Postprandial Insulin Resistance in Zucker Diabetic Fatty Rats is Associated with Parasympathetic-Nitric Oxide Axis Deficiencies

The Zucker diabetic fatty (ZDF) rat is an obesity and type 2 diabetes model. Progression to diabetes is well characterised in ZDF rats, but only in the fasted state. We evaluated the mechanisms underlying postprandial insulin resistance in young ZDF rats. We tested the hypothesis that the overall postprandial action of insulin is affected in ZDF rats as a result of impairment of the hepatic parasympathetic-nitric oxide (PSN-NO) axis and/or glutathione (GSH), resulting in decreased indirect (PSN-NO axis) and direct actions of insulin. Nine-week-old male ZDF rats and lean Zucker rats (LZR, controls) were used. The action of insulin was assessed in the fed state before and after parasympathetic antagonism atropine. Basal hepatic NO and GSH were measured, as well as NO synthase (NOS) and γ-glutamyl-cysteine synthethase (GCS) activity and expression. ZDF rats presented postprandial hyperglycaemia (ZDF, 201.4±12.9mg/dl; LZR, 107.7±4.3mg/dl), but not insulinopaenia (ZDF, 5.9±0.8ng/ml; LZR, 1.5±0.3ng/ml). Total postprandial insulin resistance was observed (ZDF, 78.6±7.5mg glucose/kg; LZR, 289.2±24.7mg glucose/kg), with a decrease in both the direct action of insulin (ZDF, 54.8±7.0mg glucose/kg; LZR, 173.3±20.5mg glucose/kg) and the PSN-NO axis (ZDF, 24.5±3.9mg glucose/kg; LZR, 115.9±19.4mg glucose/kg). Hepatic NO (ZDF, 117.2±11.4μmol/g tissue; LZR, 164.6±4.9μmol/g tissue) and GSH (ZDF, 4.9±0.3μmol/g; LZR, 5.9±0.2μmol/g) were also compromised as a result of decreased NOS and GCS activity, respectively. These results suggest a compromise of the mechanism responsible for potentiating insulin action after a meal in ZDF rats. We show that defective PSN-NO axis and GSH synthesis, together with an impaired direct action of insulin, appears to contribute to postprandial insulin resistance in this model.