Volume expansion natriuresis during servo control of systemic blood pressure in conscious dogs

The importance of arterial blood pressure (BP) and ANG II for the renal nat riuretic response (NaEx) to volume expansion (3.5% body wt) was investigate d during converting enzyme blockade (enalaprilate, 2 mg/kg). In separate ex periments, BP was clamped either 30 mmHg above or a few millimeters mercury below baseline by servo-controlled infusion of ANG II or sodium nitropruss ide, respectively, so that volume expansion did not change BP. Enalapril de creased BP by 8 mmHg. Without clamping, volume expansion returned BP to tha t of preenalapril control and increased NaEx 10-fold (40 +/- 10 to 377 +/- 69 mu mol/min). During high pressure clamping(133 +/- 2 mmHg), peak NaEx af ter volume expansion was 6% of control experiments. During low pressure cla mping, NaEx was 68% of control experiments (45 +/- 15 to 256 +/- 64 mu mol/ min). The results show that I) in absence of ANG II, volume expansion elici ted pronounced natriuresis without increases in BP beyond baseline, 2) in t he presence of hypertensive amounts of ANG Il, the volume expansion-induced natriuresis was almost eliminated, and 3) nitroprusside prevented the incr ease in BP but not sodium excretion during volume expansion. ANG II appears to dominate the control of NaEx; however, when absent, volume expansion ma y still induce marked natriuresis even at constant BP, possibly via nitric oxide-mediated mechanisms.