Factors influencing plasma renin and renin substrate in premature infants.

Concentrations of plasma renin (PRC) and plasma renin substrate (PRS) were measured during the first week of life in 52 infants born at less than 37 weeks' gestation (mean (SEM) gestation 30 (0.4) weeks, mean (SEM) birth weight 1.35 (0.08) kg). Both PRC (median 35, interquartiles 16.3, 94.5 ng/ml/hour) and PRS (median 2.3, interquartiles 1.3, 5.0 micrograms/ml) were raised compared with adults. The proportional rise in PRC was much greater than that in PRS, suggesting that PRS may be rate limiting in the generation of angiotensin I. Log10 PRC was inversely proportional to gestational age and a high urinary sodium loss was associated with a significantly raised log10 PRC. In hypoxaemic infants, there was a strong correlation between log10 PRS and haemoglobin concentration; this is a new observation in human infants but consistent with data available from other species.