Effects of early life growth restriction on the heart, kidneys and insulin sensitivity in adulthood and the potential vulnerability to diabetes

Many epidemiological studies have linked intrauterine growth restriction (IUGR) with an increased incidence of disease in adulthood, especially when there is postnatal ‘catch-up’ in growth. In this thesis, I explored how early life growth restriction can program for disease later in life. It was hypothesised that early life growth restriction, due to maternal protein restriction during pregnancy and lactation programs for increased insulin sensitivity (which implies better glucose metabolism) in adulthood when the postnatal growth trajectory remains lower than normal. It was also proposed that the reduced complement of cardiomyocytes in the heart and decreased nephron endowment in the kidney of growth restricted offspring leads to dysfunction of the heart and kidneys in adulthood and renders the heart and kidney more vulnerable to a secondary insult of hyperglycemia. When blood glucose levels are closely controlled, it was proposed that the adverse effects of hyperglycemia would be markedly attenuated. The overall aims were to examine the effect of early life growth restriction on: 1) growth of cardiomyocytes at weaning and insulin sensitivity, cardiac function and renal function in adulthood. 2) cardiac and renal function when combined with a secondary insult of mild or moderate hyperglycemia. To address these aims a model of maternal protein restriction in rats was used, whereby rat dams were fed a low protein diet (LPD; 8.7% casein) for 2 weeks prior to mating, during pregnancy and for 2 weeks during lactation. Controls were fed with a normal protein diet (NPD; 20% casein). In all experiments, growth restricted offspring that were exposed to maternal protein restriction remained smaller throughout the experimental period and blood pressure was unaffected. It was clearly shown in Chapter 2, using a euglycemic-hyperinsulinemic clamp technique that early life growth restriction programs for higher insulin sensitivity in adulthood. At 32 weeks of age, whole body insulin sensitivity was markedly increased (P < 0.0001) in LPD offspring. This is likely attributed to the failure of full ‘catch-up’ growth in the offspring; whereby the growth trajectory after birth reflects that in utero. Furthermore, at 32 weeks of age, body composition, oxygen consumption, respiratory exchange ratio and locomotor activity were not different to controls. At weaning, there was evidence of left ventricular hypertrophy (the heart volume to body weight ratio was significantly elevated; P < 0.0001) in the growth restricted offspring. Although maternal protein restriction had previously been shown to lead to a reduced number of cardiomyocytes in rat offspring at birth, when cardiomyocyte number was stereologically analysed at weaning (Chapter 3), cardiomyocyte number was no longer attenuated, suggestive of ‘catch-up’ hyperplasia and/or altered levels of apoptosis at this time. Cardiomyocyte number was directly related to the size of the heart in both growth restricted and non-growth restricted offspring (there was a significant linear correlation between cardiomyocyte number and heart volume in both LPD and NPD offspring; P = 0.0005 and P = 0.001, respectively) When IUGR offspring were followed to adulthood, echocardiographical analysis (Chapter 4) showed that the left ventricular hypertrophy persisted in IUGR offspring (LPD offspring exhibited a significant increase in heart weight to body weight ratio; P = 0.05 and relative posterior wall and septal wall thickness; P = 0.0001 and P = 0.05, respectively). This was accompanied by an increase in cardiac fibrosis (P < 0.0001), but at this stage, there was no evidence of impaired cardiac function as indicated by normal fractional shortening of the cardiac muscle. Similar to the heart, in the IUGR offspring, there was evidence of renal hypertrophy (increased relative kidney length; P = 0.008). Although overall renal function was maintained with GFR/g kidney weight not different, there was evidence of underlying renal impairment with an increase in renal vascular resistance and decreased renal plasma flow. Of concern, there was a significant increase (P = 0.03) in filtration fraction (15% increase in male and 14% increase in female LPD kidneys), indicative of glomerular hyperfiltration. In general, female offspring exhibited higher renal function than male offspring , however, there was no evidence to indicate that early life growth restriction leads to differential programming of renal function between the sexes (Chapter 5). At 24 weeks of age, diabetes was induced (via streptozotocin injection), as a secondary insult to the IUGR heart and kidneys. Blood glucose levels were stratified to be maintained at a mild (blood glucose levels between 7-10 mmol/L) or moderate level (blood glucose levels between 10-15mmol/L) by daily injection of long-acting insulin (Chapter 4 and Chapter 6). As expected, induction of hyperglycemia led to marked cardiac (Chapter 4) and renal (Chapter 6) dysfunction but the overall response to hyperglycemia was not different between the IUGR and control offspring; importantly, maintaining blood glucose levels at a mild hyperglycemic level attenuated many of the adverse effects. Of concern however, the levels of cardiac fibrosis remained high even when blood glucose levels were maintained at a mild hyperglycemic level (Chapter 4). In particular, hyperglycemic IUGR offspring exhibited the highest levels of cardiac fibrosis (23% and 54% increase in interstitial fibrosis in the LPD moderately hyperglycemic group when compared to the LPD and NPD controls, respectively). This may lead to overt cardiac muscle dysfunction later in life, even if glucose levels are well maintained. In conclusion, this thesis shows that IUGR in rats, due to maternal protein restriction, leads to increased insulin sensitivity in adulthood, which is likely due to the persistent attenuation of body growth postnatally. In adulthood, although overall renal and cardiac function were maintained with no changes in blood pressure, there was evidence of underlying renal and cardiac impairment, including left ventricular hypertrophy; this may manifest into overt cardiac and renal dysfunction later in life. The deleterious effects of a secondary insult of hyperglycemia to the heart and kidneys were not exacerbated in the IUGR offspring, however, it is to be noted that this may not reflect the response to alternative secondary insults.