Plasma volume, electrolyte, and endocrine changes during onset of laminitis hypertension in horses.

Acute laminitis-hypertension was produced by carbohydrate overloading of the gastrointestinal tract in 12 adult horses. Obel grade 3 (OG3) lameness developed 40 hours (+/- 3.5, SEM) after overfeeding. At OG3 lameness, mean plasma volume was significantly decreased (P less than 0.005) when compared with base-line values. Before OG3 lameness, transient decreases in serum phosphorus and calcium were recorded. Mild hyponatremia also developed before OG3 lameness and persisted. After establishment of OG3 lameness, persistent hypokalemia and increased plasma aldosterone concentration occurred coincidently. Transient increase in plasma hydrocortisone (cortisol) and renin activity and transient hypochloremia were also recorded during the syndromal phase. Changes in plasma volume and serum electrolytes are discussed and related to the pathogenesis of acute equine laminitis. The alterations in plasma renin activity and aldosterone concentration were interpreted as homeostatic adjustments to fluid and electrolyte imbalances. Differences between the horse and pony during onset of experimental alimentary laminitis are also discussed.