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Changes of sodium appetite during the estrous cycle of sheep
Physiology and Behavior, Vol. 14, pp. 223-226. Brain Research Publications Inc., 1975. Printed in the U.S.A.
BRIEF COMMUNICATION Changes of Sodium Appetite During the Estrous Cycle of Sheep I A. R. MICHELL
Department of Medicine, Royal Veterinary College, Hawkshead Lane N. Mimms, Hatfield, Herts, England
(Received 16 April 1974)
MICHELL, A. R. Changesof sodium appetiteduring the estrouscycleof sheep. PHYSIOL. BEHAV. 14(2) 223-226,1975. The relative preference for sodium solutions and water was observed during a succession of estrous cycles in housed sheep. The luteal phase of the cycle was associated with statistically significant rises of sodium preference compared with the 6 days following estrus. These reflected changes in the intake both of sodium and water. The similar pattern with both bicarbonate and chloride suggests that it is the acceptabifity of the sodium which changes. The results are discussed in relation to sodium appetite and electrolyte homeostasis. The possible significance of any similar changes during the menstrual cycle is considered. Sodium appetite Preference behaviour and estrous cycle Sodium appetite in sheep
MANY animals exhibit sodium appetite, whether offered the solid or solution, and in some circumstances the sodium intake appears appropriate to experimentally imposed deficits [3,8]. Yet sodium appetite is often ignored by those concered with fluids and electrolytes, for example it is not mentioned at all in a recent review of "regulation of total body sodium" [ 12]. Although the concept of sodium requirement underlies many studies of sodium appetite, its physiological basis is frequently nebulous [18]. The magnitude of an imposed deficit does not indicate a sodium requirement since various tissues can respond adaptively intil more sodium is mobilised from b o n e ; there is no inherent need for a m a t c h e d change in intake. Moreover experementally imposed deficits are often massive and acute, particularly in sheep prepared with exteriorised parotid salivary ducts [7]. Thus 4 0 0 - 8 0 0 mEq of sodium may be lost in a few days [2,8,9] whereas the suggested daily sodium intake is of the order of 50 mEq [ 1 ]. On comparable sodium intakes sheep secrete much less aldosterone than humans [5] although the adrenal gland and kidney respond swiftly and effectively to tangible sodium deficits. Thus sheep seem well adapted to their low sodium intake. Nevertheless normal sheep will take additional sodium when it is available. This
Estrous cycle and electrolyte intake
spontaneous sodium appetite was studied in order to find possible factors involved in adjusting sodium intake to the milder changes in sodium balance likely to be encountered naturally, for example during the estrous cycle [ 17]. METHOD Unmated Cheviot ewes weighing about 50 kg and in their second year were housed individually; after several weeks of settling they were observed between November and April. They were fed a 300 g concentrate ration which was always consumed and provided a daily minimum of 26 mEq of sodium; chaffed hay was fed ad lib. Estrus was assessed from examination of vaginal mucus. Water and a sodium solution were available, the position alternating daily. Liquids were offered in polythene buckets accessible to the sheep through the cage bars. Buckets were meticulously and identically cleaned every second day. Sodium preference was the percentage of the 48-hr fluid intake selected from the sodium solution. Eight sheep yielded observations of 38 complete estrous cycles during which sodium bicarbonate (40 mM) was available and 3 additional sheep were offered sodium chloride (150 raM) during 3 cycles each. The concentrations were chosen since previous
1This work was supported by the Clement Stephenson Trust and the Agricultural Research Council It formed part of a Doctoral DiR~rtafion submitted to the University of London and executed under the helpful supervi~'on of Professor F.R. Bell. The generous cooperation ofDg. M.F. Tarttelin is acknowledged. 223
224
MICHELL
work suggested that sodium preference would be in the intermediate range and permit fluctuations in either direction. Statistical comparisons employed Student's t-test.
70 +
Preference + S,E. M.
