Hemodynamics and blood volume in angiotensin II salt-dependent hypertension in dogs

The sequential effects of an increased daily NaCl intake on hemodynamics, fluid electrolyte balances, and hormonal responses were evaluated in dogs (n = 7) with fixed circulating levels of angiotensin II (ANG II). During the control period, ANG II was infused at 3 ng.kg-1.min-1 while dogs were maintained on an 8 meq NaCl/day diet. Water intake was fixed at 700 ml/day. Continuously recorded (24 h/day) changes of total body weight (TBW) were used as an index of total body water. Cardiac stroke volume and arterial pressure were recorded, and each beat was digitized to provide hourly and 24-h average cardiac output (CO), mean arterial pressure (MAP), and total peripheral resistance (TPR). After three stable control days, daily salt intake was increased to 120 meq for 7 days. TBW increased gradually to 448 +/- 111 g (2.9%, P less than 0.05) above control by day 3. An 11% expansion of blood volume (P less than 0.05) was found (51CR-labeled red blood cells) on day 2 of high NaCl. CO rose 12% and MAP 20% (P less than 0.05) in parallel with TBW by day 4. By day 7, CO remained only 5% elevated, whereas MAP had stabilized at 20% above control levels. TPR remained significantly elevated from days 3 through 7. A positive Na balance averaging 91 +/- 8 meq (P less than 0.05) occurred on day 1. Plasma Na concentration was increased 2-3 meq/l above control throughout the period of high-salt intake. Plasma renin activity and aldosterone levels decreased to nearly undetectable levels, vasopressin rose slightly, and atrial natriuretic peptide levels increased significantly. Dogs maintained at 8 meq/day NaCl during the same infusion of ANG II showed no changes in MAP, CO, TPR, or TBW. In summary, the salt-induced hypertension was consistently related to small but significant fluid retention, blood volume expansion, elevations of cardiac output, and a gradual increase in TPR.