiac function (assessed using P-V loops and echocardiography) and structure (assessed using echocardiography) were examined in female LPD and NPD offspring at 18 weeks of age. There were no apparent adverse effects on blood pressure, overall cardiac structure or fractional shortening of the cardiac muscle. However, although cardiac function was maintained, aortic peak systolic velocity was depressed suggestive of impaired systolic contractility. While there were no structural differences in the myocardium between NPD and LPD female offspring, in a different cohort of animals of the same age (Chapter 6), there was a significant increase in the absolute as well as relative thickness of the left ventricular chamber dimension during systole of the cardiac cycle. In the final experimental chapter, (Chapter 6) I have explored, how the adult IUGR heart responds to a secondary insult, hypertension. Hypertension (a sustained elevation in blood pressure) leads to hypertrophy of the left ventricle in response to the elevation in systemic blood pressure. Hence, this series of experiments was designed to see how the IUGR heart responds to this hypertrophic challenge. Hypertension was induced by infusion of Ang II which can stimulate cardiac hypertrophy directly and through its effects on blood pressure. As expected, Ang II led to an increase in blood pressure and cardiac hypertrophy in those rats that were infused with Ang II, however, the overall growth response to Ang II (as assessed echocardiographically) was not different between the NPD and LPD offspring. Furthermore, Ang II did not affect cardiac function (no difference in the fractional shortening of the cardiac muscle and no difference in the peak systolic velocity) or left ventricular chamber dimensions. Interestingly, however, there was a significant increase in the thickness of the left ventricular free wall. Unexpectedly, the levels of interstitial collagen were significantly lower in the IUGR Ang II infused hearts compared to controls, with the levels of interstitial fibrosis increased to a much greater extent in the Ang II treated NPD offspring. In conclusion, this thesis shows that IUGR, induced through maternal protein restriction, in rats leads to alterations in the biochemical composition of the heart in adulthood, although basal cardiac function is preserved. However, there was evidence of underlying cardiac impairment in response to stress. When the adaptive capabilities of the IUGR heart were challenged by induction of hypertension, the response to the challenge was not different compared to the non-growth-restricted hearts. In fact, the deposition of collagen was greater in the controls. In my model of IUGR the offspring remain smaller and normotensive throughout life, which may have inferred some protective benefit to the adult IUGR heart. It is likely that pathological changes may manifest if there was catch-up in body growth; but this is yet to be elucidated.