Influence of ruminal water-loading on renal sodium excretion and water intake following feeding in sheep
We investigated the effect of ruminal water loading before feeding on the natriuretic and drinking responses that follow feeding. Six sheep fed 800 g of chaff drank 1360 ± 150 mL during the 5 h immediately following feeding and increased renal Na excretion. Plasma Na concentration increased by 4 mmol L–1 and plasma osmolality by 9 mosmol kg–1 within 1.5 h and remained elevated. A rumen load of water administered before feeding prevented the increases in plasma Na and osmolality without affecting feeding. The natriuresis, water drinking and vasopressin secretion in response to feeding were abolished. Total sodium excreted during the experiment was halved in water-loaded animals compared with untreated animals (30.4 ± 2.1 mmol–1 cf. 63.8 ± 2.9 mmol–1;P < 0.01). Ruminal loading with isotonic saline caused a 33% reduction in postprandial drinking, however, reducing cerebrospinal fluid NaCl concentration abolished postprandial drinking and natriuresis. Intravenous infusion of isotonic dextran appeared to delay the onset of water intake without changing the total volume of water drunk, suggesting a role of plasma volume in initiating drinking. We conclude from the data that central osmoregulatory mechanisms that include increased sodium excretion as well as thirst and vasopressin release are activated following food intake by sheep.