DAILY RHYTHM OF NACL INTAKE IN RATS FED LOW-CA2- RELATION TO PLASMA AND URINARY MINERALS AND HORMONES( DIET )

To assess daily rhythms of salt appetite, we measured spontaneous 300 mM NaCl intake of male Sprague-Dawley rats fed a diet containing 150 o r 25 mmol Ca2+/kg. Both groups drank most NaCl at night, but, as the d ark period progressed, intakes of controls remained constant or dimini shed, whereas intakes of rats fed low-Ca2+ diet increased. During the late dark. period, when the difference in NaCl intake between the two dietary groups was greatest, rats fed a low-Ca2+ diet lost more cortic osterone and sodium in urine, had lower plasma osmolarity, and had hig her plasma adrenocorticotropic hormone (ACTH) and corticosterone conce ntrations than did controls. Over the 24-h cycle, rats fed the low-Ca2 + diet excreted less Ca2+ and more corticosterone in urine than did co ntrols. They also had consistently lower plasma concentrations of Ca2 and renin activity and consistently higher plasma phosphorus, arginin e vasopressin, parathyroid hormone, thyroxine, calcitonin, and 1,25-di hydroxyvitamin Da. These findings support the hypothesis that salt app etite induced by dietary Ca2+ deficiency involves a subtle dysfunction of the ACTH-corticosterone axis, but they also raise several other po ssibilities.