Ischemia Induces Surface Membrane Dysfunction

Reversible ischemia reduced renal cortical brush border membrane (BBM) Na'-dependent D-glucose uptake (336±31 vs. 138±30 pmol/mg per 2 s, P < 0.01) but hadno effect on Na'-independent glucose or Na'-dependent L-alanine uptake. The effect on D-glucose uptake was present after only 15 mm of ischemia and was due to a reduction in maximum velocity (1913±251 vs. 999±130 pmol/mg per 2 s; P < 0.01). This reduction was not due to more rapid dissipation of the Na' gradient, altered sidedness of the vesicles, or an alteration in membrane potential. Ischemia did, however, reduce the BBM sphingomyelin-to-phosphatidylcholine (SPH/PC) and cholesterol-to-phospholipid ratios and the number of specific highaffinity Na'-dependent phlorizin binding sites (390±43 vs. 146±24 pmol/mg; P < 0.01) without altering the binding dissociation constant (Kd). 20mM benzyl alcohol also reduced the number of Na'-dependent phlorizin binding sites (418±65 vs. 117±46; P < 0.01) without altering Kd. The reduction in Na+dependent D-glucose transport correlated with ischemic-induced changes in the BBM SPH/PC and cholesterol-to-phospholipid ratios and membrane fluidity. Taken together these data indicate the cellular site responsible for ischemic-induced reduction in renal cortical transcellular glucose transport is the BBM. We propose the mechanism involves marked alterations in BBM lipids leading to large increases inBBM fluidity which reduces the binding capacity of Na'-dependent glucose carriers. These data indicate that reversible ischemia has profound effects on the surface membrane function of epithelial cells.