Desensitization of /5-receptor-linked adenylate cyclase occurs after prolonged occupancy of the /8-receptors by their agonists. We have followed the development and recovery from "down'-regulation of ^-receptors in enzymatically dissociated cardiac myocytes by using the hydrophilic antagonist [ 3 H]-CGP-12177 to identify surface-bound /3-receptors. After in vitro incubation with (-)-isoproterenol, almost 50% of the /3-receptors are lost within 10 minutes. Isoproterenol-mediated cyclic adenosine

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... monophosphate accumulation by isolated myocytes was also decreased after a 15-minute preincubation with isoproterenol. 'Lost' /3-receptors can, however, be recovered when isoproterenol-pretreated, washed cardiac myocytes are incubated at 37°C, 85 ± 7% of the lost /3-receptors have returned to the cell surface after 20 minutes of incubation. The requirements for such recycling were investigated. Receptor recovery does not depend on de novo protein synthesis, since it is unaffected by prior exposure to cycloheximide. It is, however, dependent on cellular energy, because it is prevented by adenosine triphosphate depletion and involves a lysosomal step since it is inhibited by the lysomotropic agent, chloroquine. In addition, the Golgi apparatus and the microtubules are involved in the ^-receptor recycling to the cell surface, as evidenced by the inhibitory effects of monensin and colchicine, respectively. The mechanism of isoproterenol-induced down-regulation of cardiac /3-receptors involves a rapid, reversible cycling to and from the cytosol and the cell membrane. This intracellular receptor traffic is energy dependent, requires several structures, including lysosomes and microtubules, and may be modified by pathological processes involving the heart. (Circ Res 55: 524-531, 1984) Methods Experiments were carried out on adult male Sprague-Dawley rats weighing 250-350 g. Cardiac myocytes were prepared by enzymatic dissociation using a procedure previously described (Limas, 1982) . Briefly, rat ventricles were finely minced and washed free of blood with cold (4°C) dissociation buffer containing (g/liter): NaCl, 6.8;