Glucagon-induced desensitization of adenylyl cyclase in primary cultures of chick hepatocytes. Evidence for multiple pathways.
Chick hepatocytes in primary culture have been used to study the homologous and heterologous pathways of glucagon-induced desensitization of adenylyl cyclase. Scatchard analysis and guanine nucleotide effects on dissociation kinetics indicate that the initial phase of homologous desensitization, an increase in low affinity glucagon receptors due to the rapid uncoupling of the receptor from Gs, is essentially complete within 5 min. These receptors recouple within 20 min upon removal of glucagon. Upon prolonged (2 h or more) exposure of hepatocytes to glucagon, disappearance of low affinity receptors from cell surface membranes constitutes the second phase of homologous desensitization. Recovery of these lost and presumably internalized receptors requires more than 12 h following the removal of glucagon but is not dependent on new protein synthesis. The heterologous phase of desensitization is slower, requiring 20-30 min of glucagon treatment to reach completion. Stimulation of adenylyl cyclase by hormonal and nonhormonal effectors is similarly reduced, indicating a common defect in this desensitized state. Agonist occupancy of other hormone receptors coupled to adenylyl cyclase in hepatocytes, such as beta-adrenergic, prostaglandin E1, and vasoactive intestinal peptide, results in heterologous desensitization. Heterologous desensitization is rapidly reversed (within 30 min) upon partial removal of glucagon, under conditions allowing the maintenance of the homologously desensitized state. Neither onset of nor recovery from heterologous desensitization requires protein synthesis. These data indicate that homologous and heterologous desensitization occurs by independent mechanisms. Homologous desensitization involves uncoupling of the glucagon receptor from Gs, followed by removal of these uncoupled receptors from the cell surface. Heterologous desensitization represents a second level of cellular control of hormonal responsiveness to be turned on when the cell is subjected to prolonged hormonal stimulation and withdrawn when hormone levels are lowered.
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