Effect of dietary salt on hemodynamics of established renal hypertension in the rabbit. Implications for the autoregulation theory of hypertension.

Abstract

Two groups of 10 rabbits were subjected to renal cellophane wrapping and sham operation. Their initial mean arterial pressures (MAP) were similar, 92 +/- 1.5 and 90 +/- 2.9 mm Hg. Six weeks later three experimental periods began, each of 2 weeks' duration, on low, normal, and high salt (1, 9, and 50 mmole Na/100 g food) diets. Each group had two subgroups: rabbits with both kidneys, and rabbits with only one kidney and previous nephrectomy. The hemodynamic findings were similar in each group. After sham operation, the range of dietary salt produced no significant circulatory changes. After wrapping, MAP was reduced on low compared with normal and high salt diets (122 vs 132 and 136 mm Hg; p = 0.01). This was entirely due to lowering of cardiac output (CO) on low salt; on normal and high salt CO was higher than in sham-operated rabbits. Total peripheral resistance (TPR) in the wrapped animals was unaffected by diet, i.e., 21.4, 20.5, and 21.2 units on low, normal, and high salt--about 35% above values of sham-operated rabbits. Volume-related CO changes therefore produce long-term changes in MAP without alteration in TPR, which is not in conformity with the autoregulation theory of hypertension. Evidence of impaired capacity of wrapped compared with sham-operated rabbits to handle salt included diet-related hematocrit changes, lower creatinine clearance, and some differences in renin responses to salt. Giving saralasin reduced TPR while the rabbits were on low salt; the fall was twice as great in wrapped compared with sham-operated rabbits.