Embryonic cardioprotection by hydrogen sulphide: studies of isolated cardiac function and ischaemia‐reperfusion injury in the chicken embryo.
Complications during mammalian pregnancy, such as gestational diabetes, maternal smoking, pregnancy at high altitude or preeclampsia can trigger a fetal origin of heart disease (Fowden et al. 2006). However, underlying mechanisms have proven difficult to isolate because suboptimal conditions during pregnancy often affect the mother, the placenta as well as the developing offspring. In this regard, oviparous species, like the chicken, offer many advantages. Embryonic development occurs in the absence of either a mother or a placenta, nutrition is fixed within the egg, there is no need to control for within‐litter variation or for effects on lactation, and many milestones of cardiac development are similar to humans (Itani et al. 2018). Therefore, the chicken embryo is ideally placed to isolate direct effects of adverse developmental conditions and of therapy on an early origin of heart disease. Therefore, the objectives of this work were to adapt the isolated Langendorff technique using the chicken embryo to study the physiology of the developing heart under basal conditions during ischaemia‐reperfusion (IR) injury. The validity of the technique was tested by investigating potential cardioprotective agents against IR and underlying physiological mechanisms.
Growing evidence suggests that H2S is vital to cardiovascular health (Shen et al. 2015). For instance, clinical studies report that a decrease in endogenous H2S levels is linked to age‐related cardiovascular pathology (Jiang et al. 2005; Polhemus et al. 2014; Perridon et al. 2016). Moreover, work in animal models shows that supplementation with an exogenous H2S donor protects the adult heart from ischaemic reperfusion (IR) injury (Johansen et al. 2006). However, whether H2S confers protection in the developing heart before birth remains completely unknown.
Mechanisms underlying cardiac protection by H2S may include an increase in coronary blood flow enhancing coronary reserve due to its vasodilator actions (Bhatia, 2005) and/or action on myocardial KATP channels (Shen et al. 2015). Under normal physiological conditions in the adult heart, KATP channels are closed (Lu et al. 2008). In response to a decrease in the ATP:ADP ratio, as experienced during an ischaemic challenge, myocardial KATP channels open. This preserves cardiac health via limiting calcium influx and energy expenditure (Lederer et al. 1989; Burke et al. 2008). Mutations in cardiac KATP channels have been identified in patients with dilated cardiomyopathy (Bienengraeber et al. 2004). Genetic and pharmacological disruption of KATP channels impairs recovery following IR and negates the beneficial effects of ischaemic preconditioning (Suzuki et al. 2001; Gumina et al. 2003; Kane et al. 2006). Conversely, KATP overexpression confers resistance to ischaemic injury (Du et al. 2006).
Episodes of IR can also present in utero for a range of reasons. These include periods of increased uteroplacental vascular resistance, such as during preeclampsia, or during compression of the umbilical cord in late gestation, as in complicated labour and delivery (Morrison, 2008; Giussani, 2016). A recent review by Bennet (2017) shows clearly that the preterm sheep fetus is remarkably tolerant to episodes of ischaemia produced by complete occlusion of the umbilical cord, even those lasting for periods longer than 20 min. Despite the fetal heart being more resistant to ischaemia compared to the adult heart, insufficient oxygen supply to developing organs can have detrimental and long‐term effects, particularly in metabolically active tissues, such as the fetal heart in late gestation (Li et al. 2003). Growing evidence derived from human clinical studies as well as from animal models links suboptimal oxygenation during fetal development with increased cardiac susceptibility in the neonatal period, as well as increased cardiac risk in the adult offspring (Patterson & Zhang, 2010; Giussani & Davidge, 2013). Therefore, it is clinically relevant to find possible therapeutic targets and interventions to protect the developing heart against IR injury. Here, we tested the hypothesis that H2S confers protection against IR injury in the embryonic heart via opening of KATP channels and not via increasing coronary flow. This work lays the foundation to study the direct effects of H2S therapy on the embryonic heart independent of effects on the mother and the placenta in adverse development.