No relation between increases of spleen volume and reductions in circulating pro-inflammatory microparticles following fourteen days of apnea and hypoxia exposures

INTRODUCTION: Indigenous breath-hold diving populations have comparatively large spleen volumes, suggesting a functional benefit of a larger spleen in the context of diving. The spleen plays a role in clearing blood-borne microparticles (MPs). Pro-inflammatory MPs are increased with diving and have been associated with the etiology of decompression sickness. Thus, a potential benefit of a larger spleen may be enhanced splenic clearance of MPs. Repeated apneas and hypoxia exposure increases spleen volume in people without breath-hold diving ancestry. PURPOSE: Test the hypothesis that repeated apnea and hypoxia exposures reduce blood-borne pro-inflammatory MPs and the reduction is correlated with changes in spleen size. Methods: 20 healthy adults (10 women) completed 10 maximal apneas and 3 h of exposure to normobaric hypoxia (F i O 2 =0.14) for 14 days within a 16-day period. On Days 1 and 14, pre-exposure spleen volume was measured via ultrasound using the Pilström equation and venous blood was collected for blood-borne pro-inflammatory MPs analysis via flow cytometry. MPs data were log transformed for analyses. Data are presented as mean ± SD. Results: Average apnea duration increased from Day 1 (76±31 s) to Day 14 (91±30 s, p=0.004, d z =0.48). Pre-exposure spleen volume increased from Day 1 (169±36 ml) to Day 14 (182±39 ml, p=0.002, d z =0.33). Total MPs decreased from Day 1 (1407±814 #/μl) to Day 14 (1092±561#/μl, p=0.047, d z =0.45). MPs expressing CD66b+ (neutrophil origin; 194±89 vs. 156±72 #/μl, p=0.014, d z =0.47) and CD146 (endothelial origin; 337±197 vs. 250±116 #/μl, p=0.037, d z =0.53) were lower on Day 14, but MPs expressing CD41a (platelet origin) did not differ between Days 1 (82±78 #/μl) and 14 (55±38 #/μl, p=0.166, d z =0.45). Changes in MP counts from Day 1 to Day 14 were not correlated with changes in spleen volume for Total MPs (r=0.02, p=0.93) nor MPs expressing CD66b+ (r=0.30, p=0.23), CD146 (r=-0.04, p=0.86), or CD41a (r=0.31, p=0.20). CONCLUSION: Repeated apnea and hypoxia exposures increase spleen volume and reduce blood-borne pro-inflammatory MPs. However, there was no relation between exposure-induced changes in spleen size and MP counts. Thus, spleen size may not directly impact MP clearance. Rather, alterations in the MP phagocytic properties found within the spleen or another systemic change likely contribute to the observed reductions in MPs. Further work is needed to understand the role of splenic function in these observations. Support: Offce of Naval Research (N00014-20-1-2593). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.