Low baroreflex sensitivity predisposes to salt-sensitive hypertension in the rabbit

The aim of this study was to determine the effect of an increase in dietary salt on blood pressure (BP), Na⁺ balance, and plasma renin activity (PRA) in normotensive rabbits bred for differences in cardiac baroreflex sensitivity (BRS). Food and fluid intake, BP, heart rate, body weight, PRA, hematocrit, and creatinine clearance were monitored weekly and Na⁺ balance daily for 3 wk each on normal NaCl (8 meq/day) and high salt (32 meq/day) in 27 rabbits of the second and third generation of animals bred for high BRS (group I, 6.1 ± 0.3 beats · min^-1 · mmHg^-1, n = 9) or low BRS (group II, 3.61 ± 0.1 beats · min^-1 · mmHg^-1, n = 18). BRS was assessed in each animal on normal salt and at the end of the high-salt period. Both systolic and diastolic BP rose by >10 mmHg in 50% of group II and by <5 mmHg in the remainder and in all group I. The rise in BP was associated with Na⁺ and fluid retention and weight gain during the first 2 wk, which returned to presalt levels by the 3rd wk, although BP continued to rise. The lack of effect on BP in the remaining nine group II was associated with a marked sensitization of their BRS by the high salt to 6 ± 0.4 beats · min^-1 · mmHg^-1. BRS remained unchanged in the other rabbits. A highly significant correlation (P < 0.001) was found between the increment of BP and BRS after 3 wk of raised salt intake. The salt-insensitive group II had significantly higher control levels of PRA, which were markedly lowered by the diet. It is concluded that low BRS can predispose to initial Na⁺ and fluid retention on salt loading, which acts as a trigger for the development of hypertension. Sensitization of BRS by the high salt intake, possibly by a reduction in angiotensin II activity, can prevent the BP rise.