Ischemia induces surface membrane dysfunction. Mechanism of altered Na+-dependent glucose transport.

Abstract

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 had-no 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). 20 mM 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 in BBM 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.

Cholesterol modulates rat renal brush border membrane phosphate transport.

“…In renal BBM the effect of in vitro alterations in BBM fluidity on Na-glucose cotransport has been studied but the results are conflicting. Increasing ambient temperature, which increases BBM fluidity, has been shown to increase renal BBM Na-glucose cotransport, whereas incubation of BBM with either oleic acid (40), or benzyl alcohol (41,42), which also increase BBM fluidity, has been shown to decrease renal BBM Na-glucose cotransport activity. Oleic acid, however, causes a faster dissipation of the Na' gradient (40), and benzyl alcohol causes a decrease in the number of Na-dependent phloridzin binding sites (42).…”

“…Increasing ambient temperature, which increases BBM fluidity, has been shown to increase renal BBM Na-glucose cotransport, whereas incubation of BBM with either oleic acid (40), or benzyl alcohol (41,42), which also increase BBM fluidity, has been shown to decrease renal BBM Na-glucose cotransport activity. Oleic acid, however, causes a faster dissipation of the Na' gradient (40), and benzyl alcohol causes a decrease in the number of Na-dependent phloridzin binding sites (42). Therefore, the effects on Na' gradient-dissipation and the number or accessibility of glucose carrier units, rather than the effects on BBM fluidity per se, may mediate the inhibitory effects of oleic acid and benzyl alcohol on Na-glucose cotransport activity.…”

Alterations in the establishment and maintenance of epithelial cell polarity as a basis for disease processes.

“…Ischemia in proximal tubule cells and bile duct ligation (hepatocytes) both result in loss ofapical microvilli with cellular internalization and blebbing of the apical membrane into the lumen, loss ofsurface membranepolarity ofboth apical and basolateral membrane domain-specific markers (62-64), alterations in apical terminal web actin microfilaments (65,66), and reduction in vectorial transportfunctions (63,67,68). The alterations in vectorial transport functions were related to the redistribution of surface membrane phospholipids (67) and domain-specific apical and basolateral membrane enzymes (63,68).…”

Ischemia-induced loss of epithelial polarity. Role of the tight junction.

“…This was associated with the equilibration of surface membrane cholesterol and phospholipids between the two surface membrane domains. Furthermore, this loss of lipid and protein polarity was related to specific reductions in transcellular glucose and Na+ transport (8,9), and normalization of these cellular functions required the reestablishment of surface membrane polarity (10). Since cellular tight junctions are known to play a critical role in the maintenance of epithelial polarity (1 1, 12), and disruption of tight junctions results in loss of epithelial polarity (13,14), we questioned whether the documented ischemia-induced loss of epithelial polanrty was due to the disruption of cellular tight junctions.…”