Drugs acting on the sympathetic system

TTic sympathetic nervous system is impotiam in regulating organs such as the heart and peripheral vaseuluiure (Chapters 15 and I HI. The transmitter released from sympathetic nerve endings is norepinephrine INK I (noralrenaline. ) but. in response to some forms of stress, epinephrine (adrtmalinei is also re ea.sed from the adrenal medulla. These catecholamines are inactivated mainly by reuptake ). 

Sympathomimetic (left) are drugs that panially or completely mimic the actions of norepinephrine and epinephrine. They act either directly on a- and/or P-adrenoceptors ilefl. open column) or indirectly on the presynaptic terminals (top left, shaded), usually by causing the release of norepinephrine (Tlic elTecLs of adrenoceptor stimulation can be seen in the ligiire in Chapter . 

Sympathomimetic amines that act mainly by causing norepinephrine release (c.g, am fe tarn ine) have the ot /a, selectivity of norepinephrine. 

Keuptake of norepinephrine by a higli-affiniiy iranspon system (Uptake ) in the nerve terminals recaptures most of the transmitter and is the main nit thod of temiinaung its effects. A similar (extraneu-rona ) transport system (Uptake 2) exists in the tissues but is less selective and less easily satiitaied. 

Cocaine, in addition to being a local anaesthetic (Chapter 5). is a sympathomimetic because il inhibits the reuplake of norepinephrine by nerve terminals. It has an intense central stimulant effect that has made il a popular drug of abuse (Chapter. 11). 

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