Effects of hagfish insulin in the Atlantic hagfish, Myxine glutinosa the in vivo metabolism of [ 14C]glucose and [ 14C]leucine and studies on starvation and glucose-loading

Abstract

In hagfish, starved for 1 month at 4–6°, blood glucose decreased (1.9 to 0.8 m M) and serum insulin values diminished (2.2 to 1.1 n M). More than 90% of the glycogen in the liver and skeletal muscle was consumed, whereas protein and triglyceride contents were far more stable. The serum levels of amino nitrogen, triglycerides, and free fatty acids were all decreased after starvation. The results indicate that skeletal muscle glycogen was the prime source of energy. In starved hagfish, hagfish insulin (0.1 μg/g body weight) induced an approximate twofold stimulation of the synthesis of glycogen, protein, and neutral lipids from [ 14C]glucose after 33 hr at 4–6°. Most of the incorporation was detected in muscle glycogen. No insulin effects were seen in the liver. In analogous studies with [ 14C]leucine, hagfish insulin likewise stimulated the synthesis of glycogen and protein in muscle and protein synthesis in the liver. Despite the evidence of insulin-stimulated syntheses, the total glycogen and protein contents in muscle and liver were unaltered after 33 hr. Likewise, no insulin effects were seen on blood glucose, amino nitrogen, triglycerides, or free fatty acids. Only about 10% of the radioactive dose was incorporated into the muscle and liver, and the metabolic effects of insulin contributed to only about half of this fraction. Glucose-loading increased the serum insulin value from 0.7 to 1.9 n M. Pretreatment of hagfish with a mixture of glucose and amino acids for 3 days before and after injection of the above isotopes resulted in an increase of the serum insulin values. These endogenously elevated insulin levels were sufficient to stimulate the incorporation of the label into muscle glycogen and protein. The stimulations were similar to those obtained in experiments with exogenous insulin. It was concluded that the physiological role of insulin in skeletal muscle was similar to what has been observed in higher animals although the quantitative effects of insulin in the hagfish appeared smaller than in higher vertebrates.