Age influences therapeutic ultrasound effects: From increased inflammation in young to decreased inflammaging in adults
Authors: Dunham Chelsey , Frank Joseph 
Published: May 2024

BACKGROUND: Therapeutic ultrasound is a controversial topic in physical therapy because outcomes varied due to different experimental designs. At the cellular level, acoustic radiation force (i.e., pressure) from ultrasound previously showed activation of sodium/calcium channels, hyper-proliferation and increased cytokine signaling. However, few have studied the immune response after ultrasound despite the tight coupling between muscle inflammation and regeneration. Additionally, limited work has assessed ultrasound in aged populations. Therefore, a deeper understanding of how therapeutic ultrasound influences the muscle microenvironment is required to use it as a regenerative therapy. The aim of this study was to evaluate inflammatory cytokine signaling and immune cell infiltration after ultrasound in young and aging murine muscle. We hypothesized that the immune response will increase with ultrasound pressure but decrease with age. METHODS: Young (6-weeks-old) and Adult (52-weeks-old) male and female non-injured gastrocnemii were treated with a single sonication of either low intensity pulsed ultrasound (LIPUS) at ~0.243 MPa or pulsed focused ultrasound (pFUS) at ~5.96 MPa. After sonication, muscles were harvested at 1-, 8-, and 24-hours for cytokine analysis using an immunology multiplex assay (138 mice) and at 1- and 24-hours for flow cytometry (120 mice). Control mice were not treated. Sexes were combined due to minimal, if any, differences. Statistical analyses were performed using an ANOVA with post-hoc Bonferroni comparisons ( p < 0.05); only differences compared to control are described. RESULTS: In Young post-LIPUS, IL-15 was upregulated at 1-hour and likely caused the increased M1 and M2 macrophages (M2 > M1) at 1-hour. In Young post-pFUS, the upregulation of VEGF at 1-hour was counteracted by MIG at 8-hours resulting in no change in endothelial cells. Increased MCP-1 at 24-hours post-pFUS corresponded with increased neutrophils, monocytes, and M1 and M2 macrophages (M1 > M2) at 24-hours. In Adult, cytokine expression throughout the 24-hour period was downregulated after LIPUS (IL-9/-15) and pFUS (IL-1α/-2/-6/-9, M-CSF, MIP-1α). Thus, there were no changes in the immune cell response over time post-LIPUS or -pFUS. Additionally, for most of these cytokines, Adult control expression was at least two times greater than Young control. CONCLUSIONS: In Young, LIPUS caused an early, temporary pro- and anti-inflammatory response while pFUS caused a late, extended pro-inflammatory response. However, this response was lost in Adult where age-induced inflammaging was decreased after sonication. In younger muscle, ultrasound could enhance regeneration by augmenting the immune response while in aging muscle it could be used to decrease inflammation induced atrophy. Overall, therapeutic ultrasound caused a pressure dependent inflammatory response that can increase or decrease intrinsic signaling and cell recruitment in adolescent or aged muscles, respectively. This work was funded by the Intramural Research Program of the National Institutes of Health, Clinical Center and National Institutes of Biomedical Imaging and Bioengineering. 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.