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Effect of Lactose on Phosphate Transport into Rat Intestinal Brush Border Membrane Vesicles^1,2,

Hanna Debiec and Roman Lorenc

Department of Biochemistry and Experimental Medicine, Hospital Monument Child's Health Center, Warsaw, Poland

Brush border membrane vesicles (BBMV) were used to evaluate the influence of lactose on phosphate (P_i) transport in the intestine of the rat. BBMV were prepared from duodenum, jejunum and ileum of 3-, 4- and 7-wk-old rats. Na⁺-dependent active transport of P_i was investigated in the presence of an Na⁺ gradient; Na⁺-independent, diffusional transport was investigated by substitution of choline chloride for NaCl. In 3-wk-old rats active P_i transport dominated, whereas in 4-wk-old rats a decrease in active transport and an increase of the diffusional component of total P_i transport were observed. Furthermore, in 7-wk-old rats, P_i was transported by simple diffusion alone. Addition of lactose to the preincubation medium had no effect on P_i transport in duodenal and jejunal BBMV isolated from 3-wk-old rats. In the remaining BBMV preparations, lactose caused a twofold enhancement of diffusional P_i uptake. Glucose and galactose had no effect on diffusional P_i uptake into BBMV. These results indicate that lactose may directly interact with luminal membranes, thus leading to increased diffusional transport. The membranes with a decreased Na⁺-dependent P_i transport activity are most susceptible to the influence of lactose.

KEY WORDS: • lactose • phosphate transport • intestinal brush border membrane vesicles

¹ Supported in part by PL ARS-99 Grant No. J-MOA-USDA-4.

² Part of the work reported in this paper was presented in March, 1984, in Vienna, Austria, at the Second International Workshop on Calcium and Phosphate Transport across Biomembranes.

Manuscript received 15 November 1984. Revision accepted 3 June 1985.