Sampling Blood samples for analysis of plasma ET 1 and serum cortisol were drawn using vacutainer technique and col lected in evacuated NaEDTA and serum glass tubes. In each horse, the blood samples were drawn im mediately after the blood pressure measurements had been completed. All samples were kept on ice until cen trifuged for 10 min, plasma and serum subsequently harvested and stored in a ?80 C freezer until analysis. Sample analysis Plasma ET 1 concentrations were measured in duplicate with a commercial ELISA kit enzyme immunoassay, Biomedica Medizinprodukte GmbH Co KG, Wien, Austria. Serum cortisol concentrations were measured in duplicates with a radioimmunoassay kit.

Statistical methods In each horse, the lowest and highest recorded value of the five consecutive blood pressure determinations were excluded and the mean systolic and diastolic pressure was calculated from the three remaining determinations. The statistical analysis was performed using SigmaPlot software version 11. The results of repeated tests for indirect blood pressure measurement, plasma ET 1 and serum cortisol were com pared by use of a paired t test or one way repeated measures analyses of variance.Differences between individual means for the three days were tested with a tukey test. Dif ferences in the two breeds for measurement of indirect blood pressure measurement, plasma ET 1 and serum cor tisol were compared by use of a t test whereas differences between the blood pressure measurement devices were compared by use of a paired t test.

Measurement error, defined as the Drug_discovery variation between measurement of the same quantity on the same animal, blood pressure device and ELISA and radioimmunoassay kit, was expressed as the coefficient of variation. The standard deviation for du plicate or triplicate measurements was calculated according to Bland. The CV was calculated as the SD divided by the means of each set of two or three measurements and expressed in percentage. Values are expressed as mean SD. The level of significance was set at. Results Systolic and diastolic indirect blood pressure measurements The mean systolic blood pressure differed significantly be tween the two devices, but not the mean diastolic blood pressure. The CV for three repeated measurements on the same oc casion were 5. 4% in systole and 13. 9% in diastole for the Cardell device and 9. 5% in systole and 12. 1% in diastole for the HDO device. The Cardell showed a better inter assay CV in systole than the HDO, but there was no difference between the two devices in diastole. Regardless of which blood pressure measurement device that was used, there was no difference in mean systolic or diastolic blood pres sure between M1 and M2 or between the two breeds.