MODULATION OF INTESTINAL-ABSORPTION OF CALCIUM

Abstract

Absorption of ingested calcium (2 ml of a 10 mM CaCl2 solution + 45Ca) by the adult rat was facilitated by the simultaneous ingestion of an active carbohydrate, L-arabinose. As the carbohydrate concentration is increased from 10-200 mM, the absorption of Ca is maximized at a level corresponding to about twice the control absorption level. A similar doubling of Ca absorption is obtained when a 100 mM concentration of any one of a number of other carbohydrates (gluconic acid, mannose, glucosamine, sorbitol, lactose, raffinose, stachyose) is ingested simultaneously with a 10 mM CaCl2 solution. Conversely, the simultaneous ingestion of increasing doses (10-100 mM) of phosphate (NaH2PO4) with a 10 mM CaCl2 solution results in decreased 45Ca absorption and retention by the adult rat. The maximum inhibition of Ca absorption by phosphate is independent of the concentration of the ingested Ca solution (from 5-50 mM CaCl2). The simultaneous ingestion of CaCl2 (10 mM) with lactose and sodium phosphate (50 and 10 mM, respectively) shows that the activating effect of lactose upon 45Ca absorption may be partly dissimulated by the presence of phosphate. Within a large concentration range (2-50 mM CaCl2), Ca absorption appears to be a precisely modulated diffusion process. Ca absorption varies (between minimum and maximum levels) as a function of the state of saturation by the activators (carbohydrates) and inhibitors (phosphate) of the Ca transport system.