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

Six broodmares that;were genetically tested to be heterozygous (H/N) a nd 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 exper iment 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 (di et B) and 2.9% (diet C) potassium. The experimental periods were 14 da ys long, resulting in 28 meals per period. At meals 1 and 27, blood sa mples were taken every 30 minutes for 12 hours and every 10 minutes fr om 2 to 5 hours post-feeding. Total urine and fecal collections were m ade on the last 4 days of each period. Water, feed and fecal samples w ere analyzed for sodium and potassium content. Blood was analyzed for packed cell volume (PCV) and plasma potassium (K+) and sodium (Na+) co ncentrations. Apparent absorption of potassium was 99.8% across all di ets, and potassium was excreted principally in the urine. More potassi um was retained when the horses were fed diet B than diets A or C. App arent absorption of sodium was 99.6% with no difference by diet. Neith er potassium nor sodium balances were affected by HYPP status. There w as no post-prandial pattern of plasma KC concentration seen when the h orses were fed the low potassium diet. When fed the higher potassium d iets, 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 h orses, meaning that plasma K+ cannot be used as a diagnostic aid for t he presence of the KYPP mutation. By meal 27, some adaptation had occu rred resulting in lower plasma K+ values when the horses were fed the higher potassium diets. The horses had increased plasma K+ concentrati ons throughout the post-prandial period while fed diet B compared to d iet A, agreeing with the positive potassium balance seen in this exper iment. 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.