Alpha-adrenergic agents

Mechanism of Action An ocular alpha-adrenergic agent that is a relatively selective alphaj receptor agonist. Therapeutic Effect Reduces intraocular pressure. 

Alpha-adrenergic agents can cause marked hypotension after the first dose, especially in the upright position, with syncope and other postural symptoms... 

Dopamine can thus be added to the list of hormones and neurotransmitters which can stimulate or inhibit cyclic AMP formation, depending upon their tissue of action. Thus, while dopamine stimulates cyclic AMP formation in parathyroid cells, superior cervical ganglia, retina and striatal tissue (27, 58-61), it inhibits the accumulation of the cyclic nucleotide in cells of the intermediate and anterior lobes of the pituitary gland. Opposite effects on the cyclic AMP system are also found with LHRH which stimulates and inhibits cyclic AMP levels in the anterior pituitary gland (62) and ovary (63), respectively. Similarly, alpha-adrenergic agents show opposite effects on cyclic AMP formation in brain (64) and platelets (65). PGE, stimulates cyclic AMP formation in the anterior pituitary gland (62) while it inhibits the same parameter in fat cells (66). 

Bradycardia or AV block may also be a reflex response (baroreceptor reflex) to hypertension induced by alpha-adrenergic agents such as phenylpropanolamine or phenylephrine. 

Selective alpha-adrenergic agents cause hypertension with reflex (barore-ceptor-mediated) bradycardia or even AV block. 

The pharmacologic actions of these agents include Alpha-adrenergic stimulation (vasoconstriction, nasal decongestion, pressor effects) - -adrenergic stimulation (increased myocardial contractility and conduction) and 2-adrenergic stimulation (bronchial dilation and vasodilation, enhancement of mucociliary clearance, inhibition... 

Mectianism of Action An antiadrenergic, sympatholytic agent that prevents pain signal transmission to the brain and produces analgesia at pre- and post-alpha-adrenergic receptors in the spinal cord. Therapeutic Effect Reduces peripheral resistance decreases BP and heart rate. 

Geriatric Considerations - Summary Alpha-adrenergic blockers are modestly effective alone, and in combination with 5-alpha reductase inhibitors (e g, finasteride) in the treatment of urinary obstructive symptoms related to benign prostatic hyperplasia, The main side effect of these agents is orthostatic hypotension, and in hypertensive patients, these agents may increase the risk of congestive heart failure as reported in the ALLHAT study. 

Mechanism of Action An antihypertensive agent that stimulates central inhibitory alpha-adrenergic receptors, lowers arterial pressure, and reduces plasma renin activity. Therapeutic Effect Reduces BP. 

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

Table 3.3.1 Classification for alpha adrenergic blocking agents.

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. 

Mastropasqua L, Carpineto P, Ciancaglini M, et al. Effect of dapiprazole, an alpha-adrenergic blocking agent, on aqueous humor dynamics in pigmentary glaucoma. Ophthalmic Res 1996 28 312-318. 

Unnerstall JR, Kopajtic TA, Kuhar MJ. 1984. Distribution of alpha 2 agonist binding sites in the rat and human central nervous system Analysis of some functional, anatomic correlates of the pharmacologic effects of clonidine and related adrenergic agents. Brain Res 319 69-101. 

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