The effect of training and cessation of training on plasma total carbon dioxide in Standardbred horses Robert A Lehnhard 1 , Amanda M Szucsik 2 , Valarie Balaskonis 2 , Karyn Malinowski 2 and Kenneth H McKeever 2, * 1 Department of Kinesiology, University of Maine, Orono, ME, USA 2 Equine Science Center, Department of Animal Science, Rutgers, The State University of New Jersey, New Brunswick, NJ, USA * Corresponding author: mckeever@aesop.rutgers.edu Submitted 24 March 2010: Accepted 7 May 2010 – First published online 14 June 2010 Short Communication Abstract Twenty-seven mature Standardbred mares (9–27 years, , 522 kg) were used to test the hypothesis that training and short-term cessation of training would alter total plasma total carbon dioxide (tCO 2 ) concentrations. Plasma tCO 2 concentrations were measured in blood samples (20 ml) collected at rest from the same ten unfit mares that were used in two separate studies that were conducted 1 month apart. Comparisons between the samples obtained from the two trials were made to demonstrate the consistency of plasma tCO 2 concentrations in the untrained popu- lation. Another set of resting blood samples was collected from an additional 17 mares that were being utilized in a training study that was under way during the same period. All the mares were housed in groups on dry lots, and were fed approximately 12 kg of mixed alfalfa-grass hay divided into two feedings per 24 h period. During the ‘detraining’ period, the 17 horses were housed in 5 £ 5 m stalls, and were fed the same hay ration. Water and trace-mineral blocks were available ad libitum. Blood samples were collected from the mares prior to the last session of their 12 weeks of training at 60% HR max (maximum heart rate), as well as on the third day following 2 days of detraining (simulated quarantine). Plasma tCO 2 concentrations were measured in duplicate using a Beckman EL-ISE analyser. For data analysis, t-tests with the a priori level of statistical significance set at P , 0.05 were used. Resting plasma tCO 2 concentrations were lower (P , 0.05) in the trained horses (31.4 ^ 1.9 mMol l 21 ) than in the untrained horses (34.4 ^ 0.9 mMol l 21 ). There were no effects (P . 0.05) due to cessation of training. It was concluded that training causes a decrease in plasma tCO 2 concentrations that is not reversed by limited cessation of training. Keywords: plasma tCO 2 concentration; training; detraining Introduction Administering sodium bicarbonate or other alkalinizing agents via a nasogastric tube prior to a competition is a well-known (and illegal) abuse of the buffering capabilities of these compounds. This practice, referred to as ‘milkshaking’, is a threat to the health and welfare of the horse as well as to the integrity of the entire racing industry 1,2 . To counter this threat, many racing jurisdictions measure total plasma carbon dioxide (tCO 2 ) concentrations. Plasma tCO 2 concentration is an important physio- logical factor that is controlled so as to ensure the tight regulation of blood pH 3–5 . Animals transport . 90–95% of all CO 2 produced in metabolically active tissue to the lungs in the form of carbonic acid, which rapidly dissociates to bicarbonate and hydrogen ions in the presence of an aqueous environment. This renders tCO 2 an effective and reliable measure of bicarbonate concentration within blood plasma 3–5 . Multiple investigations have reported , 30 mMol l 21 to be the average plasma tCO 2 concentration for racing horses 1–9 . Thus, most racing jurisdictions have set the threshold for a positive test at 37 mMol l 21 to avoid false positives 1–9 , which is approximately four standard deviations from the mean. One area of controversy arises when one compares the data obtained from populations of very fit race- horses with those obtained from the unfit horses used at many research institutions. Training can affect Comparative Exercise Physiology 7(1); 15–17 doi:10.1017/S1755254010000164 q Cambridge University Press 2010