50 RESULTS There is a maximum of sodium preference 6 days before estrus following which preference falls (Fig. 1). The difference between 6 day periods before and after estrus is statistically significant (67.8% -+ S.E. 2.3; 54.5% -+ 2.8: p = 0.001). It reflects the rise in water intake (1207 + 89 mL; 1740 + 116 mL: p = 0.001) and fall in sodium intake (2625 + 131 mL; 2132 +- 131 mL: p = 0.01). The same preference pattern is obtained if each sheep is represented by the mean of its cycles; preference in the 6 days before estrus is higher (69.0% + 2.0; 56.1% -+ 3.2: p = 0.01). It is also obtained if the data of each cycle are related to the mean of that same cycle, i.e. within individual cycles preference is higher (p = 0.001) in the 6 days preceding estrus [19]. Total fluid intake remains steady in the two periods (3824 --- 139 mL; 3892 -+. 124 mL). The fall in total fluid intake at estrus scarcely affects water intake although when cattle or sheep are offered water alone, intake does fall [15, 20, 26]. Sodium chloride yields very similar trends (Fig. 2). Sodium preference is significantly higher in the 6 days before estrus than the 6 days after estrus (61.2% + 4.9; 44.8% +- 3.8: p = 0.02). Water intake was significantly higher in the 6 days following estrus (1751 +- 205 mL; 1017 -+ 120 mL; p = 0.01). The difference in the volume of sodium solution taken during the 2 periods did not reach statistical significance (1847 +- 209 mL; 1382 +- 144 mL). The similar patterns of preference suggest that the relative acceptability of sodium and water is changing, regardless of the anion, although this need imply no change in taste acuity. Since in the absence of sodium solutions water intake is equal before and after estrus [20] we may assume that the acceptability of sodium dictates the pattern. Although the changes in sodium preference are statistically significant, the changes in absolute sodium intake are remarkably small, the 6 days before and after estrus differing by about 10 mEq daily with bicarbonate and 35 mEq with chloride. Changes in chaff intake during the cycle are small except on the day of estrus; the slightly lower intake in the period following estrus produces a dietary sodium intake differing from the luteal phase by less than 3 mEq/day. DISCUSSION The pattern of sodium preference parallels the plasma progesterone concentration [11, 23, 25, 27] and progesterone causes sodium loss in humans by antagonising th9 renal effects of aldosterone while enhancing its secretion [10, 13, 14]. It would be plausible to suppose that the enhanced sodium appetite during the luteal phase was a response to a deficit of this type but such an explanation seems less plausible with the very low aldosterone levels reported in normal sheep. Thus daily secretion is 15 t~g ( 1 0 0 - 3 0 0 ~g in humans) and on sodium intakes associated with a plasma aldosterone of 6m~g/100 mL in humans, plasma levels in sheep fall close to the limits of the assay (< I m~g/100 mL) [4, 5, 16]. Moreover subsequent studies indicate that negative sodium balance and an aldosterone response are more likely after estrus than during the luteal phase [20]. The pattern of sodium preference does not
~" 30 Total Fluid
o =
3
t-
oo
eNaHCO3
.I .-,I
=
2
c~
I
"Water
i!
!
I
6 4 Days before 0estrus
1
#
I
I
I
20estrus 2 4 6 Days after oestrus
FIG. 1. Changes in sodium preference, total fluid intake, sodium bicarbonate intake and water intake during 38 oestrous cycles observed in 8 sheep. Sodium bicarbonate concentration is 40 mM.
match the pattern of estrogen secretion so well since the fall in preference outlasts estrogen secretion by some 6 days. Nevertheless, in the absence of experimental administ r a t i o n o f either hormone, conclusions about their importance, direct or indirect, would be premature. Interestingly, enhanced sodium appetite during pregnancy in the rabbit is induced by estrogen rather than progesterone [6]. It is possible that the changes in sodium preference are related to the fall in food and fluid intake which characterises estrus [15,26], a possibility which could be examined using pair-fed ovariectomised animals. Altered food intake can affect the balance between alimentary secretion and reabsorbtion without influencing external sodium balance. This fall in food and fluid intake occurs within 48 hours of the estrous smear in all the cycles studied here and within 24 hours in 80% of cycles. Since 48-hour units are used to calculate preference, it is not surprising that a very similar preference pattern is obtained if the day of estrus is identified by the disturbance of fluid intake instead of the smear. Indeed such a procedure eliminates the deceptive appearence of a 4 day drop in fluid intake at estrus; this merely reflects the fact that when the drop in total fluid intake did not coincide with the smear, it usually preceded it. The striking fact in the bicarbonate experiment is that the differences in sodium intake, although reproducible, are small. If they reflect changing physiological needs, the
SODIUM APPETITE AND ESTROUS CYCLE
225
+'~~.~.z~==~ 705030:
Preferent_ + S.E.M.
