Renal Control of Plasma Urea Level in Sheep: The Diuretic Effect of Urea, Potassium and Sodium Chloride

Abstract

Two merino ewes were fed 800 and then 1000 g of lucerne chaff per day and renal responses and extracellular fluid volume (E.C.F.) were determined. Urine urea nitrogen, glomerular filtration rate (G.F.R.), urine volume, effective renal plasma flow (E.R.P.F.) and E.C.F. increased in both animals with the greater nitrogen intake. In a second experiment, five merino wethers and four merino ewes were infused intraruminally with increasing quantities of urea, whilst being fed a mainly roughage ration, and the above parameters were determined. No significant changes in G.F.R., E.R.P.F. or E.C.F. were noted. Urinary excretion maintained a linear response to nitrogen input until 20·6 g of urea nitrogen per day were infused. At this level of infusion, an increased fractional reabsorption of urea resulted in a large increase in plasma urea concentration, but when 500 mmol of NaCl or KCl were added to the infusate, urine urea nitrogen excretion increased from 10·4 to 11·4 and 11·9 g/d respectively and plasma urea concentration decreased from 68·0 to 35·2 and 37·3 mg nitrogen/100 ml. Urea clearance virtually trebled with both electrolyte infusions and E.C.F. and G.F.R. increased by aproximately 10 and 25% respectively. It is concluded that urea alone has limited diuretic ability and that the increased electrolyte content of higher protein dry roughage diets aids urine urea excretion by increasing G.F.R., and the urine flow rate by an osmotic diuretic effect.

Changes in endogenous urea recycling and the handling of renal urea in pregnant and lactating Sardi sheep kept on a constant feeding level

“…The secretion of saliva increases during mastication, especially when the animal is ruminating (Church, 1979 Renal urea excretion is reported to be positively correlated with urine flow rate in sheep (Cocimano and Leng, 1967;Godwin and Williams, 1984 (Nolan and Leng, 1970) and in pregnant and lactating Merino sheep (Oddy et al, 1983). The high potential for nitrogen conservation during the two productive states could be of special importance for breeds such as the Sardi, which usually graze vegetation poor in nitrogen.…”

Effects of intraruminal sodium chloride infusion on rumen and renal nitrogen and electrolyte dynamics in sheep

“…This is a typical effect of increased NaCl intake (Dirks et al 1976;Godwin & Williams, 1984). Potter (1961) found that sheep given 13 g NaC1/1 as drinking water, through which they voluntarily consumed 1000 mmol NaCl/d for prolonged periods, have a n increased GFR and a reduced ERPF resulting in a greater renal filtration fraction.…”

“…However the salt intake from this treatment was only about 20 mmol/kg body-weight per d. In the present study this level, equivalent to about 750 mmol/d, had little effect on the indices of N metabolism measured. Previous work from this laboratory has shown that relatively small intraruminal infusions of NaCl (500 mmol/d) will reduce the high plasma urea concentrations of urea-loaded sheep by increasing the renal excretion of urea (Godwin & Williams, 1984).…”

“…The present study examines the effects of high levels of NaCl on the digestibility of food and the dynamics of the rumen and kidneys of sheep given a low-protein roughage. A summary of part of this work has been published (Godwin & Williams, 1983).…”

“…The plasma level may be affected by NaCl intake, as sheep loaded with urea and infused intraruminally with NaCl showed an increased renal excretion and a consequent lowering of the plasma concentration of urea compared with animals not given NaCl (Godwin & Williams, 1984).…”