The tricyclic antidepressants amitriptyline, nortriptyline and imipramine are weak antagonists of human and rat alpha1B-adrenoceptors

Although it is long known that the tricyclic antidepressants amitriptyline, nortriptyline and imipramine inhibit the noradrenaline transporter and alpha(1)-adrenoceptors with similar affinities, which may lead to self-cancelling actions, the selectivity of these drugs for alpha(1)-adrenoceptor subtypes is unknown. The present study investigates the selectivity of amitriptyline, nortriptyline and imipramine for human recombinant and rat native alpha(1)-adrenoceptor subtypes. The selectivity of amitriptyline, nortriptyline and imipramine was investigated in HEK-293 cells expressing each of the human alpha(1)-subtypes and in rat native receptors from the vas deferens (alpha(1A)), spleen (alpha(1B)) and aorta (alpha(1D)) through [(3)H]prazosin binding, and noradrenaline-induced intracellular Ca(2+) increases and contraction assays. Amitriptyline, nortriptyline and imipramine showed considerably higher affinities for alpha(1A)- (approximately 25- to 80-fold) and alpha(1D)-adrenoceptors (approximately 10- to 25-fold) than for alpha(1B)-adrenoceptors in both contraction and [(3)H]prazosin binding assays with rat native and human receptors, respectively. In addition, amitriptyline, nortriptyline and imipramine were substantially more potent in the inhibition of noradrenaline-induced intracellular Ca(2+) increases in HEK-293 cells expressing alpha(1A)- or a truncated version of alpha(1D)-adrenoceptors which traffics more efficiently towards the cell membrane than in cells expressing alpha(1B)-adrenoceptors. Amitriptyline, nortriptyline and imipramine are much weaker antagonists of rat and human alpha(1B)-adrenoceptors than of alpha(1A)- and alpha(1D)-adrenoceptors. The differential affinities for these receptors indicate that the alpha(1)-adrenoceptor subtype which activation is most increased by the augmented noradrenaline availability resultant from the blockade of neuronal reuptake is the alpha(1B)-adrenoceptor. This may be important for the behavioural effects of these drugs.

Multiple effects of sibutramine on ejaculation and on vas deferens and seminal vesicle contractility

Sibutramine is an inhibitor of norepinephrine and 5-HT reuptake largely used in the management of obesity. Although a fairly safe drug, postmarketing adverse effects of sibutramine were reported including abnormal ejaculation in men. This study investigates the effects of sibutramine on ejaculation and vas deferens and seminal vesicle contractility. Adult male rats received sibutramine (5; 20; or 50 mg kg(-1), ip) and after 60 min were exposed to receptive females for determination of ejaculation parameters. The vasa deferentia and seminal vesicles of untreated rats were mounted in isolated organ baths for recording of isometric contractions and HEK293 cells loaded with fluorescent calcium indicator were used to measure intracellular Ca(2+) transients. Sibutramine 5 and 20 mg kg(-1) reduced ejaculation latency whereas 50 mg kg(-1) increased ejaculation latency. Sibutramine 3 to 30 microM greatly increased the sensitivity of the seminal vesicle and vas deferens to norepinephrine, but at concentrations higher than 10 microM there were striking depressions of maximal contractions induced by norepinephrine, carbachol and CaCl(2). In HEK293 cells, sibutramine 10 to 100 microM inhibited intracellular Ca(2+) transients induced by carbachol. Depending on the doses, sibutramine either facilitates or inhibits ejaculation. Apart from its actions in the central nervous system, facilitation of ejaculation may result from augmented sensitivity of smooth muscles to norepinephrine while reductions of intracellular Ca(2+) may be involved in the delayed ejaculation observed with high doses of sibutramine.

alpha1-Adrenoceptors in proximal segments of tail arteries from control and reserpinised rats

It has been recently shown that the supersensitivity of distal segments of the rat tail artery to phenylephrine after chemical sympathectomy with reserpine results from the appearance of alpha(1D)-adrenoceptors. It is known that both alpha(1A)- and alpha(1D)-adrenoceptors are involved in the contractions of proximal portions of the rat tail artery. Therefore, this study investigated whether sympathectomy with reserpine would induce supersensitivity in proximal segments of the rat tail artery, a tissue in which alpha(1D)-adrenoceptors are already functional. Proximal segments of tail arteries from reserpinised rats were three- to sixfold more sensitive to phenylephrine and methoxamine than were arteries from control rats (n = 6-2; p < 0.05). The imidazolines N-[5-(4,5-Dihydro-1H-imidazol-2-yl)-2-hydroxy-5,6,7,8-tetrahydronaphthalen-1-yl]methanesulfonamide hydrobromide (A-61603) and oxymetazoline, which activate selectively alpha(1A)-adrenoceptors, were equipotent in tail arteries from control and reserpinised rats (n = 4-2; p < 0.05), whereas buspirone, which activates selectively alpha(1D)-adrenoceptor, was approximately 4-fold more potent in tail arteries from reserpinised rats (n = 4-6; p < 0.05). Prazosin (nonselective) and 5-methylurapidil (alpha(1A)-selective), were competitive antagonists of contractions induced by phenylephrine and were equipotent in tail arteries from control and reserpinised rats (n = 4-6). The selective alpha(1D)-adrenoceptor antagonist 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione dihydrochloride (BMY-7378) presented similar complex antagonism in tail arteries from control and reserpinised rats, with Schild slopes much lower than 1.0 (p < 0.05, n = 4-6). Semiquantitative reverse transcriptase polymerase chain reaction (RT-PCR) revealed that mRNA encoding alpha(1A)-and alpha(1B)-adrenoceptors are similarly distributed in tail arteries from control and reserpinised rats, whereas mRNA for alpha(1D)-adrenoceptors is twice more abundant in the tail artery from reserpinised rats. In conclusion, the supersensitivity induced by reserpine is related only to alpha(1D)-adrenoceptors, even in tissues where this receptor subtype is already present and functional. Only the use of subtype-selective alpha(1)-adrenoceptor agonists detected the increased alpha(1D)-adrenoceptor component after reserpinisation, as the antagonists behaved similarly in tail arteries from control and reserpinised rats.