Primary hypothesis:

Catheter-based renal denervation reduces central sympathetic activation in patients with refractory hypertension.

Secondary hypotheses:

1. The magnitude of the individual depressor response after catheter-based renal denervation depends on the extent of sympathoinhibition.
2. Both, the reduction in arterial pressure and in central sympathetic activation are sustained over time up to 24±3 months after catheter-based renal denervation.
3. Catheter-based renal denervation resets the sympathetic baroreflex to lower blood pressure values.

Inclusion Criteria:

• Refractory hypertension (as defined by the WHO: poorly controlled hypertension (>140/90 mmHg) despite adequate doses of at least three antihypertensive drugs including a diuretic)
• Men or women aged >18 years
• Intact peroneal nerve
• Written informed consent

Exclusion Criteria:

• Diseases, dysfunctions, or medications (such as NET inhibitors) that exclude a participation in the study according to the investigator.
• Legal incompetence or circumstances that interfere with the patient´s ability to fully understand scope, relevance, and/or consequences of participation in this study.
• People in custody
• hypersensitivity to clonidine, phenylephrine, or nitroprusside-sodium
• sick sinus syndrome, second or third degree AV block, bradycardia < 50 bpm (contraindication for clonidine)
• endogenous depression (contraindication for clonidine)

Patients with refractory hypertension, in whom catheter-based renal denervation is indicated and who are scheduled for this procedure, will be recruited from the outpatient clinic.