Glucose transport and microvillus membrane physical properties along the crypt-villus axis of the rabbit

Both transport function and microvillus membrane physical properties evolve as the enterocyte matures and migrates up the crypt-villus axis. We isolated enriched fractions of villus tip, mid-villus, and crypt enterocytes from which microvillus membrane vesicles were prepared. Using this material we characterized the alterations that occur in microvillus mem-brane fluidity as the rabbit enterocyte matures and correlated these with kinetic studies of glucose transport. With increasing maturity the microvillus membrane becomes more rigid due to both an increase in the cholesterol/phospholipid ratio and al-terations in individual phospholipid subclasses. Maximal rates of glucose transport were greatest in microvillus membrane vesicles prepared from mature cells. However, the glucose concentration producing half-maximal rates of transport (K.) was significantly lower in crypt microvillus membrane vesicles, suggesting that a distinct glucose transporter existed in crypt enterocytes. This distinction disappeared when differences be-tween membrane lipid environments were removed. By fluidiz-ing villus-tip microvillus membrane vesicles, in vitro, to levels seen in the crypt microvillus membrane, we observed a reduc-tion in the K. of this transport system. These data suggest that the kinetic characteristics of the sodium-dependent glucose transporter are dependent upon its local membrane environ-ment. (J. Clin. Invest. 1990. 85:1099-1107.) membrane fluid-ity- nutrient absorption- transport kinetic