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 af ter overnight water deprivation, consuming similar to 200 ml/24 h. Although such large intakes of this hypertonic solution initially elevated plasma o smolality, excretion of comparable volumes of urine more concentrated than 300 meq Na+/l ultimately appears to restore plasma osmolality to normal lev els. Rats drank similar to 100 ml of 0.5 M NaCl after overnight water depri vation, 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 i njected load and did not significantly alter 24-h intake of 0.3 or 0.5 M Na Cl. Rats with lesions of area postrema had much higher saline intakes and l ower U-Na than did intact control rats; nonetheless, they appeared to osmor egulate well while drinking 0.3 M NaCl but not while drinking 0.5 M NaCl. D etailed analyses of drinking behavior by intact rats suggest that individua l bouts were terminated by some rapid postabsorptive consequence of the ing ested NaCl load that inhibited further NaCl intake, not by a fixed intake v olume or number of licks that temporarily satiated thirst.