2.Alpha-1 blockers Prazosin哌唑嗪，terazosin特拉唑嚎，doxazosin多沙唑嚎 Mechanism of action:Competitive antagonists for a-AR.Blocking a-AR leads to relaxation of both arterial and venous smooth muscles and thereby reduce PVR. Non-selective alpha blockers:Phentolamine and phenoxybenzamine,for treatment of hypertensive emergencies resulting from pheochromocytoma Therapeutic use:These agents are indirect vasodilators,and are used for treating patients who have not responded to initial antihypertensive therapy. Combined use with propranolol or diuretics may produce additive effects. Long-term use is not likely to cause significant changes in cardiac output and renal blood flow.Thus tachycardia and increased renin release do not occur.No adverse effect on serum lipids and other cardiac risk factors. Adverse effects and toxicity: (1)For prazosin,terazosin,doxazosin:Reflex tachycardia and first-dose syncope are common.Concomitant use with a B-blocker may be necessary. (2)For phentolamine,increased cardiac stimulation(by blocking a2-AR negative feedback)can cause severe tachycardia,arrhythmias,and myocardial ischemia For phenoxybenzamine,postural hypotension may occur.CNS symptoms,such as fatigue,sedation and nausea,are also seen in patients using phenoxybenzamine. 1212 2. Alpha-1 blockers Prazosin 哌唑嗪, terazosin 特拉唑嗪, doxazosin 多沙唑嗪 Mechanism of action: Competitive antagonists for α1-AR. Blocking α1-AR leads to relaxation of both arterial and venous smooth muscles and thereby reduce PVR. Non-selective alpha blockers: Phentolamine and phenoxybenzamine, for treatment of hypertensive emergencies resulting from pheochromocytoma Therapeutic use: These agents are indirect vasodilators, and are used for treating patients who have not responded to initial antihypertensive therapy. Combined use with propranolol or diuretics may produce additive effects. Long-term use is not likely to cause significant changes in cardiac output and renal blood flow. Thus tachycardia and increased renin release do not occur. No adverse effect on serum lipids and other cardiac risk factors. Adverse effects and toxicity: (1) For prazosin, terazosin, doxazosin: Reflex tachycardia and first-dose syncope are common. Concomitant use with a β-blocker may be necessary. (2) For phentolamine, increased cardiac stimulation (by blocking α2-AR negative feedback) can cause severe tachycardia, arrhythmias, and myocardial ischemia. For phenoxybenzamine, postural hypotension may occur. CNS symptoms, such as fatigue, sedation and nausea, are also seen in patients using phenoxybenzamine