Rapid alterations induced by insulin in hepatocyte ultrastructure and glycogen levels

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Abstract
The speed with which insulin alters hepatocyte ultrastructure and glycogen levels in insulin‐deficient rats has been studied. Insulin deficiency was induced with alloxan, followed by insulin treatment with regular and NPH insulin. Rats were killed at various times after the insulin injection, blood samples were obtained, plasma glucose levels were determined, and liver samples were prepared for electron microscopy and glycogen determinations. Plasma glucose levels in insulin‐deficient rats declined to normal values by 4 hours post insulin, returning to insulin‐deficient levels by 8 hours post insulin. Hepatic glycogen was considerably reduced in the insulin‐deficient rats. By 1 hour post insulin hepatic glycogen increased, reached maximal levels by 8 hours, then declined to insulin‐deficient levels by 36 hours. The ultrastructural appearance of both centrilobular and periportal hepatocytes from insulin‐deficient rats showed abundant vesicular smooth endoplasmic reticulum (SER), decreased rough endoplasmic reticulum (RER), and enlarged RER intracisternal spaces. One‐half hour post insulin, centrilobular hepatocytes were unchanged. In periportal hepatocytes, however, vesicular SER was no longer visible, the RER intracisternal spaces appeared normal, and the amount of RER had increased. By 1 hour post insulin the centrilobular hepatocytes showed similar ultrastructural changes. These changes became more pronounced in the next few hours and remained through 24 hours. By 36 hours both centrilobular and periportal hepatocytes appeared similar to those in the insulin‐deficient rat. These results demonstrate the rapid and lobular‐specific effects insulin has on the hepatocyte.