The effects of insulin replacement and withdrawal on hepatic ultrastructure and biochemistry

Abstract

This study examines the early hepatic biochemical and ultrastructural responses to insulin replacement in streptozotocin-diabetic rats and insulin withdrawal from insulin-maintained diabetic rats. Insulin administration rapidly lowered plasma glucose and the elevated glucose-6-phosphatase (G-6-Pase) specific activity of the diabetic rats. However, hepatic glycogen did not increase until after 3 hr of insulin treatment. Hepatic ultrastructure responded to insulin replacement after the decline in glucose and G-6-Pase. This was seen in periportal hepatocytes as a reduction in the close association between smooth endoplasmic reticulum (SER) and glycogen particles in the diabetic animals. The treated rats showed hepatic SER restricted to the periphery of glycogen masses, as is characteristic of these cells from normal rats, in many cells by 6 hr and all cells by 18 hr. Insulin withdrawal from insulin-treated diabetic rats elicited nearly a total reversal of the above events. Plasma insulin declined to a value half that of the normal rats by 6 hr after withdrawal; concurrently, plasma glucose rose sharply to hyperglycemic values as hepatic glycogen content dropped. Following the rise in plasma glucose and fall in glycogen content, G-6-Pase specific activity increased and by 16 hr reached the high values characteristic of the diabetic animal. Hepatic ultrastructure was also changed as evidenced by an intrusion of elements of the SER into the dense glycogen masses; the result was dispersed glycogen closely associated with SER as seen in the diabetic animal. It is concluded that the hepatic response to insulin replacement in diabetic animals and diabetic onset in insulin-withdrawn animals is rapid and occurs through defined stages.