Improved stability of rabbit and rat intestinal brush border membrane vesicles using phospholipase inhibitors

The initial rates of Na(+)-dependent D-aspartate and D-glucose uptakes were shown to decline from the time of resuspension of brush border membrane vesicles isolated from rabbit and rat jejunum by standard divalent cation precipitation procedures. The former were however more stable than the latter and followed quite closely the decrease in the intravesicular volume, thus suggesting that the loss of transport activity may involve both nonspecific opening of the vesicles and either direct or indirect specific inactivation of the transporters. Uptake rates for both substrates did tend to stabilize at 6-24 h from resuspension, however this final 'next day' uptake activity was too low to be of practical use in kinetic studies. Freezing aliquots of rabbit jejunal vesicles in liquid N2 until the time of assay resulted in complete stabilization of D-glucose uptake. A modified homogenate buffer designed to inhibit a broad spectrum of phospholipase activities resulted in a partial stabilization of glucose transport by rabbit jejunal vesicles with, on average, an over 6-fold enrichment in the 'next day' stable specific activity of uptake as compared to unfrozen vesicles. The modified homogenate buffer also improved the stability and the 'next day' specific activities of D-glucose uptake in rat jejunal brush border vesicles and D-aspartic acid uptake in rabbit jejunal vesicles. It also completely stabilized the intravesicular volume in the latter preparation. An evaluation of the kinetic parameters of Na(+)-dependent D-glucose transport in rabbit vesicles prepared from either the standard homogenate media and frozen in liquid N2 or the modified media and allowed to stabilize overnight, revealed a single transport system with a Km of 0.31-0.32 mM as the best model to fit the data. As such the modifications to the homogenate media do not appear to effect the functional properties of D-glucose transport in the membrane. While being less efficient in stabilizing the vesicles than the rapid freezing protocol, it is shown that the modified homogenate should however be preferred when dealing with slowly permeant ions like choline since it provides in this case the only alternative to reliable measurement of uptake rates across a stable and equilibrated vesicle preparation.