title: Exposure of thymocytes to a low temperature (4 degrees C) inhibits the onset of their hormone-induced cellular refractoriness.
abstract: The hormonal response of viable mouse thymocytes is radically dependent of their ambient temperature. While at 37 degrees C the cells respond to isoproterenol by an abrupt rise (within 30 s) followed by a exponential decline in the level of intracellular cAMP, at 4 degrees C the level of cAMP remains high, i.e. there is an inhibition of the hormone-induced refractory state. These distinctly different patterns of response are reflected also in both the state of activation of cAMP-dependent protein kinase and the activity of adenylate cyclase. The inhibition of cellular refractoriness in the cold is shown to be fully reversible, lasting only as long as the hormone is present in the extracellular medium. Washing out the hormone or displacing it by a specific antagonist (propranolol) results in a decline of cAMP, of the activity ratio of the kinase, and of the activity of the adenylate cyclase back to basal values. Evidence is presented to show that at 4 degrees C there is no significant hormone-dependent decreases in cAMP degradation or efflux. On the other hand, the activity of adenylate cyclase remains persistently high, through neither the hormone-binding site of the receptor nor the active site of the catalytic subunit of the cyclase seem to be impaired. The different response pattern observed at 4 degrees C appears, therefore, to be associated with the transfer and the signal between these two sites and probably with the G/F protein (s). The possibility to dissect in a selective and reversible manner the process of hormonal stimulation (coupling) from the process of desensitization, which, under normal physiological conditions constitute consecutive and inseparable chain of events, leads us to a propose that the signal transfer which enables activation of adenylate cyclase is, somewhere along its way, distinct from the signal transfer which brings about the onset of the refractory state, and to conclude that these two processes are partially autonomous and regulated by either two different proteins or two different sites on the same protein. The postulated proteins (or sites) should, therefore, differ in their sensitivity to temperature changes, a difference which may be most useful in the identification and isolation of the molecular species involved and in the study of their properties and their mechanism of action.
mesh_terms: D000262:Adenylyl Cyclases / Q000378:metabolism; D000818:Animals; D003080:Cold Temperature; D000242:Cyclic AMP / Q000378:metabolism*; D007545:Isoproterenol / Q000494:pharmacology*; D007700:Kinetics; D051379:Mice; D008815:Mice, Inbred Strains; D011494:Protein Kinases / Q000378:metabolism*; D013950:Thymus Gland / Q000187:drug effects / Q000378:metabolism*
chemical_list: D000242:Cyclic AMP; D011494:Protein Kinases; D000262:Adenylyl Cyclases; D007545:Isoproterenol