Perinatal hypoxia causes ventricular enlargement associated with increased atrial natriuretic peptide (ANP) mRNA levels in newborn mice

Right Ventricular Involution: Big Changes in Small Hearts

BACKGROUND

Before birth, the fetal right ventricle (RV) is the pump for the systemic circulation and is about as thick as the left ventricle (LV). After birth, the RV becomes the pump for the lower pressure pulmonary circulation, and the RV chamber elongates without change in its wall thickness. We hypothesize that the fetal RV may be a model of compensated RV hypertrophy, and understanding this process may aid in discovering therapeutic strategies for RV failure.

METHODS

We performed a literature review and identified pertinent articles from 1980 to present.

RESULTS

The following topics were identified to be most pertinent in right ventricular involution: morphologic and histologic changes of the RV, cellular proliferation and terminal differentiation, the effect of stress on RV development, excitation contraction coupling and inotropic response change over time, and the amount of apoptosis through RV development.

CONCLUSIONS

The RV changes on multiple levels after its transition from systemic to pulmonary circulation. Although published literature has variable results due partly from differences between animal models, the literature shows a clear need for more research in the field.

Cortical neurogenesis enhanced by chronic perinatal hypoxia

Most regions of the mature mammalian brain, including the cerebral cortex, appear to be unable to support the genesis of new neurons. Here, we report that a low level of neurogenesis occurs in the cerebral cortex of the infant mouse brain and is enhanced by chronic perinatal hypoxia. When mice were reared in a low-oxygen environment from postnatal days 3 to 11, approximately 30% of the cortical neurons were lost after the insult; yet this damage was transient. The loss of cortical neuron number, cortical volume, and brain weight were all reversed during the recovery period. At P18, 7 days after the cessation of hypoxia, there was a marked increase in astroglial cell proliferation within the SVZ, as assessed by 5-bromodeoxyuridine (BrdU) incorporation in S-phase cells. One month after BrdU incorporation, 40% more BrdU-positive cells were found in the cerebral cortex of hypoxic-reared as compared to normoxic control mice. Among these newly generated cortical cells, approximately 45% were oligodendrocytes, 35% were astrocytes, and 10% were neurons in both hypoxic and normoxic mice. However, twice as many BrdU-labeled cells expressed neuronal markers in the neocortex in mice recovering from hypoxia as compared to controls. In both hypoxic-reared and normoxic infant/juvenile mice, putative neuroblasts could be seen detaching from the forebrain subventricular zone, migrating through the subcortical white matter and entering the lower cortical layers, 5 to 11 days after their last mitotic division. We suggest that cortical neurogenesis may play a significant role in repairing neuronal losses after neonatal injury.

Repeated daily injections and osmotic pump infusion of isoproterenol cause similar increases in cardiac mass but have different effects on blood pressure

We found in mice that repeated single daily subcutaneous (s.c.) isoproterenol (ISO) injections, like constant infusions using osmotic minipumps, caused increased biventricular mass or weight relative to body weight (VW/BW). We found that 5 (1/d) s.c. injections of 2, 10, or 20 microg/g body weight caused equivalent VW/BW increases as compared with 5-d infusions at 20 microg/(g.d)). While it is often presumed that ISO elicits hypertrophy by a direct effect on the myocytes, growth may also be secondary to systemic hemodynamic effects. The 2 modes of ISO administration had different effects on mean arterial blood pressure (MABP) and heart rate. Using telemetry we observed that single injections of ISO (0, 0.5, 2, and 10 microg/g) were associated with hypotension and tachycardia with a duration but not a magnitude that was dose dependent. MABP dropped rapidly to 60 mm Hg for more than 2 h at the highest dose. Constant s.c. infusion of ISO at 20 microg/(g.d) initially lowered MABP to about 70 mm Hg for 24 h. At 48 h MABP was normal, but rose 10 mm Hg higher than baseline by day 5. Thus, different routes of administration of ISO that cause comparable increases in VW/BW had different effects on MABP. Thus when evaluating mouse models of ISO-induced cardiac hypertrophy, both repeated daily injections or infusions can cause similar increases in VW/BW, but the daily doses that are required are not the same. Furthermore, these different routes of administration have different hemodynamic sequelae and could potentially evoke different cardiac phenotypes.

