Genetic-diet interactions in the hyperkalemic periodic paralysis syndrome in quarter horses fed varying amounts of potassium: I. Potassium and sodium balance, packed cell volume and plasma potassium and sodium concentrations
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Six broodmares that were genetically tested to be heterozygous (H/N) and six broodmares that were tested to be homozygous negative (N/N) for Equine Hyperkalemic Periodic Paralysis (HYPP), that were descendents of the same stallion, were used in a replicated 3x3 Latin square experiment to determine the genetic-diet relationships in the HYPP syndrome. 1 The mares were fed rations consisting of 65 % pelleted concentrate and 35% Coastal Bermuda grass hay that provided 1.1 (diet A), 1.9 (diet B) and 2.9% (diet C) potassium. The experimental periods were 14 days long, resulting in 28 meals per period. At meals 1 and 27, blood samples were taken every 30 minutes for 12 hours and every 10 minutes from 2 to 5 hours post-feeding. Total urine and fecal collections were made on the last 4 days of each period. Water, feed and fecal samples were analyzed for sodium and potassium content. Blood was analyzed for packed cell volume (PCV) and plasma potassium (K+) and sodium (Na+) concentrations. Apparent absorption of potassium was 99.8% across all diets, and potassium was excreted principally in the urine. More potassium was retained when the horses were fed diet B than diets A or C. Apparent absorption of sodium was 99.6% with no difference by diet. Neither potassium nor sodium balances were affected by HYPP status. There was no post-prandial pattern of plasma K+ concentration seen when the horses were fed the low potassium diet. When fed the higher potassium diets, they had greater plasma K+ values and exhibited a post-prandial peak at 2 to 5 hours after feeding. This peak occurred in all of the horses, meaning that plasma K+ cannot be used as a diagnostic aid for the presence of the HYPP mutation. By meal 27, some adaptation had occurred resulting in lower plasma K+ values when the horses were fed the higher potassium diets. The horses had increased plasma K+ concentrations throughout the post-prandial period while fed diet B compared to diet A, agreeing with the positive potassium balance seen in this experiment. The HYPP H/ N horses had lower plasma Na+ concentrations in the early post-prandial hours and continuously greater PCV values than the HYPP N/N horses.
