NEWBORN URINARY CYCLIC AMP AND DEVELOPMENTAL RENAL RESPONSIVENESS TO PARATHYROID HORMONE

Abstract

Since the renal action of parathyroid hormone is known to be mediated via 3',5'-adenosine monophosphate (cyclic AMP), urinary cyclic AMP studies were used to determine proximal tubular maturation. Ten formula fed full-term male infants showed a thirty- to sixtyfold increase in phosphate clearance and excretion with a three- to fourfold increase in urinary cyclic AMP comparing their first and third day 24-hour urines (2.37 ± 0.41 to 6.93 ± 0.96 Nm/mg creatinine M. ± S.E.M. first and third days respectively). Seven breast-fed infants showed cyclic AMP excretion of 3.6 ± 0.3 S.E.M. and 6.5 ± 0.3 S.E.M. on Day 1 and Day 3 respectively, showing an insignificant difference to the formula-fed infants. An additional 10 formula-fed infants 5 to 10 days of age excreted 7.0 ± 0.9 S.E.M. Nm/mg of creatinine which is comparable to the 3 to 4 days of age group. The threefold increase in cyclic AMP after the first day of life may possibly reflect increasing parathyroid renal responsiveness. One- and 3-day-old infants and adults were given a 1-hour parathyroid (PTH) infusion (5 U/kg/hr) with measurement of urinary cyclic AMP in time periods before and after the infusions. Peak increases in responses from baseline of cyclic AMP were 1.64 ± 0.34, 7.1 ± 1.5, and 36.0 ± 0.73 Nm/mg of creatinine M. ± range on first day, third day, and in adults respectively with similar relationships of increasing phosphate excretion. Thus, there is most likely a maturational renal proximal tubular responsiveness to PTH on the cellular cyclic AMP level.