Circadian variation of blood clotting time and circulating vitamin K in the athletic horse

In equine sport medicine, blood clotting and fibrinolysis variations are well investigated, given the practical implications of several pathophysiological conditions affecting the athlete horse such as exercise-induced pulmonary haemorrhage (EIPH) and other bleeding disorders whose etiology and pathogenesis mechanisms are not yet clearly understood. The purpose of the present investigation was to gain evidence of a daily rhythm of several blood clotting indices such as prothrombin time (PT), activated partial thromboplastin time (APTT), thrombin time (TT), plasma fibrinogen concentration and serum vitamin K concentration in the athletic horses. Blood samples from five thoroughbred mares were collected at 4-h intervals for 48 h (starting at 08:00 h on day 1 and finishing at 4:00 on day 2 via an intravenous catheter inserted into the jugular vein. Prothrombin time, activated partial thromboplastin time, thrombin time and plasma fibrinogen concentration were assessed by means of a Seac Clot 2 coagulometer (SEAC, Italy), while serum vitamin K concentration was measured by HPLC. Data analysis was conducted by one-way repeated analysis of variance (ANOVA) and by the single cosinor method. ANOVA showed a significant influence of time on all parameters investigated, in all horses, on either day. Cosinor analysis defined the periodic parameters and their acrophases (expressed in hours) during the 2 days of monitoring. PT showed a nocturnal acrophase, whereas serum vitamin K concentration acrophase occurred during the evening. The results of this study reflect the physiological peculiarities of the horse that is subjected to a number of exogenous (environmental, nutritional, physical) and endogenous stimuli capable of entraining the circadian rhythm specifically and thus producing time-dependent variations not always comparable with those observed in humans or laboratory animals.