Ca2+ transport across intestinal brush border membranes of the cichlid teleost Oreochromis mossambicus.

Brush border membranes were isolated from tilapia (Oreochromis mossambicus) intestine by the use of magnesium precipitation and differential centrifugation. The membrane preparation was enriched 17-fold in alkaline phosphatase. The membranes were 99% right-side-out oriented as indicated by the unmasking of latent glyceraldehyde-3-phosphate dehydrogenase and acetylcholine esterase activity by detergent treatment. The transport of Ca2+ in brush border membrane vesicles was analyzed. A saturable and a nonsaturable component in the uptake of Ca2+ was resolved. The saturable component is characterized by a Km much lower than the Ca2+ concentrations predicted to occur in the intestinal lumen. The nonsaturable component displays a Ca2+ permeability too high to be explained by simple diffusion. We discuss the role of the saturable component as the rate-limiting step in transmembrane Ca2+ movement, and suggest that the nonsaturable component reflects a transport mechanism operating well below its level of saturation.