Prolonged adenosine triphosphate infusion and exercise hyperemia in humans

In humans, intra-arterial ATP infusion in limbs mimics many features of exercise hyperemia. However, it remains unknown whether ATP can evoke the prolonged vasodilation seen during exercise. Therefore, we addressed two questions during a continuous 3-h brachial artery infusion of ATP [20 g100 ml forearm volume (FAV)1min1]: 1) would skeletal muscle blood flow remain robust or wane over time (tachyphylaxis); and 2) would the hyperemic response to moderate-intensity exercise performed during the ATP administration be blunted compared with that during control (saline) infusion. Nine participants (25 1 yr) performed one trial consisting of seven bouts of rhythmic handgrip exercise (20 contractions/min at 20% of maximum), two bouts during saline (control), and five bouts during 180 min of continuous ATP infusion. Five minutes of ATP infusion resulted in a 710% increase in forearm vascular conductance (FVC) from control (4.8 0.77 vs. 35.0 5.7 mlmin1100 mmHg1dl FAV1, P<0.05). Contrary to our expectations, FVC did not wane over time with values of 35.0 5.7 and 36.0 7.7 mlmin1100 mmHg1dl FAV1 (P<0.05), seen prior to the exercise bouts at 5 vs. 150 min, respectively. During superimposed exercise, FVC increased from 35.0 5.7 to 49.6 5.4 mlmin1100 mmHg1dl FAV1 at 5 min and 36.0 7.7 to 54.5 5.0 at 150 min (P<0.05). Our findings demonstrate ATP vasodilation is prolonged over time without tachyphylaxis; however, exercise hyperemia responses remain intact. Our results challenge the metabolic theory of exercise hyperemia, suggesting a disconnect between matching of blood flow and metabolic demand.