Optimal temperature for whole-body hypothermia in the newborn: An in vitro study using foreskin mitochondrial oxygen consumption

OBJECTIVES: Whole-body hypothermia (to 33.5 ± 0.5°C) is a therapeutic modality that reduces risks of death and neurodevelopmental disability in neonates subjected to hypoxic-ischemic insults. This in vitro study was designed to determine changes in neonatal cellular metabolism with temperature. Its main aim was to compare the metabolic rate at ≤33°C with that at ≥35°C. STUDY DESIGN: Foreskin specimens were used as a source of neonatal tissue. Cellular respiration (mitochondrial O₂ consumption) was used as a surrogate biomarker for the metabolic rate. Foreskin specimens from healthy newborns were collected immediately after circumcision and processed within one hour for measuring the rate of O₂ consumption at various temperatures (±0.5°C). O₂ consumption was determined as function of time from the phosphorescence decay of Pd (II) meso-tetra-(4-sulfonatophenyl)-tetrabenzoporphyrin. RESULTS: In a vial sealed from air and containing foreskin specimen in phosphate-buffered saline supplemented with 5mM glucose, [O₂] decreased linearly with time, confirming its zero-order kinetics. The rate of O₂ consumption (μM O₂.min^-1), thus, was the negative of the slope of [O₂] vs. time. Cyanide inhibited O₂ consumption, confirming the oxidation occurred in the respiratory chain. Cellular respiration at ≤33°C (n = 25) significantly differed from that at ≥35°C (n = 24), p < 0.001. The rate (μM O₂.min^-1.mg^-1) at 25°C was 0.034 ± 0.006 (n = 11, p = 0.044), at 33°C was 0.029 ± 0.008 (n = 14, reference temperature), at 35°C was 0.062 ± 0.020 (2-fold higher, n = 18, p < 0.001), and at 37°C was 0.061 ± 0.009 (2-fold higher, n =6, p < 0.001). CONCLUSIONS: Neonatal foreskin cellular respiration is highly sensitive to critical temperatures (33°C vs. 35°C).