The role of the sympathetic nervous system in the cardiovascular responses to angiotensin in the pithed rat.

1. Angiotensin (200-500 ng, intravenously) produced a biphasic pressor response accompanied by reflex bradycardia in rats anaesthetized with sodium pentobarbitone. In pithed rats and in anaesthetized animals treated with a ganglion blocking agent there was a similar pressor response but an increase in the heart rate which was coincident with the second phase of the pressor response. In pithed preparations an increase in cardiac contractile force accompanied the heart rate changes.2. Acute or chronic adrenalectomy did not alter the pressor responses and chronotropic effect of angiotensin in the pithed rat. Propranolol abolished the chronotropic and inotropic action of angiotensin, but left the biphasic pressor response unaltered. Bethanidine, or pretreatment with reserpine, altered the pressor response to a simple rise, and also abolished the chronotropic action of angiotensin. Phentolamine also abolished the second pressor component. Desmethylimipramine produced a marked potentiation of the cardiovascular effects of angiotensin at all dose levels tested (10-500 ng).3. It is concluded that in the rat only large doses of angiotensin are capable of indirectly stimulating the sympathetic nerves either at the site of their ganglionic cells or their post-ganglionic nerve ending.