Functional adaptations of the coronary microcirculation to anaemia in fetal sheep

Chronic in utero mitral inflow obstruction unloads left ventricular volume in a novel late gestation fetal lamb model

Objective

The in utero no flow/no grow hypothesis postulates that reduced inflow of blood into the left ventricle due to a stenotic mitral valve could lead to ventricular hypoplasia and hypoplastic left heart syndrome. This has been demonstrated in chick embryos, but less so in large animals. We investigated the impact of mitral obstruction on left and right ventricular growth in fetal lambs.

Methods

Twelve pregnant ewes, most bearing twins, were instrumented at 119 ± 1 days gestational age. Carotid artery and jugular vein catheters, an ascending aorta flow probe, and a left atrial deflated balloon catheter were implanted into 1 fetus (left atrial balloon group), and the twin remained an uninstrumented control. The balloon was inflated gradually over 8 days until net antegrade aortic flow was eliminated. Fetal transesophageal echocardiography was performed at the time of surgery and just before termination in both groups.

Results

Terminal fetal body weights were comparable between groups. Terminal heart/body weight ratio was higher in left atrial balloon group fetuses (6.9 ± 0.8 g/kg) compared with controls (5.9 ± 0.6 g, P = .0126). The left ventricular/right ventricular weight ratio was 24% (P = .0077) lower in left atrial balloon group fetuses than in controls. Left ventricular/heart weight (0.24 ± 0.04 g/g vs 0.30 ± 0.04 g/g, P = .0009), left ventricular end-diastolic volume (2.3 ± 0.7 mL vs 7.1 ± 0.8 mL; P = .0012), and left ventricular end-systolic volume (1.01 mL [0.95-1.95 mL] vs 3.38 mL [3.28-3.57 mL], P = .0042) were lower in left atrial balloon group fetuses compared with controls. Right ventricular weight (g/kg), right ventricular end-diastolic volume, and right ventricular end-systolic volume were similar between groups.

Conclusions

In this late-gestation fetal lamb model, in utero obstruction of mitral inflow slowed left ventricular growth and caused right ventricular remodeling.

Changes in microvascular perfusion of heart and skeletal muscle in sheep around the time of birth

NEW FINDINGS

What is the central question of this study? How does the microvascular perfusion of striated muscle change during the dynamic developmental period between the late gestation fetus and early neonate? What is the main finding and its importance? In both myocardium and skeletal muscle, perfusion of striated muscle is significantly reduced in the neonate compared to the late term fetus, but flow reserve is unchanged. The results suggest striated muscle capillary networks grow more slowly relative to the myofibres they nourish during the perinatal period.

ABSTRACT

Microvascular perfusion of striated muscle is an important determinant of health throughout life. Birth is a transition with profound effects on the growth and function of striated muscle, but the regulation of microvascular perfusion around this transition is poorly understood. We used contrast-enhanced ultrasound perfusion imaging (CEUS) to study the perfusion of left ventricular myocardium and hindlimb biceps femoris, which are populations of muscle with different degrees of change in pre- to postnatal workloads and different capacities for postnatal proliferative growth. We studied separate groups of lambs in late gestation (135 days' gestational age; 92% of term) and shortly after birth (5 days' postnatal age). We used CEUS to quantify baseline perfusion, perfusion during hyperaemia induced by adenosine infusion (myocardium) or electrically stimulated unloaded exercise (skeletal muscle), flow reserve and oxygen delivery. We found heart-to-body weight ratio was greater in neonates than fetuses. Microvascular volume and overall perfusion were lower in neonates than fetuses in both muscle groups at baseline and with hyperaemia. Flux rate differed with muscle group, with myocardial flux being faster in neonates than fetuses, but skeletal muscle flux being slower. Oxygen delivery to skeletal muscle at baseline was lower in neonates than fetuses, but was not significantly different in myocardium. Flow reserve was not different between ages. Given the significant somatic growth, and the transition from hyperplastic to hypertrophic myocyte growth occurring in the perinatal period, we postulate that the primary driver of lower neonatal striated muscle perfusion is faster growth of myofibres than their associated capillary networks.

