entirely clear. Reed and his co-workers (Reed et al. 1965; Topps et al. 1966) found that

Transcription

1 Quarterly Journal of Experimental Physiology (1985), 70, Printed in Great Britain THE EFFECTS OF FEEDING EITHER ROUGHAGE OR CONCENTRATE DIETS ON SALIVARY PHOSPHORUS SECRETION, NET INTESTINAL PHOSPHORUS ABSORPTION AND URINARY PHOSPHORUS EXCRETION IN THE SHEEP D. SCOTT AND W. BUCHAN Rowett Research Institute, Bucksburn, Aberdeen AB2 9SB (RECEIVED FOR PUBLICATION 29 AUGUST 1984) SUMMARY Mature sheep fitted with rumen and duodenal cannulae were fed either a hay or a concentrate diet and the effects on salivary phosphorus secretion, net intestinal phosphorous absorption and pathway for phosphorous excretion were examined. Route of excretion was markedly affected by diet with urine phosphorus levels being much higher and faecal levels lower when the concentrate diet was fed. This difference was not due to differences in phosphorus intake nor could it be related to differences in either plasma phosphorus levels or net intestinal phosphorus absorption. Salivary phosphorus secretion and renal tubular reabsorptive efficiency for phosphorus were, however, both lower when the concentrate diet was fed. The significance of these effects ofdiet in relation to the control of phosphorus balance in ruminants is discussed. INTRODUCTION Sheep and cattle fed concentrate diets in general seem to excrete much higher levels of phosphorus in their urine (Reed, Elliot & Topps, 1965; Topps, Reed & Elliot, 1966; Hyldgaard-Jensen, Whitelaw, Reid & Murray, 1966; Scott, 1972) than those fed roughage diets (Guegen, 1962; Scott, 1969, 1972; Manston & Vagg, 1970; Clark, Budtz-Olsen, Cross, Finnamore & Bauert, 1973; Grace, Ullyat & MacRae, 1974; Tomas, 1975; Towns, Boston & Leaver, 1978; Grace, 1982) and in some circumstances this can lead to the formation of renal calculi (Bushman, Emerick & Embry, 1965; Hoar, Emerick & Embry, 1969; Rice & McMurray, 1981; Godwin & Williams, 1982). The reason for this phosphaturia is not entirely clear. Reed and his co-workers (Reed et al. 1965; Topps et al. 1966) found that cattle fed concentrate diets tended to become mildly acidaemic and they suggested that additional phosphorus might under these conditions be excreted in the urine to serve as a vehicle for acid excretion. However, if this were so then correction of the acidosis ought to lead to a reduction in phosphorus excretion to levels seen in animals fed roughage diets but when this was tested no reduction in excretion was observed (Scott, 1972). Another possible explanation for the phosphaturia comes from studies reported by Tomas (1974) in which sheep were fed a diet containing a high proportion of either coarse or finely ground oat hulls. Changing the physical form of the diet had little effect on over-all phosphorus balance but influenced the route by which phosphorus was excreted, urinary levels being slightly higher and faecal levels lower when the ground diet was fed. Feeding the ground diet also led to a reduction in the amount of phosphorus secreted into the gut via the saliva and Tomas (1974) suggested that the changes in urine and faecal phosphorus excretion were directly linked to these changes in salivary phosphorus secretion.

