The experiments, performed in pentobarbital sodium-anesthetized rats, consisted of a 1-h equilibration period followed by two 30-min control periods. Subsequently, synthetic rat pro atrial natriuretic peptide (ANP) [proANP-(1-30)] (n = 8) was given as a bolus of 10 microg in 1 ml of 0.9% saline followed by an infusion at 30 ng/min (20 microl/min) for six additional periods. Control rats (n = 6) received only 0.45% saline in the appropriate volumes. Mean arterial pressure, renal blood flow, and glomerular filtration rate did not change significantly in either group during the proANP-(1-30) infusion. Urine flow and potassium excretion increased approximately 50% in the proANP-(1-30)-infused group only (P < 0.05). Sodium excretion and fractional excretion of sodium, expressed as the change from their own baselines, were significantly increased by the proANP-(1-30) infusion (P < 0.05), whereas cGMP excretion was similar in both groups. These results suggest that the rat sequence of proANP-(1-30) produces a natriuresis in the rat independent of changes in hemodynamics and renal cGMP production. In a second study, rats (n = 8) were prepared as above and pretreated with 0.4 ml iv of rabbit serum containing an antibody directed against proANP-(1-30) (anti-proANP group). The rats were volume expanded with 3 ml of 6% albumin in Krebs and observed for 3 h to determine if the anti-proANP would attenuate the responses to volume expansion. Control rats (n = 7) received 0.4 ml of normal rabbit serum. The elevation in potassium excretion in response to volume expansion was significantly attenuated in the anti-proANP group (P < 0.05). Sodium excretion and urine flow responses also tended to be reduced but not significantly. These results suggest that in the rat, proANP-(1-30) plays a physiological role in regulating renal excretion.

Calves fed a roughage diet (50 per cent hay+ other components) excreted an alkaline urine rich in bicarbonate but poor in phosphorus. Infusion of HCI into the rumen led to a fall in urine pH, to a decrease in bicarbonate excretion and to the excretion of acid in the urine. Between 16-25 per cent of this acid was excreted as acid phosphate (H2PO4) and the remaining 75-84 per cent as ammonium ions.In contrast, when the calves were fed a concentrate diet (85 per cent barley+other components) the urine was acid and contained appreciable amounts of phosphorus and ammonium ions. Infusion of HCI into the rumen produced a further increase in acid excretion, practically all of which appeared in the urine as ammonium ions.Excretion of calcium in the urine increased during infusion of HCI and was correlated with urine pH.There were few changes in faecal mineral excretion during acid infusion but less sodium and potassium were excreted in the faeces when the concentrate diet was fed compared to when the roughage diet was fed.Roughages such as hay, straw or grass form the basis of the more traditional diets that are fed to sheep and cattle. Although the reticulorumen is well suited to the digestion of such fibrous foods, they tend to be low in digestible energy content and hence are of limited value in intensive husbandry. In consequence, it has become common in recent years to feed both cattle and sheep concentrated diets which may contain up to 85 per cent of cereals such as barley or maize.The urine of sheep and cattle given roughage diets is usually alkaline and contains large amounts of bicarbonate with only small amounts of phosphorus [Tillman and Brethour 1958; Tillman, Brethour and Hansard, 1959; Anderson and Pickering, 1962;Clark et al., 1968;Stacy, 1969;Scott, 1969]. In contrast, Reed, Elliot and Topps [1965] and Topps, Reed and Elliot [1966] showed that cattle fed concentrated feeds excreted a urine which was markedly acid and contained large amounts of titratable acid present as acid phosphate (H2PO0).These workers suggested that such diets may tend to produce mild acidosis and this was confirmed by Hyldgaard-Jensen, Whitelaw, Reid and Murray [1966] who reported a decrease in blood pH and in plasma bicarbonate concentration in cattle changed from a roughage to an all-cereal diet. A decrease in urine pH in these experiments was associated with a marked increase in the urinary excretion of ammonia and phosphate.Since it is known [Scott, 1969] that the normal response of sheep to an imposed acid load is to increase urinary ammonia excretion with little or no increase in urinary acid phosphate excretion, these findings suggest that cattle may differ from sheep in their response to acidosis. Alternatively, the feeding 18

Measurements were made of the electrical potential gradient between the blood and the rumen contents of sheep made sodium deficient by the withdrawal of parotid saliva.Loss of saliva resulted in an increase in the concentration of potassium and a fall in the concentration of sodium in the rumen fluid, and these changes were associated with an increase in the rumen potential. The results suggest that a potential arising from the passive diffusion of potassium into the blood may contribute significantly to the total potential which is generated across the rumen wall. The change in rumen composition appears to be due to a change in the relative amounts of sodium and potassium entering the rumen. This results from both the loss of saliva and a change in the composition of saliva entering the rumen, since the ratio of Na: K in the parotid saliva falls in response to sodium deficit through the secretion of adrenal hormones.There is, so far, no indication that adrenal hormones exert any independent influence on the rumen potential other than through a change in the composition of the saliva and rumen fluid. [1958] demonstrated that the contents of the reticulorumen of the sheep were negative by 30-40 mV relative to the blood. Later it was shown by Sellers and Dobson [1960] that this potential tended to rise when sheep were fed grass which was rich in potassium and in these circumstances the concentration of potassium in the rumen fluid rose while that of sodium fell. It is possible that this increase in potential could be explained simply in terms of an increased diffusion of potassium into the blood, and Harrison, et al. [1964] showed that, in anaesthetized sheep whose rumens were filled with isotonic solutions of potassium sulphate and sucrose, the potential rose as the concentration of potassium was increased through the range 30-90 mM in much the way that would be predicted by such a theory. DOBsON and PhillipsonAnother factor which contributes to the rumen potential is the mechanism responsible for the active transport of sodium across the rumen epithelium . This in turn could be affected by changes in the concentration of sodium in the rumen.The withdrawal of parotid saliva from sheep results in changes in the composition of the rumen fluid [Goodall and Kay, 1965]. In particular, the concentrations of potassium and sodium may vary over a much greater range than those which result from change of diet or variations in the level of feed intake previously observed by Sellers and Dobson [1960] or by Warner and Stacy [1965].The purpose of the present observations was to measure rumen potential 60

