Abstract

Sibutramine is a serotonin and norepinephrine reuptake inhibitor, used in the treatment of obesity. In this study, cardiovascular effects of sibutramine (0.9, 3, or 9 mg kg^-1 i.p.) were measured in conscious Sprague-Dawley rats, in the absence and presence of β- and/or α-adrenoceptor antagonism (with propranolol and/or phentolamine, respectively). Sibutramine caused pressor and tachycardic effects, with celiac and mesenteric vasoconstrictions, and hyperemic hindquarters vasodilatation. Pretreatment with propranolol inhibited the tachycardic and hindquarters vasodilator effect of sibutramine, whereas phentolamine inhibited the pressor and vasoconstrictor effects of sibutramine. In the presence of phentolamine, sibutramine caused hyperemic mese nteric vasodilatation. In preconstricted, isolated, mesenteric vessels, sibutramine and its metabolites BTS 54 505 (N-desmethylsibutramine) and BTS 54 354 (N-didesmethylsibutramine) (10 μM) produced significant vasodilatations. Neither sibutramine nor BTS 54 505 enhanced vessel sensitivity to norepinephrine, whereas BTS 54 354 produced a significant leftward shift in the concentration-response curve to norepinephrine. Collectively, the results indicate that the overt cardiovascular effects of sibutramine involve α-adrenoceptor-mediated celiac and mesenteric vasoconstrictions, and β-adrenoceptor-mediated hindquarters vasodilatation and tachycardia. The mesenteric vasodilator response to sibutramine, seen in the presence of phentolamine, may be a direct effect of the drug and/or its metabolites, on vessel tone. The cardiovascular effects of sibutramine in vivo may be secondary to inhibition of peripheral and/or central reuptake of monoamines by the metabolites BTS 54 354 and/or BTS 54 505. It remains to explain why BTS 54 354, but not BTS 54 505, enhanced norepinephrine sensitivity in vitro, because both metabolites are potent inhibitors of the norepinephrine transporter.