Water intake and changes in plasma and CSF composition in response to acute administration of hypertonic NaCI and water deprivation in sheep

Abstract

Water intake and changes in plasma and cerebrospinal fluid (CSF) composition were measured in response to intracerebroventricular (i. e.v.) and intracarotid infusions of hypertonic NaCI solutions and after 48 h of water deprivation in sheep. Significant interindividual differences in dipsogenic sensitivity to i. e.v. NaCI were found, whereas no such differences were observed in response to intracarotid infusion of hypertonic NaCI. In the more sensitive animals, the increase in CSF [Na] at initiation of drinking during i. e.v. infusion did not differ significantly from the increase in plasma [Na] seen at the thirst threshold during intracarotid infusion of 1 M NaCI. The thirst‐eliciting infusions of hypertonic NaCI into the carotid arteries were associated with a small, significant, increase in CSF [Na], which however did not differ from that caused by an i. e. v. non‐dipsogenic ‘control’ infusion of a slightly hypertonic (0.154 M ) NaCI solution. Water deprivation for 48 h induced increases in CSF and plasma [Na] similar to those observed at the onset of drinking in response to i. e.v. and intracarotid infusions of hypertonic NaCI. However, the dehydrated animals drank about four times the amount of water consumed in response to the separate treatments with hypertonic NaCI. It is concluded that significant interindividual differences in dipsogenic sensitivity to osmotic stimuli are present in sheep, and that these differences may not necessarily be simultaneously expressed on both sides of the blood‐brain barrier. The thirst‐eliciting effect of intravascular infusion of hypertonic NaCI may be induced without concomitant increase in CSF [Na] and/or osmolality. A simultaneous increase in CSF and plasma [Na] and/or osmolality is suggested to contribute to the conspicuous water consumption seen in response to dehydration compared to that caused by acute administration of hypertonic NaCl.