Rapid effect of change in cerebrospinal fluid sodium concentration on salt appetite

Abstract

THIRST can be evoked within 1–2 min by intracarotid infusion of hypertonic NaCl. In contrast, the onset of a specific appetite for sodium salts as a result of rapid loss of body sodium is slow. A parotid fistula in naive sheep can produce a significant Na deficit (200–700 mmol) within 1–3 d but a Na appetite usually develops after 2–5 d (ref. 1). With large urinary water and Na loss induced by frusemide, water intake occurs within an hour but Na appetite is delayed until 24 h (ref. 2). Moreover, in the Na-depleted sheep or goats, increase in the Na concentration of cerebral arterial blood by 10–30 mmol l−1 by slow intracarotid infusion of 4 M NaCl for 7 min before or during presentation of Na solution does not reduce Na intake^3,4. However, rapid intravenous infusion of significant amounts of isotonic and 2–4 M NaCl solutions in Na-deficient sheep substantially reduces Na appetite after 40–120 min^5,6. In the rat, tissue fluid sequestration produced by subcutaneous polyethylene glycol injection causes water drinking within an hour, but onset of salt appetite is delayed by 6–10 h.⁷ This delay also occurs in salt appetite produced by peritoneal dialysis⁸, tourniquet release⁹ and formalin injection^10,11. New data indicate the powerful effects of the hormones involved in the reproductive process in inducing salt appetite. With pregnancy and lactation in Na-replete rabbits, avid salt appetite develops with a daily turnover of 50% or more of extracellular fluid (ECF) Na (ref. 12). This is caused by the combined action of oestradiol, cortisol, ACTH, prolactin and oxytocin¹³. ACTH seems to act directly on the brain¹⁴. Recent experimental data provide new information on the central nervous system control mechanism. Weisinger et al.¹⁵ reported that in Na-depleted sheep, the systemic influx of Hypertonie NaCl significantly reduced motivation for salt within 10–20 min. In the light of these data, we decided to evaluate the effect on Na appetite of increasing the cerebrospinal fluid (CSF) Na concentration or osmolality in Na-deficient sheep. We now report the first experiments to show that it is feasible to induce salt appetite within an hour or less by a physiological procedure.