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Sodium preference in sheep excreting sodium predominantly in urine or faeces
Physiology&Behavior,Vol. 52, pp. 285-286, 1992
0031-9384/92 $5.00 + .00 Copyright© 1992PergamonPressLtd.
Printed in the USA.
Sodium Preference in Sheep Excreting Sodium Predominantly in Urine or Faeces A. R. M I C H E L L
Department of Large Animal Medicine and Surgery, Royal Veterinary College (University of London), Hawkshead Lane, North Mymms, Herts AL9 7TA, UK Received 27 N o v e m b e r 1991 MICHELL, A. R. Sodiumpreferencein sheepexcretingsodiumpredominantly in urineor [aeces. PHYSIOL BEHAV 52(2) 285286, 1992.--On moderate sodium intakes (0.5 mmol/kg/day approx.) sheep are readily divided into those that are predominantly urinary or faecal excretors of sodium (U or F sheep). Water turnover and obligatory sodium loss are lower in F sheep. In view of these differences, sodium preference was examined in U and F sheep offered water and sodium bicarbonate solutions (40 or 300 mmol/I). The F sheep took significantlymore of the bicarbonate solutions and, therefore, had sodium intakes that were 51% higher on the lowerconcentration, 94% higher on the higherconcentration. Thus, F sheep not only conservesodium more readily when it is scarce, they select it more readily when it is available. Sodium excretion Sodiumpreference Salt appetite S h e e p Ruminants Herbivores Enteric sodium excretion Faecalsodium excretion Sodiumbalance Sodiumbicarbonate Sodium intake
SHEEP and other herbivores on moderate sodium intakes, i.e., consistent with nutritional requirement, excrete sodium predominantly in faeces, rather than urine (2). On high sodium intakes urine becomes the predominant route of sodium excretion (4,5). On sodium intakes of around 0.5 mmol/kg/day, generous compared with requirement but still not high, sheep are readily divided into a majority (F) that excrete sodium predominantly in faeces and a minority (U) that excrete sodium predominantly in urine. The latter have a consistently higher water turnover (4,8). More surprisingly, since they correspond to the textbook pattern of sodium excretion being primarily, or even exclusively, renal, U sheep are less effective at reducing their net sodium loss on low sodium intakes even when there is the added demand of pregnancy (7,8). In view of these differences in their handling of sodium and water, it seemed logical to examine the sodium preference of U and F sheep using hypertonic (300 mmol/l) and hypotonic (40 mmol/l) solutions of sodium bicarbonate as in our previous experiments on salt appetite during pregnancy (6).
consecutive 48-hour periods with a choice between sodium bicarbonate (40 mmol/I) and water, and four further consecutive periods with sodium bicarbonate (300 mmol/l) as the alternative to water. Results are expressed as means +_SEM and the statistical difference between means was assessed by Student's t-test. Sodium preference was the percentage of 48-hour fluid intake selected from the sodium solution. RESULTS Details of U and F sheep before access to sodium solutions are shown in Table l, during preference tests in Table 2. As expected, water turnover was higher in the U sheep, which had significantly higher urine output (p < 0.01) and 15% higher water intake. Although this difference was not statistically significant, it represented a 23% greater intake per bodyweight (38.1, 31.0 ml/kg). The F sheep had a higher preference for 40 mmol/l sodium bicarbonate, but this did not reach statistical significance; the same was true with the 300 mmol/l sodium bicarbonate solution. During the period of availability of sodium solutions water intake remained 15% higher in U sheep than in F sheep. In contrast, the F sheep had a significantly higher intake from the sodium solutions (p < 0.05). Thus, as a result of their higher sodium intake, the F sheep had a higher fluid intake than U sheep ( 1878, 1599 ml/day), while the sodium solutions were available. The F sheep had a higher absolute sodium intake than U sheep with both concentrations of sodium bicarbonate (differences of 51% and 94%, respectively, with 40 retool/l, 300 mmol/l bicarbonate). Although urine and faeces were not collected during the period when sodium solutions were offered, previous work shows that both U and F sheep excrete excess sodium (i.e., above nutritional requirement) mainly in their urine (7).
