Saturation kinetics of magnesium efflux across the rumen wall in heifers.

1. Absorption of magnesium from the temporarily-isolated, emptied and washed rumen of four 10-12-month-old heifers was measured at five different Mg concentrations (2.5, 5.0, 7.5, 12.5 and 20 mmol/l). 2. Net absorption of Mg was observed at all concentrations. Saturation occurred at a concentration of 12.5 mmol Mg/l. 3. Using an Eadie-Hofstee plot, the Michaelis-Menten constant (Km; 11.43 mmol/l) and maximal efflux (Vmax; 120.3 mumol/min) were estimated. 4. The phenomenon of saturation supports the assumption that Mg transport across the rumen wall of heifers is an active process.