Greater post-contraction hyperaemia below vs. above heart level: the role of active vasodilatation vs. passive mechanical distension of arterioles.

Key Points: The immediate increase in skeletal muscle blood flow following contraction is greater when the contracting muscle is below vs. above heart level. This has been attributed to muscle pump-mediated venous emptying and subsequent widening of the arterial to venous pressure gradient, which can occur below but not above heart level. However, alternative explanations could include greater rapid onset vasodilatation and/or transmural pressure-mediated mechanical distension of resistance vessels, but these remain unexplored. We demonstrate that active vasodilatation is not responsible for greater post-contraction hyperaemia below the heart. Instead, an increased transmural pressure-mediated mechanical distension of resistance vessels is a key mechanism responsible for this phenomenon. Our findings establish the importance of considering/accounting for local mechanical arteriolar distension effects when investigating exercise hyperaemia. They also inform the application of exercise for rehabilitative purposes and prompt investigation into whether arteriolar distension accompanying vasodilatation is reduced with diseases or ageing, thereby compromising exercising muscle perfusion.

Abstract: We tested the hypotheses that increased post-contraction hyperaemia in higher (H; below heart) vs. lower (L; above heart) transmural pressure conditions is due to (1) greater active vasodilatation or (2) greater transmural pressure-mediated arteriolar distension. Participants (n = 20, 12 male, 8 female; combined mean age 24.5 ± 2 years) performed a 2 s isometric handgrip contraction, where arm position was maintained within or changed between H and L during contraction, resulting in four starting-finishing arm position conditions (LL, HL, LH, HH). Post-contraction forearm blood flow (echo and Doppler ultrasound) was higher with contraction release in H vs. L environments (P < 0.05). However, contraction initiated in H did not result in greater vasodilatation (forearm vascular conductance; FVC) than contraction initiated in L, regardless of contraction release condition (peak FVC: LL 217 ± 104 vs. HL 204 ± 92 ml min (100 mmHg) , P = 0.313, LH 229 ± 8 vs. HH 225 ± 85 ml min (100 mmHg) , P = 0.391; first post-contraction cardiac cycle FVC: same comparisons, both P = 0.317). However, FVC of the first post-contraction cardiac cycle was greater for contractions released in H vs. L regardless of pre-contraction condition (LL 106 ± 67 vs. LH 152 ± 76 ml min (100 mmHg) , P < 0.05; HL 80 ± 51 vs. HH 119 ± 58 ml min (100 mmHg) , P < 0.05). These findings refute the hypothesis that greater hyperaemia following a single contraction in higher transmural pressure conditions is due to greater active vasodilatation. Instead, our findings reveal a key role for increased transmural pressure-mediated mechanical distension of arterioles in creating a greater increase in vascular conductance for a given active vasodilatation following skeletal muscle contraction.