The effect of maternal and lifelong vitamin D deficiency on the heart
2017-02-09T05:54:34Z (GMT) by
In recent years there has been a resurgence in vitamin D deficiency worldwide in subjects of all ages, ranging from infants through to the elderly. The rise in vitamin D deficiency has been attributed to a variety of causes including reduced exposure to sunlight, reduced production of cholecalciferol in the skin and/or reduced vitamin D absorption. Of concern, vitamin D deficiency is strongly linked to an increased incidence of heart disease. In this regard, the studies in this thesis explore the effects of maternal vitamin D deficiency during pregnancy and lactation on the development of the heart and the effects of subsequent lifelong vitamin D deficiency on cardiac structure and function. The specific aims were to firstly determine the effect of maternal vitamin D deficiency during pregnancy and lactation on cardiomyocyte growth in the offspring at weaning and secondly to determine the effect of lifelong vitamin D deficiency on: a) heart structure and function, b) the development of myocardial fibrosis, c) the response of the heart to a challenge (isoprenaline induced) and ischemia/reperfusion injury and d) myocardial capillarisation. To address the aims, a model of vitamin D deficiency in rats was used, whereby Sprague-Dawley rats, that were housed under incandescent lighting, were fed either a control vitamin D replete diet (1000 IU/kg) or a vitamin D deficient diet 6 weeks before conception, during pregnancy and throughout lactation. The offspring were maintained on their respective diets after weaning until early adulthood. In this model, serum 25(OH)D3 concentrations are markedly reduced but serum calcium levels remain normal. In Chapter 2, cardiomyocyte number was determined in the fixed hearts of offspring at postnatal day 3 and 4 weeks of age using an optical disector/fractionator stereological technique. In other litters, cardiomyocytes were isolated from freshly excised hearts to determine the proportion of mononucleated and binucleated cardiomyocytes. Maternal vitamin D deficiency had no effect on cardiomyocyte number, cardiomyocyte area, or the proportion of mononucleated/binucleated cardiomyocytes in the 3 day old male and female offspring. Importantly, however, vitamin D deficiency led to a marked increase in left ventricle and adjoining septum (LV+S) wall volume that was accompanied by an increase in cardiomyocyte number and size and in the proportion of mononucleated cardiomyocytes at 4 weeks of age. In Chapter 3, heart structure and function was echocardiographically examined in adult male and female rats that had been vitamin D deficient throughout life (from conception until early adulthood). The left ventricular hypertrophy (LVH) in vitamin D deficient rats at weaning persisted into adulthood with a marked increase in left ventricular (LV) mass relative to body weight in the vitamin D deficient rats (males and females) at 14 weeks of age. This was accompanied by a significant increase in relative interventricular septal thickness in diastole, absolute posterior wall thickness in diastole and absolute interventricular septal thickness in both diastole and systole. Although, the LV wall and septal thickness was increased in the vitamin D deficient offspring there was no evidence at this stage of adverse effects on cardiac muscle function, with fractional shortening (an indicator of cardiac muscle performance) not different between the vitamin D deficient and control hearts. Levels of interstitial fibrosis (an accumulation of collagen in the interstitial space) was markedly increased in the LV+S of vitamin D deficient rats (Chapter 4). There was a 2-fold increase in the level of interstitial fibrosis in the LV+S of vitamin D deficient rats compared to controls. In general, male offspring exhibited greater interstitial fibrosis than female offspring. Of concern, in a number of the vitamin D deficient rats there was evidence of reparative/replacement fibrosis within the LV indicative of injury to the myocardium, loss of cardiomyocytes and replacement with scar tissue. At 16 weeks of age cardiac function and the response to ischemia/reperfusion injury was comprehensively examined in isolated hearts using a modified Langendorff apparatus (Chapter 5). Of concern, under basal conditions there was a significant decrease in coronary blood flow in the vitamin D deficient hearts. When exposed to ischemia there was altered cardiac function in male hearts (increased heart rate and decreased rate of relaxation when expressed as a percentage of recovery) in the reperfusion period. Alarmingly, there was a marked increase in infarct size in the vitamin D deficient hearts (both males and females) when exposed to ischemia. In general, female vitamin D deficient hearts performed better during post-ischemic recovery, although females exhibited greater LVH compared to vitamin D deficient male hearts. The response to ischemia/reperfusion was not different between males and females. Given the reduced coronary blood flow and vulnerability to ischemia in the vitamin D deficient hearts, the myocardial microvasculature within the hypertrophied LV+S of rats exposed to lifelong vitamin D deficiency was subsequently examined in Chapter 6 using unbiased stereological techniques. Importantly, it was shown that capillarisation within the LV myocardium was not compromised within the enlarged LV of the vitamin D deficient rats; there were no differences in capillary length density, surface area density or diffusion radius in the myocardial capillaries in the vitamin D deficient rats compared to controls. In general, male rats (control and vitamin D deficient) were larger than females but heart size relative to body weight was increased in females compared to males. However, there was no evidence to suggest that cardiac structure or function was adversely affected in the females. Indeed, fractional shortening of the cardiac muscle was increased in females compared to males indicative of better cardiac function. Overall, the effect of vitamin D deficiency on the structure and function of the heart was not different between males and females, with very few differences detected in the way the female and male heart responded to vitamin D deficiency. Interestingly, the early gene response to vitamin D deficiency was different in the hearts of males compared to females although no differences in cardiomyocyte growth were detected. In conclusion, the findings of this thesis provide important insight into why individuals with vitamin D deficiency are more vulnerable to cardiovascular disease. The findings demonstrate that maternal vitamin D deficiency leads to abnormal cardiomyocyte growth and subsequent LVH in the heart of offspring by weaning. When vitamin D deficiency is maintained throughout life there are adverse effects on cardiac structure and function, including exacerbated collagen deposition, poor coronary blood flow and vulnerability to ischemia. These adverse effects of vitamin D deficiency on the heart are generally not different between males and females. Overall the findings shed light into the mechanisms leading to cardiac vulnerability in vitamin D deficient subjects and provide important avenues for future research.