Acute isovolemic hemodilution worsens spinal tissue oxygenation during hypoxemia in anesthetized rats

Acute intermittent hypoxia (AIH) induces a type of spinal plasticity that arises from a dynamic interplay between competing serotonin- and adenosine-driven mechanisms. Adenosine can support or inhibit plasticity, and its direction of action is tightly linked to local spinal tissue oxygenation (PtO 2 ), dictating if plasticity is expressed, blunted, or cancelled. This relationship, therefore, positions PtO 2 as a “physiological gatekeeper” that determines the operational range of these competing cellular pathways. Individuals with reduced arterial oxygen content (CaO 2 ) and spinal cord oxygen delivery experience AIH in a disadvantaged state that would cause greater hypoxemia-evoked PtO 2 decreases and augmented adenosine signaling at the same arterial partial pressure of oxygen (PaO 2 ). Such shifts in the serotonin/adenosine balance offer a plausible explanation for the substantial low-responder rate to therapeutic AIH in humans (~35-40%). Since the quantitative impact of anemia and decreased CaO 2 on PtO 2 in cervical vs thoracic segments has not been explored, we determined how reductions in CaO 2 achieved through controlled isovolemic hemodilution alter PtO 2 in these segments. We hypothesized that decreased CaO 2 would: 1) reduce baseline spinal PtO 2 and exacerbate hypoxemia-evoked deoxygenation, and 2) reveal differences in tissue hypoxia between cervical vs thoracic spinal segments. Urethane-anesthetized, vagotomized, paralyzed, and ventilated male Sprague–Dawley rats (n = 7; 3-5 months) underwent continuous physiological monitoring with arterial pressure clamped via phenylephrine. PtO 2 was measured using micro-optodes at cervical (C4) and thoracic (T10) sites. Moderate hypoxemia (PaO 2 40–50 mmHg) was delivered before and after isovolemic hemodilution (20–25% blood volume exchange with prewarmed 5% albumin). PaO 2 and hemoglobin measurements were used to calculate CaO 2 . Isovolemic hemodilution reduced hemoglobin concentration as expected (pre: 15.6±0.8 g/dL, post: 9±0.6 g/dL, P < 0.001) while maintaining arterial oxygen saturation levels (pre: 99.5±0.5%, post: 99.4±0.5%, P = 0.91). Despite similar saturation levels, CaO 2 was substantially reduced (pre: 21.6±1.3 mL O 2 /dL, post: 13±1.3 mL O 2 /dL, P < 0.001). Clamping arterial pressure to pre-hemodilution levels did not prevent decreases in spinal PtO 2 with CaO 2 reduction; baseline PtO 2 fell at both recording sites (cervical: 58±13 vs 37±13 mmHg, P < 0.001; thoracic: 52±12 vs. 29±14 mmHg, P < 0.001). Despite matched PaO 2 levels (pre-HD hypoxemia: 43±3; post-HD hypoxia: 43±3 mmHg), hypoxemia post-hemodilution produced segment-specific PtO 2 reductions vs the pre-hemodilution values (cervical: 17±9 vs 7±5 mmHg, P = 0.03; thoracic: 11±5 vs. 4±3 mmHg, P = 0.005). Hemodilution at baseline produced a larger PtO 2 reduction in cervical (−21±17 mmHg) vs thoracic segments (−13±32 mmHg), representing a ~62% greater decline and heightened cervical vulnerability to anemia. However, with combined hypoxemia and hemodilution, both segments converged to similar PtO 2 values (cervical: −51±14 mmHg; thoracic: −48±11 mmHg), suggesting that, once oxygenation reserves are sufficiently compromised, maximal deoxygenation occurs at both spinal levels. These observations advance mechanistic understanding of AIH-induced plasticity and may help explain a high prevalence of low-responders in the human population. Individual factors such as hemoglobin and PaO 2 levels should be considered when refining or adjusting AIH protocols to improve therapeutic efficacy. Funding: NIH R01HL149800 and HL147554; MA supported by UF McKnight Brain Institute Gator NeuroScholar Program This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.