METHOD Twelve Scottish Blackface ewes were housed in metabolism cages to which they were accustomed, and after a week of acclimatisation to the diet and conditions 24-hour samples of urine and faeces were collected for a week (8). These were processed as previously described (8) in order to determine the sodium concentration and net daily excretion in urine and faeces. On this basis (percentage of daily sodium excretion in urine or faeces) the sheep were classified as F (n = 8) or U (n = 4). The basal diet (7) included a supplement of 25 mmol Na ÷ per day. The sheep were then tested for sodium preference (6) during four
285
286
MICHI'.I_[ DISCUSSIOn,
Most experimenters infer sodium balance from salivary Na/ K ratios, aldosterone levels, or sodium levels in urine. Faeces are seldom analysed and. since they are often the major route of sodium excretion, conclusions based on urine alone are frequently misleading (2,4), The conventional interpretation of the preference data in these two groups, for example, would be that the F sheep, having a lower sodium excretion in urine, are more sodium deficient and, therefore, select more s o d i u m - c o n t a i n i n g solution when offered the choice. It is unreasonable, however, in normal animals, to visualise one sodium regulatory system compensating for another: rather, we should expect that all systems act in concert, as they do during the oestrous cycle, i.e., body sodium falls as a result of reduced intake and increased excretion (3). From this viewpoint, it is not surprising that those sheep (F) which are best able to curtail their sodium loss when confronted with low intakes, also take more sodium when they have the opportunity. Both responses would equip them better for survival in periods or e n v i r o n m e n t s with u n d e p e n d a b l e sources of sodium. The lower water t u r n o v e r of F sheep would also equip
I. Water intake (ml/day) (before availability of bicarbonate solutions) Urine output (ml/day) % Preference (40 mmol/l) % Preference (300 mmol/l) Na intake at 40 mmol/1 (mmol/day) Na intake at 300 mmol/I (retool/day) NaHCO3intake(ml/day) Water intake (ml/day)
2353 1181 52.9 22.7
F
± 237 _~ 179 ± 9.2% ± 6.9~
38.0 ±
7.8
109.9 ± 41.4 611 ± 129 939 ± 113
2051 596 71.9 30.3
± 129 +- 62 ± 4.8% ± 5.1~
57.4 ±
5.5
213.2 ± 39.3 1065 ± 105 814 ± 73
t h e m better for arid e n v i r o n m e n t s , as would their greater urine osmolality a n d resistance to hyperkalaemia during water restriction (5). It also avoids excessive urinary losses of urea which can impair protein metabolism in r u m i n a n t s (1). Most important, F sheep not only conserve sodium more effectively than U sheep when it is scarce, they select it more strongly when it is available.
TABLE 1 BODYWEIGHT AND Na EXCRETION PRIOR TO PREFERENCE TESTS IN U AND F SHEEP
Bodyweight (kg) Urinary Na excretion (mmol/day) Faecal Na excretion (mmol/day)
TABLE 2 RELA lIVE PREFERENCE FOR SODIUM BICARBONAII AND WATER IN U AND F SHEEP
U
F
61.7 + 2.0 12.8 ± 2.1 10.9 +_ 1.6
66.2 ± 1.4 4.4 ± 0.9 21.6 ± 4.7
ACKNOWLEDGEMENTS These experiments were supported by the Wellcome Trust and benefitted from the invaluable technical support of Mr. P. Moss and Mr. A. T. Jones. I thank Mrs. Rosemary Forster for preparation of the manuscript.
REFERENCES
1. King, J. M. Fellowship Thesis. Royal College of Veterinary Surgeons: 1987. 2. Michell, A. R. The gut, the unobtrusive regulator of sodium balance. Perspec. Biol. Med. 29:203-213: 1986. 3. Michell, A. R.: Noakes, D. E. The effect of oestrogen and progesterone on fluid and electrolyte balance in sheep. Res. Vet. Sci. 38:46-53, 1985. 4. Michell, A. R,~ Moss, P. The gut and sodium regulation in sheep and other herbivores. J. Physiol. 394:134P: 1987. 5. Michell, A. R.: Moss, P. Responses to water restriction in sheep e×-
creting sodium predominantly in urine or faeces: Observations at three levels of sodium intake. J. Physiol. 396:31P: 1988. 6. Michell, A. R.: Moss, P. Salt appetite during pregnancy in sheep. Physiol. Behav. 42:491-493:1988. 7. Michell, A. R.: Moss, P. Physiological differences between sheep excreting sodium predominantly in their urine or in their faeces: The effect of changes in sodium intake. Exp. Physiol. (in press). 8. Michel[, A. R.: Moss, P.: Hill, R.; Vincent, I. C., Noakes, D. E. The effect of pregnancy and sodium intake on water and electrolyte balance in sheep. Br. Vet. J. 144:147-157" 1988.
