Quantitative assessment of the pre- and postsynaptic alpha adrenoceptor antagonist potency of amitriptyline.
The pre- and postsynaptic alpha adrenoceptor blocking affinity of amitriptyline was determined in isolated tissues by Schild regression analysis. In the absence of uptake-1 blockade with cocaine, amitriptyline treatment (3 X 10(-8)-3 X 10(-6) M) affected only marginally norepinephrine concentration-response curves in the rat anococcygeus muscle; a result suggesting opposing pharmacological effects (uptake-1 blockade and alpha blockade). After cocaine (3 X 10(-5) M) treatment, amitriptyline (3 X 10(-8)-3 X 10(-6) M) antagonized competitively concentration-response curves to norepinephrine, yielding a postsynaptic pKb of 7.51. A similar pKb was obtained when methoxamine was the agonist. Presynaptic alpha blocking affinity was determined by using the field-stimulated rat vas deferens. Stimulus conditions were chosen which minimized the inhibition of twitch height by high concentrations of cocaine. Using these conditions (10 Hz, 200 msec duration at 100-sec intervals), amitriptyline antagonized competitively clonidine inhibition of field-stimulated twitch contractions, yielding at pKb of 5.23. The presynaptic pKb was not changed in the presence of theophylline (10(-4) M). Amitriptyline was also observed to increase the release of [3H]norepinephrine from the field-stimulated rat anococcygeus muscle pretreated with cocaine. Although this effect primarily reflects alpha blockade, other biochemical and presynaptic mechanisms may be involved. Comparing the pre- and postsynaptic alpha blocking affinities indicates that amitriptyline has 191 X greater affinity for post- than presynaptic alpha adrenoceptors (i.e., alpha-1 much greater than alpha-2). The relevance of these observations to the mechanism of action and side effects of amitriptyline are discussed.
D000317:Adrenergic alpha-Antagonists; D000639:Amitriptyline / Q000494:pharmacology*; D000818:Animals; D003042:Cocaine / Q000494:pharmacology; D066298:In Vitro Techniques; D008297:Male; D008729:Methoxamine / Q000494:pharmacology; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects; D009638:Norepinephrine / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D013569:Synapses / Q000187:drug effects*; D014649:Vas Deferens
D000317:Adrenergic alpha-Antagonists; D000639:Amitriptyline; D008729:Methoxamine; D003042:Cocaine; D009638:Norepinephrine