2 366 D. SCOTT AND W. BUCHAN It is widely believed that sheep and cattle secrete less saliva when they are fed concentrate diets than when they are fed roughage diets and it seemed to us that if this were also true for phosphorus secretion then this might explain the phosphaturia seen when concentrate diets are fed. The main object of the present study was to look at the relationship between salivary phosphorus secretion, net intestinal absorption and urinary phosphorus excretion in sheep fed either a hay or a concentrate diet. METHODS Seven mature sheep (five ewes and two castrate males) of either the Scottish Blackface or Suffolk x Scottish Blackface breed and weighing between 45 and 55 kg were used. Prior to experiment they were all fitted with a 2 5 cm diameter ebonite cannula into the rumen. In addition to the rumen cannula, three of the sheep (two castrate males and one female) were also fitted with a T-shaped Perspex cannula (1 5 cm diameter) into the duodenum about 6-10 cm from the pylorus and these animals were used to study the effects of diet on salivary phosphorus secretion, the remaining four ewes being used in a series of renal clearance studies. At least 4 weeks were allowed for recovery after surgery before starting any experiment. During experiment the sheep were housed in metabolism cages which permitted the separate collection of faeces and urine. Effects on salivary phosphorus secretion. The three sheep fitted with rumen and T-shaped duodenal cannulae were used. The experiment consisted of three consecutive 28 d periods during which the sheep were fed (via a continuous belt feeder) 900 g. d-l of either a concentrate diet containing 88% bruised barley and 12% fish meal, a mixed diet consisting of equal parts of the concentrate diet and a ground hay (2 mm sieve) or the ground hay alone. Supplements of CaHPO4. 2H20 and Na2HPO4 were added to the hay and the mixed diet to raise the levels of calcium and phosphorus in them to those present in the concentrate diet. A proprietary trace element + vitamin supplement (Norvite 317, Norvite Feed Supplements Ltd, Wardhouse, Insch, Aberdeenshire) was also added to each diet at the rate of 1 g. kg-'. All three diets with their supplements were then pelleted. Details of the sampling schedule within each 28 d period are given in Fig. 1. Samples of blood from the jugular vein were taken at h on 7 d while faeces and urine were collected over a consecutive 7 d period beginning on day 16. During the experiment radioactive markers (103ruthenium phenanthroline and [51Cr]EDTA) were used to estimate digesta flow at the duodenum. The procedure used was to give 5 4,/Ci 103ruthenium phenanthroline and 36 4aCi [51Cr]EDTA as an initial priming dose into the rumen at h on day 19 and thereafter to give the same quantity daily by continuous infusion for a further 7 d period with digesta samples being collected on the last 3 d of this period. On each of these days digesta samples were collected from the duodenum at I h intervals from to h. The hourly samples were pooled to give a composite sample of about 0-5 kg for each sheep. Each of these pooled samples was then homogenized and subsamples taken for dry matter analysis while further subsamples were centrifuged and the supernatant fraction retained for analysis. The amount of digesta and phosphorus flowing from the abomasum to the small intestine was estimated by reference to the two markers using the procedure described by Faichney (1975). Effects on renalfunction. The four ewes fitted with rumen cannulae alone were used. Two separate series of experiments were performed during periods when they were fed 900 g. d-' of either the concentrate or the hay diet. Both diets were supplemented as before to equate phosphorus intake. At the beginning of each experiment both right and left jugular veins were fitted with a Polythene catheter (PPV 190, Portex Ltd, Hythe, Kent), one catheter being used for infusion purposes and the other for the withdrawal of blood samples. Urine was collected using an 18 gauge self-retaining urethral catheter (Warner Surgical Products, Andover, Hants). Each experiment consisted of three 30 min control periods during which a 0-15 mol. 1-1 solution of NaCl was infused at the rate of 2 ml. min-' followed by eight 30 min periods during which a 0 33 mol. 1-1 solution of Na2HPO4 was infused at the same rate of 2 ml. ml-', urine samples being collected throughout each period and blood samples being drawn at each mid-point. Inulin was used as a marker to estimate glomerular filtration rate (Scott & Mason, 1970).

3 PHOSPHORUS HOMOEOSTASIS IN SHEEP 367 Treatment Period Duration (d) Hay Concentrate Sampling schedule within each period Day Il Blood [IL] L LI] Digesta 11] Urine + faeces [ 103Ru + 51Cr Fig. 1. Sampling schedule used in studies on the effects of diet on salivary phosphorus secretion. Chemical methods. Inorganic phosphorus in the plasma and duodenal supernatant was estimated using the automated procedure described by Young (1966) while total phosphorus in food, digesta, faeces and urine was estimated using the method of Roach (1965). Inulin in plasma and urine was estimated using the procedure of Wilson, Stacy & Thorburn (1969). The amounts of radioactive ruthenium and chromium in whole digesta and in the digesta supernatant were measured using an automatic gamma counter (Wallac DECEM-GTL system). Counts were expressed in relation to sample weight but using a fixed volume of 3 ml for both standards and unknowns. Statistical methods. The effects of diet on salivary phosphorus secretion and intestinal phosphorus absorption were analysed using a standard analysis of variance. This included testing whether the results from the 50:50 concentrate/hay diet differed significantly from the mean for the two separate diets. A significant difference would indicate non-linearity in the change produced by progressive addition of hay to the concentrate diet. In testing the effects of diet on renal function it was clear (Fig. 2) that as the amount of phosphorus filtered by the kidney (x) rose, little increase in excretion was at first observed but that above a threshold value (7) the amount reabsorbed (y) fell away from x to approach a ceiling value (T+ S). A suitable mathematical model to describe these changes is: y=x, whenx<t y = T+ S (I-exp [(T-x)/s]), when x > T. Equations of this form were fitted to the sets of results for each sheep on each diet in order to obtain smoothed estimates of the efficiency of reabsorption (100 y/x) for a range ofplasma phosphorus levels. These smoothed estimates were then subjected to analysis of variance to compare the two diets. Units. Concentrations of phosphorus in plasma and digesta supernatant are expressed in mmol. 1-1; the renal clearance data shown in Fig. 2 are given as mmol. 1-1 glomerular filtrate. Daily amounts of phosphorus secreted in the saliva or excreted in faeces or urine are expressed in g. d-l; I mol phosphorus is equivalent to g phosphorus.