Totalfluid
=Water
o~Cl 1 i
I
I
,
~k
I
I
I
8 6 4 20eslrus 2 4 6 Daysbeforeoestrus Daysafteroestrus FIG. 2. Changes in sodium preference, total fluid intake, sodium chloride intake and water intake during 9 oestrous cycles observed in 3 sheep. Sodium chloride concentration is 150 mM.
regulatory mechanisms involved must be exquisitely sensitive. Knowledge of the changing balance and distribution of electrolytes during the estrous cycle may provide clues concerning the possible needs and mechanisms underlying the alterations of sodium intake. Changes in sodium appetite and sodium excretion have been observed during the estrous cycle of the rat [22,24] but this animal is much less suitable because its cycle, particularly the luteal phase, is short. In sheep, the estrous cycle offers the opportunity to study sodium appetite in a physiological situation without recourse to sodium depletion, and to see whether varia-
tions reflect any tangible changes in sodium requirement. A behavioural contribution to electrolyte homeostasis is inherently no less likely than the existence of thirst. In conclusion, and with the normal reservations about extrapolation to different species, it is interesting that a similar luteal predominance of sodium appetite, if it occurred in humans, would lead to maximum salt intake during the week preceding menstruation. This time is characteristically associated with premenstrual tension, a syndrome in which sodium retention has often been implicated [211.
REFERENCES 1. Agricultural Research Council, Nutrient Requirements o f Farm Livestock, No. 2 Ruminants. London: A. R. C., 1965. 2. Blair-West, J. R., J. P. Coghlan, D. A. Denton, J. R. Goding, M. Wintout and R. D. Wright. The direct effect of increased sodium concentration in adrenal arterial blood on corticosteroid secretion in sodium deficient sheep. Aust. J. exp. Biol. meal. Sci. 44: 455-474, 1966. 3. Blair-West,J. R., J. P. Coghlan, D. A. Denton, J. F. Nelson, E. Orchard, B. A. Scoggins, R. D. Wright, K. Myers and C. L. Junqueria. Physiological, morphological and behavioural adaptation to a sodium deficient environment by wild native Australian and introduced species of animals. Nature, Lend. 217: 922-928, 1968.
4. Blair-West,J. R., J. P. Coghlan, D. A. Denton, D. Scott and R. D. Wright. The role of aldosterone in renal sodium conservation during sodium depletion. Aust. J. exp. Biol. reed. Sci. 46: 529-539, 1968. 5. Coghlan, J. P. and B. A. Scoggins.Measurement of aldosterone in peripheral blood of man and sheep. J. clin. Endocr. Metab. 27: 1470-1486, 1967. 6. Covelli, M. D., D. A. Denton, J. F. Nelson and A. A. Shulkes. Hormonal factors influencing salt appetite in pregnancy. Endocrinology 93: 423--429, 1973. 7. Denton, D. A. The study of sheep with permanent unilateral parotid fistulae. Q. Jl exp. Physiol. 42: 72-95, 1957. 8. Denton, D. A. Salt appetite. In: Handbook of Physiology. Section 6. Alimentary Canal Vol. I, edited by C. F. Code. Washington, DC: American Physiological Society, 1967, pp. 433-459.
226 9. Denton, D. A., J. F. Nelson, E. Orchard and S. Weller. The role of adrenocortical hormone secretion in salt appetite. In: Olfaction and Taste, edited by C. Pfaffmann. New York: Rockefeller University Press, 1969, pp. 535-547. 10. Gray, M. J., K. S. Strausfield, M. Watanabe, E.A.H. Sims and S. Soloman. Aldosterone secretory rates in normal menstrual cycle. J. clin. Endocr. Metab. 28: 1269-1275, 1968. 11. Hutchinson, J. S. M. and H. A. Robertson. The growth of the follicle and corpus luteum in the ovary of the sheep. Res. vet. Sci. 7: 17-25, 1966. 12. Kramer, K., J. W. Boylan and W. Keck. Regulation of total body sodium in the mammalian organism. Nephron 6 : 3 7 9 388, 1969. 13. Laidlaw, J. E., J. L. Rose and A. G. Gornall. The influence of oestrogen and progesterone on aldosterone excretion. J. clin. Endocr. Metab. 22: 161-171,1962. 14. Landau, R. L. and K. Lugibuhl. Inhibition of the sodium retaining influence of aldosterone by progesterone. J. ctin. Endocr. Metab. 18: 1237-1245, 1958. 15. MacFarlane, J. S. The effect of oestrus on free water intake in zebu type heifers. Vet. Rec. 80: 361-362, 1967. 16. Maxwell, M. H. and C. R. Kleeman. ClinicalDisorders o f Fluid and Electrolyte Metabolism. London: McGraw Hill, 1962, p. 350. 17. Michell, A. R. A study of salt appetite in the sheep with special reference to the role of taste. PhD. Thesis, University of London, 1969.
