Pulmonary alterations induced by intermittent hypoxia in castrated C57BL/6J male mice

Abstract

Sleep apnea (SA) is characterized by airway obstructions leading to intermittent hypoxia (IH). In addition to inducing physiological disturbances, IH affects the mechanical properties of the airways such as increasing resistance, and there are growing concerns that SA might worsen pulmonary pathologies. In men, SA patients have less circulating testosterone than the rest of the population and the severity of SA in overweight patient is negatively correlated with testosterone levels. We tested the hypothesis that testosterone could modulate the pulmonary responses to IH. For this, we used intact or castrated male mice (C57BL/6J) exposed to IH (14 days, 12h/day, 10cycles/h, 30s 6% oxygen nadir) or to normoxia (14 days in room air). Freely behaving mice were tested in the whole body plethysmography to measure minute ventilation (Ve), oxygen consumption (VO2) and oxygen extraction coefficient (OEF = VO2 / [Ve × FiO2]) under resting conditions in normoxia. At the end of the recordings, mice were anesthetized, tracheotomized, connected to a flexiVent and paralyzed to be ventilated mechanically and to assess respiratory mechanics. We used selected protocols to measure respiratory system resistance (Rrs), Newtonian resistance (Rn: which is a surrogate for the resistance of the large airways) and tissue resistance (G). Castrated mice exposed to IH had an increased Ve (1840±620 µl.min-1.g-1) for the same oxygen consumption (51.4±11.4 µl.min-1 g-1) and a tendency toward reduction of the oxygen extraction coefficient (0.14±0.02) compared to intact mice exposed to IH (Ve=1130±610 µl.min-1.g-1, VO2=49.9±6.6 µl.min-1.g-1, OEF=0.28±0.17) or normoxia (Ve=940±250 µl.min-1.g-1, VO2=43.1±12.9 µl.min-1.g-1, OEF=0.26±0.09) and castrated mice exposed to normoxia (Ve= 1100±280 µl.min-1.g-1 VO2=53.7±7.6 µl.min-1/.g-1 OEF=0.25±0.09). In addition, castration increases Rrs in normoxia and in IH (1.17±0.22 cmH2O.s.ml-1) compared to intact mice (0.94±0.08 cmH2O.s.ml-1). However, the mechanism underlying this increase in Rrs differs depending on the ambient oxygen level. In normoxia, there was an increase in Rn (0.37±0.09 cmH2O.s.ml-1) compared to other groups (intact in normoxia: 0.26±0.04 cmH2O.s.ml-1 or IH: 0.23±0.07 cmH2O.s.ml-1 and castrated in IH: 0.23±0.06 cmH2O.s.ml-1), while in IH there was a non-significant tendency toward an increase in G (9.16±2.50 cmH2O.ml-1) compared to other groups (intact in IH: 7.26±1.52 cmH2O.s.ml-1 or in normoxia: 7.31±1.29 cmH2O.s.ml-1 and castrated in normoxia: 8.06±2.26 cmH2O.s.ml-1). These preliminary data demonstrate that pulmonary changes in mice exposed to IH are enhanced by castration in male mice, and that testosterone deficiency in sleep apnea patients might worsen the pulmonary consequences.