The involvement of noradrenergic nerves in the cardiovascular reflex responses to lower body negative pressure in the anaesthetised rabbit.

1. Resting cardiovascular status and the reflex responses to lower body negative pressure (LBNP) were examined in control rabbits and in rabbits sympathectomised with 6-hydroxydopamine (6-OHDA). 2. Resting systemic arterial pressure and total peripheral resistance were significantly higher in control than in 6-OHDA treated animals. 3. Control animals responded to LBNP at -25 mm Hg with a peripheral vasoconstriction and tachycardia. These responses were absent in 6-OHDA-treated animals. 4. Control animals responded to LBNP at -70 mm Hg with a peripheral vasoconstriction and a bradycardia followed by a tachycardia. There was no significant change in heart rate in response to this stimulus in 6-OHDA-treated animals, although a slight vasoconstriction was seen. 5. Bethanidine blocked the tachycardia and vasoconstriction seen in response to LBNP in control and 6-OHDA-treated rabbits. Under these conditions there was no significant difference between the total peripheral resistance of the control and treated animals. 6. The results indicated that neither the maintenance of the resting systemic arterial pressure nor the reflexes elicited by LBNP in control or in chronically sympathectomised rabbits involved hormonal mechanisms, but LBNP was maintained for 1 min only. These findings are discussed in the context of previous observations.