| Abstract: | A possible modulation of ion permeabilities of rat intestinal brush-border membrane vesicles by Ca2+, a putative second messenger of salt secretion, was explored by three independent methods: (1) measurements of 3H]glucose accumulation driven by a Na+ gradient; (2) stopped-flow spectrophotometry of salt-induced osmotic swelling; (3) 86Rb+, 22Na+ and 36Cl- flux measurements. Cytoskeleton-deprived membrane vesicles were prepared from isolated brushborders by thiocyanate treatment. Intravescicular Ca2+ levels were varied by preincubating vesicles in Ca-EGTA buffers in the presence of the Ca2+-ionophore A23187. At Ca2+free greater than 10(-5) M, initial Na+-dependent glucose uptake in the presence of a 0.1 M NaSCN gradient (but not in its absence) was inhibited by about 50 per cent as compared to EGTA alone (ED50 approximately equal to 10(-6) M Ca2+). By contrast, initial rates of 22Na+ uptake and reswelling rates of vesicles exposed to a NaSCN gradient were increased at least 2-fold by 10(-5) M Ca2+free. Both observations are compatible with a Ca2+-induced increase of the Na+-permeability of the vesicle membrane. The modulation of ion transport was fully reversible and critically dependent on internal Ca2+, suggesting a localization of Ca2+-sensor sites at the inner surface of the microvillous membrane. As shown by radiotracer and osmotic swelling measurements, micromolar Ca2+ additionally increased the flux rate of K+, Rb+, Cl- and NO-3 but did not change the membrane permeability for small uncharged molecules, including glucose and mannitol. The effect of Ca2+ on ion permeabilities could be blocked by Ba2+ (10(-3) M) or Mg2+ (10(-2) M), but not by amiloride (10(-3) M), apamin (2 X 10(-7) M), trifluoperazine (10(-4) M) or quinine (5 X 10(-4) M). At present it is unclear whether Ca2+ activates a nonselective cation and anion channel or multiple highly selective channels in the vesicle membrane. |
