The effect of ruminal phosphate concentration on the absorption of calcium, phosphorus and magnesium from the reticulo-rumen of the sheep.

1. The absorption rates of calcium, inorganic phosphate (Pi) and magnesium were determined from buffered solutions placed in the temporarily isolated and washed reticulo-rumen of conscious sheep. The basic composition of these solutions was similar to that found in supernatant fractions of ultracentrifuged rumen contents. 2. The Pi concentrations studied in these solutions were 2, 8.7, 14, 17.3 and 38 mmol/l. The initial concentration of Ca was 2.0 mmol/l and that of Mg was 2.5 mmol/l in all experiments. 3. Increasing the Pi concentration in the rumen solution from 2 to 38 mmol/l resulted in increases in the net absorption rates of both Ca and Pi, and a decrease in the potential difference across the wall of the rumen. 4. Similarly, increasing the Pi concentration from 2 to 17.3 mmol/l resulted in an increase in the net absorption rate of Mg from the rumen. 5. Mineral analysis of strained rumen fluid or a 30,000 g centrifugate of strained rumen fluid revealed a reduced Pi concentration in sheep fed on frozen spring grass as opposed to the pellet + hay diet. The values obtained were within the range studied.