0031-9384/92 $5.00 + .00 Copyright© 1992PergamonPressLtd.
Printed in the USA.
Sodium Preference in Sheep Excreting Sodium Predominantly in Urine or Faeces A. R. M I C H E L L
Department of Large Animal Medicine and Surgery, Royal Veterinary College (University of London), Hawkshead Lane, North Mymms, Herts AL9 7TA, UK Received 27 N o v e m b e r 1991 MICHELL, A. R. Sodiumpreferencein sheepexcretingsodiumpredominantly in urineor [aeces. PHYSIOL BEHAV 52(2) 285286, 1992.--On moderate sodium intakes (0.5 mmol/kg/day approx.) sheep are readily divided into those that are predominantly urinary or faecal excretors of sodium (U or F sheep). Water turnover and obligatory sodium loss are lower in F sheep. In view of these differences, sodium preference was examined in U and F sheep offered water and sodium bicarbonate solutions (40 or 300 mmol/I). The F sheep took significantlymore of the bicarbonate solutions and, therefore, had sodium intakes that were 51% higher on the lowerconcentration, 94% higher on the higherconcentration. Thus, F sheep not only conservesodium more readily when it is scarce, they select it more readily when it is available. Sodium excretion Sodiumpreference Salt appetite S h e e p Ruminants Herbivores Enteric sodium excretion Faecalsodium excretion Sodiumbalance Sodiumbicarbonate Sodium intake
SHEEP and other herbivores on moderate sodium intakes, i.e., consistent with nutritional requirement, excrete sodium predominantly in faeces, rather than urine (2). On high sodium intakes urine becomes the predominant route of sodium excretion (4,5). On sodium intakes of around 0.5 mmol/kg/day, generous compared with requirement but still not high, sheep are readily divided into a majority (F) that excrete sodium predominantly in faeces and a minority (U) that excrete sodium predominantly in urine. The latter have a consistently higher water turnover (4,8). More surprisingly, since they correspond to the textbook pattern of sodium excretion being primarily, or even exclusively, renal, U sheep are less effective at reducing their net sodium loss on low sodium intakes even when there is the added demand of pregnancy (7,8). In view of these differences in their handling of sodium and water, it seemed logical to examine the sodium preference of U and F sheep using hypertonic (300 mmol/l) and hypotonic (40 mmol/l) solutions of sodium bicarbonate as in our previous experiments on salt appetite during pregnancy (6).
consecutive 48-hour periods with a choice between sodium bicarbonate (40 mmol/I) and water, and four further consecutive periods with sodium bicarbonate (300 mmol/l) as the alternative to water. Results are expressed as means +_SEM and the statistical difference between means was assessed by Student's t-test. Sodium preference was the percentage of 48-hour fluid intake selected from the sodium solution. RESULTS Details of U and F sheep before access to sodium solutions are shown in Table l, during preference tests in Table 2. As expected, water turnover was higher in the U sheep, which had significantly higher urine output (p < 0.01) and 15% higher water intake. Although this difference was not statistically significant, it represented a 23% greater intake per bodyweight (38.1, 31.0 ml/kg). The F sheep had a higher preference for 40 mmol/l sodium bicarbonate, but this did not reach statistical significance; the same was true with the 300 mmol/l sodium bicarbonate solution. During the period of availability of sodium solutions water intake remained 15% higher in U sheep than in F sheep. In contrast, the F sheep had a significantly higher intake from the sodium solutions (p < 0.05). Thus, as a result of their higher sodium intake, the F sheep had a higher fluid intake than U sheep ( 1878, 1599 ml/day), while the sodium solutions were available. The F sheep had a higher absolute sodium intake than U sheep with both concentrations of sodium bicarbonate (differences of 51% and 94%, respectively, with 40 retool/l, 300 mmol/l bicarbonate). Although urine and faeces were not collected during the period when sodium solutions were offered, previous work shows that both U and F sheep excrete excess sodium (i.e., above nutritional requirement) mainly in their urine (7).
