Ephedrine
Ephedrine is a naturally occurring alkaloid obtained from the Chinese shrub Ephedra vulgaris. It is now synthesized chemically and the clinically used ephedrine is levo form.
Mechanism of action
It is mixed acting sympathomimmetic which can directly stimulate both α and β receptors and cause release of endogenous nor adrenaline.
Pharmacological effects
Similar to epinephrine, but is many times less potent than adrenaline. However it is 7-10 times long acting and produces the effects even after oral administration. It produced mild CNS stimulation that is evident as increased alertness, decreased fatigue and insomnia.
Unlike adrenaline and or adrenaline repeated injetions of ephedrine produces progressively diminishing response and this phenomenon is called “tachyphylaxis”.
- Tachyphylaxis is attributes to the following-
- Repeated administration, gradually diminish the available stores of endogenous noradrenaline.
- When cardiovascular system is stimulated, reflex mechanisms are pressed into action to return the heamodynamics towards normally.
- As adrenergic receptors are already occupied by previously administered long acting ephedrine, less and less number of adrenergic receptors are available for subsequent doses of ephedrine.
Pharmacokinetics
Rapidly absorbed after oral or parenteral administration. Also absorbed from topical instillation in the eye to produce mydriasis. It is a poor substrate for both MAO or COMT and is largely excreted unchanged.
Clinical uses
It lacks selectively and its efficacy is low. Mainly used in urinary incontinence and bronchoconstriction. Advantages over other sympathomimetics are long duration of action (hence no need of infusion) and oral administration
Pseudoephedrine
Pseudoephedrine is a stereoisomer of ephedrine. It has fewer CNS and CVS effects. It is also a poor bronchodilator, because of its little β-agonistic activity.
It is mainly used as a decongestant in upper respiratory tract problems associated with profuse secretion.