4 368 D. SCOTT AND W. BUCHAN 4. C) C> rl en a, 00 V V V C) M tr) Wl C> f.j r-n L. C> C> L. to C.l op ol r) r-n u ca... Z v v C) C> C>,.j C') a, 00 V V v C14 0 C> r.ol C) CA 00 "tt C> V v v 14 ob (.o 6 z 7:1 00 C> C) C> c. C.l z z z z Q-4 00 "t oc, 0 OP 1.1 v v v in cd + Q + el CZ 0 14.).z QL7 > u U 0 C> V,) u 4- u 0 qu cl

5 PHOSPHORUS HOMOEOSTASIS IN SHEEP 369 A;C -. n x,4 op t.v ro~~~~oz 0~~~~~~~~0r qj c~ cn - Q. a ~~~~~C4, n ra (A CA 64 4 o0 6 - co ~ ~ ~~)

6 370 D. SCOTT AND W. BUCHAN 6 t::/^~~~~~~0 0 5 _ Hay Filtered X4 - /. Reabsorbed 0 Excreted oo x o6 Filtered Concentrate D / 4/ Excreted 3 e A Reabsorbed o Plasma phosphorus (mmol.1) Fig. 2. The relationship between the amount of phosphorus excreted in the urine, that reabsorbed by the renal tubule and the level of phosphorus in the plasma. Values are for four sheep fed either a hay or a concentrate diet, details of which are given in the text. Included are calculated lines of best fit, details of which are given in statistical methods. RESULTS Effect on salivary phosphorus secretion. The effects of feeding either the concentrate diet or the hay diet on the concentration of phosphorus in the plasma and the levels excreted in urine and faeces is shown in Table 1. Although the concentration of phosphorus in the plasma was not affected by diet the amounts excreted in urine and faeces were markedly affected, urinary excretion being much higher (P < 0-01) and faecal excretion much lower (P < 0 01) when the concentrate diet was fed compared to when the hay diet was fed. Table 2 shows the effects of feeding either the concentrate or the hay diet on the amount of phosphorus flowing from the abomasum to the duodenum, the amount estimated to have been added to the digesta via the saliva (duodenal phosphorus -phosphorus intake) and on the net amount absorbed from the intestine. The amount of phosphorus secreted in the

7 PHOSPHORUS HOMOEOSTASIS IN SHEEP 371 Table 4. Changes in the proportion offiltered phosphorus reabsorbed by the renal tubule in relation to plasma phosphorus concentration in sheep fed either hay or concentrate diets. Mean values for four sheep Plasma P concentration (mmol.1-1) Mean proportion of filtered P reabsorbed Hay Concentrate S.E.D Significance of difference N.s. P < 0 05 P < 0 01 P < 0 01 P < saliva was much lower (P < 0-01) when the concentrate diet was fed compared to when the hay diet was fed. Total net absorption of phosphorus in the intestine differed between diets being slightly higher (P < 0 1) when the hay diet was fed compared to when the concentrate diet was fed. The results given in Table 3 indicate the distribution of phosphorus within the digesta collected from the duodenal cannula. Most of the phosphorus was present in the liquid phase with the total amount in this phase being much higher when the hay diet was fed compared to when the concentrate diet was fed. The amount of phosphorus associated with the particulate phase was in contrast unaffected by diet. Effects on renalfunction. The results shown in Fig. 2 illustrate the relationships between renal tubular phosphorus absorption and excretion and plasma phosphorus concentration seen in sheep fed either the concentrate diet or the hay diet. Diet appeared to markedly influence phosphorus handling by the kidney such that over a wide range of filtered load the rate at which phosphorus was excreted in the urine was higher when the concentrate diet was fed compared to when the hay diet was fed. This difference in response was not due to differences in glomerular filtration rate which was similar for both diets (mean values + s.e.; hay *3 ml. min'; concentrate 92* ml. min-) but instead appeared to be largely due to differences in renal tubular reabsorptive efficiency which was lower in periods when the concentrate diet was fed compared to when the hay diet was fed (Table 4). DISCUSSION The main object of this study was to examine the relationships between salivary phosphorus secretion, net intestinal phosphorus absorption and urinary phosphorus excretion in sheep fed either roughage or concentrate diets. In doing so we have assumed that the difference between the amount of phosphorus flowing into the duodenum and intake provides a reasonable estimate of the amount of phosphorus added to the digesta via the saliva. As argued previously (Scott, McLean & Buchan, 1984a) we have based this assumption on isolated organ studies in the sheep which suggest that little or no secretion or absorption of phosphorus occurs either in the reticulo-rumen (Scarisbrick & Ewer, 1951; Sperber & Hyden, 1951; Parthasarathy, Garton & Phillipson, 1952), the omasum (Engelhardt & Hauffe, 1975) or the abomasum (Garton, 1951). Further support for these results can be seen in the work of Pfeffer (1968) who added radioactive phosphorus to the rumen

