Characterization of pulsatile secretion and clearance of plasma cortisol in premature and term neonates using deconvolution analysis

Pulsatile secretion of cortisol (F) has not been documented in the newborn infant. Using repeated blood sampling and deconvolution analysis, we investigated F secretion and elimination dynamics in a group of five premature (gestational age, 24-34 weeks) and five term neonates. These infants had required placement of an umbilical arterial cannula for monitoring respiratory status, but were otherwise clinically stable. Blood samples were obtained at 15-min intervals for a 6-h period. All plasma F determinations were 58 nmol/L (2.1 μg/dL) or more, and pulsatile F secretion was observed in all infants. No significant differences were noted between the two groups with regard to 6-h mean plasma F concentration [350 ± 129 (premature) vs. 277 ± 54 nmol/L (term)], plasma corticosteroid-binding globulin (14 ± 0 vs. 13 ± 1 mg/L), F secretory burst frequency (4 ± 0 vs. 5 ± 1 bursts/6 h), mass of F secreted per burst [760 ± 480 vs. 310 ± 100 nmol/L_v [L_v, liter of F distribution volume)], F production rate (FPR; 2.7 ± 1.4 vs. 1.1 ± 0.2 μmol/L_v·6 h), or plasma F half-life (45 ± 6 vs. 56 ± 4 min). However, the premature infants had a significantly longer F secretory burst half-duration (63 ± 18 vs. 6.7 ± 4.0 min; P < 0.01) and a significantly lower maximal F secretory rate (9.4 ± 3.4 vs. 100 ± 26 nmol/L_v·min; P < 0.02) than the term infants. Body surface area and body weight were inversely correlated with F secretory burst half-duration (r = -0.74 and -0.75, respectively); both were also positively correlated with the maximal F secretory rate (r = 0.66 and 0.72). The two most premature infants had significantly greater mean plasma F and FPR than the other three premature and all of the term infants. Extrapolating to 24 h and correcting for the distribution volume of F and body surface area, we estimate FPR to be approximately 17-24 μmol/m²·24 h (6.6-8.8 mg/m²·24 h) for newborn infants of 34 weeks or more gestational age. These values are consistent with newer estimates of FPR in older children and adults determined using either deconvolution analysis or stable isotope dilution methods.