WATER, SODIUM AND POTASSIUM MOVEMENTS ACROSS THE RUMEN WALL OF SHEEP

Abstract

Saliva was entirely diverted from the rumen of sheep via an oesophageal fistula and usually replaced by infusion of a synthetic saliva of known composition. Water movements in the rumen were measured by the use of soluble markers, and sodium and potassium movements were calculated. Water and various solutions were added to the rumen to vary its osmolality over the range 180–550 m‐osmole/kg. When saliva was not replaced, recovery of the normal rumen osmolality following water loading was very slow indeed. When saliva was replaced by a synthetic solution the rate of water movement across the rumen wall ( y, [l./hr]) was a linear function of the osmotic pressure ( x, [m‐osmole/kg]), such that y = 0·404–0·001206 x ( r = ‐0·90). This suggests that the filtration permeability of the rumen wall was similar to that of frogskin or toad bladder. Potassium absorption across the rumen wall was also slow but positively related to the potassium concentration. Sodium absorption appeared to depend on the potassium concentration and the osmotic pressure as well as on the sodium concentration. While moderate levels of potassium increased the rate of sodium absorption, grossly high levels, sufficient to produce a substantial entry of water across the rumen wall, decreased it, sometimes markedly. The rate of potasium and perhaps sodium absorption seemed to depend on the immediate state of nutrition of the animal.