Recurrent episodes of tying-up in Quarter Horses may be related to mutations in a gene called called GSY1, glycogen synthase type 1, which leads to build up of an abnormal form of glycogen within their muscle cells. However, research published in the December 2008 Neuromuscular Disorders showed some of these horses are getting a double hit by also having a mutation in a gene called RYR1.
RYR1 codes for a structure called the ryanodine receptor. The ryanodine receptor controls the release of calcium from within storage sites in the muscle cell. Calcium release is what triggers contraction. This mutation causes severe recurrent tying-up in Thoroughbreds and Standardbreds. When a Quarter Horse has both mutations present, their tying-up is more severe than with only the GYS1 mutation.
Bottom Line
Reduced simple-carbohydrate diets help horses with tying-up, but if it’s not enough, there is an option for horses with the RYR1 mutation. The drug dantrolene sodium (Dantrium) can be used intravenously by your vet to break a severe episode of tying-up. Genetic testing is available through the Neuromuscular Diagnostic Laboratory at the University of Minnesota (www.cvm.umn.edu, 800-605-8787).
