Nitric oxide and central antihypertensive drugs: one more difference between catecholamines and imidazolines.
NO is known to be involved in the peripheral and central regulation of the cardiovascular function. It plays a neuromodulatory role via a direct action on presynaptic nerve terminals, stimulating the release of gamma-aminobutyric acid, glutamate, and norepinephrine. Our aim was to study the possible role of NO in the cardiovascular effects of the central antihypertensive drugs clonidine, rilmenidine, and alpha-methyl-norepinephrine (alpha-MNA). Sites and mechanisms of the hypotensive action of these drugs were different; clonidine and rilmenidine acted on imidazoline receptors in the nucleus reticularis lateralis, whereas alpha-MNA acted upon alpha(2)-adrenoceptors in the nucleus tractus solitarius. The influence of N:(G)-nitro-L-arginine, an NO synthase inhibitor, on the central hypotensive effects of these drugs was investigated in pentobarbital-anesthetized rabbits. The intracisternal (IC) administration of alpha-MNA (30 microg/kg) induced hypotension (79+/-2 versus 103+/-4 mm Hg) and bradycardia (222+/-8 versus 278+/-4 bpm) (P:<0.05) (n=5). Clonidine (0.07 microg/kg IC) also induced hypotension (69+/-5 versus 99+/-4 mm Hg) and bradycardia (266+/-7 versus 306+/-10 bpm) (P:<0.05) (n=5). In addition to clonidine, rilmenidine (1 microg/kg IC) induced hypotension (64+/-4 versus 97+/-4 mm Hg) and bradycardia (264+/-11 versus 310+/-4 bpm) (P:<0.05) (n=5). Pretreatment with N:(G)-nitro-L-arginine (900 microg/kg IC) completely prevented the hypotensive effect of alpha-MNA but influenced the cardiovascular effects of neither clonidine nor rilmenidine. These results confirm that imidazoline drugs, such as clonidine, rilmenidine, and the catecholamine alpha(2)-adrenoceptor agonist alpha-MNA, have distinct mechanisms of action.
Laboratoire de Neurobiologie et Pharmacologie Cardiovasculaire, Faculté de Médecine, Université Louis Pasteur, Strasbourg, France.
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D000818:Animals; D000959:Antihypertensive Agents / Q000494:pharmacology*; D001794:Blood Pressure / Q000187:drug effects; D002319:Cardiovascular System / Q000187:drug effects* / Q000503:physiopathology; D002395:Catecholamines / Q000494:pharmacology; D003000:Clonidine / Q000494:pharmacology; D006339:Heart Rate / Q000187:drug effects; D007022:Hypotension / Q000517:prevention & control; D007093:Imidazoles / Q000494:pharmacology; D008297:Male; D009569:Nitric Oxide / Q000502:physiology*; D019335:Nitroarginine / Q000494:pharmacology; D008771:Nordefrin / Q000494:pharmacology; D010080:Oxazoles / Q000494:pharmacology; D011817:Rabbits; D000077769:Rilmenidine
D000959:Antihypertensive Agents; D002395:Catecholamines; D007093:Imidazoles; D010080:Oxazoles; D019335:Nitroarginine; D009569:Nitric Oxide; D003000:Clonidine; D000077769:Rilmenidine; D008771:Nordefrin