Journal of Animal Science, Vol 73, Issue 3 728-737, Copyright © 1995 by American Society of Animal Science

JOURNAL ARTICLE

Glucagon binding to receptors on the surface of chicken adipocytes

T. P. Oscar
Agricultural Research Service, USDA, Georgetown, DE 19947, USA.

Adipocytes isolated from abdominal fat of chickens were used 1) to demonstrate that [125I]-tyr10-glucagon binds with all the characteristics of a hormone-receptor interaction and 2) to determine whether glucagon binds to one or more receptor sites. Binding of [125I]-tyr10-glucagon was characterized at 12 degrees C, a temperature found to completely inhibit internalization of the radioligand. Culturing adipocytes increased radioligand binding by 60 to 100% and, therefore, fat cells were incubated for 72 h before characterizing binding. Binding of [125I]-tyr10-glucagon to chicken adipocytes reached steady-state, was reversible, was specific for glucagon, was saturable, and increased linearly as a function of fat cell concentration. Both kinetic and steady-state experiments indicated that [125I]-tyr10-glucagon bound to two sites on chicken adipocytes. Equilibrium dissociation constants (Kd) of 54 +/- 16 pM and 3.3 +/- 1.3 nM were obtained for [125I]-tyr10-glucagon, whereas Kd of .49 and 81 nM were calculated for mammalian glucagon. There were 4,831 +/- 1,057 high-affinity receptors and 200,780 +/- 63,404 low-affinity receptors per adipocyte. Thus, only 2.3% of the glucagon binding sites were of high affinity. In addition to higher binding affinity, [125I]-tyr10-glucagon stimulated glycerol release from chicken adipocytes with greater potency than porcine glucagon. Therefore, it was concluded that [125I]-tyr10-glucagon was a supra-agonist that bound to high- and low-affinity receptors on the surface of chicken adipocytes with all the characteristics of a hormone-receptor interaction.