title: Alteration of sensitivity of adrenergic vascular responses after prolonged exposure to agonists via osmotic minipump.
abstract: A study was performed to determine whether a constant 1-week exposure to either alpha or beta agonists in vivo would allow alteration or manipulation of the responses of rat aortic alpha- and beta-adrenergic receptors. Osmotic minipumps delivering either phenylephrine, isoproterenol, or propranolol for 7 days at a dose of 3.2, 4.2, or 5.2 mg/kg/day, respectively, were implanted in male Holtzman rats under halothane anesthesia. Seven days later, rats were killed and aortic ring preparations were used to measure alpha- and beta-adrenergic responses. In phenylephrine-pretreated rats, alpha-adrenergic responses, as measured by contractions induced by phenylephrine, were markedly reduced (P less than 0.05) across a dose range of 10(-9) to 10(-6) M. In contrast, in these same phenylephrine-pretreated preparations, the beta-adrenergic responses involving isoproterenol-induced relaxation were significantly increased (P less than 0.05) across a dose range of 10(-7) to 10(-5) M. Isoproterenol pretreatment for 7 days resulted in a statistically significant reduction of beta-adrenergic aortic relaxation, whereas the alpha-adrenergic responses to phenylephrine remained unchanged compared with controls. Propranolol pretreatment had no effect on either alpha- or beta-adrenergic responses. These findings indicate that the alpha agonist-induced response after in vivo pretreatment induces reciprocal changes in the functionally related beta-adrenergic apparatus, and also suggest linkage between these two receptors. In contrast, the beta response appears to desensitize or downregulate in response to beta agonist exposure in a manner that seems to be independent of or to operate in the absence of an alteration of the alpha response.
mesh_terms: D000818:Animals; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D010656:Phenylephrine / Q000494:pharmacology; D011433:Propranolol / Q000494:pharmacology; D051381:Rats; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects; D011943:Receptors, Adrenergic, beta / Q000187:drug effects; D013566:Sympathomimetics / Q000008:administration & dosage*; D013997:Time Factors; D014655:Vascular Resistance / Q000187:drug effects*
chemical_list: D011942:Receptors, Adrenergic, alpha; D011943:Receptors, Adrenergic, beta; D013566:Sympathomimetics; D010656:Phenylephrine; D011433:Propranolol; D007545:Isoproterenol