Experimental Physiology (1992), 77, 799-805 Printed in Great Britain
DIFFERENCES BETWEEN SHEEP EXCRETING SODIUM PREDOMINANTLY IN THEIR URINE OR IN THEIR FAECES: THE EFFECT OF CHANGES IN SODIUM INTAKE A. R. MICHELL AND P. MOSS Department of Large Animal Medicine and Surgery, Royal Veterinary College (University of London), Hawkshead Lane, North Mymms, Hatfield, Herts AL9 7TA (MANUSCRIPT RECEIVED 21 DECEMBER 1991, ACCEPTED 7 JULY 1992)
SUMMARY
Sheep receiving a total of about 31 mmol day-1 (0-5 mmol kg-') of sodium were classified according to the predominant route of sodium excretion; urinary (U) or faecal (F). U sheep had a greater water turnover than F sheep; their intake was 41 % higher and they produced 133 % more urine but there was little difference in faecal water loss. Most faecal sodium was readily exchangeable with water in both groups. When sodium intake was reduced by 80 % (to 6 mmol day-1; 0'1 mmol kg-'), the reduction in total sodium excretion was equally effective in F sheep and U sheep after 48 h and after 2 weeks the overall losses of sodium were smaller in F sheep. On sodium intakes close to requirement (0-1 mmol kg-' or less) the majority of the sheep excreted most of their sodium in faeces and did so on intakes up to 0 5 mmol kg-' day-'. Excess dietary sodium is mainly excreted renally. When sodium intake is increased abruptly (by 20 mmol day-', 0-3 mmol kg-'), total sodium excretion only increases gradually but after about 3 days it 'overshoots' as in humans. INTRODUCTION
A number of workers have observed that faecal sodium excretion often exceeds urinary sodium excretion in sheep and other herbivores but have not commented on this surprising observation (Michell & Noakes, 1980 and English 1966; Jones, Miles & Sinclair, 1967; Tasker, 1967; Dewhurst, Harrison & Keynes, 1968; Lomba, Paquay, Bienfet & Lousse, 1969; Michell, 1969; Morris & Gartner, 1971; Aitken, 1976; Boehncke, Gropp & Wandl, 1976; Alexander, 1977; Maloiy & Clemens, 1980; all cited in Michell, 1986). This may be because it is so widely accepted that if animals are normal, the kidney is the dominant or sole regulator of sodium balance (e.g. Daniels, Cortell & Leonard, 1985). We were particularly struck, however, by the fact that in sheep undergoing repeated pregnancies and lactations on exceptionally low sodium intakes (Michell, Moss, Hill, Vincent & Noakes, 1988), those sheep (F) which excreted sodium predominantly in faeces were more successful at limiting their total daily sodium output than sheep (U) which conformed to the conventionally accepted pattern and excreted sodium predominantly in urine. We therefore examined some aspects of this difference further. Three experiments are described: (1) some physiological comparisons on basal sodium intakes (phase I); (2) a comparison of the response to low sodium intake (phase II); (3) a comparison of the response to sodium loading (phase III). METHODS
Eighteen Scottish Blackface ewes (average weight around 60 kg), previously accustomed to periods in metabolism cages of the type described by Michell (1979), were reintroduced to cages and allowed to acclimatize for one week. The basal diet comprised 0 5 kg oats, 0 3 kg straw and 0-2 kg bran,
800
A. R. MICHELL AND P. MOSS
Table 1. Comparison of baseline data between predominantly urinary (U) andfaecal (F) excretors of sodium on 25 mmol day-' sodium supplement (phase I, Methods)
Body weight (kg) Urinary Na+ excretion (mmol day-') Faecal Na+ excretion (mmol day-') Faecal Na+ (% of total Na+ excreted) Faecal water (ml day-') Faecal water content (%) Urine volume (ml day-') Total water output (ml day-') Water intake (ml day-') Unmeasured water loss (ml) Urinary K+ excretion (mmol day-') Faecal K+ excretion (mmol day-1) Urinary Na+ concentration (mmol 1') Faecal Na+ concentration (mmol kg-') Residual Na+* in faeces (% of faecal Na+) Residual Na+* in faeces (mmol day-') *
i.e. not
Significance
U (n = 6)
F (n = 6)
61-9+2-0
61-7+2-3
12-9+1 9 77 + 1-6 37-6+ 8-2 325 + 31 555+ 1-2 1602+224 1927 2848+203 921 109 + 64 87+ 18-4 10 9+ 15
10-6+0-9 32-0+2-7 25 + 05
3-1+004
