M.R. Ely1, S.M. Ratchford2, H.L. Clifton2, D.T. La Salle2, J.D. Trinity2, D.W. Wray2, J.R. Halliwill1FACSM.

1University of Oregon. Eugene, OR; 2University of Utah, Salt Lake City, UT.

Histamine mediated vasodilator pathways elevate microcirculatory blood flow during inflammation and immune responses as well as skeletal muscle blood flow following endurance exercises. During exercise, intramuscular histamine concentration increases, however, the contribution of histamine to exercise hyperemia is unknown. The production of intramuscular histamine is reported to be positively correlated with exercise intensity and duration. Therefore, elevations in intramuscular histamine may contribute to skeletal muscle blood flow as exercise increases in intensity and duration. PURPOSE: To compare limb blood flow during a ramped increase in exercise intensity before and after prolonged exercise under normal conditions and when histamine signaling is blocked. It was hypothesized that H1/H2antihistamines would decrease limb blood flow and the effect would be greater at high exercise intensities and following prolonged exercise. METHODS: Sixteen (7F) volunteers performed unilateral knee-extension exercise after consuming either Placebo or histamine (H1/H2) receptor antagonists (Blockade). The exercise consisted of two incremental ramp protocols at 20, 40, 60, and 80% of peak work rate, which were separated by 60 min of knee-extension exercise (60% of peak). Femoral artery blood flow (Logiq e9, GE Medical Systems) was measured during each exercise intensity and every 10 min during the 60 min of prolonged exercise. Data were analyzed with a 3-way RM ANOVA and are presented as Means±SEM. RESULTS: Femoral artery blood flow increased with exercise intensity from 1676±57, 2008±68, 2272±75, to 2660±97 ml/min at 20, 40, 60 to 80% of peak work rate during Placebo (P<0.05). Blood flow was further elevated with Blockade 1750±79, 2155±88, 2542±88, to 2836±124 ml/min (P<0.05) but was not different before versus after the 60 min exercise bout (P=0.129). Femoral blood flow was constant over the 60 min of knee-extension exercise in both Placebo (2533±43 ml/min) and Blockade (2823±32 ml/min) but was significantly elevated with Blockade over Placebo (+291±26 ml/min)(P<0.05). CONCLUSION: Contrary to the hypothesis, these results suggest that blocking histamine’s actions during exercise, regardless of the intensity or duration, increased skeletal muscle blood flow.

Support provided by The Eugene & Clarissa Evonuk Memorial Graduate Fellowship.

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