Coronary conductance in the normal development of sheep during the perinatal period

Birth is associated with substantial shifts in cardiovascular physiology. Little is known about coronary vascular adaptations during this period. We used fetal and neonatal lambs to measure coronary function at late gestation (92% of term) and shortly after birth (5-6 days postnatal age). In each animal we measured unanesthetized myocardial perfusion and oxygen delivery using a circumflex artery flow probe. We used inflatable occluders and adenosine to determine coronary conductance and flow reserve. In a subset of animals, we used myocardial contrast echocardiography (MCE, anesthetized) to assess its utility as a tool for studying changes in regional myocardial perfusion in normal development. Separate age-matched animals were instrumented with aortic and coronary sinus sampling catheters to determine myocardial oxygen extraction (unanesthetized). With an average of 17 days of developmental time separating our neonatal and fetal cohorts we found that heart-to-body weight ratio was significantly greater in neonates than fetuses. In resting animals, we found significant decreases in weight-normalized perfusion of, and oxygen delivery to, neonatal relative to fetal myocardium. Similar results were seen when measuring baseline MCE-derived perfusion. Pressure-flow relationship studies revealed lower baseline and maximal coronary conductance in neonates than fetuses, with similar coronary flow reserve between groups. There was greater oxygen extraction in neonates than fetuses. Combined analysis of oxygen extraction with coronary flow suggested greater oxygen consumption by the fetal than neonatal myocardium. We conclude that, during the immediate perinatal period, cardiac growth outpaces coronary microvascular growth resulting in lower capacity for microvascular perfusion in the early neonate.

Coronary microvascular dysfunction is associated with degree of anaemia in end-stage renal disease

BACKGROUND

Coronary microvascular dysfunction (CMD) is common in end-stage renal disease (ESRD) and is an adverse prognostic marker. Coronary flow velocity reserve (CFVR) is a measure of coronary microvascular function and can be assessed using Doppler echocardiography. Reduced CFVR in ESRD has been attributed to factors such as diabetes, hypertension and left ventricular hypertrophy. The contributory role of other mediators important in the development of cardiovascular disease in ESRD has not been studied. The aim of this study was to examine the prevalence of CMD in a cohort of kidney transplant candidates and to look for associations of CMD with markers of anaemia, bone mineral metabolism and chronic inflammation.

METHODS

Twenty-two kidney transplant candidates with ESRD were studied with myocardial contrast echocardiography, Doppler CFVR assessment and serum multiplex immunoassay analysis. Individuals with diabetes, uncontrolled hypertension or ischaemic heart disease were excluded.

RESULTS

7/22 subjects had CMD (defined as CFVR < 2). Demographic, laboratory and echocardiographic parameters and serum biomarkers were similar between subjects with and without CMD. Subjects with CMD had significantly lower haemoglobin than subjects without CMD (102 g/L ± 12 vs. 117 g/L ± 11, p = 0.008). There was a positive correlation between haemoglobin and CFVR (r = 0.7, p = 0.001). Similar results were seen for haematocrit. In regression analyses, haemoglobin was an independent predictor of CFVR (β = 0.041 95% confidence interval 0.012-0.071, p = 0.009) and of CFVR < 2 (odds ratio 0.85 95% confidence interval 0.74-0.98, p = 0.022).

CONCLUSIONS

Among kidney transplant candidates with ESRD, there is a high prevalence of CMD, despite the absence of traditional risk factors. Anaemia may be a potential driver of microvascular dysfunction in this population and requires further investigation.

A 1 week IGF-1 infusion decreases arterial insulin concentrations but increases pancreatic insulin content and islet vascularity in fetal sheep

Fetal insulin is critical for regulation of growth. Insulin concentrations are partly determined by the amount of β-cells present and their insulin content. Insulin-like growth factor-1 (IGF-1) is a fetal anabolic growth factor which also impacts β-cell mass in models of β-cell injury and diabetes. The extent to which circulating concentrations of IGF-1 impact fetal β-cell mass and pancreatic insulin content is unknown. We hypothesized that an infusion of an IGF-1 analog for 1 week into the late gestation fetal sheep circulation would increase β-cell mass, pancreatic islet size, and pancreatic insulin content. After the 1-week infusion, pancreatic insulin concentrations were 80% higher than control fetuses (P < 0.05), but there were no differences in β-cell area, β-cell mass, or pancreatic vascularity. However, pancreatic islet vascularity was 15% higher in IGF-1 fetuses and pancreatic VEGFA, HGF, IGF1, and IGF2 mRNA expressions were 70-90% higher in IGF-1 fetuses compared to control fetuses (P < 0.05). Plasma oxygen, glucose, and insulin concentrations were 25%, 22%, and 84% lower in IGF-1 fetuses, respectively (P < 0.05). The previously described role for IGF-1 as a β-cell growth factor may be more relevant for local paracrine signaling in the pancreas compared to circulating endocrine signaling.

