Changes of water and electrolyte metabolism in ruminants after water deprivation and NaCl load.

The regulation of Na+ and water metabolism was studied in sheep with rumen fistula after water deprivation for 48 hours. Na+ and K+ concentrations were determined in blood plasma, blood, urine and saliva, with or without NaCl load. The results showed that the water content of the ruminal fluid plays an important role in the control of electrolyte and water metabolism. The absorption of water and Na+ from the rumen favours the maintenance of isosmolality and isovolaemia until the concentration of Na+ in the ruminal fluid has exceeded that in blood plasma. Thereafter the balance of water movement between rumen and extravascular space is shifted towards the rumen inhibiting thereby a further increase of Na+ concentration and ensuring the life conditions for the ruminal microbial population. During water deprivation not only water is lost but as a result of Na+ excretion relative hyponatraemia also ensues as can readily be assessed from the ratio of salivary Na+ and K+ concentrations. Upon NaCl load the salivary Na+/K+ ratio was increased indicating the extent of Na+ supply.