Session duration affects the magnitude of post-exercise hypervolemia but not the erythropoietin response to acute high-intensity interval exercise

The increase in plasma volume ~24 hours post-exercise may act as an erythropoietic signal, but this mechanism's responsiveness to different exercise prescription variables is poorly understood. The purpose of this study was to determine the impact of high-intensity interval exercise duration on plasma volume and related responses. On separate days, 16 healthy, recreationally active participants (n=8 males; n=8 females) performed four (4x4) or eight intervals (8x4) consisting of 4 min at 105% critical power with 3 min recovery. Venous blood samples collected before, immediately after, and 24 hours after each HIIT session were used to measure hemoglobin concentration and hematocrit to calculate plasma volume changes. Erythropoietic and plasma volume regulating hormone concentrations were measured using ELISA kits. Plasma volume decreased immediately after both protocols (4x4: -4.4 ±3.5%, p<0.05; 8x4: -4.4 ±3.6%, p<0.05) but was only significantly elevated above baseline 24 hours after the 8x4 protocol (4x4: +1.0 ±7.1%, p>0.05; 8x4: +5.6 ±4.6%, p<0.05). Erythropoietin concentration ([EPO]) was higher than baseline 24 hours after the HIIT protocols (4x4: Pre vs 24 h post: 6.5 ±3.1 vs. 7.1 ±3.3 mIU/mL; 8x4: 6.9 ±3.7 vs. 7.3 ±3.7 mIU/mL; main effect of time, p<0.05) with no difference between protocols (p>0.05). [Aldosterone] was elevated immediately post-exercise after both protocols (4x4: Pre vs 0 h post: 295 ±151 vs. 544 ±259 pg/mL; 8x4: 335 ±235 vs. 821 ±553 pg/mL), but the 8x4 protocol caused a larger increase (interaction effect, p<0.05). That post-exercise hypervolemia may be exercise duration-dependent but is not required for increases in circulating EPO has important implications for endurance training aiming to increase oxygen delivery to active tissues.