Mechanism of IsoprenalineInduced Refractoriness of the βAdrenoceptor—Adenylate Cyclase Syste in Chick Embryo Cardiac Cells

We examined the role of β-adrenoceptors in regulating cardiac cell cyclic adenosine monophosphate (cyclic AMP) concentrations during β-receptor activation. Chick embryo cardiac cells in primary tissue culture respond to 50 JUM isoprenaline with a rapid, but transient, increase in intracellular cyclic AMP. Continued exposure of these cells to this concentration of isoprenaline results in a rapid time-dependent decrease in maximum β-receptor responsiveness. In vitrodetermination of adenylate cyclase and 125I-iodohydroxybenzyl pindoiol specific β-receptor binding sites in 35,000 × gpaniculate cell fractions and phosphodiesterase activity in cell homogenates suggest that the decrease in the cells' ability to raise intracellular cyclic AMP concentration is mainly due to an initial “uncoupling” of the β-receptor-adenylate cyclase system. During 16 hr exposure of cells to varying isoprenaline concentrations, the magnitude of the cells' ability to increase intracellular cyclic AMP and the concentration of β-receptor binding sites were inversely related to the isoprenaline concentration to which they had been previously exposed. On removal of isoprenaline, both β-receptor response and receptor concentration returned slowly towards control levels over the next 24 hr. These results suggest that loss of β-receptor responsiveness of cardiac cells during prolonged exposure to isoprenaline involves both alterations in receptor number and alterations of the adenylate cyclase system distal to the receptor.