Surfactant-Associated Protein B Kinetics In Vivo in Newborn Infants by Stable Isotopes

Abstract

Surfactant-associated protein B (SP-B) is critical to the biophysical function of pulmonary surfactant. No information is available on SP-B synthesis and kinetics in humans. We administered a 24-h i.v. infusion of 13 C-valine as metabolic precursor of SP-B to six newborn infants (weight 3.5 Ϯ 0.5 kg; age 12 d, range 1-43 d). Three of the study infants also received i.v. 2 H-palmitate to label surfactant disaturated phosphatidylcholine (DSPC). SP-B and DSPC were isolated from tracheal aspirates, and their respective 13 C and 2 H enrichments were measured by gas chromatography-mass spectrometry. SP-B kinetics was measured successfully in all six infants. SP-B median (range) fractional synthesis rate was 30% per day (20 -78% per day), secretion time was 4.5 h (1-9 h), time to peak was 24 h (12-36 h), and half-life was 21 h (8 -35 h). The ascending part of the SP-B kinetic curve was similar to the DSPC curve, suggesting similar secretion pathways. SP-B half-life seemed to be shorter than DSPC half-life. These results agree with existing animal data. We conclude that the measurement of SP-B kinetics is feasible in vivo in humans by stable isotope technology. Surfactant-associated proteins B and C (SP-B and SP-C, respectively) represent Ͻ5% of surfactant by weight, but they play a pivotal role in maintaining surfactant function (1). Surfactant deficiency is the hallmark of respiratory distress syndrome in preterm infants. In respiratory distress syndrome, there is a reduced amount of phospholipids (2) and of SP-B and SP-C (3). The key role of SP-B became evident in congenital SP-B deficiency, which is characterized by severe respiratory failure leading ultimately to death unless lung transplant is performed (1,4). To date, there are no data on SP-B kinetics in humans, but information is available in adult and newborn animals, in which radioactive labels can be used. These studies showed differences in SP-B clearance and kinetics depending on animal species and postnatal age (5-7).We recently described methods based on stable isotopes to measure surfactant disaturated-phosphatidylcholine (DSPC) kinetics in humans (8,9). The main objective of this study was to demonstrate the feasibility of measuring SP-B kinetics in vivo in newborn infants by means of stable isotopes. (Table 1) who were admitted to the neonatal intensive care unit, Department of Paediatrics, University of Padua. Inclusion criteria were 1) gestational age Ͼ37 wk, 2) respiratory failure requiring endotracheal intubation for an estimated length of at least 48 h, 3) arterial and venous lines placed for clinical monitoring, and 4) written parental consent. Exclusion criteria were congenital chromosomal abnormalities and exogenous surfactant given at Ͻ48 h before study start. The local Ethics Committee approved the study protocol. METHODS SP-B kinetics was studied in six infantsAll patients received a 24-h constant i.v. infusion of 2 mg/kg/h 1-13 C valine (Mass Trace, Woburn, MA), dissolved in normal saline, while they were on a lipid-free i.v. infusion. In ...

Pilot trial of late booster doses of surfactant for ventilated premature infants

“…The transient increase in SP-B content of surfactant after each Infasurf dose (return to baseline by 3 days) is consistent with the reported turnover kinetics observed for endogenous SP-B in stable isotope studies. 37 These observations support the concept that booster surfactant doses should be administered every 2 to 3 days to maintain sufficient SP-B content. This approach increases the alveolar load of surfactant lipid; however, clearance is apparently rapid, as we did not observe increased recovery of surfactant PL in TAF 1 day after a booster dose.…”

Impaired surfactant protein B synthesis in infants with congenital diaphragmatic hernia

“…Isolation of SP-B from tracheal aspirates SP-B was isolated from tracheal aspirates and hydrolysed as previously described [20]. Individual amino acids were derivatised into their N-acetyl-n-propyl derivatives [21] and the 13 C enrichment of leucine was measured by mass spectrometry (GC-MS; Agilent, Milan, Italy).…”

“…The 2 H enrichment of palmitate-DSPC was analysed by gas chromatography-isotope ratio mass spectrometer (IRMS) and expressed in Dø after correction for isotopic contribution of the derivative group [20]. Urine deuterium enrichment was analysed by a high-temperature conversion elemental analyser coupled with an IRMS (TC-EA-IRMS; Thermo Scientific, Bremen, Germany) [20].…”

“…Urine deuterium enrichment was analysed by a high-temperature conversion elemental analyser coupled with an IRMS (TC-EA-IRMS; Thermo Scientific, Bremen, Germany) [20].…”

“…Fractional synthesis rate was calculated from the linear portion of the enrichment curve rise over time relative to the plateau enrichment of the respective precursors [19,20].…”