The basis for the antihypertensive activity of the ganglionic blockers lies in their ability to block transmission through autonomic ganglia (Fig. 20.2C). This action, 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 receptors 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, paralytic 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 hypertension. However, certain intravenous preparations, such as the short-acting agent trimethaphan camsylate (Arfonad), are used occasionally for hypertensive emergencies and in surgical procedures in which hypotension is desirable to reduce the possibility of hemorrhage.
A more complete description of trimethaphan and other ganglionic blocking agents can be found in Chapter 14.
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