Alpha-1 -adrenergic receptor-blocking agents

Tolazoline, an alpha-adrenergic-receptor-blocking agent with vasodilating properties (1 to 2 mg/kg IV via a scalp vein over 10 minutes), is indicated in persistent pulmonary vasoconstriction and hypertension of the newborn (persistent fetal circulation). 

Phenothiazine derivatives cause postural or orthostatic hypotension. This may be more pronounced in patients with reduced vascular volume resulting from acute hemorrhage or dehydration, and when used with diuretic agents. Hypotension is more frequent with phenothiazine derivatives having either an aliphatic substitution on NIO (e.g., chlorpromazine) or a piperidine substitution on NIO (e.g., mesoridazine or thioridazine). It occurs less frequently with compounds containing a piperazine substitution (e.g., trifluoperazine). The hypotension is due to direct vasodilation and an alpha-adrenergic-receptor-blocking effect. The pressor effects of epinephrine can be reduced, blocked, or reversed by appropriate doses of chlorpromazine. 

Mechanism of Action An antihypertensive and benign prostatic hyperplasia agent that blocks alpha-adrenergic receptors. Produces vasodilation, decreases peripheral... 

These are the agents, which block the action of adrenaline and noradrenaline. They block either alpha or beta or both adrenergic receptors. They are classified as in table 3.3.1 and 3.3.2. 

It is a potent alpha-adrenergic blocking agent and only haloalkylamine used clinically. It effectively prevents the responses mediated by alpha receptors and diastolic blood pressure tends to decrease. It interferes with the reflex adjustment of blood pressure and produces postural hypotension. It increases the cardiac output and decreases the total peripheral resistance. It also antagonizes cardiac arrhythmias provoked by catecholamines. Apart from these effects, phenoxybenzamine has other actions also e.g. antagonism of acetylcholine, histamine, 5-hydroxytryptamine (serotonin). However, the vasodilatation produced by phenoxybenzamine is because of alpha blockage. Adverse reactions are miosis, dryness of mouth, inhibition of ejaculation, palpitation, nasal stuffiness and in higher doses, postural hypotension and reflex bradycardia. 

In contrast to the actions of ganglionic blocking agents, guanethidine suppresses equally the responses mediated by alpha- and beta-adrenergic receptors but does not produce parasympathetic blockade. It reduces both the systolic and diastolic pressures, and the effect is especially pronounced when the patient is standing. 

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