22-9 + 3-6 86-4+3 1 442+29 61-3 + 1 3 687+ 80
(P
DIFFERENCES BETWEEN SHEEP EXCRETING SODIUM PREDOMINANTLY IN THEIR URINE OR IN THEIR FAECES: THE EFFECT OF CHANGES IN SODIUM INTAKE A. R. MICHELL AND P. MOSS Department of Large Animal Medicine and Surgery, Royal Veterinary College (University of London), Hawkshead Lane, North Mymms, Hatfield, Herts AL9 7TA (MANUSCRIPT RECEIVED 21 DECEMBER 1991, ACCEPTED 7 JULY 1992)
SUMMARY
Sheep receiving a total of about 31 mmol day-1 (0-5 mmol kg-') of sodium were classified according to the predominant route of sodium excretion; urinary (U) or faecal (F). U sheep had a greater water turnover than F sheep; their intake was 41 % higher and they produced 133 % more urine but there was little difference in faecal water loss. Most faecal sodium was readily exchangeable with water in both groups. When sodium intake was reduced by 80 % (to 6 mmol day-1; 0'1 mmol kg-'), the reduction in total sodium excretion was equally effective in F sheep and U sheep after 48 h and after 2 weeks the overall losses of sodium were smaller in F sheep. On sodium intakes close to requirement (0-1 mmol kg-' or less) the majority of the sheep excreted most of their sodium in faeces and did so on intakes up to 0 5 mmol kg-' day-'. Excess dietary sodium is mainly excreted renally. When sodium intake is increased abruptly (by 20 mmol day-', 0-3 mmol kg-'), total sodium excretion only increases gradually but after about 3 days it 'overshoots' as in humans. INTRODUCTION
A number of workers have observed that faecal sodium excretion often exceeds urinary sodium excretion in sheep and other herbivores but have not commented on this surprising observation (Michell & Noakes, 1980 and English 1966; Jones, Miles & Sinclair, 1967; Tasker, 1967; Dewhurst, Harrison & Keynes, 1968; Lomba, Paquay, Bienfet & Lousse, 1969; Michell, 1969; Morris & Gartner, 1971; Aitken, 1976; Boehncke, Gropp & Wandl, 1976; Alexander, 1977; Maloiy & Clemens, 1980; all cited in Michell, 1986). This may be because it is so widely accepted that if animals are normal, the kidney is the dominant or sole regulator of sodium balance (e.g. Daniels, Cortell & Leonard, 1985). We were particularly struck, however, by the fact that in sheep undergoing repeated pregnancies and lactations on exceptionally low sodium intakes (Michell, Moss, Hill, Vincent & Noakes, 1988), those sheep (F) which excreted sodium predominantly in faeces were more successful at limiting their total daily sodium output than sheep (U) which conformed to the conventionally accepted pattern and excreted sodium predominantly in urine. We therefore examined some aspects of this difference further. Three experiments are described: (1) some physiological comparisons on basal sodium intakes (phase I); (2) a comparison of the response to low sodium intake (phase II); (3) a comparison of the response to sodium loading (phase III). METHODS
Eighteen Scottish Blackface ewes (average weight around 60 kg), previously accustomed to periods in metabolism cages of the type described by Michell (1979), were reintroduced to cages and allowed to acclimatize for one week. The basal diet comprised 0 5 kg oats, 0 3 kg straw and 0-2 kg bran,
800
A. R. MICHELL AND P. MOSS
Table 1. Comparison of baseline data between predominantly urinary (U) andfaecal (F) excretors of sodium on 25 mmol day-' sodium supplement (phase I, Methods)
Body weight (kg) Urinary Na+ excretion (mmol day-') Faecal Na+ excretion (mmol day-') Faecal Na+ (% of total Na+ excreted) Faecal water (ml day-') Faecal water content (%) Urine volume (ml day-') Total water output (ml day-') Water intake (ml day-') Unmeasured water loss (ml) Urinary K+ excretion (mmol day-') Faecal K+ excretion (mmol day-1) Urinary Na+ concentration (mmol 1') Faecal Na+ concentration (mmol kg-') Residual Na+* in faeces (% of faecal Na+) Residual Na+* in faeces (mmol day-') *
i.e. not
Significance
U (n = 6)
F (n = 6)
61-9+2-0
61-7+2-3
12-9+1 9 77 + 1-6 37-6+ 8-2 325 + 31 555+ 1-2 1602+224 1927 2848+203 921 109 + 64 87+ 18-4 10 9+ 15
10-6+0-9 32-0+2-7 25 + 05
3-1+004
22-9 + 3-6 86-4+3 1 442+29 61-3 + 1 3 687+ 80
(P
