GANGLIONIC BLOCKING AGENTS
The basis for the antihypertensive activity of the gan-glionic blockers lies in their ability to block transmis-sion through autonomic ganglia (Fig. 20.2C). This ac-tion, which results in a decrease in the number of impulses passing down the postganglionic sympathetic (and parasympathetic) nerves, decreases vascular tone, cardiac output, and blood pressure. These drugs prevent the interaction of acetylcholine (the transmitter of the preganglionic autonomic nerves) with the nicotinic re-ceptors on postsynaptic neuronal membranes of both the sympathetic and parasympathetic nervous systems.
The ganglionic blocking agents are extremely potent antihypertensive agents and can reduce blood pressure regardless of the extent of hypertension. Unfortunately, blockade of transmission in both the sympathetic and parasympathetic systems produces numerous untoward responses, including marked postural hypotension, blurred vision, and dryness of mouth, constipation, par-alytic ileus, urinary retention, and impotence. Owing to the frequency and severity of these side effects and to the development of other powerful antihypertensive agents, the ganglionic blocking agents are rarely used.
The orally effective ganglionic blocking agents in fact are not recommended for the treatment of primary hy-pertension. However, certain intravenous preparations, such as the short-acting agent trimethaphan camsylate (Arfonad), are used occasionally for hypertensive emer-gencies and in surgical procedures in which hypotension is desirable to reduce the possibility of hemorrhage.
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