Osmoregulation in water-deprived rats drinking hypertonic saline: Effect of area postrema lesions

Rats drank rapidly when 0.3 M NaCl was the only drinking fluid available after overnight water deprivation, consuming ∼200 ml/24 h. Although such large intakes of this hypertonic solution initially elevated plasma osmolality, excretion of comparable volumes of urine more concentrated than 300 meq Na⁺/l ultimately appears to restore plasma osmolality to normal levels. Rats drank ∼100 ml of 0.5 M NaCl after overnight water deprivation, but urine Na⁺ concentration (U_Na) did not increase sufficiently to achieve osmoregulation. When an injected salt load exacerbated the initial dehydration caused by water deprivation, rats increased U_Na to void the injected load and did not significantly alter 24-h intake of 0.3 or 0.5 M NaCl. Rats with lesions of area postrema had much higher saline intakes and lower U_Na than did intact control rats; nonetheless, they appeared to osmoregulate well while drinking 0.3 M NaCl but not while drinking 0.5 M NaCl. Detailed analyses of drinking behavior by intact rats suggest that individual bouts were terminated by some rapid postabsorptive consequence of the ingested NaCl load that inhibited further NaCl intake, not by a fixed intake volume or number of licks that temporarily satiated thirst.