MICHELL 18. Michell, A. R. Biochemical observations on spontaneous salt appetite in sheep. Proc. 78th A. Cony. Am. psychol. Ass., 1970, pp. 211-212. 19. Michell, A. R. Changes in voluntary sodium intake during the oestrus cycle of sheep. J. Physiol. 231:42 44, I973. 20. Michell, A. R. Changes in urinary electrolyte excretion during the oestrus cycle in sheep. J. Physiol., in press. 21. Paschkin, K. E., A. E. Rakoff, A. Cantarow and 1. J. Rupp. Clinical Endocrinology, 3rd ed. New York: Harper and Row, 1967, pp. 689-691. 22. Pye, R. G. and A. J. Matty. Cyclic variations of the urinary Na: K ratio in female rats fed on a 'low iodine' diet. J. Endoer. 40: x-xi, 1968. 23. Robinson, T. J. The Control o f the Ovarian Cycle in the Sheep. Sydney: Sydney University Press, 1967. 24. Rodrigues, Antunes, J., and M. R. Covian. Hypothalamic control of sodium chloride and water intake. Acta Physiol. latinoamer 13: 94-100, 1963. 25. Stabenfeldt, G. H., J. A. Holt and L. L. Ewing. Peripheral plasma progesterone levels during the ovine oestrous cycle. Endocrinology 85:11 15, 1969. 26. Tarttelin, M. F. A physiological investigation of the medial hypothalamus of the sheep with special reference to food and water intake. Ph.D. Thesis, University of London, 1969. 27. Thorhurn, G.D., J. M. Bassett and I.D. Smith. Progesterone concentration in peripheral plasma of sheep during the oestrus cycle. J. Endocr. 45: 459-469, 1969.
BRIEF COMMUNICATION Changes of Sodium Appetite During the Estrous Cycle of Sheep I A. R. MICHELL
Department of Medicine, Royal Veterinary College, Hawkshead Lane N. Mimms, Hatfield, Herts, England
(Received 16 April 1974)
MICHELL, A. R. Changesof sodium appetiteduring the estrouscycleof sheep. PHYSIOL. BEHAV. 14(2) 223-226,1975. The relative preference for sodium solutions and water was observed during a succession of estrous cycles in housed sheep. The luteal phase of the cycle was associated with statistically significant rises of sodium preference compared with the 6 days following estrus. These reflected changes in the intake both of sodium and water. The similar pattern with both bicarbonate and chloride suggests that it is the acceptabifity of the sodium which changes. The results are discussed in relation to sodium appetite and electrolyte homeostasis. The possible significance of any similar changes during the menstrual cycle is considered. Sodium appetite Preference behaviour and estrous cycle Sodium appetite in sheep
MANY animals exhibit sodium appetite, whether offered the solid or solution, and in some circumstances the sodium intake appears appropriate to experimentally imposed deficits [3,8]. Yet sodium appetite is often ignored by those concered with fluids and electrolytes, for example it is not mentioned at all in a recent review of "regulation of total body sodium" [ 12]. Although the concept of sodium requirement underlies many studies of sodium appetite, its physiological basis is frequently nebulous [18]. The magnitude of an imposed deficit does not indicate a sodium requirement since various tissues can respond adaptively intil more sodium is mobilised from b o n e ; there is no inherent need for a m a t c h e d change in intake. Moreover experementally imposed deficits are often massive and acute, particularly in sheep prepared with exteriorised parotid salivary ducts [7]. Thus 4 0 0 - 8 0 0 mEq of sodium may be lost in a few days [2,8,9] whereas the suggested daily sodium intake is of the order of 50 mEq [ 1 ]. On comparable sodium intakes sheep secrete much less aldosterone than humans [5] although the adrenal gland and kidney respond swiftly and effectively to tangible sodium deficits. Thus sheep seem well adapted to their low sodium intake. Nevertheless normal sheep will take additional sodium when it is available. This
Estrous cycle and electrolyte intake
