Parthasarathy and Phillipson, 1953] and Dobson [1959] showed that the. only necessitate active transport if the potential difference between the

Transcription

1 Quart. J. exp. Physiol. (1967) 52, THE EFFECTS OF POTASSIUM SUPPLEMENTS UPON THE ABSORP- TION OF POTASSIUM AND SODIUM FROM THE SHEEP RUMEN By D. SCOTT. From the Physiology Department, Rowett Research Institute, Bucksburn, Aberdeen. (Received for publication 15th December 1966). The infusion of potassium salts into the rumen of sheep led to an increase in both the concentration and amount of potassium in the fluid in the rumen and to a decrease in the concentration and amount of sodium. The amount of potassium absorbed from the rumen increased as the intake of potassium was increased and was related to the concentration of potassium in the rumen fluid. The amount of potassium flowing out of the rumen increased and the amount of sodium flowing out decreased with increase in potassium intake but neither the volume of fluid in the rumen nor the rate at which it flowed on to the omasum was affected. There was no evidence that potassium supplements reduced the amount of sodium entering the rumen in the saliva. These results indicate that the amount of sodium absorbed from the rumen is increased when potassium supplements are given. IN experiments designed to study the absorption of sodium and chloride from the rumen of the sheep, Dobson and Phillipson [1958] demonstrated that the contents of the rumen were 3 to 4 mv electrically negative with respect to the blood. It is well known that large amounts of sodium move from the rumen to the blood [Danielli et al., 1945; Sperber and Hyden, 1952; Parthasarathy and Phillipson, 1953] and Dobson [1959] showed that the process is an 'active' one since absorption occurred against the electrochemical gradient for this ion. On the other hand, rather meagre evidence suggests that potassium may be absorbed from the rumen by passive diffusion. Sperber and Hyden [1952] found that potassium accumulated in a rumen pouch in a conscious goat to a level of about 5 times higher than that in the plasma, but this would only necessitate active transport if the potential difference between the rumen contents and the blood were less than 4 mv. Parthasarathy and Phillipson [1953] studied absorption from the isolated rumen of the sheep and suggested that potassium may be absorbed by passive diffusion. Large amounts of potassium are normally ingested by the ruminant and the purpose of the present experiments was to examine the quantitative aspects of absorption and outflow of potassium from the rumen in conscious sheep receiving a varied potassium intake. METHODS Animals and Diets. - Two adult Scottish Blackface ewes (Sheep 1 and 2) weighing 38 and 4 kg. were used in the absorption studies. Both sheep were fitted with permanent ebonite cannulas into the rumen several months before observations were begun. During the experiments the sheep were kept in metabolism cages. Four 382

2 Potassium and Sodium Absorption in Sheep 383 other Blackface ewes averaging 5 kg. (Sheep 414, 418, 419, 42) were used in experiments to determine the effects of potassium supplements upon the flow of parotid saliva. In the majority of experiments the sheep were fed a diet of grass-cubes but in one group of experiments hay-cubes were used. The diet was well mixed and offered TABLE I. COMPOSITION OF RATIONS. Concentration Intake Na K Na K Dry matter m.equiv./1 g. m.equiv./day g./1 g. Grass-cubes I L II III b Hay-cubes *8 in 8 g. amounts given each day from a continuous belt feeder. The composition of the various diets used is given in Table I. In addition to the basic ration, varying amounts of a potassium solution were given by continuous infusion into the rumen. This solution was used to increase the total intake of potassium per day and was designed to be similar in anion composition and tonicity to parotid saliva but contained only potassium as cation. The composition of this solution is given in Table II. TABLE II. COMPOSITION OF POTASSIUM SUPPLEMENT GIVEN INTO RUMENS. Concentration m.equiv. /1. K+ HCO_ HPO4 C Measurement of Absorption. - The amount of potassium absorbed from the rumen was calculated from the difference between the amount of potassium entering the rumen each day in the food + supplement and the amount of potassium leaving the rumen per day in the fluid flowing to the omasum. This will be a minimal value as it takes no account of the potassium entering the rumen in saliva. In each experiment the concentration and total amount of potassium in the rumen fluid and the rate at which potassium was flowing to the omasum were estimated on each of 2 or 3 consecutive days while the basic diet alone was fed. On the morning of the fourth day known amounts of the potassium solution (Table II) were given as a continuous infusion into the rumen. This was continued for seven days; measurements of the concentration and amount of potassium in the rumen fluid and the rate at which potassium was flowing to the omasum were made on the last 2 or 3 days of the infusion period. The only exception to this procedure was in the experiments when haycubes were fed. In these potassium absorption was calculated when the basic ration was fed and no potassium supplement was given. Measurement of Rumen Fluid Volume and Outflow. - The volume of fluid in the rumen and its rate of outflow to the omasum were calculated as described by Hyden [1961]. Either 5 or 1 g. of polyethylene glycol (PEG) in 1 ml. of water were given into the rumen each day at 9 a.m. and samples of the rumen contents were drawn at 2, 4, 6, 8, 12 and 24 hours after dosing. If the volume of rumen fluid and the rate of flow of fluid into and out of the rumen are constant during the period of observation then the concentration of PEG will decline exponentially with time. A straight line should result when the natural logarithm of marker concentration is plotted against time. Extrapolation of this

