Modulation of rat distal colonic brush-border membrane Na+-H+ exchange by dexamethasone: role of lipid fluidity.

Earlier studies by our laboratory have suggested a relationship between an amiloride-sensitive Na+-H+ exchange process and the physical state of the lipids of rat colonic brush-border membrane vesicles. To further assess this possible relationship, a series of experiments were performed to examine the effect of dexamethasone administration (100 micrograms/100 g body wt. per day) subcutaneously for 4 days on Na+-H+ exchange, lipid composition and lipid fluidity of rat distal colonic brush-border membrane vesicles. The results of these studies demonstrate that dexamethasone treatment significantly: (1) increased the Vmax of the Na+-H+ exchange without altering the Km for sodium of this exchange process, utilizing the fluorescent pH-sensitive dye, acridine orange. 22Na flux experiments also demonstrated an increase in amiloride-sensitive proton-stimulated sodium influx across dexamethasone-treated brush-border membrane vesicles; (2) increased the lipid fluidity of treated-membrane vesicles compared to their control counterparts, as assessed by steady-state fluorescence polarization techniques using three different lipid-soluble fluorophores; and (3) increased the phospholipid content of treated-membrane vesicles thereby, decreasing the cholesterol/phospholipid molar ratio of treated compared to control preparations. This data, therefore, demonstrates that dexamethasone administration can modulate amiloride-sensitive Na+-H+ exchange in rat colonic distal brush-border membrane vesicles. Moreover, it adds support to the contention that a direct relationship exists between Na+-H+ exchange activity and the physical state of the lipids of rat colonic apical plasma membranes.