32-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...

Prolonged exposure of -adrenoceptor agonists down-regulates -adrenoceptors, ie, their number decreases and they become less responsive. On the other hand, prolonged exposure to -adrenoceptor antagonists (those without ISA) upregulates -adrenoceptors, ie, their numbers increase and they become more responsive. Therefore, patients on -adrenoceptor blocker therapy should be withdrawn from this medication gradually (40). 

Isoproterenol. Isoproterenol hydrochloride is an nonselective P-adrenoceptor agonist that is chemically related to NE. It mimics the effects of stimulation of the sympathetic innervation to the heart which are mediated by NE. It increases heart rate by increasing automaticity of the SA and AV nodes by increasing the rate of phase 4 diastoHc depolarization. It is used in the treatment of acute heart block and supraventricular bradyarrhythmias, although use of atropine is safer for bradyarrhythmias foUowing MI (86). 

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). 

Rilmenidine. RiLmenidine is a central a2 adrenoceptor agonist and has been shown to be a potent centrally acting antihypertensive agent without the prominent side effect of sedation. 

FIGURE 2.4 Occupancy-response curves for P-adrenoceptor agonists in transfected CHO cells. Occupancy (abscissae) calculated from binding affinity measured by displacement of [125I]iodocyanopindolol. Response measured as increases in cyclic AMP. Drawn from [3],... 

Kenakin, T. P., and Beek, D. (1980). Is prenalterol (H 133/80) really a selective beta-1 adrenoceptor agonist Tissue selectivity resulting selective beta-1 adrenoceptor agonist Tissue selectivity resulting from difference in stimulus-response relationships. J. Pharmacol. Exp. Ther. 213 406—413. 

FIGURE 4.19 Correlation of the GTP shift for P-adrenoceptor agonists in turkey erythocytes (ordinates) and intrinsic activity of the agonists in functional studies (abscissae). Data redrawn from [16]. 

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. 

The release of NO from the endothelium is induced by various chemical substances, including acetylcholine polypeptides such as substance P, bradykinin, and arginine vasopressin histamine ATP/ADP a2-adrenoceptor agonists thrombin and Ca2+ iono-phores. NO formed in response to mechanical stimuli like shear stress or transmural pressure plays an important role in maintaining basal blood flow. Endothelial NO causes vasodilatation, decreased... 

Tocolytics are drugs such as the selective (32-adrenoceptor agonists, e.g. salbutamol or terbutaline, which inhibit both the spontaneous and oxytocin-induced contractions of the pregnant uterus by relaxing... 

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