Vasopressin in dehydrated and rehydrated ponies.

This research article explored changes in the hormone vasopressin and hydration levels in pony mares after being deprived of water and then rehydrated. The researchers found that both vasopressin and osmolality increased during dehydration, and that consuming water led to a rapid decrease in these levels. However, the vasopressin levels did not return to normal until 15 minutes after rehydration, indicating the horse body regulates this hormone differently than some other species.

Research Context

• The research team aimed to determine how vasopressin levels fluctuate in response to hydration levels in horses, specifically in pony mares. Vasopressin is a hormone responsible for maintaining the body’s water balance by reducing water loss in the urine. This is particularly important in situations where the body is dehydrated.

Research Methodology

• Six pony mares were deprived of water for a period of 24 hours. The scientists then measured the amount of vasopressin in their blood and their plasma osmolality. Osmolality is a measure of the concentration of particles in a solution, and in this case, refers to the concentration of particles in the mares’ blood plasma.
• After the dehydration period, the ponies were given access to water. Their drinking behavior was observed, and changes in vasopressin levels and osmolality were measured once again.

Research Findings

• The results showed a significant increase in both plasma vasopressin and osmolality after the mares were deprived of water for 24 hours. Once they were given access to water, the mares drank quickly, with 5 of the 6 drinking to repletion within 90 seconds.
• However, while the mares’ fluid levels normalized rapidly, their vasopressin levels remained elevated and did not return to normal until 15 minutes after rehydration.
• The researchers suggest these findings indicate that, unlike rodents and humans, horses’ vasopressin levels do not begin to decrease until osmolality returns to normal.

Research Implications

• The research suggests that the regulation of vasopressin in response to changes in hydration status differs in horses compared to other species. In particular, the delay in the reduction of vasopressin levels after rehydration is seen in horses and pigs, but not in rodents and humans.

Conclusion

• The team concluded that the release of vasopressin in horses may be less influenced by factors in the mouth and throat (oropharyngeal factors) than in other species, which could be due to the specific biologic mechanisms unique to horses.