8 372 D. SCOTT AND W. BUCHAN contents of sheep fed a pelleted grass diet. The sheep were fitted with a re-entrant cannula in the duodenum close to the pylorus and digesta was collected through this cannula for 30 h after dosing, unlabelled donor material being used to replace that removed. Appearance of radioactive phosphorus in the blood was used to gauge the importance of the forestomach area as an absorptive site but throughout the collection period no measurable amount of radioactivity was detected in the blood suggesting that little or no absorption had occurred proximal to the cannula. The omasum, while of little importance as a site for phosphorus absorption in the sheep (Engelhardt & Hauffe, 1975), has been suggested as an important site for absorption in cattle (Smith & Edrise, 1978; Banks & Smith, 1984). In cattle the omasum is, however, much larger relative to the other stomach compartments than in sheep and as such may favour its enhanced role in mineral absorption (Smith, 1984). In this study, diet was shown to markedly affect urinary phosphorus excretion, excretion being much higher when the concentrate diet was fed than when the hay diet was fed. This difference in renal response could not be ascribed to differences in phosphorus intake or plasma phosphorus levels (Table 1) nor could it be attributed simply to differences in net phosphorus absorption from the intestine (Table 2). There were, however, marked differences between diets in the balance between that absorbed from the gut and that secreted in the saliva (Table 2), secretion of phosphorus in the saliva being much reduced when the all-concentrate diet was fed. Differences in renal function were also evident and it seems pertinent to consider how these changes may be linked. A number of factors are known to influence salivary flow rate but the one which appears to have the greatest effect is the physical nature of the diet, flow rates being highest when poor quality roughage diets are fed. Pelleted (Putnam, Yarns & Davis, 1966) or finely ground diets (Wilson & Tribe, 1963), by reducing both eating and rumination times have both been shown to lead to a reduction in secretion rate and highly digestible concentrate diets have been reported to have similar effects (Bailey, 1961; Lawlor, Giesecke & Walser -Kirst, 1966; Sato, Kato & Tsuda, 1976). Factors affecting the amount of phosphorus secreted in the saliva are less well defined. Phosphorus concentration in the saliva is known to be dependent on plasma concentration (Denton, 1956; Scott & Beastall, 1978; Mands-Almendros, Ross & Care, 1982) though on a daily basis the amount secreted appears to be largely determined by salivary flow rate (Bailey & Balch, 1961; Hawkins, Autrey & Huff, 1965; Lawlor et al. 1966) and as such one would expect a higher rate of phosphorus secretion in sheep fed roughage diets compared to those fed concentrate diets (Sato et al. 1976). The significance of this in relation to the present study derives from results reported by Tomas (1974) in which sheep were fed the same diet in either a coarse or finely ground form. Changing the physical form of the diet had little effect on over-all phosphorus balance but did alter the pathway of excretion with urinary excretion being higher and faecal excretion lower when the ground diet was fed. Grinding the diet also led to reduced rumination, reduced saliva secretion and hence to a reduction in the amount of phosphorus secreted into the gut via this route. The effect on renal function was also examined and it appeared that the increased urinary excretion of phosphorus seen when the ground diet was fed was not due to differences in plasma phosphorus concentration or glomerular filtration rate but rather was the result of small changes in renal tubular reabsorptive efficiency. The similarity between Tomas' results and those of the present study are clearly very striking in that the increased urinary phosphorus excretion seen when the concentrate diet was fed compared with hay feeding was on the one hand associated with a reduction in

