title: Desensitization of beta-adrenergic receptors by pheochromocytoma.
abstract: Prolonged stimulation of cells by beta-adrenergic receptor agonists may lead to diminished responsiveness of the cells to subsequent activation by catecholamines. This phenomenon has been termed desensitization; the mechanism(s) for desensitization may involve an apparent loss in the number of beta-adrenergic receptors or an alteration in receptor-effector coupling. We have examined the consequences of prolonged stimulation of beta-adrenergic receptors in an interesting rat model harboring pheochromocytoma. New England Deaconess Hospital rats with transplanted pheochromocytomas developed systolic hypertension and plasma norepinephrine concentrations approximately 40-fold greater than controls. beta-Adrenergic receptors were quantitated in several tissues from controls and rats with transplanted pheochromocytoma using the beta-adrenergic receptor antagonist [125I]iodocyanopindolol. Down-regulation of beta 1-receptors was found in heart tissue (22.8 vs. 13.6 fmol/mg protein; P less than 0.001) and adipocytes (29,400 vs. 2,800 sites/cell; P less than 0.001). Also, maximal isoproterenol-stimulated cAMP accumulation in isolated adipocytes was diminished in pheochromocytomic animals (13.1 vs. 4.9 pmol cAMP/10(5) cells/min; P less than 0.05). Interestingly, there was no change in beta-receptors in lung and mesenteric artery, which predominantly contain beta 2-receptors. Furthermore, the competition curves of isoproterenol in the heart membranes from control and pheochromocytomic rats in the absence and presence of guanylylimidodiphosphate indicated uncoupling of the beta-adrenergic receptors in pheochromocytomic animals. Rats with pheochromocytoma secreting large amounts of norepinephrine provide a valuable model system for studying the in vivo development of desensitization.
mesh_terms: D000273:Adipose Tissue / Q000187:drug effects / Q000378:metabolism; D000310:Adrenal Gland Neoplasms / Q000503:physiopathology*; D000818:Animals; D001835:Body Weight; D002462:Cell Membrane / Q000378:metabolism; D000242:Cyclic AMP / Q000378:metabolism; D002149:Energy Intake; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D006352:Heart Ventricles / Q000378:metabolism; D020120:Iodocyanopindolol; D007545:Isoproterenol / Q000494:pharmacology; D007700:Kinetics; D008168:Lung / Q000378:metabolism; D008638:Mesenteric Arteries / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism; D010673:Pheochromocytoma / Q000503:physiopathology*; D010869:Pindolol / Q000031:analogs & derivatives / Q000378:metabolism; D051381:Rats; D011943:Receptors, Adrenergic, beta / Q000378:metabolism*
chemical_list: D011943:Receptors, Adrenergic, beta; D006165:Guanylyl Imidodiphosphate; D020120:Iodocyanopindolol; D010869:Pindolol; D000242:Cyclic AMP; D007545:Isoproterenol