title: Increased alpha 1- and decreased alpha 2-adrenoceptor sensitivities upon chronic treatment with imipramine in mediating cardiovascular responses in pithed rats.
abstract: After chronic treatment with imipramine (20 mg/kg, i.p., twice daily for 14 days) the pressor dose-response curves to phenylephrine, methoxamine and cirazoline (alpha 1-adrenoceptor agonists) significantly shifted to the left with decreased PD50 values in pithed rats; however, the dose-response curve to Sgd 101/75, a selective alpha 1-adrenoceptor agonist was not affected. On the other hand, the alpha 2-adrenoceptor agonists such as B-HT 920, xylazine and clonidine produced a rightward shift for both the pressor (increased PD50) and cardioinhibition (increased ID50) dose-response curves in these rats. These results required treatment with imipramine over 2 weeks. Chronic treatment with imipramine has reduced the antagonism by prazosin of the pressor effect of phenylephrine when compared with the dose-ratios between the 2 groups. On the contrary, the antagonism by piperoxan of the cardioinhibitory effect of B-HT 920 was rather enhanced by the treatment, but that of the pressor effect of B-HT 920 was little changed. In cerebrocortical membrane fractions obtained from rats pretreated with imipramine, Ki of phenylephrine to displace [3H]prazosin was decreased, whereas that of clonidine and yohimbine to displace [3H]yohimbine was increased. In conclusion, it is demonstrated that after chronic imipramine treatment the peripheral alpha 2-adrenoceptors (both presynaptic and postsynaptic sites) as well as central alpha 2-adrenoceptors respond with a decreased sensitivity to the alpha 2-adrenoceptor agonists, and moreover, this treatment produces an increased sensitivity of the central and peripheral alpha 1-adrenoceptors to the alpha 1-adrenoceptor full agonists.
mesh_terms: D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D001794:Blood Pressure / Q000187:drug effects; D002490:Central Nervous System / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D005260:Female; D006339:Heart Rate / Q000187:drug effects; D006439:Hemodynamics / Q000187:drug effects*; D007099:Imipramine / Q000494:pharmacology*; D008297:Male; D010525:Peripheral Nerves / Q000187:drug effects; D051381:Rats; D011919:Rats, Inbred Strains; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects*; D013116:Spinal Cord / Q000502:physiology; D013997:Time Factors
chemical_list: D000317:Adrenergic alpha-Antagonists; D011942:Receptors, Adrenergic, alpha; D007099:Imipramine