Sympathetic nervous system peripheral blockers

The actions of p-blockers on blood pressure are complex. After acute administration, blood pressure is only slightly altered. This is because of the compensatory reflex increase in peripheral vascular resistance that results from a (3-blocker-induced decrease in cardiac output. Vasoconstriction is mediated by a-receptors, and a-receptors are not antagonized by (3-receptor blocking agents. Chronic administration of (3-blockers, however, results in a reduction of blood pressure, and this is the reason for their use in primary hypertension (see Chapter 20). The mechanism of this effect is not well understood, but it may include such actions as a reduction in renin release, antagonism of (3-receptors in the central nervous system, or antagonism of presynaptic facilita-tory (3-receptors on sympathetic nerves. 

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