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    • As our earlier study investigated glut mRNA levels in respon

    2021-10-19

    As our earlier study investigated glut mRNA levels in response to a protein load contained in a meal and the studies that used glucose and insulin injections did not examine GLUT expression, this study sought to investigate the effects of hyperglycaemia and insulin on mRNA levels in the spiny dogfish (S. suckleyi). Dogfish were subjected to a glucose load or XL413 hydrochloride intra-arterial injection of bovine insulin and the following were determined: 1) how long it takes to reduce plasma glucose, 2) the effect on glut 1 and 4 and glycogen synthase mRNA expression in muscle and liver, and 3) the effect on muscle and liver glycogen content. It was hypothesised that both hyperglycaemia and insulin would increase glut mRNA levels and cause glycogen deposition in muscle and liver, thus suggesting a conserved mechanism for glucose homeostasis in vertebrates.
    Methods
    Results
    Discussion This study investigated the effects of glucose and insulin administration on plasma glucose concentration and glucose transporter mRNA levels in the spiny dogfish shark (S. suckleyi). Previous work has indicated that elasmobranchs are slow to clear an injected glucose load, with plasma glucose levels remaining elevated for as long as 48h post-injection (Hartman et al., 1944, Patent, 1970). Although the restoration of plasma glucose levels took much longer than it would in mammals, the majority of the glucose was cleared within 6h by dogfish in the present study with factors such as a lower metabolic rate and temperature maybe playing a role in this observed delay for complete clearance compared to mammals. What was interesting was that the restoration of plasma glucose levels in this study corresponded to significant increases in both liver and muscle glycogen, as well as increases in glut mRNA levels. This is the first study to measure these parameters in an elasmobranch following a glucose load and results parallel the well-documented response in mammals. Significant increases in glut1 and glut4 mRNA expression were observed in dogfish white skeletal muscle within 24h of the glucose injection, which appears to be delayed relative to the decreases in plasma glucose, however, it should be noted that the GLUT4 protein is known to be stored within intracellular vesicles in the muscle and adipose tissue of mammals and teleosts and, when stimulated by insulin, it translocates to the cell membrane to enhance glucose uptake (Diaz et al., 2007, Huang and Czech, 2007). Once plasma glucose and insulin levels decrease, the transporters are removed from the membrane and either broken down or recycled. Thus, in the first few hours following a glucose injection, glucose uptake may be enhanced by the insertion of GLUTs into the muscle cell membrane and then as plasma glucose decreases (Huang and Czech, 2007). However, glucose uptake will then decrease as the GLUT4 proteins are depleted (Pessin and Bell, 1992), and thus an increase in transcription would likely be required to replace the intracellular GLUT4 stores, hence the need for additional mRNA. Further, in regards to the time frame for changes in glut mRNA levels, glucose transporters are more constitutively expressed in the membrane of hepatocytes and as such, increases in glucose uptake can occur immediately as there is less of a need for insulin-stimulated translocation (Mueckler and Thorens, 2013). Indeed, we did observe significant increases in the glycogen content of the dogfish liver in response to a glucose load much earlier than in the muscle, suggesting that the liver may play a predominate role in glucose homeostasis. Although not typically responsible for GLUT translocation in the liver, insulin will stimulate transcription and glycogen synthesis while inhibiting glycogenolysis and gluconeogenesis in mammals (reviewed by Saltiel and Kahn, 2001, Navarro et al., 2002). Studies in teleosts have indicated that insulin induces hypoglycaemia in a number of species as well as changes in GLUT and glycogen levels, as is typically the case in mammals (see review by Polakof et al., 2012). For instance, significant increases in muscle glut1 and 4 mRNA levels and glycogen content were observed in rainbow trout (O. mykiss) in response to infusions with bovine insulin (Polakof et al., 2010) and both GLUT4 protein and mRNA levels increased in response to insulin in brown trout (S. trutta; Diaz et al., 2007, Diaz et al., 2009). However, the effects on glucose transporters and glycogen stores appear to vary between different teleost species (see Polakof et al., 2012) which could be due to differences in life history traits or the dose and method of administration for the insulin itself as this varies widely between studies.