Sympathomimetics, a-Sympatholytics

Vasoconstriction induced by an a-sympathomimetic is followed by a phase of enhanced blood flow (reactive hyperemia, A). This reaction can be observed after the application of a-sympa-thomimetics (naphazoline, tetrahydro-zoline, xylometazoline) to the nasal mucosa. Initially, vasoconstriction reduces mucosal blood flow and, hence, capillary pressure. Fluid exuded into the interstitial space is drained through the veins, thus shrinking the nasal mucosa. 

Due to the reduced supply of fluid, secretion of nasal mucus decreases. In coryza, nasal patency is restored. However, after vasoconstriction subsides, reactive hyperemia causes renewed exudation of plasma fluid into the interstitial space, the nose is stuffy again, and the patient feels a need to reapply decongestant. In this way, a vicious cycle threatens. Besides rebound congestion, persistent use of a decongestant entails the risk of atrophic damage caused by prolonged hypoxia of the nasal mucosa. 

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Presynaptic a2-adrenoceptors function like sensors that enable norepinephrine concentration outside the axolemma to be monitored, thus regulating its release via a local feedback mechanism. When presynaptic a2-re-ceptors are stimulated, further release of norepinephrine is inhibited. Conversely, their blockade leads to uncontrolled release of norepinephrine with an overt enhancement of sympathetic effects at Pi-adrenoceptor-mediated myocardial neuroeffector junctions, resulting in tachycardia and tachyarrhythmia. 

In benign hyperplasia of the prostate, a-blockers (terazosin, alfuzosin) may serve to lower tonus of smooth musculature in the prostatic region and thereby facilitate micturition (p. 252). 

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