C20-09 Respiration-circulation Micromechanics at the Capillary Level Across Lifespan in Mice and Under Positive and Negative Pressure Ventilation

Abstract Rationale The mechanical, biological, and immunological properties of the lung undergo progressive remodeling throughout the lifespan and in positive- vs negative-pressure ventilation. Although histology provides static snapshots of structural changes, quantifying age-related changes in the lung’s dynamic response to mechanical stimuli, particularly in a functioning lung, remains elusive. Capillary and alveolar distention – how the diameter of capillaries and alveoli change in response to pressure – are two such dynamic processes. To overcome these limitations, we utilize the crystal ribcage (Nature Methods, 2023) to image functional mouse lungs at a range of biologically relevant vascular and transpulmonary pressures. We directly quantify capillary and alveolar distention as the pulmonary pressure conditions are precisely adjusted, combining these experimental findings with mathematical modeling to estimate the axial and radial Young’s moduli of the intact capillaries. Additionally, we probe the effect of positive (mechanical ventilation) versus negative (spontaneous breathing) pressure ventilation at the capillary scale. Together, this data offers a valuable measure of age-related and ventilation mode-dependent mechanical changes in respiration-circulation coupling, with direct impact on trafficking and activities of circulating cells. Methods C57BL/6 mice are categorized by age as postnatal (7 days), young (12-18 weeks), and aged (20+ months). Lungs are resected and imaged using a crystal ribcage (Fig. 1A). Evans Blue dye is perfused to label the vascular lumen (Fig. 1B). 3D image stacks are acquired at each pressure via fluorescent confocal microscopy, and alveolar and septal diameters are quantified using custom-written Python scripts. Mathematical modeling calculates the axial and radial Young’s modulus components of the vessels. Results Postnatal vessels distend less than those in older counterparts in response to changes in vascular and transpulmonary pressures (Fig. 1C). Subsequent mathematical modeling quantifies the radial and axial Young’s moduli for vessels, with postnatal lungs significantly stiffer than aged counterparts (Fig. 1D). Alveoli responded similarly to both positive- and negative-pressure ventilation (Fig. 1E(i)). However, we find that increased positive transpulmonary pressure constricted capillaries more so than did negative-pressure increases (Fig. 1E(ii)), with notable reductions across the 9 cmH2O pressure regime. Conclusions Using the crystal ribcage, we capture how the micromechanical response to alveolar and vascular pressures varies across the murine lifespan at the capillary and alveolar scale. We find lung micromechanics vary with age and postnatal capillaries and septa are stiffest radially and axially. Capillary lumens are less responsive under negative pressure compared to positive pressure, potentially implying heightened compression under positive pressure. This abstract is funded by: DP2HL168562, Beckman Young Investigator Award, NSF CAREER Award, Hevolution/AFAR New Investigator Award