METHOD Twelve Scottish Blackface ewes were housed in metabolism cages to which they were accustomed, and after a week of acclimatisation to the diet and conditions 24-hour samples of urine and faeces were collected for a week (8). These were processed as previously described (8) in order to determine the sodium concentration and net daily excretion in urine and faeces. On this basis (percentage of daily sodium excretion in urine or faeces) the sheep were classified as F (n = 8) or U (n = 4). The basal diet (7) included a supplement of 25 mmol Na ÷ per day. The sheep were then tested for sodium preference (6) during four
285
286
MICHI'.I_[ DISCUSSIOn,
Most experimenters infer sodium balance from salivary Na/ K ratios, aldosterone levels, or sodium levels in urine. Faeces are seldom analysed and. since they are often the major route of sodium excretion, conclusions based on urine alone are frequently misleading (2,4), The conventional interpretation of the preference data in these two groups, for example, would be that the F sheep, having a lower sodium excretion in urine, are more sodium deficient and, therefore, select more s o d i u m - c o n t a i n i n g solution when offered the choice. It is unreasonable, however, in normal animals, to visualise one sodium regulatory system compensating for another: rather, we should expect that all systems act in concert, as they do during the oestrous cycle, i.e., body sodium falls as a result of reduced intake and increased excretion (3). From this viewpoint, it is not surprising that those sheep (F) which are best able to curtail their sodium loss when confronted with low intakes, also take more sodium when they have the opportunity. Both responses would equip them better for survival in periods or e n v i r o n m e n t s with u n d e p e n d a b l e sources of sodium. The lower water t u r n o v e r of F sheep would also equip
I. Water intake (ml/day) (before availability of bicarbonate solutions) Urine output (ml/day) % Preference (40 mmol/l) % Preference (300 mmol/l) Na intake at 40 mmol/1 (mmol/day) Na intake at 300 mmol/I (retool/day) NaHCO3intake(ml/day) Water intake (ml/day)
2353 1181 52.9 22.7
F
± 237 _~ 179 ± 9.2% ± 6.9~
38.0 ±
7.8
109.9 ± 41.4 611 ± 129 939 ± 113
2051 596 71.9 30.3
± 129 +- 62 ± 4.8% ± 5.1~
57.4 ±
5.5
213.2 ± 39.3 1065 ± 105 814 ± 73
t h e m better for arid e n v i r o n m e n t s , as would their greater urine osmolality a n d resistance to hyperkalaemia during water restriction (5). It also avoids excessive urinary losses of urea which can impair protein metabolism in r u m i n a n t s (1). Most important, F sheep not only conserve sodium more effectively than U sheep when it is scarce, they select it more strongly when it is available.
TABLE 1 BODYWEIGHT AND Na EXCRETION PRIOR TO PREFERENCE TESTS IN U AND F SHEEP
Bodyweight (kg) Urinary Na excretion (mmol/day) Faecal Na excretion (mmol/day)
TABLE 2 RELA lIVE PREFERENCE FOR SODIUM BICARBONAII AND WATER IN U AND F SHEEP
U
F
61.7 + 2.0 12.8 ± 2.1 10.9 +_ 1.6
66.2 ± 1.4 4.4 ± 0.9 21.6 ± 4.7
ACKNOWLEDGEMENTS These experiments were supported by the Wellcome Trust and benefitted from the invaluable technical support of Mr. P. Moss and Mr. A. T. Jones. I thank Mrs. Rosemary Forster for preparation of the manuscript.
REFERENCES
1. King, J. M. Fellowship Thesis. Royal College of Veterinary Surgeons: 1987. 2. Michell, A. R. The gut, the unobtrusive regulator of sodium balance. Perspec. Biol. Med. 29:203-213: 1986. 3. Michell, A. R.: Noakes, D. E. The effect of oestrogen and progesterone on fluid and electrolyte balance in sheep. Res. Vet. Sci. 38:46-53, 1985. 4. Michell, A. R,~ Moss, P. The gut and sodium regulation in sheep and other herbivores. J. Physiol. 394:134P: 1987. 5. Michell, A. R.: Moss, P. Responses to water restriction in sheep e×-
creting sodium predominantly in urine or faeces: Observations at three levels of sodium intake. J. Physiol. 396:31P: 1988. 6. Michell, A. R.: Moss, P. Salt appetite during pregnancy in sheep. Physiol. Behav. 42:491-493:1988. 7. Michell, A. R.: Moss, P. Physiological differences between sheep excreting sodium predominantly in their urine or in their faeces: The effect of changes in sodium intake. Exp. Physiol. (in press). 8. Michel[, A. R.: Moss, P.: Hill, R.; Vincent, I. C., Noakes, D. E. The effect of pregnancy and sodium intake on water and electrolyte balance in sheep. Br. Vet. J. 144:147-157" 1988.