Response to slow, graded bleeding in salt-depleted rats.

Abstract

The response to slow bleeding was studied in rats on low sodium intake (0.04%) versus 'ordinary' (0.4%) intake. After 12 days on the diets acute experiments were performed on paired, conscious rats. Initially the salt-depleted rats had slightly lower mean arterial pressure (121 +/- 2 versus 129 +/- 3 mmHg, n = 30 pairs, P less than 0.02). Heart rate (HR), cardiac output, plasma volume, extracellular volume and plasma renin activity (PRA) did not differ significantly, while haematocrit was slightly higher in the salt-depleted rats (46.5 +/- 0.3 versus 45.1 +/- 0.5%, P less than 0.05). The animals were then slowly bled 1 ml every 5 min until mean arterial pressure (MAP) fell and remained below 50 mmHg. The controls tolerated up to a 55 +/- 1% loss of initial blood volume, while the salt-depleted rats turned into irreversible shock already after losing 38 +/- 2% (P less than 0.001). At this stage compensatory haemodilution, HR and PRA increases were less pronounced than in the controls, suggesting that chronic sodium restriction had somehow interfered with the neurohormonal defences against accidental salt fluid losses.