The effect of initial arteriolar transmural pressure on vasodilation at the onset of muscle contraction
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Muscle contraction initiates a rapid vasodilation (widening) of small blood vessels in the skeletal muscle called arterioles, increasing muscle blood flow, which is greater when the contracting muscle is below, compared to above heart level. The mechanisms responsible for this effect of arm position on blood flow are unclear. Below the heart, blood pressure is larger in the arterial system due to gravity (called transmural pressure). Arterioles vasodilate in response to a reduction in transmural pressure, termed myogenic vasodilation. During muscle contraction, transmural pressure is reduced. This thesis tested the hypothesis that there is an increased myogenic vasodilation and corresponding blood flow after release of forearm contraction if precontraction transmural pressure was higher, as it is below vs. above heart level. To test this hypothesis, participants laid supine with their arm placed ~20cm either above (low transmural pressure condition; L) or below (high transmural pressure condition; H) heart level. During a 2s forearm contraction of a handheld force transducer, the arm position was either maintained or changed to the opposite limb position. This was repeated for all four possible starting-ending position combinations (L-L, H-L, L-H, H-H). Comparing different starting position effects by measuring the response in the same ending position ensured that only differences in active vasodilation could contribute to differences in post-contraction blood flow. As expected, postcontraction blood flow response was greater when measured below vs. above the heart. However, within the same ending position, starting arm position did not affect the magnitude of blood flow during either the first or peak cardiac cycle post-contraction (i.e. LL vs. HL, or LH vs. HH; P = 1.00). Similarly, there were no differences in dilation of the arterioles (calculated as; forearm vascular conductance = blood flow ÷ local forearm arterial blood pressure). These findings REFUTE the hypothesis that a greater transmural pressure environment upon contraction would result in increased vasodilation. Secondary analysis determined that the higher post-contraction blood flow below vs. above the heart could be explained by greater local arterial blood pressure and/or greater mechanical distension of arterioles due to greater transmural pressure below the heart.