3 384 Scott line back to zero time by linear regression provides the concentration of PEG at the time of dosing. Rumen volume (V) can thus be calculated from v = A C -C' where A is the amount of PEG added to the rumen and C' and C" are the concentrations of PEG before and after dosing. The rate of flow of fluid out of the rumen per unit time (u) is expressed by the relationship u =kv where k is the slope of the semi-logarithmic plot of the marker dilution curve. Collection of Saliva. - Parotid saliva was collected from a single parotid duct as described by Scott [1966]. Analytical Methods. - Samples of whole rumen contents drawn through the cannulas were centrifuged for 15 min. at 2, g. The supernatant fluid was then removed and centrifuged for 3 min. at 25, g. The clear supernatant obtained was then stored for analysis. Sodium and potassium in the supernatant fluid of digesta and in parotid saliva were estimated by flame photometry [Scott, 1966]. Polyethylene glycol in the supernatant fluid of digesta was estimated by the turbidimetric method of Hyden [1955]. RESULTS The relationships between the volume of fluid in the rumen, its rate of outflow to the omasum and potassium intake are shown in fig. 1. The continuous infusion of potassium supplements into the rumen did not have any effect 1O- ~1 E_ 6 > a a X, E 2 ~~~~~~~2- a) E K intake m-equiv/day FIG. 1. The relationships between the volume of fluid in the rumen, its rate of outflow and the potassium intake. *, sheep 1; +, sheep 2. on the volume of fluid in the rumen or its rate of outflow to the omasum. The rate of flow of fluid to the omasum was higher (5.1 to 82 L./24 hr.) in the one series of experiments in which hay-cubes were fed than in those experiments in which grass-cubes were fed (3- to /24 hr.). The relationships between the concentrations of potassium and sodium in the rumen fluid and potassium intake are shown in fig. 2. As the total intake of potassium was varied through the range m.equiv./day, the concentration of potassium in the fluid in the rumen increased (p < 1)

4 Potassium and Sodium Absorption in Sheep 385 and the concentration of sodium decreased (p <.1). This decrease in sodium concentration was reciprocal to the rise in the concentration of potassium so that the sum of these two ions remained approximately constant l _ 91- go9~ + *.' _ 7t C' c 5 _ + t *~~~~~~~~ ~ ± o K intake m-equ;v/day FIG. 2. The relationships between the concentrations of potassium and sodium in the fluid in the rumen and the potassium intake. Symbols as in fig. 1. The effect of potassium supplements on the total amounts of sodium and potassium in the rumen fluid are illustrated in fig. 3. Increasing the amount of potassium entering the rumen each day through a potassium supplement led to a decrease in the total amount of sodium in the rumen fluid (p < 1) and to an increase in the total amount of potassium (p < -1). > 4 > 4 r -3~~~~~~~~~ c C 2 - E ± + _ + z 1 l -o C 4 G GOO 8 1 K intake m quiv/day FIG. 3. The relationships between the amounts of sodium and potassium in the fluid in the rumen and the potassium intake. Symbols as in fig. 1. The effects of potassium supplements upon the amount of potassium flowing out of the rumen are illustrated in fig. 4. The amount of potassium flowing out of the rumen to the omasum increased as the intake of potassium increased (p < '1). The relationship between the amount of potassium estimated to be absorbed from the rumen and the intake of potassium is illustrated in

