An estimate of adenosine triphosphate release into the venous effluent from exercising human forearm muscle.

1. Human subjects performed a sustained contraction of the forearm muscles for 4 min in the presence of arterial and venous occlusion.2. The contraction was maintained at 5% of the maximum voluntary contraction, a tension during which the muscle blood flow might be expected to increase by about three times (Lind & McNicol, 1967).3. Adenosine triphosphate (ATP) was identified in the venous effluent from occluded exercising forearm, but not in the venous effluent from occluded forearm without exercise.4. The rate of degradation of ATP was assessed in plasma at 37 degrees C, with an estimate of the percentage loss occurring between sampling and testing. This enabled the rate of appearance of ATP in the blood at the time of exercise to be calculated as approximately 7.5-10.5 mug/min (14-20 n-mole/min). These amounts are compared with 16 mug/min that was infused intra-arterially into human forearm to cause a threefold increase in blood flow (Duff, Patterson & Shepherd, 1954).5. It is likely that the ATP detected in the venous effluent has active muscle as the source; if so, then the amounts calculated to be released could satisfy the vasodilator requirements of active skeletal muscle.6. The effects of circulating ATP on respiration and coronary blood flow during exercise is discussed, including the role it may play locally in the production of ischaemic pain.