9 PHOSPHORUS HOMOEOSTASIS IN SHEEP 373 salivary phosphorus secretion (Table 2) and on the other with changes in renal tubular reabsorptive efficiency (Table 4). Taken together this suggests that the factor common to both studies is the effect of diet on salivary secretion rate such that changing from either a coarse to a finely ground diet (Tomas, 1974) or from the less digestible (0-52) hay diet to the more highly digestible (0-84) concentrate diet both resulted in a decrease in salivary flow rate and thus reduced the rate at which phosphorus was removed from the plasma via the saliva relative to that at which it was absorbed from the gut, the inbalance being reflected in a higher level of phosphorus excretion in urine which was primarily achieved through a change in renal tubular reabsorptive efficiency. Further evidence for such a model comes from earlier studies (Scott, McLean & Buchan, 1984b) in which sheep given an intravascular phosphate load were seen to excrete a much higher proportion of this load in the urine when fed a grass diet compared to when fed a hay diet. Here again there was evidence that this difference in renal response was linked to reduced salivary secretion in sheep fed the more digestible grass diet. The changes in renal tubular reabsorption of phosphate seen here are similar to those reported in the dog either in response to feeding (Foulks, 1955) or to variation in phosphorus intake (Hellman, Baird & Bartter, 1964) while similar changes have also been seen in both the rat (Trohler, Bonjour & Fleisch, 1976) and the pig (Bonjour, Care & Pickard, 1979) as an adaptive response to low phosphorus intake. Feeding low phosphorus diets has also been shown to result in enhanced phosphate absorption by the intestine in the rat (Cramer & McMillan, 1980) and there is recent evidence that the response of salivary gland to intravenous phosphate loading is lower in sheep fed low phosphorus diets (Manas-Almendros et al. 1982; Wright Blair-West, Nelson & Tregear, 1984). The mechanism responsible for these changes is not known though the fact that they still occur after thyroparathyroidectomy (Trohler et al. 1976; Bonjour et al. 1979; Manas-Almendros et al. 1982) would indicate that they are not dependent upon changes in parathyroid hormone or calcitonin secretion. REFERENCES BAILEY, C. B. (1961). Saliva secretion and its relation to feeding in cattle. 4. The relationship between the concentrations of sodium potassium, chloride and inorganic phosphate in mixed saliva and rumen fluid. British Journal of Nutrition 15, BAILEY, C. B. & BALCH, C. C. (1961). Salivary secretion and its relation to feeding in cattle. I. The composition and rate of secretion of parotid saliva in the small steer. British Journal of Nutrition 15, BANKS, J. N. & SMITH, R. H. (1984). Sites of absorption of magnesium and phosphate in the stomach of the ruminating calf. Proceedings of the Nutrition Society 43, 8A. BONJOUR, J. P., CARE, A. D. & PICKARD, D. W. (1979). Renal adaptation to changes in dietary phosphorus intake by thyroparathyroidectomized pigs. Journal of Physiology 290, 27-28P. BUSHMAN, D. H., EMERICK, R. J. & EMBRY, L. B. (1965). Experimentally induced ovine phosphatic urolithiasis: Relationships involving dietary calcium, phosphorus and magnesium. Journal of Nutrition 87, CLARK, R. C., BUDTZ-OLSEN, 0. E., CROSS, R. B., FINNAMORE, P. & BAUERT, P. A. (1973). The importance of the salivary glands in the maintenance of phosphorus homeostasis in the sheep. Australian Journal of Agricultural Research 24, CRAMER, C. F. & MCMILLAN, J. (1980). Phosphorus adaptation in rats in absence of vitamin D or parathyroid glands. American Journal of Physiology 239, G DENTON, D. A. (1956). The effect of Na+ depletion on the Na+: K+ ratio of the parotid saliva of the sheep. Journal of Physiology 131, ENGELHARDT, W. V. & HAUFFE, R. (1975). Role of the omasum in absorption and secretion of water and electrolytes in sheep and goats. In Digestion and Metabolism in the Ruminant, ed. McDONALD, I. W. & WARNER, A. C. I., pp Armidale: University of New England Publishing Unit.

