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MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

Glucose transporter in bovine mammary epithelial cells is an asymmetric carrier that exhibits cooperativity and trans-stimulation

Department of Animal and Poultry Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1

Submitted 8 May 2003 ; accepted in final form 4 July 2003

Glucose transport kinetics were quantified in isolated bovine mammary epithelial cells using 3-O-methyl-D-glucose. Isolated cells retained satisfactory viability and glucose uptake activity, which was inhibited by cytochalasin B, phloretin, HgCl₂, and low temperature. Initial rates of entry were measured over a 15-s interval at 37°C under zero-trans, equilibrium-exchange, high-cis, and high-trans concentrations of 3-O-methyl-D-glucose between 0 and 20 mM. The combined set of rate measurements from all experimental conditions was fit to the fixed-site carrier model by nonlinear regression to estimate parameters of transport. For the regression between predicted and observed initial rates, r² was 0.97. Forward V_max was estimated at 18.2 nmol·min^-1·mg protein^-1, and the Michaelis constant was 8.29 mM. The cooperativity parameter was 1.63, trans-stimulation was 2.13-fold, and asymmetry was 2.06-fold. On the basis of the kinetic parameters, variations in intracellular glucose concentrations are not responsible for the range of glucose uptakes by bovine mammary glands observed in vivo.

model; cow; mammary epithelial cell

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