Contrasting cardiovascular responses from intrathecal administration of epinephrine and norepinephrine in conscious rats: role of alpha 1- and alpha 2-adrenoceptors.

In conscious rats, intrathecal (i.t.) administration of norepinephrine (NE) produced pressor responses, whereas i.t. epinephrine (Epi) caused depressor responses at low doses (0.1-1 microgram) and pressor responses at a higher dose (10 micrograms). Epi administered i.t. produced bradycardia; however, NE caused tachycardia at low doses and bradycardia at high doses. The cardiovascular responses were dissimilar to those observed after intravenous (i.v.) administration of these doses of NE and Epi. When [3H]NE or [3H]Epi (1.0 microgram, 10 mCi) was injected i.t., minimal radioactivity was detected in peripheral blood (PB) samples, indicating that the effects of i.t.-injected catecholamines on blood pressure (BP) and heart rate (HR) are due to stimulation of central spinal adrenoceptors and not to peripheral effects after leakage. Pretreatment with i.t. administration of the alpha 1-antagonist prazosin (1.0 microgram) attenuated pressor responses and tachycardia produced by i.t. NE (1.0 microgram), whereas i.t. pretreatment with the alpha 2-antagonist yohimbine (10 micrograms) counteracted depressor responses and bradycardia produced by i.t. Epi. Therefore, these spinally released catecholamines appear to produce opposite cardiovascular effects whereby sympathetic preganglionic neurons are excited by NE through spinal alpha 1-adrenoceptors and are inhibited by Epi through spinal alpha 2-adrenoceptors.