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Department of Medicine, Royal Veterinary College, London, N.W.1

Department of Endocrinology, Medical College of St Bartholomew's Hospital, London, E.C.1

1. Sodium depletion which occurred in cattle following exteriorization of a parotid duct produced depression of both plasma and salivary sodium, acidosis, elevated plasma aldosterone and renin activity. Increased sodium appetite, characteristic of sodium depletion, was assessed by operant behaviour where scoring of panel pressing for NaHCO₃ rewards showed change in sodium appetite.

2. Sodium-depleted calves readily drank the calculated ionic deficit as a hypertonic solution (4 l.) in a few minutes, or as an isotonic solution (16 l.) usually within 30 min.

3. When the ionic deficit was restored by either I.V. infusion or drinking, sodium appetite was reduced significantly. The suppression of sodium appetite was more rapid when the depleted ions were replaced by drinking (30 min) than by I.V. infusion (2 hr) but in both circumstances the effect was short lived since sodium appetite redeveloped within 3 hr.

4. The rapid return of sodium appetite following restoration of the ionic deficit occurred even when the plasma sodium level was normal. Other biochemical changes resulting from sodium depletion, such as acidosis and reduced salivary sodium, could not be correlated with variation in sodium appetite.

5. Rapid infusion of Ringer saline (4 l.) did not inhibit the sodium appetite, which suggests that neither vascular volume changes per se nor vascular baroreceptors control sodium appetite in sodium-deficient calves.

Plasma aldosterone fell rapidly following infusion of the hypertonic solution but only slightly with the isotonic infusion. The change in plasma hormone level was not related to changes in sodium appetite.

6. Drinking the hypertonic solution produced a marked reduction in panel pressing for NaHCO₃ with a rapid rise in plasma sodium. Consumption of the larger volume of isotonic solution also inhibited sodium intake but plasma sodium remained low. A secondary increase in plasma renin activity (p.r.a.) occurred following ingestion of the hypertonic solution, but both p.r.a. and aldosterone fell to normal levels over the next 6 hr when the cattle again showed marked sodium appetite. It is possible that these effects may be due to ion and fluid movement between gut and extracellular fluid and reflect osmolality changes or tissue dehydration.

7. It is concluded that the sodium appetite of sodium deficient cattle is only temporarily alleviated by restoration of the depleted ionic loss, and that the behavioural response to seek sodium rewards is independent of plasma sodium, p.r.a., aldosterone and volume changes in the gut and vascular system.

8. Recent reports suggest that sodium appetite may be controlled by receptors in the hypothalamus or by angiotensin II in the brain. In cattle the capacious gut may also be involved, since sodium appetite is inhibited more rapidly when the depleted ions are taken orally than by I.V. infusion.