Rats exhibit aldosterone-dependent sodium appetite during 24 h hindlimb unloading

Hindlimb unloading (HU) is an animal model of microgravity and bed rest. In these studies, we examined the role of ingestive behaviours in regulating body fluid balance during 24 h HU. In the first experiment, all rats were given distilled water to drink while two groups were also given access to a sodium chloride solution (0.9% or 1.8%). Water and saline intakes were measured before, during and after 24 h of HU. Rats reduced water intake during 24 h HU in all conditions. During HU, rats increased their intakes of both saline solutions (0.9% NaCl (n = 11): control 7.8 ± 3 ml; HU 18.2 ± 4 ml; recovery 8.9 ± 2.5 ml; 1.8% NaCl (n = 7): control 1.0 ± 0.4 ml; HU 3.8 ± 0.3 ml; recovery 1.2 ± 0.5 ml). Although water intake decreased there was no reduction in total fluid intake when saline was available. Plasma volumes were reduced during HU compared to rats in a normal posture when only water was available to drink (control (n = 11) versus HU (n = 11): 4.0 ± 0.2 versus 3.4 ± 0.2 ml (100 g bodyweight)^-1). When 0.9% saline was available in addition to water, plasma volumes after 24 h HU were not different from rats in a normal posture (control (n = 11) versus HU (n = 12): 4.3 ± 0.4 versus 4.3 ± 0.1 ml (100 g body weight)^-1). Plasma aldosterone but not plasma renin activity was significantly elevated after 24 h HU. Central infusions of spironolactone blocked the increased intake of 1.8% saline that was associated with 24 h HU. Thus, HU results in an aldosterone-dependent sodium appetite and the ingestion of sodium may help maintain plasma volume.