Effects of high pressure on glucose transport in the human erythrocyte

The effects of raised hydraulic pressure on D-glucose exit from human red cells at 25 degrees C were determined using light scattering measurements in a sealed pressurized spectrofluorimeter cuvette. The reduction in the rates of glucose exit with raised pressure provides an index of the activation volume, deltaV++ (delta ln k/deltaP)(T) = -deltaV++/RT. Raised pressure decreased the rate constant of glucose exit from 0.077 +/- 0.003 s(-1) to 0.050 +/- 0.002 s(-1) (n = 5, P < 0.003). The Ki for glucose binding to the external site was 2.7 +/- 0.4 mm (0.1 MPa) and was reduced to 1.45 +/- 0.15 mm (40 MPa), (P < 0.01, Student's t test). Maltose had a biphasic effect on deltaV++. At [maltose] <250 microM, deltaV++ of glucose exit increased above that with [maltose = 0 mM], at >1 mm maltose, deltaV++ was reduced below that with [maltose = 0 mM]. Pentobarbital (2 mM) decreased the deltaV++ of net glucose exit into glucose-free solution from 30 +/- 5 ml mol(-1) (control) to 2 +/- 0.5 ml mol(-1) (P < 0.01). Raised pressure had a negligible effect on L-sorbose exit. These findings suggest that stable hydrated and liganded forms of GLUT with lower affinity towards glucose permit higher glucose mobilities across the transporter and are modelled equally well with one-alternating or a two-fixed-site kinetic models.