Transport and metabolism of glucose in an insulin-secreting cell line, beta TC-1.
Kinetic characteristics of glucose transport and glucose phosphorylation were studied in the islet cell line beta TC-1 to explore the roles of these processes in determining the dependence of glucose metabolism and insulin secretion on external glucose. The predominant glucose transporter present was the rat brain/erythrocyte type (Glut1), as determined by RNA and immunoblot analysis. The liver/islet glucose transporter (Glut2) RNA was not detected. The functional parameters of zero-trans glucose entry were Km = 9.5 +/- 2 mM and Vmax = 15.2 +/- 2 nmol min-1 (microL of cell water)-1. Phosphorylation kinetics of two hexokinase activities were characterized in situ. A low-Km (0.036 mM) hexokinase with a Vmax of 0.40 nmol min-1 (microL of cell water)-1 was present along with a high-Km (10 mM) hexokinase, which appeared to conform to a cooperative model with a Hill coefficient of about 1.4 and a Vmax of 0.3 nmol min-1 (microL of cell water)-1. Intracellular glucose at steady state was about 80% of the extracellular glucose from 3 to 15 mM, and transport did not limit metabolism in this range. In this static (nonperifusion) system, 2-3 times more immunoreactive insulin was secreted into the medium at 15 mM glucose than at 3 mM. The dependence of insulin secretion on external glucose roughly paralleled the dependence of glucose metabolism on external glucose. Simulations with a model demonstrated the degree to which changes in transport activity would affect intracellular glucose levels and the rate of the high-Km hexokinase (with the potential to affect insulin release).
Whitesell
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Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, Tennessee 37232.
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D000818:Animals; D001692:Biological Transport; D015152:Blotting, Northern; D002460:Cell Line; D005110:Extracellular Space / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000378:metabolism; D005947:Glucose / Q000378:metabolism*; D006019:Glycolysis; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007340:Insulinoma / Q000378:metabolism*; D007424:Intracellular Fluid / Q000378:metabolism; D007700:Kinetics; D051379:Mice; D009004:Monosaccharide Transport Proteins / Q000378:metabolism; D010766:Phosphorylation
D007328:Insulin; D009004:Monosaccharide Transport Proteins; D019204:GTP-Binding Proteins; D005947:Glucose