10 374 D. SCOTT AND W. BUCHAN FAICHNEY, G. J. (1975). The use of markers to partition digestion within the gastro-intestinal tract of ruminants. In Digestion and Metabolism in the Ruminant, ed. McDONALD, I. W. & WARNER, A. C. I., pp Armidale: University of New England Publishing Unit. FOULKS, J. G. (1955). Homeostatic adjustment in the renal tubular transport of inorganic phosphate in the dog. Canadian Journal of Biochemistry and Physiology 33, GARTON, G. A. (1951). Observations on the distribution of inorganic phosphorus, soluble calcium and soluble magnesium in the stomach of the sheep. Journal of Experimental Biology 28, GODWIN, I. R. & WILLIAMS, V. J. (1982). Urinary calculi formation in sheep on high wheat grain diets. Australian Journal of Agricultural Research 33, GRACE, N. D. (1982). Phosphorus kinetics in the sheep. British Journal of Nutrition 45, GRACE, N. D., ULLYAT, M. J. & MACRAE, J. C. (1974). Quantitative digestion of fresh herbage by sheep. III. The movement of Mg, Ca, P, K and Na in the digestive tract. Journal of Agricultural Science 82, GUIGEN, L. (1962). L'utilisation digestive reelle du phosphore du foin de lucerne par le mouton, measuree a l'aide du 32p. Annales de biologie animale, biochimie biophysique 2, HAWKINS, G. E., AUTREY, K. M. & HUFF, J. W. (1965). Effect of partial deprivation of parotid saliva on physiological responses of steers fed three levels of dietary sodium. Journal of Dairy Science 48, HELLMAN, D., BAIRD, H. R. & BARTTER, F. G. (1964). Relationship of maximal tubular phosphate reabsorption to dietary phosphate in the dog. American Journal of Physiology 207, HOAR, D. W., EMERICK, R. J. & EMBRY, L. B. (1969). Ovine phosphatic urolithiasis as related to the phosphorus and calcium contents and acid-base forming effects of all-concentrate diets. Journal of Animal Science 29, HYLDGAARD-JENSEN, J., WHITELAW, F. G., REID, R. S. & MURRAY, M. (1966). Some changes in rumen fluid, blood and urine of cattle given a high-concentrate diet. In Programme and abstracts of the Ninth International Congress of Animal Production, pp Edinburgh: Oliver & Boyd. LAWLOR, M. J., GIESECKE, D. & WALSER-KARST, K. (1966). Comparative studies on the digestive physiology of sheep fed on semi-purified or roughage-concentrate diets. 1. Food and water intake, rumen volume and rates of parotid secretion. British Journal of Nutrition. 20, MANAS-ALMENDROS, M., Ross, R. & CARE, A. D. (1982). Factors affecting the secretion of phosphate in parotid saliva in the sheep and goat. Quarterly Journal ofexperimental Physiology 67, MANSTON, R. & VAGG, M. J. (1970). Urinary phosphate excretion in the dairy cow. Journal of Agricultural Science 74, PARTHASARATHY, D., GARTON, G. A. & PHILLIPSON, A. T. (1952). The passage of phosphorus across the rumen epithelium of sheep. Biochemical Journal 52, xvi. PFEFFER, E. (1968). Untersuchungen uiber mineralstojfbewegungen im verdauungskanal von ausgewachsenen Hameln. University of G6ttingen: Habilitations-Schrift. PUTNAM, P. A., YARNS, D. A. & DAVIS, R. E. (1966). Effect of pelleting rations and hay: grain ratio on salivary secretion and ruminal characteristics of steers. Journal ofanimal Science 25, REED, W. D. C., ELLIOT, R. C. & ToPPs, J. H. (1965). Phosphorus excretion of cattle fed high energy diets. Nature 208, RICE, D. A. & MCMURRAY, C. H. (1981). Urolithiasis in calves. Veterinary Record 109, 88. ROACH, A. G. (1965). Application of Technicon Autoanalyzer equipment to the routine determination of calcium and phosphorus in animal