The kidneys and arterial pressure in immature and adult rabbits.

1. The effect of stepwise haemorrhage on arterial pressure in adult and immature (9-15 days old) rabbits lightly anaesthetized with sodium pentobarbitone is described.2. Bethanidine (1 or 3 mg/kg) lowered initial arterial pressure but did not impair the maintenance of arterial pressure in immature or adult rabbits during stepwise haemorrhage.3. Arterial pressure was only slightly lower in nephrectomized than in intact adult rabbits; in some immature rabbits nephrectomy caused a substantial fall of resting arterial pressure. This fall was largest in animals of low body weight and low haematocrit. Nephrectomy reduced the ability of both immature and adult rabbits to maintain arterial pressure during haemorrhage.4. Arterial pressure fell more precipitately on stepwise haemorrhage in adult rabbits in which the carotid sinus and depressor nerves had been cut than in intact rabbits. No such difference was seen in immature rabbits despite the fact that initial arterial pressure was higher in denervated animals at all ages.5. The responses to stepwise haemorrhage were compared in dummy operated and nephrectomized immature and adult rabbits with carotid sinus and depressor nerves cut to minimize changes of sympathetic tone. Under these conditions the presence or absence of kidneys made no significant difference in the response to stepwise bleeding in adult rabbits. In immature rabbits maintenance of arterial pressure was greatly impaired in the absence of the kidneys.6. The results suggest that a pressor mechanism of renal origin may be relatively more important in the maintenance of arterial pressure in the face of haemorrhage in immature than in adult rabbits.7. Resting arterial pressure in small and anaemic immature rabbits may partly depend on the presence of the kidneys. At birth arterial pressure is higher in rabbits of higher haematocrit but this relationship reverses in the second week of life when the haematocrit level is falling.