Disparity between renal venous norepinephrine and renin responses to sodium depletion

To evaluate the hypothesis that sodium depletion produces a chronic increase in renal nerve activity, arterial and renal venous plasma norepinephrine (NE) concentrations were measured in conscious dogs subjected to various degrees of sodium depletion. After 9 days of sodium depletion (LS), there was a net loss of 69 +/- 10 meq sodium, and mean arterial pressure (MAP) was reduced from 94 +/- 5 to 88 +/- 4 mmHg. At this time plasma renin activity (PRA) was increased from a control level (sodium intake = 45 meq/day) of 0.34 +/- 0.08 to 1.47 +/- 0.26 ng angiotensin I (ANG I).ml-1.h-1 in association with an approximately sixfold increase in the PRA gradient across the kidneys. Subsequently, when captopril was infused during an additional 7 days of sodium deprivation [(LS + converting enzyme inhibitor CEI)], there was further sodium depletion (31 +/- 11 meq) and hypotension (MAP = 65 +/- 6 mmHg) and PRA and the renal PRA gradient increased even further. In marked contrast, there were no significant changes in either arterial plasma NE concentration (control = 102 +/- 5 pg/ml) or the renal arteriovenous gradient of plasma NE concentration during either LS or LS + CEI. These experiments show a distinct disparity between changes in the PRA and the plasma NE concentration gradient across the kidneys during LS and fail to support the contention that increased renal nerve activity is an important long-term adaptive response to sodium depletion.