spontaneous sodium appetite was studied in order to find possible factors involved in adjusting sodium intake to the milder changes in sodium balance likely to be encountered naturally, for example during the estrous cycle [ 17]. METHOD Unmated Cheviot ewes weighing about 50 kg and in their second year were housed individually; after several weeks of settling they were observed between November and April. They were fed a 300 g concentrate ration which was always consumed and provided a daily minimum of 26 mEq of sodium; chaffed hay was fed ad lib. Estrus was assessed from examination of vaginal mucus. Water and a sodium solution were available, the position alternating daily. Liquids were offered in polythene buckets accessible to the sheep through the cage bars. Buckets were meticulously and identically cleaned every second day. Sodium preference was the percentage of the 48-hr fluid intake selected from the sodium solution. Eight sheep yielded observations of 38 complete estrous cycles during which sodium bicarbonate (40 mM) was available and 3 additional sheep were offered sodium chloride (150 raM) during 3 cycles each. The concentrations were chosen since previous
1This work was supported by the Clement Stephenson Trust and the Agricultural Research Council It formed part of a Doctoral DiR~rtafion submitted to the University of London and executed under the helpful supervi~'on of Professor F.R. Bell. The generous cooperation ofDg. M.F. Tarttelin is acknowledged. 223
224
MICHELL
work suggested that sodium preference would be in the intermediate range and permit fluctuations in either direction. Statistical comparisons employed Student's t-test.
70 +
Preference + S,E. M.
50 RESULTS There is a maximum of sodium preference 6 days before estrus following which preference falls (Fig. 1). The difference between 6 day periods before and after estrus is statistically significant (67.8% -+ S.E. 2.3; 54.5% -+ 2.8: p = 0.001). It reflects the rise in water intake (1207 + 89 mL; 1740 + 116 mL: p = 0.001) and fall in sodium intake (2625 + 131 mL; 2132 +- 131 mL: p = 0.01). The same preference pattern is obtained if each sheep is represented by the mean of its cycles; preference in the 6 days before estrus is higher (69.0% + 2.0; 56.1% -+ 3.2: p = 0.01). It is also obtained if the data of each cycle are related to the mean of that same cycle, i.e. within individual cycles preference is higher (p = 0.001) in the 6 days preceding estrus [19]. Total fluid intake remains steady in the two periods (3824 --- 139 mL; 3892 -+. 124 mL). The fall in total fluid intake at estrus scarcely affects water intake although when cattle or sheep are offered water alone, intake does fall [15, 20, 26]. Sodium chloride yields very similar trends (Fig. 2). Sodium preference is significantly higher in the 6 days before estrus than the 6 days after estrus (61.2% + 4.9; 44.8% +- 3.8: p = 0.02). Water intake was significantly higher in the 6 days following estrus (1751 +- 205 mL; 1017 -+ 120 mL; p = 0.01). The difference in the volume of sodium solution taken during the 2 periods did not reach statistical significance (1847 +- 209 mL; 1382 +- 144 mL). The similar patterns of preference suggest that the relative acceptability of sodium and water is changing, regardless of the anion, although this need imply no change in taste acuity. Since in the absence of sodium solutions water intake is equal before and after estrus [20] we may assume that the acceptability of sodium dictates the pattern. Although the changes in sodium preference are statistically significant, the changes in absolute sodium intake are remarkably small, the 6 days before and after estrus differing by about 10 mEq daily with bicarbonate and 35 mEq with chloride. Changes in chaff intake during the cycle are small except on the day of estrus; the slightly lower intake in the period following estrus produces a dietary sodium intake differing from the luteal phase by less than 3 mEq/day. DISCUSSION The pattern of sodium preference parallels the plasma progesterone concentration [11, 23, 25, 27] and progesterone causes sodium loss in humans by antagonising th9 renal effects of aldosterone while enhancing its secretion [10, 13, 14]. It would be plausible to suppose that the enhanced sodium appetite during the luteal phase was a response to a deficit of this type but such an explanation seems less plausible with the very low aldosterone levels reported in normal sheep. Thus daily secretion is 15 t~g ( 1 0 0 - 3 0 0 ~g in humans) and on sodium intakes associated with a plasma aldosterone of 6m~g/100 mL in humans, plasma levels in sheep fall close to the limits of the assay (< I m~g/100 mL) [4, 5, 16]. Moreover subsequent studies indicate that negative sodium balance and an aldosterone response are more likely after estrus than during the luteal phase [20]. The pattern of sodium preference does not
~" 30 Total Fluid
o =
3
t-
oo
eNaHCO3
.I .-,I
=
2
c~
I
"Water
i!
