Hierarchy of blood pressure control mechanisms after spinal sympathectomy

We have investigated the relative importance of angiotensin and vasopressin in the regulation of arterial pressure following permanent interruption of supraspinal sympathetic influences. To accomplish this aim, the spinal cord of 12 dogs was transected just above the intervertebral foramen of C-6; several days later, we gave first a potent blocker of the vasculotropic actions of vasopressin and 40 min later captopril. The same experiment was performed in other dogs with the drug order reversed. Mean arterial pressure and heart rate were recorded continuously and blood samples were taken to measure plasma renin activity and plasma catecholamines. All studies were carried out at three levels of hydration: normal, after 36 h water deprivation and following an overnight infusion of 0.9% saline. Conscious dogs with complete surgical sympathectomy by spinal cord section had normal mean arterial pressure, heart rate and plasma renin activity but undetectable levels of catecholamines. Captopril produced significant falls in mean arterial pressure that were greatest in water deprivation and least in volume loading, whether the drug was given before or after treatment with the vasopressin antagonist. On the other hand, the vasopressin antagonist modified mean arterial pressure only in the water deprived state. In spinal dogs the renin angiotensin system assumes a primary role in maintaining normal mean arterial pressure at various extremes of body fluid volumes. Vasopressin plays a role only after removal of the two dominant systems.