Carbohydrate metabolism in isolated, working newborn pig heart

Optocardiography and Electrophysiology Studies of Ex Vivo Langendorff-perfused Hearts

Small animal models are most commonly used in cardiovascular research due to the availability of genetically modified species and lower cost compared to larger animals. Yet, larger mammals are better suited for translational research questions related to normal cardiac physiology, pathophysiology, and preclinical testing of therapeutic agents. To overcome the technical barriers associated with employing a larger animal model in cardiac research, we describe an approach to measure physiological parameters in an isolated, Langendorff-perfused piglet heart. This approach combines two powerful experimental tools to evaluate the state of the heart: electrophysiology (EP) study and simultaneous optical mapping of transmembrane voltage and intracellular calcium using parameter sensitive dyes (RH237, Rhod2-AM). The described methodologies are well suited for translational studies investigating the cardiac conduction system, alterations in action potential morphology, calcium handling, excitation-contraction coupling and the incidence of cardiac alternans or arrhythmias.

Postnatal Cardiac Development and Regenerative Potential in Large Mammals

The neonatal capacity for cardiac regeneration in mice is well studied and has been used to develop many potential strategies for adult cardiac regenerative repair following injury. However, translating these findings from rodents to designing regenerative therapeutics for adult human heart disease remains elusive. Large mammals including pigs, dogs, and sheep are widely used as animal models of humans in preclinical trials of new cardiac drugs and devices. However, very little is known about the fundamental cardiac cell biology and the timing of postnatal cardiac events that influence cardiomyocyte proliferation in these animals. There is emerging evidence that external physiological and environmental cues could be the key to understanding cardiomyocyte proliferative behavior. In this review, we survey available literature on postnatal development in various large mammal models to offer a perspective on the physiological and cellular characteristics that could be regulating cardiomyocyte proliferation. Similarities and differences between developmental milestones, cardiomyocyte maturational events, as well as environmental cues regulating cardiac development, are discussed for various large mammals, with a focus on postnatal cardiac regenerative potential and translatability to the human heart.

Left ventricular output and aortic blood flow in response to changes in preload and afterload in the preterm piglet heart

Low systemic blood flow occurs in up to 30% of infants born at less than 30 wk gestation. It is associated with increased morbidity and mortality, and current treatments are ineffective in 40% of cases. The aim of this study was to assess the ability of the preterm heart to respond to the acute shifts in preload and afterload that occur at the time of birth. Myocardial and coronary vascular function was assessed using an isolated working heart model in term (115 days) and preterm (92 days) piglets. Cardiac output/kg body wt in preterm hearts was ∼50% lower than that of term hearts (P = 0.001). Pressure development was similar in term and preterm hearts. Elevations in preload increased cardiac output and aortic flow similarly in term and preterm hearts, demonstrating significant preload "reserve". By contrast, elevations in afterload markedly depressed aortic flow, with a greater proportion of cardiac output being distributed to coronary flow in preterm hearts at high afterloads. The demands of increased workload were associated with greater increases in coronary flow in preterm hearts compared with term hearts. In preterm hearts, exposure to maternal glucocorticoids resulted in increased aortic flow when afterload was below 25 mmHg. These data suggest the preterm heart lacks the functional capacity to acutely adapt to postnatal afterload. To maximize systemic blood flow in preterm infants, treatments limiting afterload, while harnessing significant preload reserve, should be further explored.

Newborn hearts are at greater 'metabolic risk' during global ischemia--advantages of continuous coronary washout

BACKGROUND

Altered metabolic responses of the newborn heart to ischemia, which may increase irreversible injury, may at least partially explain the greater morbidity and mortality experienced by some children undergoing congenital cardiac repair. The present study compared newborn heart metabolic responses to global ischemia with those of adult, and evaluated whether continuous coronary artery washout in the newborn heart during 'ischemia' could favourably affect these responses.

METHODS

Adult (n=12) and newborn (n=12) pigs were anesthetized, and right ventricular biopsies were taken before global ischemia and at set intervals during ischemia. Another 12 newborns were subdivided into groups of nonperfused hearts and hearts receiving continuous perfusion. Time to onset of and time to peak of ischemic contracture were recorded. Biopsies were assayed for lactate, myocardial glycogen, glucose-6-phosphate and ATP.

RESULTS

Newborn hearts were more sensitive to global ischemia than adult hearts, based on shorter time to onset of and time to peak of ischemic contracture, and had a significantly greater rate of ATP decline (P<0.01). This was due in part to a more rapid accumulation of lactate (P<0.05) and only a 50% use of glycogen, compared with 93% by adult hearts. Continuous washout of newborn hearts prevented lactate accumulation, allowing a 90% use of glycogen and delaying time to ischemic contracture by twofold. This was accompanied by lower levels of glucose-6-phosphate accumulation (P<0.05) and a threefold reduction in the rate of ATP decline.

CONCLUSIONS

Significant differences in myocardial metabolism during ischemia in newborns compared with adults could predispose them to earlier ischemic injury, which can be eliminated by the removal of end products. Perfusion strategies taking these differences into account may further optimize pediatric myocardial protection and improve outcomes in newborn children undergoing cardiac procedures.

The effects of insulin on glucose uptake and lactate release in perfused working rat heart preparations

The effects of insulin on glucose uptake and lactate release in the perfused working rat heart have been investigated in three types of preparation: (i) a control low-workload preparation; (ii) an increased-pressure-workload preparation, simulating conditions of aortic pressure encountered in vivo; (iii) an increased-volume-workload preparation, where pumping work done is approximately the same as (ii) but coronary flow is restricted because of the decreased aortic pressure. Insulin stimulated glucose uptake and lactate release in preparations (i) and (ii), but failed to do so in preparation (iii). It was considered possible that preparation (iii) was hypoxic, thus necessitating a maximal stimulation of glucose uptake. This was confirmed by improving cardiac oxygenation by addition of stroma-free haemoglobin to the perfusate in preparation (iii). Under these conditions in the absence of insulin, glucose uptake and lactate release were decreased compared with perfusions in the absence of haemoglobin. Insulin stimulation of both processes was restored. We conclude that the failure of other workers to observe insulin effects on glucose uptake and lactate release under physiological workloads [preparation (ii)] may be a consequence of intracellular hypoxia in their preparations.