the ductus arteriosus (DA) is a vital component of the fetal circulation diverting blood flow away from the pulmonary circulation and the nonventilated lungs, directly into the aorta. With the first breath at birth increasing O2 levels in the blood, the DA constricts, now allowing the blood to flow into the expanded and ventilated lungs, marking the transition from fetal to adult circulation. Failure to achieve closure results in an extracardiac shunt, cyanosis, and failure to thrive; this persistent DA (PDA) occurs in about 70% of preterm babies and has an overall incidence of 1/2,000 live births, i.e., it is relatively common (6). Since vasodilating prostaglandins (PGs) are important regulators of human DA tone, cyclooxygenase (COX) inhibitors are often used to close the PDA, and when this fails, surgical measures have to be taken (5). Conversely, in certain types of congenital heart disease, where maintenance of the fetal circulation is desired until corrective surgery, the DA is prevented from closing by infusion of PGs (8). Although relatively effective, both the COX inhibitors and the systemic PG infusion have many undesired effects due to lack of selectivity for the DA. Therefore, an understanding of the mechanisms that regulate vascular tone in the DA, its response to O2, and its developmental biology have significant and direct clinical significance.
The DA belongs to a specialized system of O2-sensitive organs and tissues in the body that includes the pulmonary arteries, the carotid body, and the neuroepithelial body among others, which share striking similarities in the ways they respond to changes in O2 tension (25). Therefore, advancing our knowledge on the vascular biology of the DA is important for our understanding of the mechanisms by which cells or blood vessels respond to changes in O2; this has far reaching implications for diverse conditions like ischemia or cancer vascular biology.
Significant progress in our understanding of the DA pathobiology has been achieved with the use of animal models, including the sheep (14, 15), the rabbit (18, 22, 23), or primates (20). Few reports have also used human DAs, obtained at the time of complex corrective surgery for congenital heart disease (16, 17). An evolving proposed mechanism for DA closure includes an acute phase, in which within minutes of exposure to normal O2 levels, the DA (which normally exists in hypoxic conditions in the fetus) constricts. This mechanism is thought to be intrinsic to the DA smooth muscle cells (DASMC) (9, 12) and, at least in the human or rabbit DA, includes a sensor (i.e., the electron transport chain of the mitochondria), which changes the production of activated oxygen species (AOS; like H2O2) in response to changes in O2 levels (Fig. 1). This mediator (i.e., the freely diffusible H2O2) can reach the cell membrane and decrease the opening of O2- and redox-sensitive voltage-dependent K+ (Kv) channels (like Kv1.5, Kv2.1, etc.). This causes DASMC depolarization; opening of the voltage-gated Ca2+ channels, increase in intracellular Ca2+ concentration, and vasoconstriction (16, 17, 23). This mitochondria-AOS-K+ channel axis is the basis of O2 sensing in many other O2-sensitive tissues (reviewed in Ref. 25), suggesting that its central role might be preserved during evolution.
This acute phase is followed by a subacute phase in which the production of endothelin (and other endothelium-derived modulators) is altered (7, 16, 20), further promoting and sustaining constriction, leading to a functional DA closure. In a more chronic phase (i.e., days) significant changes in the DA vascular wall, including endothelial and DASMC proliferation, extracellular matrix remodeling, or vasa vasora constriction (2, 3, 11, 15) (reviewed in Ref. 6) complete the structural remodeling of the DA and lead to complete, anatomical closure. None of these theories has reached the bedside yet. Furthermore, developmental changes have been described in the elastic laminae (27), Kv channel pathways (22), PG axis, and others (6) that take place prior to birth, “preparing the DA” for its appropriate response upon exposure to normal O2 levels. However, if the fetus is born prematurely, these changes are incomplete and prevent the normal DA response at birth.
An important limitation in our knowledge of these O2 sensing systems and their developmental regulation is the complexity of the available models. All of these include mammals, where the fetal/placental circulation has to be exposed to intervention only through complex surgery; manipulation of the ambient O2 levels affect both the mother and the fetus and the duration of pregnancy is long (31 days for rabbit, 141–151 days for sheep). Therefore, there is a need for additional models addressing these limitations.
