Effects of progesterone on cardiovascular responses to amines and to sympathetic stimulation in the pithed rat.

1. Blood pressure and heart rate responses to adrenaline, noradrenaline, tyramine, 5-hydroxytryptamine and stimulation of the spinal sympathetic outflow were measured in pithed rats pretreated either with progesterone (20 mg/kg daily for 14 days) or the vehicle solution of ethyl oleate.2. Pretreatment with progesterone increased the durations but not the magnitudes of the blood pressure and heart rate responses to adrenaline and that phase of the response to sympathetic stimulation attributable to amine release from the adrenal medulla.3. Responses to noradrenaline, tyramine, 5-hydroxytryptamine and that phase of the response to sympathetic stimulation associated with amine release from the sympathetic nerves were not significantly different in the two groups.4. Pyrogallol (5 mg/kg) increased the duration but not the magnitude of responses to adrenaline, noradrenaline and sympathetic stimulation in both experimental groups. The increases in duration were consistently less in animals pretreated with progesterone than in controls.5. Pretreatment with progesterone did not affect the total amount of radioactivity nor the proportion of catechol to non-catechol metabolites excreted in the urine during a period of 7.25 h following an intraperitoneal injection of (+/-) isoprenaline-7-(3)H.6. It is concluded that the effects of progesterone may result from a localized decrease in catechol O-methyl transferase activity within the cardiovascular system.