!
I
6 4 Days before 0estrus
1
#
I
I
I
20estrus 2 4 6 Days after oestrus
FIG. 1. Changes in sodium preference, total fluid intake, sodium bicarbonate intake and water intake during 38 oestrous cycles observed in 8 sheep. Sodium bicarbonate concentration is 40 mM.
match the pattern of estrogen secretion so well since the fall in preference outlasts estrogen secretion by some 6 days. Nevertheless, in the absence of experimental administ r a t i o n o f either hormone, conclusions about their importance, direct or indirect, would be premature. Interestingly, enhanced sodium appetite during pregnancy in the rabbit is induced by estrogen rather than progesterone [6]. It is possible that the changes in sodium preference are related to the fall in food and fluid intake which characterises estrus [15,26], a possibility which could be examined using pair-fed ovariectomised animals. Altered food intake can affect the balance between alimentary secretion and reabsorbtion without influencing external sodium balance. This fall in food and fluid intake occurs within 48 hours of the estrous smear in all the cycles studied here and within 24 hours in 80% of cycles. Since 48-hour units are used to calculate preference, it is not surprising that a very similar preference pattern is obtained if the day of estrus is identified by the disturbance of fluid intake instead of the smear. Indeed such a procedure eliminates the deceptive appearence of a 4 day drop in fluid intake at estrus; this merely reflects the fact that when the drop in total fluid intake did not coincide with the smear, it usually preceded it. The striking fact in the bicarbonate experiment is that the differences in sodium intake, although reproducible, are small. If they reflect changing physiological needs, the
SODIUM APPETITE AND ESTROUS CYCLE
225
+'~~.~.z~==~ 705030:
Preferent_ + S.E.M.
Totalfluid
=Water
o~Cl 1 i
I
I
,
~k
I
I
I
8 6 4 20eslrus 2 4 6 Daysbeforeoestrus Daysafteroestrus FIG. 2. Changes in sodium preference, total fluid intake, sodium chloride intake and water intake during 9 oestrous cycles observed in 3 sheep. Sodium chloride concentration is 150 mM.
regulatory mechanisms involved must be exquisitely sensitive. Knowledge of the changing balance and distribution of electrolytes during the estrous cycle may provide clues concerning the possible needs and mechanisms underlying the alterations of sodium intake. Changes in sodium appetite and sodium excretion have been observed during the estrous cycle of the rat [22,24] but this animal is much less suitable because its cycle, particularly the luteal phase, is short. In sheep, the estrous cycle offers the opportunity to study sodium appetite in a physiological situation without recourse to sodium depletion, and to see whether varia-
tions reflect any tangible changes in sodium requirement. A behavioural contribution to electrolyte homeostasis is inherently no less likely than the existence of thirst. In conclusion, and with the normal reservations about extrapolation to different species, it is interesting that a similar luteal predominance of sodium appetite, if it occurred in humans, would lead to maximum salt intake during the week preceding menstruation. This time is characteristically associated with premenstrual tension, a syndrome in which sodium retention has often been implicated [211.
REFERENCES 1. Agricultural Research Council, Nutrient Requirements o f Farm Livestock, No. 2 Ruminants. London: A. R. C., 1965. 2. Blair-West, J. R., J. P. Coghlan, D. A. Denton, J. R. Goding, M. Wintout and R. D. Wright. The direct effect of increased sodium concentration in adrenal arterial blood on corticosteroid secretion in sodium deficient sheep. Aust. J. exp. Biol. meal. Sci. 44: 455-474, 1966. 3. Blair-West,J. R., J. P. Coghlan, D. A. Denton, J. F. Nelson, E. Orchard, B. A. Scoggins, R. D. Wright, K. Myers and C. L. Junqueria. Physiological, morphological and behavioural adaptation to a sodium deficient environment by wild native Australian and introduced species of animals. Nature, Lend. 217: 922-928, 1968.