feedstuffs. In Automation in Analytical Chemistry, Technicon Symposia, ed. SKEGGS, L. T. J., pp New York: Mediad Incorporated. SATO, H., KATO, S. & TSUDA, T. (1976). Effect of hay to concentrate ratio on parotid secretion and its sodium, potassium and phosphorus levels in sheep. Japanese Journal of Veterinary Science 38, SCARISBRICK, R. & EWER, T. K. (1951). The absorption of inorganic phosphate from the rumen of the sheep. Biochemical Journal 49, lxxix. SCOTT, D. (1969). Renal excretion of potassium and acid by sheep. Quarterly Journal ofexperimental Physiology 54, SCOTT, D. (1972). Excretion of phosphorus and acid in the urine of sheep and calves fed either roughage or concentrate diets. Quarterly Journal of Experimental Physiology 57, SCOTT, D. & BEASTALL, G. (1978). The effects of intravenous phosphate loading on salivary phosphate secretion and plasma parathyroid hormone levels in sheep. Quarterly Journal of Experimental Physiology 63,

11 PHOSPHORUS HOMOEOSTASIS IN SHEEP 375 SCOTT, D., MCLEAN, A. F. & BUCHAN, W. (1984a). The effect of variation in phosphorus intake on net intestinal phosphorus absorption, salivary phosphorus secretion and pathway of excretion in sheep fed roughage diets. Quarterly Journal of Experimental Physiology 69, SCOTT, D., MCLEAN, A. F. & BUCHAN, W. (1984b). The effect of intravenous phosphate loading on salivary phosphorus secretion, net intestinal phosphorus absorption and pathway of excretion in sheep fed roughage diets. Quarterly Journal of Experimental Physiology 69, SCOTT, D. & MASON, G. (1970). Renal tubular reabsorption of urea in sheep. Quarterly Journal of Experimental Physiology 55, SMITH, R. H. (1984). Microbial activity in the omasum. Proceedings ofthe Nutrition Society 43, SMITH, R. H. & EDRISE, B. M. (1978). Absorption of magnesium and phosphate in the omasum of the young steer. Proceedings of the Nutrition Society 37, 60A. SPERBER, I. & HYDEN, S. (1951). Transport of chloride through the rumen mucosa. Nature 169, 587. TOMAS, F. M. (1974). Phosphorus homeostasis in sheep. II. Influence of diet on pathway of excretion of phosphorus. Australian Journal of Agricultural Research 25, TOMAS, F. M. (1975). Renal response to intravenous phosphate infusion in sheep. Australian Journal of Biological Science 28, Topps, J. H., REED, W. D. C. & ELLIOT, R. C. (1966). Studies on the metabolism of cattle given high concentrate diets. Journal of Agricultural Science 66, TOWNS, K. M., BOSTON, R. C. & LEAVER, D. D. (1978). The effect of intravenous administration of phosphorus on phosphorus and calcium metabolism in sheep. Australian Journal of Agricultural Research 29, TROHLER, U., BONJOUR, J. P. & FLEISCH, H. (1976). Inorganic phosphate homeostasis: Renal adaptation to the dietary intake in intact and thyroparathyroidectomised rats. Journal of Clinical Investigation 57, WILSON, A. D. & TRIBE, D. E. (1963). The effect of diet on the secretion of parotid saliva by sheep. 1. The daily secretion of saliva by caged sheep. Australian Journal of Agricultural Research 14, WILSON, B. W., STACEY, B. D. & THORBURN, G. D. (1969). Automated determination of inulin in the estimation of glomerular filtration rate. Australian Journal of Biological and Medical Sciences 47, WRIGHT, R. D., BLAIR-WEST, J. R., NELSON, J. F. & TREGEAR, G. W. (1984). Handling of phosphate by a parotid gland (ovine). American Journal of Physiology 246, F YOUNG, D. S. (1966). An improved method for the automatic determination of serum inorganic phosphate. Journal of Clinical Pathology 19,