Abstract
Fasting causes insulin resistance in liver and fat, and increases insulin sensitivity in muscle. We studied the response in vitro and in vivo to insulin of the insulin receptor tyrosine kinase in muscle and liver from 72 h fasted and control rats. Insulin was injected intraperitoneally together with glucose, and blood and tissue samples were obtained 0, 5, 15 and 30 min later. Basal serum glucose and insulin levels were significantly higher in control than in fasting rats. Serum glucose rose to approximately 300 mg/dl at 5 min and then progressively declined without hypoglycaemia. Receptors were prepared from whole tissue by wheat germ lectin affinity chromatography. 125I-insulin binding to purified receptors was increased by fasting in both muscle (18%) and liver (50%). In untreated fasting and control animals, muscle and liver insulin receptor tyrosine kinase activity was stimulated to similar levels by insulin added in vitro. With only insulin treatment in vivo, muscle receptor tyrosine kinase behaved similarly in fasting and control animals with maximal activation at 15 min post injection. In liver, insulin in vivo stimulated receptor tyrosine kinase activity maximally at 5 min post injection in both fasting and control, but in fasting animals the treatment in vivo caused a significantly larger and more prolonged activation of the enzymic activity, possibly due to a decrease in the rate of dephosphorylation and deactivation of the beta subunits.
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