SUMMARY1. The effect of feeding on the circulation, on renin release and on renal function has been examined in the sheep.2. There was a rapid reduction of plasma volume within 15 min of starting to eat dry feed.3. This was accompanied by an abrupt rise in systolic blood pressure and by a more gradual and prolonged elevation of heart rate.4. Within 30 min of feeding there occurred renal conservation of sodium and water which continued for at least 2-3 hr. Renin was released into the circulation of sheep that ate rapidly;slow feeders did not show this response. 6. The rise in plasma renin concentration was inhibited by infusing a solution similar in composition to mixed saliva at a rate that prevented the change in plasma volume.7. It is concluded that the rapid transfer of sodium and water from the Circulation to the rumen via the saliva during ingestion of dry feed is sufficient to activate the renin/angiotensin system.

SUMMARY1. The effect ofrestricted water intake and rapid rehydration was studied in three conscious sheep with respect to plasma renin concentration (PRO), blood corticosteroid levels, plasma protein and electrolyte concentrations, and the renal and faecal excretion of sodium and potassium.2. During water restriction the plasma concentrations of renin, protein and sodium rose while aldosterone levels were low or undetectable. Plasma potassium levels were unchanged. External sodium and potassium balance appeared to be unaffected.3. During rehydration the sheep drank more than their estimated water deficit in 3-4 min with the following effects: PRC rose three-to fourfold during the ensuing 12 hr. Aldosterone levels too rose, while plasma protein, sodium and potassium concentrations fell. Urinary sodium excretion virtually ceased for 24 hr, and urine flow rate increased only little during this period.4. If there was a single stimulus to renin release during water restriction and rehydration, it was not an alteration in vascular or extravascular volume, total body sodium, systemic B.P. or plasma sodium concentration.5. It is concluded that the rise in PRC in these experiments is compatible with the theory that altered sodium transport at the macula densa was the stimulus for renin release.

Pen-fed sheep increased the concentration of their urine after receiving their daily ration, and evidence is presented to indicate that the change was brought about by the action of antidiuretic hormone (ADH). Post-prandial urine, but not urine from unfed animals, caused inhibition of urine flow when injected into water-loaded sheep. The antidiuretic material in post-prandial urine was assumed to be ADH because (a) it could be extracted by a specific procedure, (b) it was similar to synthetic arginine vasopressin in its action on water and electrolyte excretion in water-loaded sheep, and (c) its biological activity was destroyed by thioglycollate or by trypsin. It was also found that there was an increase in plasma osmotic pressure after feeding, so that conditions favoured the release of ADH. It is therefore concluded that normal, pen-fed sheep regularly experience conditions that classically stimulate the neurohypophysis to control the renal excretion of water by secreting ADH. The action of ADH on electrolytes is discussed, and it is concluded that the sheep, like other animals, does not depend upon the hormone to regulate the renal excretion of Na and K.SEVERAL workers have referred, in passing, to the effect of feeding on urine excretion in ruminants [Andersson, 1955;Schmidt-Nielsen et at., 1958]. The reports arose from the incidental observation of animals undergoing artificial diuresis during experiments on renal function; it was not assumed that normal animals showed similar effects, nor was it conjectured how the kidney achieved an acute reduction in urine flow in response to feeding. Stacy and Broolk [1964] studied the urinary changes in undisturbed sheep, some of which had chronic ureteral fistulas , and from an assessment of renal function it seemed most likely that the consistent diminution in urine flow in the post-prandial period was caused by the action of ADH. Evidence to support this suggestion is described in the present paper.In the experiments to be described, ADH always inhibited urine flow but had no single consistent effect on electrolyte excretion. The results indicate that ADH plays the same renal role in the sheep as in the dog [Verney, 1947], and although this may not be altogether unexpected, the work nevertheless assumes significance in view of the claim by Macfarlane and his colleagues that the sheep differs from man and most laboratory animals in its renal response to ADH [Kinne et al., 1961;Macfarlane et al., 1963]. MATERIALS AND METHODSMerino ewes were used in all experiments. They weighed 30-35 kg. and were fed once a day in the afternoon on 600-800 g. of an equal mixture of lucerne and wheaten chaff plus 10 g. of added sodium chloride. The food was usually eaten within 2-3 hr.

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