Sodium deprivation in rats: salt thresholds are related to salivary sodium concentrations

Abstract

Stimulation of the taste receptors by sodium in sodium-deprived rats triggers the appetitive behavior known as salt appetite. The salivary sodium concentration bathing the receptors presumably influences indirectly salt intake by affecting the thresholds and intensity of sodium solutions. To further assess the role of gustatory sensibility in controlling salt intake, we examined the relationship between salivary sodium levels and sodium chloride intake in male Sprague-Dawley rats following 9 to 26 days of sodium deprivation. Whole saliva was collected from etherized rats injected with pilocarpine (7.5 mg/kg body wt). Sodium deprivation reduced the molar concentration of sodium, but not of potassium, from 0.05 to 0.02. This corresponded to the animals' ability to recognize weak concentrations of sodium chloride solution (0.01–0.03 M ) in a one-bottle test over 10 min. Sodium-deprived rats also showed an enhanced intake of suprathreshold concentrations of sodium salts (0.1, 0.3, and 0.51 M NaCL; 0.1 Na 2 CO 3 ). Comparable changes in the intake of other nonsodium salts (0.1 CaCl 2 , 0.1 KCl, and 0.1 K 2 CO 3 ) and other gustatory stimuli (0.0001 quinine, 0.1 sucrose, and 0.001 HCl) were not found. It was suggested that (1) salivary sodium levels influence the ingestion of sodium chloride solutions by varying the adaptation of the receptors; (2) there were species differences in the abilities of rats to recognize weak saline concentrations; and (3) although salt taste receptors may not have evolved as part of a mechanism responsive to sodium need, a recent adaptation is a more specific sodium receptor utilized by some animals for sodium preference.