DOWN-REGULATION OF GLUCAGON RECEPTORS ON THE SURFACE OF BROILER ADIPOCYTES

A radioreceptor assay that specifically measures the affinity and conc entration of glucagon receptors on the surface of broiler adipocytes w as used to determine whether down-regulation of glucagon receptors is involved in the mechanism whereby glucagon induces desensitization of its ability to acutely stimulate lipolysis. Adipocytes from abdominal fat of 42- to 49-d-old female broilers were preincubated in vitro with 0 to 8 nM glucagon for 0 to 24 h before removal of glucagon by washin g and assessment of lipolysis during a subsequent 1 h incubation at 37 C and binding of ([(125)]iodo-Tyr(10))glucagon during an ensuing 6 h incubation at 12 C. In Experiment 1, pretreatment of adipocytes with 0 to 8 nM glucagon for 24 h reduced lipolysis 70% with a preincubation dose of glucagon (ED(50)) of 0.4 +/- 0.04 nM, whereas glucagon binding was decreased 75% with an ED(50) of 0.7 +/- 0.1 nM. In Experiment 2, pretreatment of adipocytes with 5.4 nM glucagon for 0 to 2 h decreased lipolysis 60% with a time of preincubation with glucagon (T-1/2) of < 1 min, whereas glucagon binding was reduced 40% with a T-1/2 of 2 +/- 0.6 min. In Experiment 3, detailed analysis of lipolysis and glucagon binding to adipocytes pretreated with 5.4 NM glucagon for 5 min, 2 h, or 24 h revealed that sensitivity of adipocytes to acute stimulation of lipolysis by glucagon and the number of glucagon receptors on the a dipocyte surface were decreased by glucagon pretreatment. The assay co ncentration of glucagon (EC(50)) for glucagon stimulation of lipolysis , a measure of adipocyte sensitivity, increased 0.8-fold at 5 min, 1.6 -fold at 2 h, and 4.9-fold at 24 h of preincubation. The number of low -affinity glucagon receptors decreased 12% at 5 min, 20% at 2 h, and 5 0% at 24 h of preincubation with glucagon. Throughout this study, decr eased glucagon binding was closely associated with reduced lipolysis, implicating down-regulation of cell-surface glucagon receptors in the mechanism whereby glucagon induces desensitization of its ability to a cutely stimulate lipolysis in broiler adipocytes.