AII concentrations on day 0 showed no significant differences among the three different birth weight groups. AII concentrations on day 7 were lower in the NBW group than those on day 0, and higher in the VLBW group than in the MLBW or NBW group. Specific physiological factors associated with VLBW were therefore thought to be responsible for the increase in the AII concentrations.
A few limited reports have focused on AII concentrations in childhood, and have indicated that AII concentrations are high in early childhood and then decrease gradually with age.3
There have been several reports on angiotensin converting enzyme (ACE) activity in the neonatal period. One study showed that ACE activity in 2–4 day old infants with respiratory distress syndrome was higher than that in healthy immature or mature infants, or mature infants with other acute illnesses.5
Another study has shown that ACE activity in immature infants with respiratory distress syndrome was equivalent to that in healthy immature infants on day 0, and a negative association was observed between birth weight and ACE activity.6
The same study also showed that ACE activity in immature infants was higher than in mature infants, whereas ACE activity in immature infants was equivalent to that in their mothers or in normal adult controls.
Another study that investigated plasma aldosterone concentrations in children aged from 2 hours to 15 years showed a maximum concentration at 2 hours of age, and then a gradual decrease during the first year of life.7
After 1 year of age, the plasma aldosterone concentration stabilised, but remained higher than in adults. Other studies of the renin‐angiotensin system in mature infants or children have shown that plasma aldosterone concentrations are not associated with potassium or sodium concentrations in serum.8
Our study also showed that AII concentrations were not associated with serum potassium or sodium concentrations.
AII has been reported to be related to organ fibrosis.10
A previous study that used immunohistochemical staining using polyclonal antibody to human AII type 1 receptor in human fetal lung fibroblasts (HFL‐1) showed that the receptor was stained.11
In the present study, the AII concentrations on day 7 were significantly higher in the VLBW group than in the other two heavier birthweight groups. Taken together, these results indicate that an increased AII concentration may affect AII type 1 receptor in the lungs of VLBW infants and promote lung fibrosis. Furthermore, this may be implicated in the progression of chronic lung disease in neonates.
On the other hand, it has been reported that the increase in plasma AII may play a role in organogenesis and neonatal growth in rats.12
Furthermore, the increased AII concentration in the VLBW group on day 7 may reflect a defence mechanism in infants. VLBW infants tend to be in a salt‐losing state. Furthermore, VLBW infants with respiratory disturbances often have limited water intake and are administered diuretics at the same time. Therefore, to maintain circulating blood volume and blood pressure concentrations, the AII concentration may rise. However, no obvious factors affecting the increase in the AII concentration could be identified in our study. It will therefore be necessary to measure the AII concentration for a longer period after birth and study the factors that mainly influence it.
What is already known on this topic
- The renin‐angiotensin system is more activated in the neonatal or infantile period than in later childhood
- Angiotensin II concentrations have not been studied in low birthweight infants
What this study adds
- Angiotensin II concentrations on day 7 in VLBW infants were significantly higher than those in NBW and MLBW infants
- There were significant correlations between angiotensin II concentration on day 7 and gestational age, and angiotensin II concentration on day 7 and birth weight