5 386 Scott fig. 5. As the intake of potassium was increased through the range m.equiv./day the amount of potassium absorbed from the rumen increased (p < 1). S \ 5 E 4 o ; 31 4 G 8 1 K intake mequi%day FIG. 4. The relationship between the amount of potassium flowing out of the rumen and the potassium intake. Symbols as in fig. 1. The relationship between the amount of potassium absorbed fromn the rumen and the potassium concentration in the fluid in the rumen is illustrated in fig. 6. Absorption of potassium increased as the concentration of potassium / E±+ D 2l o 8 1 K intake m.equi;.d'day FIG. 5. The relationship between the amount of potassium absorbed from the rumen and the potassium intake. Symbols as in fig. 1. in the rumen fluid increased through the range 35-5 to 1-7 m.equiv./l. (p < 1). This relationship can be expressed in the form K absorbed = 5 1 Rumen K concn m.equiv./24 hr.

6 Potassium and Sodium Absorption in Sheep 387 where the standard error of the regression coefficient was + 5 and the residual standard deviation was + 6'6. An estimate of the variability in the concentration of potassium in the rumen fluid within each 24 hr. flow observation resulted in a standard error of per cent. The precision with which the rate of flow of potassium from the rumen was measured was dependent upon the calculated precisions associated with the measurement of rumen volume, the rate of flow of fluid out of the rumen and the concentration of potassium in the rumen fluid. The * l.5 + < E o / + IDO -~~~~~~~~~~~~~~~~~~~~ Rumen K concm mr-equi/l. FiG. 6. The relationship between the amount of potassium absorbed from the rumen and the concentration of potassium in the rumen fluid. Symbols as in fig. 1. standard error of the rates of flow of potassium froxn the rumen calculated on this basis was per cent. The amount of potassium estimated to be entering the rumen each day makes no allowance for that supplied by the saliva. In the sodium ieplete sheep the concentration of potassium in parotid saliva is less than 1 m.equiv./1., and it may be estimated that the contribution of potassium to the rumen in the parotid saliva would be less than 5 m.equiv./day. Supporting this in an experiment when sheep 1 and 2 were fed on a grasscube diet and during which in each sheep a single parotid salivary duct was cannulated, the measured rates of secretion during a 24 hr. period were and containing 16 m.equiv. and 13 m.equiv. of potassium respectively. A further contribution of potassium would also be supplied in the other salivary secretions. Samples of whole rumen contents were dried and then ashed and acid extracts were analysed for sodium and potassium. When these analyses were compared with the sodium and potassium concentrations found to be present in the supernatant fluid from these same whole rumen contents it was found that 98-2 per cent (97.1 to 99 5 per cent, 8 observations) of the sodium and 93-7 per cent (91-2 to 96X4 per cent, 8 observations) of the potassium in the rumen were present in the supernatant fluid. The amount of potassium flowing to the omnasum is thus likely to have been underestimated and absorption overestimated owing to this small amount of

7 388 Scott c 4' - a E S 3-2 < ; 1 _ /. 1 2,^ 3 ^ 4 ^ 5 ^e^ 6 -Tn 7 A i 1 ^ ^ ^ Rumen Na concn. m-equiv/t. FIG. 7. The relationship between the amount of sodium leaving the rumen in the fluid flowing to the omasum and the sodium concentration in the rumen fluid. Symbols as in fig. 1. >1 a 6o I' 5k + E E -t 8 a z z 3: 3 4k 3k 2k e 1 oo o. An v -I 4 tq 8 1 +I + + K intake m-equiv/day FIG. 8. The relationship between the amount of sodium flowing out of the rumen and the potassium intake. Symbols as in fig. 1. TABLE III. Sheep A COMPARISON BETWEEN THE RATES OF SECRETION OF PAROTID COLLECTED SALIVA FROM A SINGLE GLAND IN UNSUPPLEMENTED AND SUPPLEMENTED POTASSIUM SHEEP. No Supplement K+ Supplement, 36 m.equiv. K intake, 52 m.equiv. /day K intake, 88 m.equiv. /day t A-- Volume Na Content 1. /48 hr. m.equiv Volume Na Content 1. /48 hr. m.equiv * '6 96

