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The effects of intracisternal pretreatment with alpha 2-adrenoceptor antagonists which possess imidazoline (idazoxan) or nonimidazoline (yohimbine) structures on the sympatho-inhibitory action of rilmenidine were investigated in conscious sinoaortic denervated rats. Intracisternal injection of rilmendine significantly decreased renal sympathetic nerve activity. Idazoxan pretreatment prevented the sympathoinhibitory action of rilmenidine, whereas yohimbine pretreatment failed. It is concluded that the sympathoinhibitory action of rilmenidine relates more to the interaction with imidazoline receptors than alpha 2-adrenoceptors.

The effects of yohimbine and idazoxane (0.5 and 5 mg/kg, intravenously) on the systemic and regional hemodynamics were studied in conscious Wistar rats by the microsphere technique. The both drugs induced an elevation of the arterial blood pressure and the heart rate. The administration of yohimbine and idazoxane in a dose of 0.5 mg/kg produced a more pronounced increase in the cerebral and coronary blood flow that a higher dose (5.0 mg/kg). The blood flow in the small intestine, kidneys and testis was decreased by a high dose of the studied alpha-blockers. It was suggested that the direct action of alpha 2-blockers on the vessel smooth muscle cells in responsible for the increase in the blood flow whereas blockade of alpha 2-receptors on the sympathetic terminals produces the decrease in the blood flow.