Infusion of HCI into the rumen of sheep led to a fall in the pH of the urine and to the reabsorption of bicarbonate from the urine. Excretion of acid in the urine rose until it was about equal to the amount of acid given into the rumen. There was very little titratable acid present in the urine and over 90 per cent of the acid excreted was in the form of ammonium ions. The limitation in the excretion of titratable acid seems to be related to the observation that sheep normally excrete very little phosphorus (as phosphate) in the urine.The renal response to acidosis in sheep was not influenced by the amount of potassium in the diet. THERE is convincing evidence from studies in man, rat and dog that the renal tubule is able to secrete potassium and that distal regions of the nephron may be important in this process [Black and Emery, 1957;Pitts et al., 1958; Sullivan et al., 1960;Berliner, 1961]. Tubular secretion of potassium has also been shown in cattle [Anderson and Pickering, 1962] and in sheep [Scott, 1969b] in response to intravenous infusion of potassium salts.Sheep normally consume in their diet amounts of potassium which are considerably in excess of requirement and the surplus is largely excreted in an alkaline urine [Ward, 1966;Dewhurst and Harrison, 1967; Beal and BudzOlsen, 1968; Dewhurst, Harrison and Keynes, 1968;Scott, 1969a].Berliner [1961] in studies with man and dog suggested that potassium may compete with hydrogen ions in the tubular cell in an exchange for sodium ions in the tubular urine; conditions favouring the accumulation of potassium in the cell lead to the excretion of an alkaline urine rich in potassium while accumulation of hydrogen ion in the cell favours an acid urine.Sheep made acidotic through the infusion of hydrochloric acid into the rumen were shown in previous observations [Scott, 1969b] to be able to excrete appreciable loads of acid and potassium in the urine, a large proportion of the acid appearing as ammonia. It seemed possible that excretion of potassium may have limited the rate of formation of titratable acidity in the urine but the relationship between acid excretion and potassium intake was not examined. The purpose of the present experiments was to examine in greater detail the nature of the renal response to acidosis in sheep fed diets containing differing amounts of potassium. METHODSAnimals. --Seven adult Scottish Blackface ewes weighing 36-40 kg were used. The sheep were fitted with permanent ebonite cannulas into the rumen several months before observations began. During experiments the sheep were kept in metabolism cages which separated the fseces and the urine. Three experimental diets were used: 412

Mature sheep receiving supplements of sodium chloride into the rumen were given intravenous infusions of arginine vasopressin at rates varying from 4-6-23 pmol/min (2-10 mU/min). Infusion of the hormone led to an increase in urine flow and to increases in the amounts of sodium and chloride excreted, the effect on flow was, however, the greater so that the osmolality of the urine fell during the infusions.In sheep given intravenous infusions of a hypertonic sodium chloride solution addition of vasopressin to the infusate led to the formation of a larger volume of urine containing a higher proportion of the infused salt load compared to when the salt solution alone was given. As before the effect on flow was the greater and hence the osmolality of the urine was lower when the hormone was given.In other experiments intravenous infusion of a hypertonic sodium chloride solution at rates providing 2-8 mnol NaCl/min led to increases in urine flow and increases in sodium and chloride excretion, the size of these increases being proportional to infusion rate. Plasma vasopressin levels markedly increased during these infusions, the levels seen being similar to those seen in sheep given vasopressin in amounts which increased both urine flow and electrolyte excretion. This suggests that during hypertonic salt loading vasopressin probably contributes directly to the increases in urine flow and the increases in electrolyte excretion which are seen. Further evidence in support of this was obtained in experiments in which a greater natriuretic response was seen in sheep given a hypertonic sodium chloride solution into the carotid artery as opposed to the jugular vein and where it was shown that plasma vasopressin levels were indeed higher when the solution was given into the artery.In 1961 Kinne, Macfarlane and Budtz-Olsen reported experiments in which infusion of vasopressin in sheep produced an unexpected increase in both water and electrolyte excretion in the urine and they suggested that this hormone may be more concerned in the regulation of electrolyte balance than water balance in this species. The diuretic response to vasopressin in sheep was subsequently confirmed [Macfarlane, 1964] and a similar effect was noted in both cattle and camels [Macfarlane, Kinne, Walmsley, Siebert and Peter, 1967].In other studies Cross, Thornton and Tweddell [1963] and Cross and Thornton [1966] found that the diuretic response to vasopressin in sheep occurred only if the osmotic concentration of the urine were initially high and SchmidtNielsen [1964] argued that in these circumstances the increase in urine flow is a consequence of the increase in electrolyte excretion due to the vasopressin. Against these observations Brook, Radford and Stacy [1968] have produced evidence suggesting that vasopressin has no special role in regulating electrolyte excretion in the sheep and they suggest that the effects seen by these other workers were possibly the result of infusing unphysiologically large amounts of the hormone.