Fig. 4. Interplay between the sympathetic, renin–angiotensin, and vasopressin systems. (  A ) Maximum change in arterial blood pressure from baseline in conscious dogs after epidural saline (  open column ), vasopressin V1 receptor blockade alone (  column with crosses ), sympathetic blockade by epidural anesthesia alone (  striped column ), and epidural anesthesia in the presence of vasopressin V1 receptor blockade (  solid column ). Vasopressin receptor blockade alone has no impact on arterial pressure, whereas vasopressin receptor blockade markedly augments the decrease in arterial pressure during sympathetic blockade. Together with increased vasopressin concentrations observed with widespread sympathetic blockade, this indicates that the vasopressin system supports arterial pressure when both the sympathetic and the renin–angiotensin systems are impaired by sympathetic blockade. PDA = peridural anesthesia. From Peters  et al. 22; with permission.  (B ) Changes in plasma renin and vasopressin concentrations in response to induced arterial hypotension both before and during sympathetic block by epidural anesthesia (sensory blockade T1–T11) in humans. Hypotension was induced by intravenous infusion of sodium nitroprusside titrated to decrease mean arterial blood pressure by at least 25%. An increase in renin concentration is seen with the sympathetic nervous system intact, whereas sympathetic blockade suppresses renin release in response to hypotension but evokes vasopressin release. From Hopf  et al. 66; with permission.  Stars indicate statistically significant differences.