Evidence for a rapid vasodilatory contribution to immediate hyperemia in rest-to-mild and mild-to-moderate forearm exercise transitions in humans

Controversy exists regarding the contribution of a rapid vasodilatory mechanism(s) to immediate exercise hyperemia. Previous in vivo investigations have exclusively examined rest-to-exercise (R-E) transitions where both the muscle pump and early vasodilator mechanisms may be activated. To isolate vasodilatory onset, the present study investigated the onset of exercise hyperemia in an exercise-to-exercise (E-E) transition, where no further increase in muscle pump contribution would occur. Eleven subjects lay supine and performed a step increase from rest to 3 min of mild (10% maximal voluntary contraction), rhythmic, dynamic forearm handgrip exercise, followed by a further step to moderate exercise (20% maximal voluntary contraction) in each of arm above ( condition A) or below ( condition B) heart level. Beat-by-beat measures of brachial arterial blood flow (Doppler ultrasound) and blood pressure (arterial tonometry) were performed. We observed an immediate increase in forearm vascular conductance in E-E transitions, and the magnitude of this increase matched that of the R-E transitions within each of the arm positions ( condition A: E-E, 52.8 ± 10.7 vs. R-E, 60.3 ± 11.7 ml·min−1·100 mmHg−1, P = 0.66; condition B: E-E, 43.2 ± 12.8 vs. R-E, 33.9 ± 8.2 ml·min−1·100 mmHg−1, P = 0.52). Furthermore, changes in forearm vascular conductance were identical between R-E and E-E transitions over the first nine contraction-relaxation cycles in condition A. The immediate and identical increase in forearm vascular conductance in R-E and E-E transitions within arm positions provides strong evidence that rapid vasodilation contributes to immediate exercise hyperemia in humans. Specific vasodilatory mechanisms responsible remain to be determined.