Hypertonic and hypovolemic stimulation of thirst in pigs

Young female pigs weighing from 15 to 45 kg were used. Plasma osmolality was slowly raised by an intravenous infusion of 15% NaCl. The behavioral threshold was when the pigs began to drink water, and the rise of osmolality was the stimulus. In 23 measurements on 8 pigs the rise of osmolality to initiate drinking was 10.4 +/- 1.4 mosmol/kg (mean +/- SE). There was also an estimated 3.8 +/- 1.4% rise of blood volume. Control infusions of 0.9% NaCl for 1 h usually resulted in no drinking, and plasma osmolality fell by 6.7 +/- 2.1 mosmol/kg, while blood volume did not change. Hypovolemia was effected using furosemide (1 mg/kg body wt) to cause excretion of nearly isotonic urine. Blood volume changes were estimated from plasma protein and packed cell volume data. In 28 measurements on 6 pigs, drinking occurred when blood volume had decreased by 6.7 +/- 1.8%. During 2-h control periods, blood volume did not change appreciably. Plasma osmolality decreased during both the furosemide treatment (by 3.5 +/- 0.7 mosmol/kg) and the controls (by 4.1 +/- 0.8 mosmol/kg).