We have studied the effects of alioxan-induced diabetes and subsequent insulin replacement on albumin and total hepatic protein synthesis. Diabetes resulted in a reduction to approximately 20% of normal in albumin synthesis relative to the rate of total protein synthesis in vivo and a reduction to 10% in the absolute rate of albumin secretion by perfused livers. In contrast, the synthesis of total secretory protein and retained hepatic protein was affected to a lesser extent by diabetes.Treatment of diabetic rats with insulin restored rates of albumin and total hepatic protein synthesis to normal levels. The molecular basis of these alterations in albumin synthesis was investigated by examining albumin mRNA levels in livers of normal, diabetic, and insulin-treated diabetic animals. The level of albumin mRNA, whether assayed by cell-free translation or by hybridization to a specific complementary DNA probe, was markedly decreased in livers of diabetic animals and was restored to normal by insulin treatment. These changes occurred in parallel with changes in the rates of albumin secretion observed in perfused liver, suggesting that albumin mRNA content is the primary factor responsible for altering rates of albumin synthesis under these conditions.A role for insulin in regulating protein synthesis in liver has not been clearly established. In rats, experimentally induced diabetes caused no change in liver protein synthesis rates in vivo, while synthesis rates in skeletal muscle and heart were markedly impaired (1). These findings are supported by studies using perfused tissues from normal rats in which measurements of protein synthesis showed that addition of insulin had no effect on the synthesis of hepatic proteins (2) but stimulated the synthesis of skeletal muscle (3) and heart (4) proteins. While some workers have reported that insulin stimulates liver protein synthesis (5), rates of protein synthesis were not determined rigorously in these experiments.Studies of protein synthesis in liver are complicated by the fact that this tissue synthesizes both proteins for export and intracellular proteins. The studies mentioned above measured primarily the synthesis of intracellular proteins. Other studies with perfused livers (6) or isolated hepatocytes (7) indicated an impaired secretion of proteins in diabetic rats. Such a defect may relate to the marked disruption and disorganization of the rough endoplasmic reticulum observed in liver of diabetic rats (8-10), as export proteins are synthesized almost exclusively on the rough endoplasmic reticulum (11).In the studies presented here, we have investigated the effect of alloxan-induced diabetes and subsequent insulin replacement on albumin and total protein production in vivo and in perfused livers, and have correlated the observed changes in albumin synthesis with changes in albumin mRNA levels. Diabetes resulted in a marked decrease in albumin synthesis and secretion in vivo and in perfused livers, with a corresponding decrease in the albumin mRNA level. T...

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Vanadate has been shown to have a number of insulin-like effects in various cells, including isolated rat adipocytes. In the present study we compared the activities of vanadate and insulin in isolated fat cells using a number of different assays of insulin-like activity. Both insulin and vanadate stimulated [2-3H]glucose incorporation into fat cell lipid in a dose-dependent manner, but the maximal effect of vanadate was markedly greater than that of insulin. At 10(-2) M vanadate the effect was 3-4 times as great as the maximal effect of insulin. This effect was dependent on specific glucose transport. Combinations of insulin and vanadate were not more effective than vanadate alone. Vanadate also produced antilipolysis with an effect somewhat greater than that of insulin. Using [U-14C]glucose both vanadate and insulin stimulated 14CO2 production and [14C]glucose incorporation into lipid, and again the effect of vanadate was greater than that of insulin. Vanadate had a greater effect on 14CO2 production than on [14C]glucose incorporation into lipid. When [1-14C]glucose was used vanadate again had a significantly greater effect on 14CO2 production than did insulin, but when [6-14C]glucose was used the effects of vanadate and insulin were equal. These results demonstrate that vanadate has insulin-like effects in isolated fat cells, but it selectively stimulates certain pathways to a greater extent than does insulin. The greater effect of vanadate than insulin appears to be primarily on the pentose phosphate shunt, suggesting that this agent may be useful for examination of this intracellular pathway in fat cells.