The effect of sildenafil citrate administration on selected physiological parameters of exercising Thoroughbred horses

Summary Reasons for performing study: Sildenafil, a phosphodiesterase‐5 inhibitor vasodilator, increases cGMP concentrations by inhibiting enzymatic degradation. Marketed to treat erectile dysfunction in men, it also reduces pulmonary arterial pressure (PAP). Because it reduces PAP, sildenafil may enhance performance and/or prevent exercise induced‐pulmonary haemorrhage (EIPH). Objective: To determine if sildenafil citrate administration altered commonly measured indices of performance or reduced EIPH in exercised horses. Methods: Thirteen athletically conditioned Thoroughbred horses (2 mares and 11 geldings, age 3–12 years) were administered sildenafil citrate or placebo in 2 crossover design exercise testing studies. In a step‐wise test to exhaustion, inspired/expired gas analysis, blood lactate, heart rate, runtime and bronchoalveolar lavage (BAL) cytology were measured. In a 13 m/s test to exhaustion, blood lactate, heart rate, runtime, BAL cytology and pulmonary arterial pressure were measured. Data were analysed with paired and unpaired t tests, one‐way ANOVA and Tukey's pair‐wise multiple comparison and Friedman repeated measure analysis of variance on ranks. Results: The administration of sildenafil did not alter mean inspired/expired gas measurements, plasma lactate concentrations or acute pulmonary haemorrhage in either exercise test or pulmonary arterial pressure measurement in the 13 m/s trial. Heart rates in both stress tests were significantly different at submaximal speeds and during the early recovery period. Run time was not affected by sildenafil administration in the step‐wise trial (P = 0.622) or in the 13 m/s trial (P = 0.059). Conclusions: Sildenafil did not alleviate pulmonary haemorrhage or enhance performance‐related indices in these trials. Sildenafil administration altered cardiovascular adaptation to intense exercise as evidenced by altered heart rates at submaximal speeds and post exercise. The effect of these alterations on other performance perimeters was not evident.