EuropeanJournal of Eur J Pediatr (1982)139:139-141 Pediatrics 9 Springer-Verlag 1982 Plasma Renin Activity and Plasma Aldosterone Concentration in Infants with Various Potassium Intakes* G. Gjuri61, D Mardegi61, V. Plavgid2, and M. Dumi61 Departments of Pediatrics 1 and Medicine 2, University Hospital Rebro, Zagreb, Yugoslavia Abstract. Plasma renin activity (PRA) and aldosterone concen- tration (PA) were measured in 20 healthy infants from 1 to 12 months of age during normal (3.03 mmol/kg/d) and high (5.20 mmol/kg/d) potassium intake. PRA did not change be- tween regimes. PA rose significantly from 618 pg/ml during normal to 1136 pg/ml during high potassium intake. It is con- cluded that the potassium intake should be taken into con- sideration when studying plasma aldosterone in infancy. Key words: Renin - Aldosterone - Potassium- Infants Introduction In order to gain insight into the regulatory mechanisms of the renin-angiotensin-aldosterone system in infancy, we studied the relationship between potassium intake, plasma renin activ- ity (PRA) and plasma aldosterone concentration (PA) in healthy infants. There are only few such data in the pediatric literature [2, 20, 22, 24] and they are slightly different from the corresponding data in adults. Material and Methods Twelve boys and eight girls aged from 1 to 12 months (mean age 4.12 months) were examined during two successive five- day study periods. All of them were in normal water and elec- trolyte balance and did not exhibit any condition which could alter their adrenal or renal function. During the first study period the children were given a milk formula containing 20 mmol/1 of potassium and 8.2 mmol/l of sodium. During the second period a potassium chloride solution contributing 2 mmol/kg/day was added to the same formula. The average daily potassium intake was calculated from the total five day period intake, on the basis of the recorded daily volume of food with known potassium concentration. The same was done for the estimation of the daily sodium intake. *Supported by a research grant from Interest Association for Science of the Socialist Republik of Croatia, contract No. V-11/78 Off)~rint request to: Dr. Gorjana Gjuri~, Department of Pediatrics, Uni- versity Hospital Rebro, Kigpatideva 12, 41000 Zagrab, Yugoslavia Three ml of venous blood was drawn at 8 a.m, at least 3 h after a feed, in the supine position, for the determination of PRA, PA, potassium and sodium at the end of each five day period. At the same time a 24h urine sample was collected for the determination of daily electrolyte and creatinine excretion. Plasma renin activity and plasma aldosterone concentra- tion were determined by angiotensin I radioimmunoassay and aldosterone radioimmunoassay, using commercially available kits from "CIS" International and following the procedures recommended by the manufacturer. The within-assay coeffic- ient of variation for PA and PRA in our laboratory were 5% and 3% respectively. The corresponding between-assay coeffi- cients of variation were 7% and 5% respectively. The sensitivity was 10pg/tube (41pg/ml) for aldosterone and 6 pg/tube for angiotensin I (0.08 ng/ml/h PRA). The plasma and urine potassium and sodium concentra- tions were determined by flame photometry. The urine crea- tinine was determined by the Mercko-test creatinine method and was used as a rough control of the completeness of urine collection. Informed parental consent was obtained for the dietary manipulation, urine collection and venepuncture. The mean differences between the individual children's paired values of sodium and potassium intake and excretion were tested by Student's t-test. Because of the skewness of the distri- bution curve of the PA and PRA values, the arithmetic means of their logarithms were calculated and the significance bet- ween them was tested by the conventional t- test. Results The results are summarized in Table 1. The mean difference between the individual children's paired values for PA during normal and high potassium intake was highly significant (P<0.01), indicating an increase in PA with the increase in potassium intake. On the contrary, in this study, there was no difference be- tween PRA during normal and high potassium intake. Correlations between the potassium excretion versus PRA or PA and between the serum potassium values versus PRA or PA were not significant. There was no correlation between PRA and PA separately for the normal and for the high potassium intake or for both groups pooled together. 0340-6199/82/0139/0139/$ 01.00