Effects of chronic treatment with ramipril, a new ACE blocking agent, on presynaptic sympathetic nervous system of SHR.

Acute administration of captopril has been reported to decrease noradrenaline (NA) release from the sympathetic nerves. In this study the chronic effects of ramipril on the sympathetic nervous system of male spontaneously hypertensive rats (SHR) have been investigated and compared to those of captopril and enalapril. As parameters for catecholamine biosynthesis and storage, the activity of tyrosine hydroxylase and the catecholamine content of the hearts and the adrenal medulla were measured by HPLC in treated and control SHR. To assess sympathetic outflow plasma NA and adrenaline (A) levels were determined during preganglionic stimulation (PS) of the spinal cord. Under none of these drugs could differences be observed between the treated and control animals, neither in the biosynthesis and storage of catecholamines in the heart and adrenal medulla nor in the sympathetic outflow. However, the dose response curves of blood pressure vs PS were significantly shifted to the right when ACE-inhibitors were administered, most strongly by ramipril. In view of the unaltered presynaptic sympathetic function long term treatment with ACE-inhibitors is suggested to increase bradykinin and angiotensin I. Bradykinin and angiotensin I are capable of releasing NA and A from the adrenal medulla, like angiotensin II.