Enhancement revisited: the effects of multiple depletions on sodium intake in rats vary with strain, substrain, and gender

Repeated sodium depletion enhances spontaneous sodium intake in animals and humans, and may be a significant determinant of lifelong sodium intake. In rats, even a single sodium depletion is reported to enhance both need-induced and spontaneous sodium intakes enduringly. Both types of increases are reported to plateau after two to three depletions, and to be greater in females. Here, in two strains of rats, Wistar (W) and Sprague–Dawley (SD), and in two laboratories using different W substrains, in Israel and France, we studied the influence of repeated sodium depletions on need-induced and spontaneous sodium intake. Need-induced intake in W increased incrementally, but in SD, need-induced intake occurred fully at the first depletion. In both cases, the response was not related to changes in the diuretic efficacy of furosemide. In all strains, depletions enhanced spontaneous sodium intake, but the enhancement dissipated within days following each depletion, and only attained significance over the whole 7-week experiment in male SD. Enhancement was not greater in females, despite their greater spontaneous sodium intakes. Finally, in rats given a choice of NaCl and CaCl 2, depletions enhanced only NaCl intake, both need-induced and spontaneous, attesting to the specificity of the appetite in both its forms. Our findings show that enhancement of need-induced and spontaneous sodium intake resulting from repeated sodium depletions depends upon the gender and strain of rats, and it does not result from an altered diuretic response. Persistent enhancement of spontaneous sodium intake is not a ubiquitous phenomenon.