Viable mouse thymocytes as a model system for studying the onset of hormone-induced cellular refractoriness.

Mouse thymocytes are characterized as a model cellular system for studying the onset of hormone-induced cellular refractoriness (desensitization). This system has the following combination of useful features. (a) The cells can be isolated without the use of digestive enzymes, avoiding possible damage to surface receptors or to other exposed membranal constituents. (b) They can be kept viable for several hours, a period during which both stimulation and desensitization get well under way. (c) They can be stimulated by a variety of hormones which function via cAMP (beta-agonists, prostaglandin E1 and specific thymic humoral factors). (d) Their desensitization is receptor-specific. (e) They can be readily ruptured under mild conditions so as to allow a physiologically relevant biochemical analysis of hormonal stimulation and desensitization. (f) The hormonal response of these cells can be monitored simultaneously by the activation of adenylate cyclase, by the intracellular level of cAMP, and by the activation of cAMP-dependent protein kinase (which functions as a metabolic sensor for cAMP). In this cellular system, desensitization does not involve processes such as the efflux of cAMP, the activation of cAMP-phosphodiesterase or the synthesis of a protein mediator. On the other hand, desensitization can be accounted for by a hormone-triggered inactivation of the adenylate cyclase system. The immediate desensitization of thymocytes is reversible and occurs without apparent loss of functional receptors. Continuous presence of hormone is shown to be required not only for triggering the chain of events which leads to the readily reversible desensitization, but also for the process which transfers the cells to the subsequent, 'locked' desensitized state.