Correlation between ANP concentrations in atria, plasma and cerebral structures and sodium chloride preference in Wistar rats

We determined whether ANP (atrial natriuretic peptide) concentrations, measured by radioimmunoassay, in the ANPergic cerebral regions involved in regulation of sodium intake and excretion and pituitary gland correlated with differences in sodium preference among 40 Wistar male rats (l80-220 g). Sodium preference was measured as mean spontaneous ingestion of 1.5 per cent NaCl solution during a test period of 12 days. The relevant tissues included the olfactory bulb (OB), the posterior and anterior lobes of the pituitary gland (PP and AP, respectively), the median eminence (ME), the medial basal hypothalamus (MBH), and the region anteroventral to the third ventricle (AV3V). We also measured ANP contens in the right (RA) and left atrium (LA) and plasma. The concentrations of ANP in the OB and the AP were correlated with sodium ingestion during the preceding 24 h, since an increase of ANP in these structures was associated with a reduced ingestion and vice-versa (OB: r = -0.3649, P<0.05; AP: r = -0.3291, P<0.05). Moreover, the AP exhibited correlation between ANP concentration and mean NaCl intake (r = -0.4165, P<0.05), but this was not the case for the OB (r = 0.2422. This suggests that differences in sodium preference among individu male rats can be related to variations of AP ANP level. Earlier studies indicated that the OB is involved in the control of NaCl ingestion. Our data suggest that the OB ANP level may play a role mainly in day-today variations of sodium ingestion in the individual rat.

Sodium balance of hyper and hyponatriophilic rats under basal conditions and during sodium deprivation

Sodium and water balance was determined in two strains of Wistar rats selectively bred for high (hypernatriophilic, HR) or low salt preference (hyponatriophilic, HO) under basal conditions and during sodium deprivation. Male rats from each strain were selected for an average ingestion of 1.5 per cent NaCl solution of more than (HR) or less than (HO) 4 ml 100 g body weight-1 day-l, during a 10-day period. HR rats (N = 17) presented markedly higher sodium intake under basal conditions (2.983 ñ 0.316 mEq 100 g body weight-1 day-l) than HO rats (N = 12; 0.406 ñ 0,076 mEq 100 g body weight-1 day-l; Mann-Whitney test, P<0.01). Water (HR: 8.6 ñ 0.57; HO: 7.7 ñ 0.32 ml 100 g body weight-1 day-1) and sodium balances (HR: 0.936 ñ 0.153; HO: 0.873 ñ 0.078 mEq 100 g body weight-1 day-l) were similar in both strains, despite a higher sodium and total fluid (HR: 16.3 ñ 1.06; HO: 10.8 + 0.49 ml 100 g body weight-1 day-l; P<0.01) ingestion in HR rats. During sodium deprivation HR rats (N = 13) exhibited a sodium balance similar to that of HO rats (N = 13) (HR: -0.159 ñ 0.011; HO: -0.129 ñ 0.019 mEq 100 g body weight-1 day-1), and, in addition, an adequate suppression of natriuresis (HR: O.049 ñ 0.011; HO: 0.026 ñ 0.004 mEq 100 g body weight-1 day-1). These data show that HR rats present hypernatriophilia as a primary trait, since their sodium-conserving mechanisms are intact. Therefore, these rats provide an adequate model to study factors that determine innate sodium preference.