Objective: Doxazosin, a selective [alpha] 1-adrenoceptor antagonist, lowers blood pressure by reducing peripheral vascular resistance without causing reflex tachycardia. To discover whether antihypertensive treatment with an [alpha] 1-adrenoceptor blocker is accompanied by an increase in sympathoadrenomedullary activity, we studied plasma catecholamine kinetics before and during treatment with doxazosin.

Patients and methods: Eleven patients with essential hypertension were studied before and after 3 months' treatment with doxazosin (4-8 mg a day). 3H-noradrenaline and 3H-adrenaline were infused simultaneously and blood samples were collected to calculate plasma catecholamine kinetics before and during sympatho-adrenomedullary stimulation (lower-body negative pressure).

Results: Doxazosin decreased systolic and diastolic blood pressure and forearm vascular resistance, whereas the heart rate did not change significantly. During doxazosin, baseline arterial plasma noradrenaline increased from 0.97+/-0.07 to 1.21+/-0.07 nmol/l, and this appeared to be due to an increase in total body noradrenaline spillover from 1.54+/-0.15 to 1.84+/-0.16 nmol/min; noradrenaline clearance did not change significantly. Forearm noradrenaline spillover also increased, from 0.89+/-0.18 to 1.48+/-0.23 pmol/100 ml per min. In contrast, arterial plasma adrenaline, total body adrenaline spillover and adrenaline clearance were not significantly affected by doxazosin treatment. The response of plasma noradrenaline and total body and forearm spillover of noradrenaline to lower-body negative pressure (-40 mmHg) was significantly increased during doxazosin administration, whereas the response of the adrenaline kinetic parameters were not altered.

Conclusions: The blood pressure reduction induced by a chronic administration of the [alpha] 1-adrenoceptor blocker doxazosin elicits a baroreceptor-mediated reflexive increase in sympathoneural but not in adrenomedullary activity. The latter finding might partly explain why the heart rate is not increased during chronic treatment with this [alpha] 1-adrenoceptor blocking drug.