Phosphate transport by basolateral plasma membranes of human small intestine.

The characteristics of phosphate transport across the human intestinal basolateral membrane were determined using an enriched preparation in which uphill Na1+-dependent D-glucose transport could not be demonstrated but adenosine triphosphate-dependent calcium transport was present. The human basolateral membrane vesicles were oriented as follows: 64% inside-out vesicles and 36% rightside-out vesicles or sheets, or both. Phosphate transport showed a 3.4-fold transient "overshoot" phenomena in the presence of an inwardly directed sodium gradient. Computerized Michaelis-Menten kinetics for the net active transport component indicated a Vmax of 65 +/- 5 pmol/mg protein per 8 s and a Km of 93 +/- 15 microM. The transport process was electroneutral. Gramicidin D and transstimulation studies confirmed the presence of a Na+-phosphate carrier at the basolateral membrane. Adenosine triphosphate enhanced phosphate uptake, indicating an adenosine triphosphate-dependent phosphate transport process. These studies are the first to determine the presence of a phosphate carrier in human intestinal basolateral membrane.