Responses to glucagon from the adenylate cyclasecyclic adenosine monophosphate (cAMP) system in liver slices from control and streptozotocin-induced diabetic rats were compared. Tissue cAMP levels were similar in the basal state but responded poorly to glucagon (20 pg/ml-2 μg/ml) in diabetic rats. Insulin treatment of diabetic rats in vivo led to a reversal of the glucagon stimulation towards the values in the control rats. The basal and glucagon-stimulated activities of adenylate cyclase in crude membrane fractions were similar in both groups.
Plasma immunoreactive glucagon levels in diabetic rats were approximately three times higher than those in normal rats.
Liver slices obtained from normal rats, which were injected with glucagon (0.2 mg, i.m.) 45 min previously, also showed an impaired responsiveness to glucagon of tissue cAMP levels, while no significant difference in adenylate cyclase activity was observed between the normal and glucagon-treated rats.
These results suggest that the responsiveness of liver slices from the streptozotocin-induced diabetic rat has been modified by the preceding hyperglucagonemia. The reason for the observed differences between slices and crude membranes is not known.