Role of adenosine signaling in coordinating cardiomyocyte function and coronary vascular growth in chronic fetal anemia

Fetal anemia causes rapid and profound changes in cardiac structure and function, stimulating proliferation of the cardiac myocytes, expansion of the coronary vascular tree, and impairing early contraction and relaxation. Although hypoxia-inducible factor-1α is sure to play a role, adenosine, a metabolic byproduct that increases coronary flow and growth, is implicated as a major stimulus for these adaptations. We hypothesized that genes involved in myocardial adenosine signaling would be upregulated in chronically anemic fetuses and that calcium-handling genes would be downregulated. After sterile surgical instrumentation under anesthesia, gestationally timed fetal sheep were made anemic by isovolumetric hemorrhage for 1 wk (16% vs. 35% hematocrit). At 87% of gestation, necropsy was performed to collect heart tissue for PCR and immunohistochemical analysis. Anemia increased mRNA expression levels of adenosine receptors ADORA 1, ADORA2A, and ADORA2B in the left and right ventricles (adenosine receptor ADORA3 was unchanged). In both ventricles, anemia also increased expression of ectonucleoside triphosphate diphosphohydrolase 1 and ecto-5'-nucleotidase. The genes for both equilibrative nucleoside transporters 1 and 2 were expressed more abundantly in the anemic right ventricle but were not different in the left ventricle. Neither adenosine deaminase nor adenosine kinase cardiac levels were significantly changed by chronic fetal anemia. Chronic fetal anemia did not significantly change cardiac mRNA expression levels of the voltage-dependent L-type calcium channel, ryanodine receptor 1, sodium-calcium exchanger, sarcoplasmic/endoplasmic reticulum calcium transporting ATPase 2, phospholamban, or cardiac calsequestrin. These data support local metabolic integration of vascular and myocyte function through adenosine signaling in the anemic fetal heart.

Chronic hypoxia alters maternal uterine and fetal hemodynamics in the full-term pregnant guinea pig

Placental hypoxia is associated with maternal hypertension, placental insufficiency, and fetal growth restriction. In the pregnant guinea pig, prenatal hypoxia during early gestation inhibits cytotrophoblast invasion of spiral arteries, increases maternal blood pressure, and induces fetal growth restriction. In this study the impact of chronic maternal hypoxia on fetal heart structure was evaluated using four-dimensional echocardiography with spatiotemporal image correlation and tomographic ultrasound, and uterine and umbilical artery resistance/pulsatility indexes and fetal heart function were evaluated using pulsed-wave Doppler ultrasound. Pregnant guinea pigs were exposed to normoxia (n = 7) or hypoxia (10.5% O₂, n = 9) at 28-30 days gestation, which was maintained until full term (65 days). At full term, fetal heart structure and outflow tracts were evaluated in the four-chamber view. Fetal heart diastolic function was assessed by E wave-to-A wave diastolic filling ratios (E/A ratios) of both ventricles and systolic function by the myocardial performance index (or Tie) of left ventricles of normoxic (n = 21) and hypoxic (n = 17) fetuses. There were no structural abnormalities in fetal hearts. However, hypoxia induced asymmetric fetal growth restriction and increased the placental/fetal weight compared with normoxic controls. Hypoxia increased Doppler resistance and pulsatility indexes in the uterine, but not umbilical, arteries, had no effect on the Tie index, and increased the E/A ratio in left, but not right, ventricles. Thus, prolonged hypoxia, starting at midgestation, increases uterine artery resistance and generates fetal growth restriction at full term. Furthermore, the enhanced cardiac diastolic filling with no changes in systolic function or umbilical artery resistance suggests that the fetal guinea pig systemic circulation undergoes a compensated, adaptive response to prolonged hypoxia exposure.