Physiological stimuli of thirst and drinking patterns in ponies

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Abstract
The stimuli that elicit thirst were studied in 4 ponies. Nineteen hours of water deprivation produced an increase in plasma protein from 67 .+-. 0.1 g/l to 72 .+-. 2 g/l, a mean (.+-. SE) increase in plasma Na from 139 .+-. 3 to 145 .+-. 2 mmol/l and an increase in plasma osmolality from 297 .+-. 1 to 306 .+-. 2 mosmol/l. Undeprived ponies drank 1.5 .+-. 0.9 kg/30 min; 19 h deprived ponies drank 10.2 .+-. 2.5 kg/30 min and corrected the deficits in plasma protein, plasma sodium and plasma osmolality as well as compensating for the water they would have drunk during the deprivation period. To determine if an increase in plasma osmolality would stimulate thirst, 250 ml of 15% NaCl was infused i.v. The ponies drank when osmolality increased 3% and when plasma Na rose from 136 .+-. 3 mmol/l to 143 .+-. 3 mmol/l. Ponies infused with 15% NaCl drank 2.9 .+-. 0.7 kg/ those infused with 0.9% NaCl drank 0.7 .+-. 0.5 kg. To determine if a decrease in plasma volume would stimulate thirst, ponies were injected with 1 or 2 mg/kg body wt (bwt) frusemide. Plasma protein rose from 68 .+-. 2 g/l preinjection to 75 .+-. 2 g/l 1 h after 1 mg/kg bwt frusemide and to 81 .+-. 1 g/l 1 h after 2 mg/kg bwt frusemide. Ponies treated with 1 mg/kg frusemide voided 5.3 .+-. 0.81 kg urine and drank 3.0 .+-. 2 kg water. Ponies injected with 2 mg frusemide voided 6.4 .+-. 5 kg urine and drank 4.3 .+-. 1.4 kg water. Controls injected with 0.9% NaCl voided 1.5 .+-. 0.5 kg urine and drank 0.9 .+-. 0.6 kg water. When food and water were freely available the ponies drank 27 .+-. 8 min/day, 89% of which occurred within a period from 10 min before to 30 min after feeding. They drank 8.7 .+-. 0.9 kg/day at a rate of 552 .+-. 232 ml/min. Ponies usually drink periprandially and when challenged with an increase in the osmotic pressure of their body fluids or a decrease in the volume of their body fluids they responded by increasing their water intake.

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