ROLE OF CYCLIC-AMP IN CARDIAC BETA-ADRENOCEPTOR DESENSITIZATION - STUDIES USING PRENALTEROL AND INHIBITORS OF PHOSPHODIESTERASE

The effects of prolonged exposure of cardiac cells in primary culture to the partial .beta.-adrenoceptor agonist prenalterol and inhibitors of phosphodiesterase on their subsequent ability to increase intracellular cAMP during a 5-min exposure to 50 .mu.M isoprenaline (receptor responsiveness) was studied. Although prenalterol possesses 7% of the agonist activity of isoprenaline on adenylate cyclase, it induces extensive .beta.-adrenoceptor desensitization. After exposing the cells to 1 .mu.M prenalterol for 3 h, .beta.-adrenoceptor responsiveness was reduced 40% (P < 0.05): after 12 h the reduction averaged 55%. Prolonging the incubation time to 48 h had no further effect on receptor desensitization. The desensitization magnitude was concentration dependent. Cells to exposed 10-8 M prenalterol for 16 h, reduced receptor responsiveness was by 19%, and at concentrations of 1 .mu.M and higher reduced responsiveness by 60% (P < 0.01). Receptor desensitization appeared to be due to an inability of receptors to activate adenylate cyclase as well as to receptor loss. To investigate if .beta.-adrenoceptor desensitization and receptor loss could be mediated by cAMP, the cells were exposed for 16 h to phosphodiesterase inhibitors. Cells exposed to the phosphodiesterase inhibitor isobutylmethylxanthine (0.1 mM) (which increased intracellular cAMP by between 50 and 150%) also induced receptor desensitization. Reduction in receptor responsiveness averaged 62% (P < 0.01). The responsiveness loss could be due by to an inability of receptors to activate adenylate cyclase was by receptor loss. .beta.-Adrenoceptor numbers were reduced by 24% (P < 0.025). A similar reduction in receptor numbers could be induced by exposing the cells to the phosphodiesterase inhibitor ICI 63197 (0.5 mM). In both instances, the affinity of the .beta.-adrenoceptor for the radioligand [125I]iodocyanopindolol was unaltered. .beta.-Adrenoceptor-induced desensitization of cardiac .beta.-adrenoceptors is due to an inability of receptors to activate adenylate cyclase and to an apparent loss of membrane receptors.