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Low-field, high-gradient diffusion exchange spectroscopy (DEXSY) nuclear magnetic resonance (NMR) measurements on viable\n                  <jats:italic>ex vivo<\/jats:italic>\n                  neonatal mouse spinal cords show that water exchange is primarily passive. The apparent exchange rate constant (AXR) depends on osmotic conditions because it reflects multiple exchange pathways, each weighted by the exchanging compartments\u2019 volume fractions. A faster transmembrane path that becomes more visible with increasing extracellular space (ECS) fraction has a high activation energy but is ion-independent, suggesting passive transport through the phospholipid bilayer but not active or passive transport through co-transporter or channel proteins. A slower pathway which dominates when the extracelluar space shrinks has a low activation energy, consistent with geometric exchange between intracellular environments. Moreover, we show how DEXSY can be used to non-invasively measure tonicity in tissue, and inform us about the status of the tissue milieu. These findings may inform future translation to clinical MRI.\n                <\/jats:p>\n                <jats:sec>\n                  <jats:title>Significance<\/jats:title>\n                  <jats:p>We use advanced nuclear magnetic resonance methods to address two unanswered questions in cellular biology: How does water exchange between tissue microenvironments under steady-state conditions, and do these processes involve active water cycling?<\/jats:p>\n                <\/jats:sec>","DOI":"10.1101\/2024.12.12.628254","type":"posted-content","created":{"date-parts":[[2024,12,14]],"date-time":"2024-12-14T05:16:10Z","timestamp":1734153370000},"source":"Crossref","is-referenced-by-count":1,"title":["Steady-state water exchange in neural tissue is primarily passive and through the phospholipid bilayer"],"prefix":"10.64898","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-0221-9121","authenticated-orcid":false,"given":"Nathan H.","family":"Williamson","sequence":"first","affiliation":[{"name":"Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA"},{"name":"Military Traumatic Brain Injury Initiative (MTBI2), Bethesda, Maryland 20814, USA"},{"name":"Uniformed Services University of the Health Sciences (USU), Bethesda, Maryland 20814, USA"},{"name":"The Henry M. 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