glycogen and 2-aminoisobutyric-acid

Previous studies have shown that Wistar rats injected at birth (n0) with STZ (n0-STZ) develop as adults a noninsulin-dependent diabetic state characterized by a lack of insulin response to glucose in vivo, a mild basal hyperglycemia, and an impaired glucose tolerance. Our former in vivo studies using the insulin-glucose clamp technique revealed an increased insulin action upon hepatic glucose production in these animals. We have now cultured hepatocytes from these mildly diabetic rats in parallel with hepatocytes from control rats, to examine more closely basal and insulin-regulated glucose production and glucose incorporation into glycogen. In addition, we extended our investigation to other hepatic functions such as lipid synthesis and amino acid transport, which could not be studied in vivo. Although glucose production from glycogenolysis or gluconeogenesis in absence or presence of glucagon was identical in the two cell populations, glucagon-stimulated glycogenolysis was more sensitive to insulin action in diabetic hepatocytes. Similarly, insulin action on glucose incorporation into glycogen, lipogenesis, and amino acid transport were enhanced in diabetic hepatocytes. The hormone effect was manifested by an increase in the sensitivity and/or in the responsiveness, reflecting the multiplicity of the pathways whereby the insulin signal is transduced through the insulin receptor to multiple postreceptor sites. To gain insight into the possible mechanism of these disturbances, we evaluated the initial insulin receptor interaction and the kinase activity of the receptor beta-subunit. In accordance with our previous study on intact livers, we found no alteration in either of these parameters in n0-STZ rat hepatocytes. Thus, the present study clearly demonstrates that these diabetic rats exhibit a postreceptor hyperresponsiveness to insulin at the cellular level. It strengthens the notion that a beta-cell deficiency with glucose intolerance does not necessarily lead to a hepatic insulin resistance.

Topics: Aminoisobutyric Acids; Animals; Animals, Newborn; Blood Glucose; Cells, Cultured; Diabetes Mellitus, Experimental; Female; Glucagon; Gluconeogenesis; Glucose; Glycogen; Insulin; Lipids; Liver; Protein-Tyrosine Kinases; Rats; Rats, Inbred Strains; Receptor, Insulin

In Madin-Darby canine kidney (MDCK) cells, specific plasma membrane binding of [125I]insulin was undetectable. Correspondingly, neither insulin-stimulated incorporation of [14C]glucose into glycogen nor insulin-induced uptake of radiolabeled alpha-aminoisobutyrate ([ 3H]AIB) could be demonstrated. These results suggested that MDCK cells lack specific cell surface insulin receptors. To further examine this question intact MDCK cells were preincubated with antireceptor serum and subsequently labeled with [125I]protein A; however, insulin receptors were not detected. Control H4 hepatoma cells bound insulin, responded with increased glycogen synthesis and amino acid uptake, and possessed immunologically recognizable insulin receptor components. The insulin-associated response of stimulated [3H]AIB uptake was induced in MDCK cells by the insulinomimetic lectins concanavalin A (130-140% of basal value at concentrations of 10-40 micrograms/ml) and wheat germ agglutinin (140-160% of basal value at concentrations of 10-30 micrograms/ml). This stimulation was abolished by the respective lectin-specific monosaccharides D-mannose and N-acetyl-D-glucosamine. Together, these data indicate that the insulin-like activity of concanavalin A and wheat germ agglutinin can be elicited in MDCK cells even in the apparent absence of specific plasma membrane insulin-binding sites.

Topics: Aminoisobutyric Acids; Animals; Cells, Cultured; Concanavalin A; Dogs; Glucose; Glycogen; Insulin; Kidney; Liver Neoplasms, Experimental; Protein Binding; Receptor, Insulin; Tumor Cells, Cultured; Wheat Germ Agglutinins