Figure 1. Experimental trial.

Each trial consisted of a 2-min rest (baseline) period. After this time period, subjects either began rhythmic forearm exercise or received intra-arterial adenosine to elevate resting forearm blood flow to similar levels observed during exercise (control non-exercise vasodilator). During minutes 5 and 6 (pre-vasoconstrictor), the dose of the α-adrenergic agonist was calculated on the basis of steady state hyperaemic forearm blood flow and forearm volume. Subsequently, the α-agonist (tyramine, phenylephrine or clonidine) was infused at minute 6 and lasted for 3 min. An average of the forearm blood flow and mean arterial blood pressure during the last 30 s of α-agonist infusion was used to calculate the vasoconstrictor effect during both hyperaemic conditions.