A-adrenoceptor agonists

Fig. 10. The postulated interaction of a-adrenoceptor agonists with the receptor. The Easson-Stedman hypothesis suggests that (R)-noradrenaline is most potent owing to its three points of attachment () to the adrenoceptor, whereas dopamine and (5)-noradrenaline are equal in activity, but less active...

Phenylephrine. Phenylephrine hydrochloride is an a -adrenoceptor agonist. Phenylephrine produces powerful vasoconstrictor and hypertensive responses. This results in baroreceptor activation of a reflex bradycardia and thus is useful in the treatment of supraventricular tachyarrhythmias. Unlike epinephrine [51-43-4] the actions of which are relatively transient, phenylephrine responses are more sustained (20 min after iv dosing and 50 min after subcutaneous dosing) (86). 

Hypotension is defined as abnormally low blood pressure. In most cases, hypotension is adequately treated with general measures (e.g. physical exercise), dtug treatment is rarely required. Drugs used for the treatment of hypotension include a-adrenoceptor agonists and compounds which activate both a and (3 adrenoceptors. 

Several clinically used drags, e.g. salbirtamol (a/ -adrenoceptor agonist), propanol (a j3-adrenoceptor antagonist) and the 2-arylpropiotric acids (NSAIDs) are employed in... 

The major impediment to using the a-adrenoceptor agonist class is the extensive list of contraindications. 

Norepinephrine and epinephrine are potent a-adrenoceptor agonists, while isoproterenol, a synthetic... 

Tab. 2.2 Apparent partition coefficients of a-adrenoceptor agonists in liposome-buffer (log K, ) systems as indicated and in n-octanol buffer (log P) at 37°C. (Adapted from Table 1 of ref. 37)...

Q2 Mydriasis means the dilation of the pupil. Miosis is the constriction of the pupil. Muscarinic antagonists and alpha-adrenoceptor (a-adrenoceptor) agonists cause dilation of the pupil. However, muscarinic agonists and a-adrenoceptor antagonists cause contraction of the pupil. 

Q4 Yes. Rob has used both atropine and phenylephrine this afternoon. Muscarinic antagonists such as atropine, tropicamide and cyclopentolate cause dilation of the pupils. The a-adrenoceptor agonists, such as phenylephrine, also produce my driasis. Mydriasis may cause acute closed-angle glaucoma in some patients. It is unlikely that a very small amount of cocaine in the eye would cause problems, but in cocaine overdose pupils become widely dilated. This is due to blockade of uptake 1, a process normally involved in terminating the effects of noradrenaline. In the presence of cocaine the effects of sympathetic stimulation on the eye would be prolonged and the pupil would dilate. Morphine causes constriction of the pupils via opiate receptors. 

Mydriasis (dilation of the pupil) can be caused by muscarinic antagonists and a-adrenoceptor agonists. Mydriasis may cause acute closed-angle glaucoma in some patients. 

Unwanted interactions include the loss of the antihypertensive effect of p-blockers when common cold remedies containing ephedrine, phenylpropanolamine or phenylephrine are taken, usually unknown to the doctor their a-adrenoceptor agonist action is unrestrained in the p-blocked patient. 

Eigot alkaloids. The naturally occurring alkaloids with effective a-adrenoceptor blocking actions are also powerful a-adrenoceptor agonists, i.e. they are... 

Horie K, Obika K, Foglar R, Tsujimoto G. Selectivity of the imidazoline a-adrenoceptor agonists (oxymetazoline and cirazoline) for human cloned a,-adrenoceptor subtypes. Br J Pharmacol 1995 116 1611-1618. 

Vlahakos D, Gavras I, Gavras H. a-Adrenoceptor agonists applied in the area of the nucleus tractus solitarii in the rat effect of anesthesia on cardiovascular responses. Brain Res 1985 347 372-375. 

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