Transport and Metabolism of Glucose in an Insulin-Secreting Cell Line, βtc-1

Kinetic characteristics of glucose transport and glucose phosphorylation were studied in the islet cell line βTC-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 K_m = 9.5 ± 2 mM and V_max = 15.2 ± 2 nmol min¹ (μL of cell water)^-1. Phosphorylation kinetics of two hexokinase activities were characterized in situ. A low-K_m (0.036 mM) hexokinase with a V_max of 0.40 nmol min^-1 (μL of cell water)^-1 was present along with a high-K_m (10 mM) hexokinase, which appeared to conform to a cooperative model with a Hill coefficient of about 1.4 and a V_max of 0.3 nmol min^-1 (μL 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-K_m hexokinase (with the potential to affect insulin release).