title: Pharmacological characterization of spinal alpha adrenoceptors related to blood pressure control in rats.
abstract: The effects of various alpha adrenoceptor agonists and antagonists injected into the spinal subarachnoid space on blood pressure and heart rate were investigated in pentobarbital-anesthetized male rats. A dose-dependent decrease in blood pressure and heart rate was induced by intrathecal injections of clonidine (0.3-3 micrograms), at the T6-T7 level. Guanabenz (3-30 micrograms) and an azepine derivative B-HT 920 (1-3 micrograms) also reduced blood pressure and heart rate. In contrast, an imidazolidine derivative St 587 (10 micrograms) was ineffective. The clonidine-induced hypotensive effect was antagonized by yohimbine but not by prazosin. Yohimbine (3-10 micrograms) alone caused an increase in blood pressure and heart rate while only a weak hypotenion occurred with prazosin (10 micrograms). Pretreatment with intrathecal 6-hydroxydopamine (50 micrograms X 2) did not impair the hypotensive action of clonidine (1 microgram) whereas the hypertensive effect of yohimbine was reduced markedly by this treatment. When injected i.v. or intrathecally at the C1-C2 level, clonidine (1 microgram) produced only a slight decrease in blood pressure. It is concluded that, in rat spinal cord, alpha-2 adrenoceptors located postsynaptically are involved in blood pressure control. Endogenous catecholamines, especially norepinephrine in the spinal cord seem to activate tonically the alpha-2 adrenoceptors.
mesh_terms: D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D001381:Azepines / Q000008:administration & dosage / Q000494:pharmacology; D001794:Blood Pressure / Q000187:drug effects; D001933:Brain Stem / Q000378:metabolism; D003000:Clonidine / Q000008:administration & dosage / Q000031:analogs & derivatives / Q000494:pharmacology; D004298:Dopamine / Q000378:metabolism; D006143:Guanabenz / Q000008:administration & dosage / Q000494:pharmacology; D006892:Hydroxydopamines / Q000494:pharmacology; D007278:Injections, Spinal; D008297:Male; D009638:Norepinephrine / Q000378:metabolism; D016627:Oxidopamine; D011224:Prazosin / Q000008:administration & dosage / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D013116:Spinal Cord / Q000378:metabolism*; D015016:Yohimbine / Q000008:administration & dosage / Q000494:pharmacology
chemical_list: D000317:Adrenergic alpha-Antagonists; D001381:Azepines; D006892:Hydroxydopamines; D011942:Receptors, Adrenergic, alpha; C030655:St 587; D015016:Yohimbine; C024763:talipexole; D016627:Oxidopamine; D006143:Guanabenz; D003000:Clonidine; D004298:Dopamine; D009638:Norepinephrine; D011224:Prazosin