Atrial natriuretic peptide in hypoxia

A growing number of mammalian genes whose expression is inducible by hypoxia have been identified. Among them, atrial natriuretic peptide (ANP) synthesis and secretion is increased during hypoxic exposure and plays an important role in the normal adaptation to hypoxia and in the pathogenesis of cardiopulmonary diseases, including chronic hypoxia-induced pulmonary hypertension and vascular remodeling, and right ventricular hypertrophy and right heart failure. This review discusses the roles of ANP and its receptors in hypoxia-induced pulmonary hypertension. We and other investigators have demonstrated that ANP gene expression is enhanced by exposure to hypoxia and that the ANP so generated protects against the development of hypoxic pulmonary hypertension. Results also show that hypoxia directly stimulates ANP gene expression and ANP release in cardiac myocytes in vitro. Several cis-responsive elements of the ANP promoter are involved in the response to changes in oxygen tension. Further, the ANP clearance receptor NPR-C, but not the biological active NPR-A and NPR-B receptors, is downregulated in hypoxia adapted lung. Hypoxia-sensitive tyrosine kinase receptor-associated growth factors, including fibroblast growth factor (FGF) and platelet derived growth factor (PDGF)-BB, but not hypoxia per se, inhibit NPR-C gene expression in pulmonary arterial smooth muscle cells in vitro. The reductions in NPR-C in the hypoxic lung retard the clearance of ANP and allow more ANP to bind to biological active NPR-A and NPR-B in the pulmonary circulation, relaxing preconstricted pulmonary vessels, reducing pulmonary arterial pressure, and attenuating the development of hypoxia-induced pulmonary hypertension and vascular remodeling.

Cellular life histories and bow tie biology

Considering the life-long influences of fetal growth biology, it is of interest to further elucidate the nature of the fetal growth process itself. Previous analyses of longitudinal fetal ultrasound data led to the hypothesis that hypoxia signals were important aspects of normal growth biology and directed attention to the place of oxygen as a basic nutrient. From the perspective of the cell, both hypoxia and lack of energy substrate trigger a common adaptive pathway through their effects on ATP availability. Comparative data from animal studies and cell culture provide evidence for an integrated energy/oxygen signaling system that acts redundantly and hierarchically with cellular differentiation programs, providing opportunities for developmental flexibility in response to variable ecologic or environmental challenge. The multinodal and interactive design of the fetal growth process suggests that it follows what has been described as the "bow tie" model of metabolism, with implications for robust and inventive approaches to cell, organ, and whole organism construction.

Perinatal hypoxia causes ventricular enlargement associated with increased atrial natriuretic peptide (ANP) mRNA levels in newborn mice

We sought to examine both the short-term and residual effects of perinatal hypoxia on ventricular mass and function of mice. We postulated that the magnitude of the ventricular hypertrophy would be determined by the timing of the exposure, be linked to augmented atrial natriuretic peptide (ANP) expression, and would persist to young adulthood. Furthermore, mice deficient in the ANP receptor type A (ANPRA) would have even greater hypertrophy. Newborns were placed in a 12% oxygen (O(2)) chamber either shortly after birth or at 8 days of age. Controls were raised in room air. After 8 or 16 days, pups were terminated and the right ventricle (RV) and left ventricle including the septum (LVS) were excised and weighed and total RNA was extracted. Hypoxia caused a reduction in body weight (BW) with an increase in right ventricle (RV) weight, rendering an increased RV to BW ratio and increased LVS/BW, albeit less. Hypertrophy was most pronounced in pups exposed to hypoxia in the first days of extrauterine life. A rapid postnatal decline in both RV and LVS ANP mRNA levels was observed in control animals, while the hypoxia elevated ANP mRNA. In mice missing the ANPRA, both ventricles were more massive than in wild type and hypoxia further augmented RV/BW and LVS/BW. In normal adult animals returned to room air after 16 days of hypoxia, RV but not LVS hypertrophy persisted in both sexes; there was an interaction between gender and the perinatal hypoxic stress on LVS dimension and perhaps on contractility. Thus perinatal hypoxia may "program" the adult mouse heart and vasculature.