Responses to reduced water intake, including dehydration natriuresis, in sheep excreting sodium predominantly in urine or in faeces

Abstract

SUMMARYSheep which were predominantly urinary excretors (U) or faecal excretors (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 intakes (0-4, 0-05 or 1-2 mmol kg-' day-') to test the hypothesis that dehydration natriuresis was not a cause of sodium depletion but a defence against hypernatraemia. Dehydration caused elevation of plasma sodium concentration, osmolality, antidiuretic hormone (ADH) and oxytocin 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 sodium excretion increased in either group of sheep although the rise in plasma sodium concentration caused by dehydration was similar. Thus, when there is a risk of sodium depletion, due to low sodium intake, dehydration natriuresis does not occur, consistent with the hypothesis. Active sodium transport inhibitor (ASTI) and atrial natriuretic peptide (ANP) fell rather than rose during dehydration. Since aldosterone is suppressed by the higher levels of sodium intake, none of these hormones is likely to mediate dehydration natriuresis in sheep. F sheep showed more effective renal and faecal water conservation when dehydrated. During water restriction, the urinary potassium excretion of U sheep was significantly reduced, unlike that of F sheep; moreover, the latter maintained an identical plasma potassium concentration between baseline and restriction period, whereas in U sheep it was 0-3 mmol l`higher during water restriction. Increased drinking rather than reduced urine output was the basis of rehydration when ad lib. water intake was restored.

Is osmolality a long-term regulator of renal sympathetic nerve activity in conscious water-deprived rats?

“…Nevertheless, the fact that elevated osmolality and vasopressin do not seem to chronically influence RSNA and that RSNA as a function of reflex maximum is not altered by water deprivation indirectly suggests that the basal level of RSNA may not be significantly altered. In addition, natriuresis is observed after dehydration and is thought to be an important physiological mechanism to help maintain normal osmolality when water intake is limited (14,15,22). However, the natriuretic response is apparently not due to suppression of RSNA, since it is not prevented by renal denervation (18).…”

Effect of water restriction on feeding and metabolism in dairy cows

“…It reflects the activation of compensatory endocrine mechanisms (renin-angiotensin system, aldosterone, vasopressin) by the reduction in plasma volume and the increase in plasma osmolality (5,8,23). With an increase in plasma sodium, renal sodium excretion eventually increases again (29,31) and stabilizes plasma sodium at a higher level. Plasma sodium is also recycled and concentrated in the saliva (36).…”