No differences in the erythropoietin response to acute high-intensity exercise when duration is doubled, despite augmented post-exercise plasma volume expansion

The increase in plasma volume ∼24 h post-exercise may act as an erythropoietic signal; however, the responsiveness of this mechanism 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 erythropoietic signalling and plasma volume expansion. On separate days, 16 healthy, recreationally active participants (<i>n</i> = 8 males; <i>n</i> = 8 females) performed four (4 × 4 HIIT) or eight intervals (8 × 4 HIIT) consisting of 4 min at 105% critical power with 3 min recovery. Venous blood samples were collected before, immediately after, and 24 h after each HIIT session to measure hemoglobin concentration and hematocrit to calculate plasma volume changes. Erythropoietic and plasma volume regulating hormone concentrations were measured using ELISA kits. Circulating erythropoietin mass was elevated 24 h after HIIT (4 × 4: Pre vs. 4 h post: 22.2 ± 13.0 vs. 24.2 ± 13.6 IU; 8 × 4: 23.6 ± 14.6 vs. 24.9 ± 14.4 IU; <i>p</i> < 0.05) with no difference between protocols (<i>p</i> > 0.05). Plasma volume decreased immediately after both protocols (4 × 4: -4.4 ± 3.5%, <i>p</i> < 0.05; 8 × 4: -4.4 ± 3.6%, <i>p</i> < 0.05) but was only significantly elevated above baseline 24 h after the 8 × 4 protocol (4 × 4: +1.1 ± 7.1%, <i>p</i> > 0.05; 8 × 4: +5.6 ± 4.6%, <i>p</i> < 0.05). Aldosterone concentration was elevated post-exercise after both protocols (4 × 4: Pre vs. 0 h post: 295 ± 151 vs. 570 ± 271 pg/mL; 8 × 4: 335 ± 235 vs. 854 ± 566 pg/mL), but the 8 × 4 protocol caused a larger increase (<i>p</i> < 0.05). Post-exercise hypervolemia depends on exercise duration, but hypervolemia is not required for increases in circulating erythropoietin, challenging the importance of this mechanism for endurance training-induced increases in hemoglobin mass and oxygen-carrying capacity.