Balances of Water and Sodium in the Rumen During Feeding: Osmotic Stimulation of Sodium Absorption in the Sheep

When sheep ate a dry ration containing added sodium chloride the concentration of sodium in the rumen liquor did not increase. When solutions containing amounts of sodium similar to those eaten with the feed were administered via rumen fistulæ to fasting sheep, they caused a rise in sodium concentration that was sustained for at least 4 hr. Different results were obtained when the same dose of sodium chloride was given to sheep in which the rumen contents were made hypertonic by the addition of other solutes (potassium chloride or mannitol‐urea). Instead of rising, the sodium concentration fell 4 hr. after treatment. Changes in the water content of the rumen were traced with a soluble marker, the 51Cr complex of ethylenediaminetetra‐acetic acid. After feeding, but not after solute administration, there was clear‐cut evidence of increased entry of body water into the rumen, the dilution rate increasing from 0·07–0·11 hr.‐1 to 0·19–0·28 hr.‐1. Total salivary secretion was measured in an animal with an œsophageal fistula and the results indicated that the increased water entry into the rumen was due to stimulation of saliva production on feeding. By making several assumptions, sodium and water balances in the rumen were derived from the experimental data. In the fasting state the calculated sodium absorption was 6–16 m. equiv/ hr.; over the 2 hr. period after feeding or solute administration the value was 36–62 m.equiv/hr. It was concluded that hypertonic conditions, as foumd after feeding or solute loading, stimulate the absorption of sodium from the rumen. The physiological implications of the findings are discussed in terms of the sheep's response to the diversion of sodium and water into the rumen on feeding.