Effect of feeding on renin-angiotensin-aldosterone system of the horse.

This study investigated the impact of feeding frequency and meal size on the renin-angiotensin-aldosterone system (RAAS) in horses. The research found that a single large feed stimulates significant diurnal variations in the RAAS, while steady-state feeding does not demonstrate spontaneous circadian activity of the RAAS in horses.

Objective and Methodology

• The objective of the study was to understand the effects of feeding patterns – specifically feeding frequency and meal size – on the RAAS in horses. The RAAS is a hormone system that helps regulate long-term blood pressure and balance extracellular fluid.
• Seven horses were used in the experiment. In one scenario (MF), their daily food ration was divided into six equal portions and given at four-hour intervals. In another scenario (SF), they were given a single large feeding.
• The horses were confined in specific spaces during the experiment to prevent strenuous exercise or changes in posture affecting the RAAS.
• Measurements were made for several elements, including plasma renin activity, aldosterone, cortisol, protein concentration, packed cell volume, and serum sodium and potassium.
• Additional factors like ensuring a consistent light-dark interval, banning other sources of salt intake etc were followed to create a controlled environment.

Results and Interpretation

• The study found that only serum sodium changed diurnally (over the course of the day) under the multiple feeding regimen, with concentrations at their lowest in the early morning and highest just before midday.
• In contrast, during the single large feeding, the activity of plasma renin and aldosterone increased at specific times after the onset of feeding. This shows a significant impact of the single large feeding pattern on the RAAS. Therefore, the time of feeding and food intake relates to variations in these hormone levels.
• Transient increases in protein concentration and packed cell volume suggested that a temporary decrease in the blood volume was responsible for triggering the release of renin.
• Changes in sodium and potassium were also observed during eating, with sodium increasing and potassium decreasing. However, potassium level later increased post feeding timing with the level of aldosterone.
• Except for a transient surge during feeding under the large feed bias, cortisol exhibited a similar, unchanged pattern under both the feeding patterns.

Conclusion

• The main conclusions drawn from this research were that single large feeding causes significant variations in the RAAS throughout the day, and the absence of spontaneous circadian activity of the RAAS in horses under a steady-state feeding regimen.
• This implies that the feeding pattern can affect the hormonal regulation of blood pressure and fluid balance in horses, thus offering insights that could be considered in their management and care.