BASOLATERAL MEMBRANE LIPID DYNAMICS ALTER NA-K ATPASE ACTIVITY IN RABBIT SMALL-INTESTINE

The role of basolateral membrane fluidity in regulating Na - K ATPase activity along the crypt - villus axis in rabbit distal small intestine was assessed. Basolateral membranes were prepared from isolated villus and crypt enterocytes at 24- to 28-fold enhancement. Villus basolateral membranes were significantly (p < 0.001) more fluid than crypt basolateral membranes as measured by 1,6-diphenyl-1,3,5-hexatriene. No difference was seen between the two groups as measured by either 2-(9-anthroyloxy)-stearic fatty acid or 16-(9-anthroyloxy)-palmitic acid. Fluidity alterations were accompanied by an increased phospholipid content in villus membranes, which resulted in a decreased cholesterol:phospholipid ratio and an increased lipid:protein molar ratio. Na-K ATPase activity was significantly (p < 0.01) greater in villus basolateral membranes than in crypt membranes, and demonstrated a greater sensitivity to ouabain inhibition. Ouabain inhibition curves calculated from villus data fit well (p < 0.001) with a two binding site model, with a high affinity (K(i) 16 nM) and a low affinity (K(i) 4.2 muM) ouabain binding site. In crypt basolateral membranes, only a low affinity site was apparent (K(i) 3.0 muM). Fluidizing crypt basolateral membranes in vitro with benzyl alcohol to levels seen in villus basolateral membranes resulted in the appearance of a high affinity ouabain binding site (K(i) 110 nM) and an increased sensitivity of Na - K ATPase to ouabain inhibition. The fluidization of villus basolateral membranes eliminated the binding associated with the high affinity site. Treatment with methanol, as a control, did not alter Na - K ATPase activity. These results indicate that changes in basolateral membrane lipid dynamics may be involved in modulating Na-K ATPase activity and transport function.