8 Potassium and Sodium Absorption in Sheep 389 potassium apparently associated with the solid and cellular material in the rumen. The relationship between the amount of sodium leaving the rumen each day in the fluid flowing to the omasum and the concentration of sodium in the rumen fluid is given in fig. 7. The amount of sodium flowing out of the rumen decreased as the concentration of sodium decreased (p <.1). The effects of potassium supplements upon the amount of sodium leaving the rumen each day in the fluid flowing to the omasum is illustrated in fig. 8. The amount of sodium leaving the rumen in the fluid flowing to the omasum decreased as the potassium intake increased (p < -1). In order to examine the effects of potassium supplements upon the rate of flow of parotid saliva, experiments were performed on sheep 414, 418, 419 and 42. In these the amount of saliva produced in a 48 hr. period was compared when the sheep were fed 8 g. of grass-cubes/day containing 52 m.equiv. of potassium and when receiving, in addition, 36 m.equiv. of the potassium supplement into the rumen each day. The results are shown in Table III. There was no evidence that potassium supplements given into the rumen caused any reduction in the amount of saliva collected or in its sodium content. DiscUSSION An increase in the concentration of potassium in the rumen fluid with increasing potassium intake confirms the previous observation that the potassium concentration in the rumen under different dietary conditions reflects the potassium content of the diet [Dobson, Scott and Bruce, 1966]. Over a range of potassium intake of from m.equiv./day the amount of potassium contained in the rumen fluid was per cent of the total intake. By comparison, from 2-5 to 5-2 times as much sodium was present in the rumen fluid as was present in the diet each day indicating the importance of saliva as the main source of sodium in the rumen. A relationship between the concentration of potassium in the rumen fluid and the content of potassium in the diet is also indicated in the experiments of Sellers and Dobson [196], Dobson and McDonald [1963], Chou and Walker [1964] and Warner and Stacy [1965]. The amount of potassium estimated to be absorbed from the rumen increased as the concentration of potassium in the rumen fluid was increased. A relationship between the concentration of potassium in the rumen fluid and the electrical potential has been demonstrated by Sellers and Dobson [196] who showed that in sheep fed grass rich in potassium the electrical potential across the rumen epithelium increased as the concentration of potassium in the rumen fluid increased. Similar results were obtained by Harrison et al. [1964] and Ferreira et at. [1966 a] who demonstrated that in the presence of a fixed sodium concentration the electrical potential between the blood and the contents of the rumen of anaesthetized sheep increased with the logarithm of the potassium concentration. A relationship between the concentration of VOL. LII, NO

9 3ti9 Scott potassium in the rumen fluid and the electrical potential through the range 3 to 7 mv was demonstrated in the conscious sheep by Scott [1966]. The conclusion from these potential studies is that rumen epithelium appears to have a higher permeability to potassium ions than frog skin in which variations in the concentration of potassium on the mucosal surface have little effect upon the potential [Koefoed-Johnsen and Ussing, 19158; Ussing, 1965]. In the rumen the concentration gradient for potassium between the rumen fluid and the blood is large and likely to provide a driving force which is greater than the effects of the electrical potential gradient so that a net movement of potassium from the rumen contents to the blood may be anticipated. The present experiments indicate that appreciable absorption of potassium from the rumen does occur and that the rate of absorption increases when the concentration of potassium in the fluid in the rumen is increased as a result of an increased potassium intake. The addition of the potassium supplements had little or no influence on the volume of rumen fluid flowing on to the omasum, but depressed the concentration of sodium in the rumen fluid from about 8-9 m.equiv. /1. when no supplement was given to 3-4 m.equiv./1. with the maximum amount of supplement. Consequently the amount of sodium flowing on to the omasum fell from about m.equiv./day to about 1-15 m.equiv./day. This reduction in the outflow of sodium poses a problem since it implies that either the saliva was carrying less sodium into the rumen when potassium was given or that more sodium was being absorbed from the rumen each day. A decrease of about 2 1. daily in total salivary secretion would be required to account for the diminished outflow of sodium. However, when 36 m.equiv. of the potassium supplement was given to four sheep there was no evidence that the addition of potassium to the rumen caused any decrease at all in the sodium content or the volume of parotid saliva secreted. If this applied to the other salivary secretions also, then the amount of sodium entering the rumen must have been approximately constant and the reduction in outflow of sodiumn must have been due to an increased absorption of sodium through the rumen wall. Dobson [1959] showed that appreciable quantities of sodium are absorbed fromii the rumen against the electrochemical gradient for this ion and suggested that an 'active' transport mechanism similar to that in frog skin might be responsible for this transport. Little is known of the factors which regulate the rate of absorption of sodium although Stacy and Warner [1966] consider that immediately following a feed, when the rumen fluid is hypertonic to the blood, the rate of absorption of sodium from the rumen is increased. Observations with isolated frog skin show a relationship between sodium transport and potassium concenitration. Koefoed-Johnsen and Ussing [1958] demonstrated that the short-circuit current measured across frog skin is accounted for by the active transport of sodium from the mucosal to the serosal surface while Ussing [1955; 1965] reported that a marked reduction in sodium transport occurs if potassium is absent from the solution bathing the

