Binding and action of glucagon in cultured mouse astrocytes

This study investigates glucagon binding in primary cultures of differentiated mouse astrocytes and the effect of glucagon on intracellular cAMP accumulation. Binding of 125I-glucagon (0.53 nM) to mouse astrocyte suspensions reached equilibrium after 10 min at 22 degrees C. Equilibrium binding corresponded to 46 +/- 15 pmol/mg protein (n = 3) representing approximately 10,000 occupied sites per cell at the tracer concentration used. Dissociation occurred with a half-time of 2.5 min at 22 degrees C and was not accelerated in the presence of unlabelled glucagon (1 microM). Scatchard analysis suggested the presence of more than one class of binding site. The Ka for the higher affinity sites was 5.7-7.4 x 10(6) M-1. The Ka for the lower affinity sites was 3.6-5.3 x 10(4) M-1. The results suggest the presence of approximately 43,000 high affinity sites per cell. Binding was inhibited by unlabelled glucagon with an IC50 of 50 nM but unaffected by insulin and somatostatin. However, no 125I-glucagon binding could be detected when intact monolayer cells attached to culture dishes were used. Glucagon stimulated cyclic-AMP accumulation in both cell suspensions and intact monolayer cells in a dose-dependent fashion. However high concentrations were required when compared to the receptor-binding studies. Marked degradation of 125I-glucagon by astrocytes during binding experiments was observed and this was inhibited by unlabelled glucagon but also by insulin and desoctapeptide insulin.