Adrenergic neuron blocking drugs are selectively taken up into adrenergic nerve endings by the active, energy-requiring, saturable amine (noradrenaline) pump mechanism (uptake-1). They accumulate in the noradrenaline storage vesicles from which they are released in response to nerve impulses, diminishing the release of noradrenaline and so all sympathetic function. They do not adequately control supine blood pressure and are prone to interactions with other drugs affecting adrenergic function, e.g. tricyclic antidepressants and topical nasal decongestants. They are virtually obsolete in hypertension.
Guanethidine has been used to reduce intraocular pressure in open angle glaucoma and to reduce thyrotoxic eyelid retraction for cosmetic effect. Other members of the group are debrisoquine and bethanidine. Metaiodobenzylguanidine (MIBG) is used diagnostically as a radioiodinated tracer, to locate chromaffin tumours (mainly phaeochromo-cytoma) which accumulate drugs in this class (p. 495).
DEPLETION OF STORED TRANSMITTER (NORADRENALINE)
Reserpine is an alkaloid from plants of the genus Rauivolfia, used in medicine since ancient times in southern Asia, particularly for insanity; more recently, reserpine was extensively used in psychiatry but is now obsolete. Reserpine depletes adrenergic nerves of noradrenaline primarily by blocking amine storage within vesicles present in the nerve ending, so reducing stores of releasable transmitter. Its antihypertensive action is due chiefly to peripheral action, but it enters the CNS and depletes catecholamine stores there too; this explains the sedation, depression and parkinsonian (extrapyramidal) side effects that can accompany its use. The effects on catecholamine storage persist for days to weeks after it is withdrawn.
It has also had an important veterinary use in preventing the death of domestic male turkeys, which are liable to fatal hypertensive dissecting aortic aneurysms. This can cause serious economic loss. The addition of reserpine to their drinking water reduces their blood pressure and preserves their lives without noticeably moderating their natural rage as may (3-adrenoceptor blockers.22
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