1. Stressful sympathetic stimulation (cold pressor test) was applied to 18 patients with essential hypertension and 15 normotensive subjects. Intra-arterial blood pressure, heart rate, plasma adrenaline and noradrenaline concentrations as well as forearm blood flow were measured before and during the cold pressor test; tests were repeated after regional postsynaptic α1-adrenoceptor blockade with prazosin.

2. Under basal conditions mean blood pressure (P < 0.001), heart rate (P < 0.01), forearm blood flow (P < 0.001) as well as adrenaline concentration (P < 0.01), but not noradrenaline, was higher in patients with essential hypertension.

3. During the cold pressor test, mean blood pressure, heart rate, plasma adrenaline and noradrenaline concentrations increased and forearm flow decreased (all P < 0.001).

4. Stress-stimulated plasma adrenaline was higher in essential hypertensive patients than in normotensive subjects (P < 0.01). In the former the stress-induced increase in plasma adrenaline correlated with the increase in mean blood pressure (r = 0.514; P < 0.05).

5. Prazosin increased forearm blood flow more in essential hypertension (P < 0.001). This increase correlated with the resting plasma adrenaline in the hypertensive (r = 0.710; P < 0.001), but not in normotensive, subjects.

6. When the cold pressor test was repeated during postsynaptic α1-adrenoceptor blockade forearm blood flow did not decrease; instead it increased further in both groups (P < 0.05).

7. Thus in essential hypertension elevated plasma adrenaline concentration reflects sympathetic overactivity as also expressed by enhanced α-adrenoceptor-mediated vasoconstriction.

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