10 Potassium and Sodium Absorption in Sheep 391 serosal surface. By comparison, variations in the concentration of potassium on the mucosal surface appear to have little effect on the potential [Koefoed- Johnsen and Ussing, 1958]. The results of Ferreira et al. [1966 b] indicate differences between frog skin and rumen epithelium since the latter appears to have a substantial permeability to potassium ions at both faces of the tissue. It seems possible that active transport of sodium across rumen epithelium may be dependent upon the concentration of potassium in the fluid in the rumen. The lack of effect of potassium supplemenits upon the volume of fluid in the rumen and its rate of onward flow may be accounted for on the basis of substitution of potassium for sodium in the rumen fluid and the increased absorption of potassium and sodium which would furthei tend to comnpensate for the increased potassium intake. The substitution of potassium for sodium was on an equimolar basis so that the sum of the concentrations of sodium and potassium remained unaltered. These effects combined with changes in the amount of water flowing across the rumen epithelium would tend to maintain a uniform osmotic pressure of the fluid in the rumen without any change in volume or increased outflow. ACKNOOWLEDGMENTS The author wishes to thank Dr. R. N. B. Kay, Professor A. T. Phillipson and Dr. A. Dobson for advice and criticism of the manuscript. Appreciation is also due to Mr. J. Ingram for skilled technical assistance during the course of these experiments. REFERENCES C'HoU, K. C. and WALKER, D. M. (1964). J. agric. SCi. 62, 15. DANI:ELLI, J. F., HITCHOCK, M. W. S., MARSHALL, R. A. and PHILLIPSON, A. T. (1945). J. exp. Biol. 22, 75. DOBSON, A. (1956). Ph.D. Thesis, University of Aberdeen. DOBsoN, A. (1956). J. Physiol. 146, 235. DOBSON, A. and PHILLIPSoN, A. T. (1958). J. Physiol. 14, 94. DOBSON, A. and McDONALD, I. (1963). Res. vet. Sci., 4, 247. DOBSON, A., SCOTT, D. and BRUCE, J. (1966). Quart. J. exp. Physiol. 51, 311. FERREIRA, H. G., HARRISON, F. A., KEYNES, R. D. and NAUSS, A. H. (1966 a). J. Physiol. 187, 615. FERREIRA, H. G., HARRISON, F. A. and KEYNES, R. D. (1966 a). J. Physiol. 187, 631. HARRISON, F. A., KEYNES, R. D. and NAUSS, A. H. (1964). J. Physiol. 171, 18P. HYDEN, S. (1955). Kungl. Lantbrhdgsk. Annlr. 22, 139. HYDJEN, S. (1961). Kungl. Lantbrhdgsk. Annlr. 27, 51. KOEFOED-JOHNSEN, V. and USSING, H. H. (1958). Acta. physiol. scand. 42, 298. PARTHASARATHY, D. and PHILLIPSON, A. T. (1953). J. Physiol. 121, 452. SCOTT, D. (1966). Quart. J. exp. Physiol. 51, 6. SELLERS, A. F. and DOBSON, A. (196). Res. vet. Sci. 1, 95. SPERBER, I. and HYDEN, S. (1952). N\Vature, Lond. 169, 587. STACY, B. D. and WARNER, A. C. I. (1966). Quart. J. exp. Physiol. 51, 79. USSINGc, H. H. (1955). In Ion Transport across Membranes, Ed. by H. T. Clarke and D. Nachmansoln, p. 3. London: Academic Press Inc. Ltd. USSING, H. H. (1965). Harvey Lect London: Academic Press Inc. Ltd. WARNER, A. C. I. and STACY, B. D. (1965). Quart. J. exp. Physsiol. 5, 169.