4. Blair-West,J. R., J. P. Coghlan, D. A. Denton, D. Scott and R. D. Wright. The role of aldosterone in renal sodium conservation during sodium depletion. Aust. J. exp. Biol. reed. Sci. 46: 529-539, 1968. 5. Coghlan, J. P. and B. A. Scoggins.Measurement of aldosterone in peripheral blood of man and sheep. J. clin. Endocr. Metab. 27: 1470-1486, 1967. 6. Covelli, M. D., D. A. Denton, J. F. Nelson and A. A. Shulkes. Hormonal factors influencing salt appetite in pregnancy. Endocrinology 93: 423--429, 1973. 7. Denton, D. A. The study of sheep with permanent unilateral parotid fistulae. Q. Jl exp. Physiol. 42: 72-95, 1957. 8. Denton, D. A. Salt appetite. In: Handbook of Physiology. Section 6. Alimentary Canal Vol. I, edited by C. F. Code. Washington, DC: American Physiological Society, 1967, pp. 433-459.
226 9. Denton, D. A., J. F. Nelson, E. Orchard and S. Weller. The role of adrenocortical hormone secretion in salt appetite. In: Olfaction and Taste, edited by C. Pfaffmann. New York: Rockefeller University Press, 1969, pp. 535-547. 10. Gray, M. J., K. S. Strausfield, M. Watanabe, E.A.H. Sims and S. Soloman. Aldosterone secretory rates in normal menstrual cycle. J. clin. Endocr. Metab. 28: 1269-1275, 1968. 11. Hutchinson, J. S. M. and H. A. Robertson. The growth of the follicle and corpus luteum in the ovary of the sheep. Res. vet. Sci. 7: 17-25, 1966. 12. Kramer, K., J. W. Boylan and W. Keck. Regulation of total body sodium in the mammalian organism. Nephron 6 : 3 7 9 388, 1969. 13. Laidlaw, J. E., J. L. Rose and A. G. Gornall. The influence of oestrogen and progesterone on aldosterone excretion. J. clin. Endocr. Metab. 22: 161-171,1962. 14. Landau, R. L. and K. Lugibuhl. Inhibition of the sodium retaining influence of aldosterone by progesterone. J. ctin. Endocr. Metab. 18: 1237-1245, 1958. 15. MacFarlane, J. S. The effect of oestrus on free water intake in zebu type heifers. Vet. Rec. 80: 361-362, 1967. 16. Maxwell, M. H. and C. R. Kleeman. ClinicalDisorders o f Fluid and Electrolyte Metabolism. London: McGraw Hill, 1962, p. 350. 17. Michell, A. R. A study of salt appetite in the sheep with special reference to the role of taste. PhD. Thesis, University of London, 1969.
MICHELL 18. Michell, A. R. Biochemical observations on spontaneous salt appetite in sheep. Proc. 78th A. Cony. Am. psychol. Ass., 1970, pp. 211-212. 19. Michell, A. R. Changes in voluntary sodium intake during the oestrus cycle of sheep. J. Physiol. 231:42 44, I973. 20. Michell, A. R. Changes in urinary electrolyte excretion during the oestrus cycle in sheep. J. Physiol., in press. 21. Paschkin, K. E., A. E. Rakoff, A. Cantarow and 1. J. Rupp. Clinical Endocrinology, 3rd ed. New York: Harper and Row, 1967, pp. 689-691. 22. Pye, R. G. and A. J. Matty. Cyclic variations of the urinary Na: K ratio in female rats fed on a 'low iodine' diet. J. Endoer. 40: x-xi, 1968. 23. Robinson, T. J. The Control o f the Ovarian Cycle in the Sheep. Sydney: Sydney University Press, 1967. 24. Rodrigues, Antunes, J., and M. R. Covian. Hypothalamic control of sodium chloride and water intake. Acta Physiol. latinoamer 13: 94-100, 1963. 25. Stabenfeldt, G. H., J. A. Holt and L. L. Ewing. Peripheral plasma progesterone levels during the ovine oestrous cycle. Endocrinology 85:11 15, 1969. 26. Tarttelin, M. F. A physiological investigation of the medial hypothalamus of the sheep with special reference to food and water intake. Ph.D. Thesis, University of London, 1969. 27. Thorhurn, G.D., J. M. Bassett and I.D. Smith. Progesterone concentration in peripheral plasma of sheep during the oestrus cycle. J. Endocr. 45: 459-469, 1969.