RESPONSES TO REDUCED WATER-INTAKE, INCLUDING DEHYDRATION NATRIURESIS,IN SHEEP EXCRETING SODIUM PREDOMINANTLY IN URINE OR IN FECES

Sheep which were predominantly urinary excretors (U) or faecal excreto rs (F) of sodium were exposed to a 75 % reduction of water intake for 72 h. The experiment was performed on moderate, low or high sodium int akes (0.4, 0.05 or 1.2 mmol kg(-1) day(-1)) to test the hypothesis tha t dehydration natriuresis was not a cause of sodium depletion but a de fence against hypernatraemia. Dehydration caused elevation of plasma s odium concentration, osmolality, antidiuretic hormone (ADH) and oxytoc in but, as in other experiments, a fall in haematocrit. The two higher levels of sodium intake were associated with dehydration natriuresis but also a smaller increase in faecal sodium excretion in both U and F sheep. On low sodium intake, however, neither urinary nor faecal sodi um excretion increased in either group of sheep although the rise in p lasma sodium concentration caused by dehydration was similar. Thus, wh en there is a risk of sodium depletion, due to low sodium intake, dehy dration natriuresis does not occur, consistent with the hypothesis. Ac tive sodium transport inhibitor (ASTI) and atrial natriuretic peptide (ANP) fell rather than rose during dehydration. Since aldosterone is s uppressed by the higher levels of sodium intake, none of these hormone s is likely to mediate dehydration natriuresis in sheep. F sheep showe d more effective renal and faecal water conservation when dehydrated. During water restriction, the urinary potassium excretion of U sheep w as significantly reduced, unlike that of F sheep; moreover, the latter maintained an identical plasma potassium concentration between baseli ne and restriction period, whereas in U sheep it was 0.3 mmol l(-1) hi gher during water restriction. Increased drinking rather than reduced urine output was the basis of rehydration when ad lib. water intake wa s restored.