The work by Ågren et al. (1) comes to fulfill this need. The authors used the chicken (Gallus gallus) embryo model to study DA biology. They characterized the basic vascular reactivity and response to O2 in the chicken DA, as well as developmental changes prior to term birth. While at 15 days of gestation (5–6 days prior to pecking) the DA does not yet constrict to O2; it constricts in response to K+ channel inhibition, a pathway that is augmented later in development. On day 19, the magnitude of the constriction to O2 is similar to KCl, a drug that inhibits the gradient of K+ efflux out of DASMC, i.e., functionally inhibiting all K+ channels. In addition, while there is contraction to O2 in DAs preconstricted to norepinephrine, there is no additional contraction in DAs preconstricted with KCl, suggesting that the constriction to O2 involves the closure of K+ channels. Pharmacological dissection of the response to KCl, revealed that the predominant K+ channels are 4-aminopyridine- (and not glyburide or tetra-ethylammonium-) sensitive; in other words, they belong to the family of Kv channels (and not the KATP or KCa channels). The augmentation of the response to 4-aminopyridine and O2 toward pecking was similar, suggesting that the two mechanisms are similarly regulated, or they represent a single system. Specific O2-sensitive Kv channels, like Kv1.5 and Kv2.1, have been shown to be the effectors of O2-constriction in the human DA (16, 17, 22, 23). Interestingly, the developmental regulation of Kv channel expression that has recently been described in DA (22), resembles the one implied by the authors’ work in the chicken. In human and rabbit DA, Kv channel expression increases toward term delivery and the weak response to O2 in premature (and Kv-poor) DA can be rescued by gene transfer of Kv1.5 or Kv2.1 (22).
In another intriguing similarity to human DA (16) (Figure 1), the constriction to O2 was not significantly affected by the presence of an NO synthase and an endothelin receptor blocker, in keeping with the observation that the predominant mechanism in acute O2 constriction involves Kv channel inhibition. However, in contrast to human DA, the chicken DA was completely unresponsive to several COX inhibitors. This suggests that the Kv channel mechanism might have preceded the PG mechanisms in evolution.
The chicken is believed to be a descendent of the Archaeopteryx, a dinosaur from the Jurassic period over 150 million years ago (4). The draft sequence of the chicken genome consists of ∼20,000–23,000 genes (10), which is comparable to the human genome (13). Approximately 60% of the chicken protein-coding genes have a single human orthologue (10). As we discussed above, the similarities between the chicken and the human DA are striking, suggesting that the Kv channel inhibition in response to O2 is preserved during evolution; and that there might also be similar developmental regulatory mechanisms. The authors suggest that, like in humans, one or more Kv channels are the effectors in O2-induced DA constriction. It is striking that the critical parts for the function of Kv1.5, i.e., the voltage sensor and the pore of the channel, show a remarkable similarity in terms of sequence across birds, worms, flies, and humans and are identical between the human and chicken (Fig. 1).
The chicken egg may be a good model for studying the fetal circulation, as the chicken embryo can be manipulated quite easily in exposing it to different experimental conditions, such as hypoxia (24). This condition can be directly related to the developmental physiology of a human fetus confined to a uterus with a poorly developed placenta resulting in a relatively hypoxic environment. Establishing a similar environment in a mammalian model would confine the mother to a hypoxic chamber, and the relative Po2 of the fetus may still be varied to that of the mother; the fetal organs might also be exposed to maternal-derived circulating factors, released in response to environmental stress. Additionally, the lung development that is also dysregulated in prematurity can also be studied in this model (21). Important questions that need to be answered include: 1) what is the molecular identity of the chicken DASMC Kv channels; 2) do isolated chicken DASMC show Kv channel inhibition (patch clamping) to O2 and do they respond in an opposing manner compared to the pulmonary artery smooth muscle cells, which are inhibited by hypoxia in all mammals (25); and 3) is a mitochondria or other redox-based mechanism involved in this response?
If these studies show similarities to the mammalian and human DA, then a new model will be available for translational studies for the human DA and, moreover, to the biology of vascular O2 sensing in vivo. The egg-chicken model will be easier to manipulate (for example, expose the whole egg into a hypoxic chamber), or intervene on [easily inject drugs (19) or gene therapies (26) through small holes in the eggshell]. Advances in imaging studies will also allow the direct imaging of vascular structures, lungs, and hearts of the chicken embryo before birth. A glimpse into the future is the CT scan image of a 17-day-old chicken embryo in the egg, where the bone structures are seen and the brain and lungs are 3D-reconstructed and shown in blue and red, respectively, in Fig. 1. Looking at this image, one cannot fail to visualize a “baby dinosaur egg” from…“Jurassic Park.” Looking into mechanisms preserved during evolution of O2 sensing from the past might allow us to look into the future of translational developmental vascular biology.
- Copyright © 2007 the American Physiological Society