Breathing and Respiratory Muscle Function in Senescent Wild‐type and Ts65Dn Male Mice

Abstract

Down Syndrome (DS) results from trisomy 21 and is the most common cause of mental retardation in the US. People with DS display reduced muscle tone and cardiorespiratory complications that may contribute to breathing disturbances. Ts65Dn (Ts) mice, a model of DS, exhibit increased apnea frequency at 3 months of age (7±9 vs. 36±28; wild‐type (WT) vs. Ts) that persists at 6 (19± 7 vs. 77±58) and 12 months (5±6 vs. 64±77; p<0.001 for age, strain, and age by strain). People with DS have increased life expectancy and it is unknown how breathing changes across the lifespan in this population. We tested the hypothesis that ~20‐month‐old Ts mice would have altered ventilation compared to age‐matched WT mice. To test this hypothesis, unrestrained barometric plethysmography was used to quantify breathing frequency (breaths/min), tidal volume (VT; mL), VT/inspiratory time (VT/Ti; mL/sec), minute ventilation (VE; mL/min) and ventilatory equivalent for CO2 (VE/VCO2) in 18 to 22‐month‐old male WT (n=12) and Ts (n=13) mice. Data Sciences International Ponemah software was used to analyze flow tracings during exposure to air (20.93% O2, balanced N2). Data are expressed as mean±S.D., WT vs. Ts. A main effect of strain was uncovered using MANOVA (p=0.033). Breathing frequency (171±33 vs. 136±41 breaths/min; p=0.025) and VE (74.6±25.7 vs. 50.3±15.5 mL/min; p=0.010) approached the a priori cutoff (Bonferroni correction, p<0.01). VT (0.439±0.112 vs. 0.377±0.075 mL/breath; p=0.123) and the ratio of VE/VCO2 (45.5 ±22.0 vs. 42.4±19.8; p=0.715) were not different. VT/Ti (3.65±1.03 vs. 2.49±0.759 mL/sec; p=0.004) was reduced in Ts vs. WT, which suggests Ts mice have a reduced ventilatory drive. Since people with DS demonstrate evidence of muscle weakness, we examined diaphragm (DIA) and sternohyoid (SH; representative pharyngeal dilator) muscle function in WT (DIA n=10, SH n=9) and Ts (DIA n=7, SH n=7) mice. Twitch (2.2±1.1 vs 2.3±1.0 N/cm2; p=0.835) and tetanic (11.2±7.3 vs. 11.6±4.8 N/cm2; p=0.900) force, and the force frequency‐relationship (p=.766) for SH muscle were equivalent between groups. Similarly, no differences were observed for DIA twitch (6.0±2.7 vs. 5.2±2.1 N/cm2; p=0.438) and tetanic (20.7±6.0 vs. 19.9±2.8 N/cm2; p=0.758) force, or the force‐frequency relationship (p=0.339). Diaphragm function was preserved in aged Ts mice which is in line with similar VT between groups. These data reveal the increased propensity for apnea in Ts mice is independent of upper airway muscle weakness and may alternatively be of neural origin. Current studies are examining motor control of the upper airway in Ts mice.Support or Funding InformationThis work was funded by 1R15HD076379 (LRD), 3R15HD076379 (LRD). Additional support was granted by the Le Moyne College McDevitt Center Undergraduate Research Fellowship in Natural Science (MS‐N, MR), and the Le Moyne College Biological Sciences Department.