Promoting Glycogen Synthesis Research Articles

Insulin in Invertebrates and Cyclostomes

It seems increasingly clear that insulin is a hormone that does not occur exclusively in vertebrates. Several independent reports now exist giving evidence of insulin production in the digestive tissues of both deuterostomian and protostomian invertebrates. Cells with some light-microscopical and ultrastructural characteristics of vertebrate B-cells have also been observed. Recently, evidence has been obtained that insulin can act as a hypoglycemic hormone, promoting glycogen synthesis, also in a protostomian invertebrate, the gastropod mollusc, Strophocheilus oblongus . The endocrine pancreas of the cyclostomes occupies a key position in the comparative endocrinology of the islet parenchyma and in the evolution of insulin production. It may represent an evolutionary link between the presumably gut-connected dispersed insulin-producing cells of deuterostomian invertebrates and the pancreatic islets of gnathostomian vertebrates. This hypothesis was supported by the fact that cells with light-microscopical and ultrastructural similarities to the islet B-cells were observed in the bile duct mucosa of the hagfish, Myxine glutinosa . However, immunofluorescent studies with antisera against human insulin and C-peptide did not show any immunoreactive material outside the B-cells of the endocrine pancreas. Particular attention was paid to elucidate the biological significance of the large cystic cavities that are so typical for the cyclostomian islet parenchyma. The working hypothesis that they may contain stored insulin, proinsulin (or even “proto-proinsulin”) was not supported by immunofluorescence, autoradiographic, or ultrastructural investigations, nor by proinsulin assays. It is possible that hagfish islet B-cells contain zinc, despite the fact that the amino acid residue in B10-position is aspartic acid instead of histidine. The biosynthesis of hagfish insulin shows a pattern similar to that in gnathostomes. Its rate is related to the ambient temperature and at 11 C the conversion of proinsulin to insulin lasts several days.

Stimulation of Glucose Uptake in the Rat Diaphragm by Hydroxy‐L‐proline and L‐lysine

AbstractBorrebæk, B. and O. Walaas. Stimulation of glucose uptake in the rat diaphragm by hydroxy‐L‐proline and L‐lysine. Acta physiol. scand. 1963. 58. 274–284. — The influence of hydroxyproline, lysine and several other amino acids on glucose uptake of rat diaphragm in vitro has been investigated. Hydroxyproline increased glucose uptake and stimulated the incorporation of glucose‐C14 into glucose‐6‐phosphate of the diaphragm. The effect occurred at a concentration level of 1.10‐3 M‐5.10‐5 M hydroxyproline in the medium. At higher concentration of hydroxyproline, glucose uptake was decreased. In contrast to insulin, hydroxyproline did not promote glycogen synthesis and did not stimulate incorporation of glucose‐C14 into glycogen. When the diaphragm was incubated in a medium containing both hydroxyproline and insulin, the insulin effect on glucose uptake was depressed.Lysine increased glucose uptake of the diaphragm, but less than hydroxyproline. No significant effect by any other single amino acids was found. However, a mixture of 13 natural amino acids, each at a concentration of 1.10‐4 M to 1.10‐3 M markedly depressed glucose uptake. The mechanism of a possible effect of hydroxyproline on the membrane carrier system for glucose uptake in muscle is discussed. Injection of hydroxyproline intraperitoneally in vivo exerted an hyperglycemic effect in the intact rat.