Sympathetic Activity and Renal Denervation

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)