NICHOLSON, GS; RODGERS, BD; KELLEY, KM: Peripheral Glucoregulation in the Longjaw Mudsucker: Insulin Regulation of Muscle Glucose Transport
Teleost fishes have often been referred to as “glucose intolerant”; and at one point doubt was cast on whether they possessed an insulin-stimulated glucose transport system in peripheral tissues (e.g., in muscle) comparable to that of mammalian GLUT4. However, our studies in the goby, the longjaw mudsucker Gillichthys mirabilis, demonstrate that this fish is both glucose tolerant and that a muscle glucose transporter system is responsive to insulin. Glucose tolerance tests (GTTs) in intact gobies show that hyperglycemia at 1.5 hr post-injection of a 250 mg/kg glucose bolus is significantly reduced by 5 hr, with normoglycemia being reached by 12 hr. When insulin (1 IU/kg) is co-injected with the same glucose load, normoglycemia is reached 1.5 hr post-injection, while muscle from the insulin-injected individuals exhibits significantly stimulated rates of 3H-2-deoxyglucose (3H-2-DG) transport. When the same experiments are carried out on gobies with insulin-dependent diabetes mellitus (IDDM; generated by pancreatic endocrine isletectomy), glucose clearance is substantially impaired, but correctable if the animals are given prior insulin replacement therapy. In addition to its in vivo effects, insulin also directly stimulates 3H-2-DG transport in muscle explants in vitro. In muscle explants from gobies with untreated IDDM, however, the glucose transporter system becomes insulin resistant, whereas muscle from insulin-treated diabetic gobies show a restored response. These data support the existence of a functional insulin-regulated glucose transport system in this teleost fish. [Support by NSF grant IBN-0115975]
