title: Mechanisms and time course of beta 1 adrenoceptor desensitisation in mammalian cardiac myocytes.
abstract: To study the mechanisms and time course of beta1 adrenoceptor desensitisation in mammalian heart tissue neonatal rat cardiac myocytes (greater than 90% pure) were cultured in serum free medium. Cells were exposed to 1 mumol X litre-1 (-)-isoprenaline for 30 min, 4 h, and 16 h. In myocyte membranes mean(SEM)) 125I-iodocyanopindolol binding was 167(46) pmol X litre-1 (n = 5) and did not differ at 30 min, 4 h, or 16 h in control compared with (-)-isoprenaline treated cells. The maximum number of binding sites was 84(32) fmol X mg protein-1 and was unchanged at 30 min, but (-)-isoprenaline stimulated adenylate cyclase activity significantly decreased from 221(62) to 103(37) pmol X mg protein-1 30 min-1. (-)-Isoprenaline competition curves at 30 min showed a significant increase in the proportion of low affinity binding sites from 46% to 62% (n = 5). By 4 h the maximum number of binding sites was significantly decreased by 54%, adenylate cyclase activity remained depressed, and agonist affinity decreased threefold in the (-)-isoprenaline treated cells. At 16 h (-)-isoprenaline treated cells showed alterations similar to the 4 h values in the maximum number of binding sites, adenylate cyclase activity, and affinity for (-)-isoprenaline. (-)-Isoprenaline stimulated adenylate cyclase activity took 72 h to recover after desensitisation. Overnight ultracentrifugation of the cytosol showed a significant 40% increase in beta adrenoceptor density in cells exposed to (-)-isoprenaline for 4 h (n = 5), suggesting receptor internalisation.(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D000262:Adenylyl Cyclases / Q000378:metabolism; D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D006321:Heart / Q000187:drug effects; D066298:In Vitro Techniques; D007545:Isoproterenol / Q000494:pharmacology; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D011869:Radioligand Assay; D051381:Rats; D011943:Receptors, Adrenergic, beta / Q000187:drug effects / Q000378:metabolism*; D013997:Time Factors
chemical_list: D011943:Receptors, Adrenergic, beta; D000262:Adenylyl Cyclases; D007545:Isoproterenol