Adipose tissue in diabetes

Cyclic AMP, cyclic GMP and glucose utilization by diabetic rat fat cells

Fat cells from epididymal adipose tissue from normal and streptozotocin-diabetic rats were studied to determine glucose utilization and cyclic nucleotide levels. Diabetic rat fat cells present a higher cAMP content (P less than 0.05) compared with controls. Addition of insulin decreases within 10-min incubation the cAMP content in both normal and diabetic cells (P less than 0.05). However, the value obtained in the latter remains by 25% higher than that of normal cells not exposed to insulin. No changes in cGMP were detected. Pretreatment of the diabetic animals during two days with propranolol (1 mg kg body wt-1 day-1) induces the decrease to normal levels of the fat cell cAMP content. However, it persists the impairment on glucose utilization observed in fat cells from diabetic animals. It seems that the increase in the intracellular amount of cAMP found in fat cells from diabetic rats is not involved, at least directly, to the impaired glucose utilization found in the diabetic state. Furthermore, through an unknown mechanism, pretreatment with propranolol can induce a drop in fat tissue cAMP toward normal values without normalizing glucose utilization.

Increased sensitivity of diabetic rat adipose tissue towards the lipolytic action of epinephrine

Adipose tissue from streptozotocin-diabetic rats exhibits half-maximal lipolytic responses (FFA, glycerol release, increase in tissue FFA) to epinephrine at hormone concentrations 5-10 times lowere than those required for half-maximal stimulation of lipolysis in adipose tissue from normal rats. The lipolytic response to epinephrine also occurs more promptly and the antilipolytic effect of insulin in the presence of submaximal epinephrine conceptrations is much less pronounced than in normal tissue. In contrast, diabetic adipose tissue is less responsive to ACTH and glucagon than normal tissue. Half-maximal lipolytic responses are elicited by similar dibutyryl cyclic AMP concentrations in both tissues. Insulin treatment of diabetic rats during 24 hrs restores the lipolytic response of their adipose tissue to epinephrine to nearly normal. Our findings point to an abnormality of diabetic adipose tissue possibly related to the hypersensitivity of catecholamines encountered in denervated organs which are adrenergically innvervated. They are consistent with present concept of different hormone discriminators on the fat cell membrane and offer a further explanation for increased FFA mobilization in the diabetic state.