The ability to actively transport nutrients is maintained in intestinal tissues of hibernating ground squirrels compared with their active counterparts, and shows apparent upregulation in hibernators when transport rates are normalized to tissue mass. To identify the mechanisms responsible for the preservation of transport function during the extended fast of hibernation, we studied D-glucose uptake into jejunal brush border membrane vesicles prepared from active and hibernating 13-lined ground squirrels. Hibernators were without food and showing regular bouts of torpor for at least 6 weeks before sacrifice. Electron micrographs indicated similar microvillus heights of jejunal enterocytes in the two activity states, whereas microvillus density was slightly greater in the hibernators. Glucose uptake into brush border membrane vesicles was inversely related to medium osmolarity, indicating negligible binding of substrate to brush border membrane vesicles surfaces, and intravesicular spaces were similar in hibernating and active squirrels. Glucose uptake showed strong Na+ dependency in both groups, with equivalent overshoot values in the presence of Na+. Kinetic analysis revealed a significant increase in the maximal velocity of transport (Jmax) in hibernators (55.9 +/- 5.6 nmol.min-1. mg-1) compared with active squirrels (36.7 +/- 5.1 nmol.min-1. mg-1, P < 0.05), with no change in K(m). Thus, the structure and absorptive capacity of the intestinal brush border persists in fasted hibernators, and the increase in Jmax for glucose uptake during hibernation likely contributes to the enhanced Na(+)-dependent glucose absorption previously observed at the tissue level.