Lung region and racing affect mechanical properties of equine pulmonary microvasculature

Exercise-induced pulmonary hemorrhage is a performance-limiting condition of racehorses associated with severe pathology, including small pulmonary vein remodeling. Pathology is limited to caudodorsal (CD) lung. Mechanical properties of equine pulmonary microvasculature have not been studied. We hypothesized that regional differences in pulmonary artery and vein mechanical characteristics do not exist in control animals, and that racing and venous remodeling impact pulmonary vein mechanical properties in CD lung. Pulmonary arteries and veins [range of internal diameters 207-386 ± 67 μm (mean ± SD)] were harvested from eight control and seven raced horses. With the use of wire myography, CD and cranioventral (CV) vessels were stretched in 10-μm increments. Peak wall tension was plotted against changes in diameter (length). Length-tension data were compared between vessel type, lung region, and horse status (control and raced). Pulmonary veins are stiffer walled than arteries. CD pulmonary arteries are stiffer than CV arteries, whereas CV veins are stiffer than CD veins. Racing is associated with increased stiffness of CD pulmonary veins and, to a lesser extent, CV arteries. For example, at 305 μm, tension in raced and control CD veins is 27.74 ± 2.91 and 19.67 ± 2.63 mN/mm (means ± SE; P < 0.05, Bonferroni's multiple-comparisons test after two-way ANOVA), and 16.12 ± 2.04 and 15.07 ± 2.47 mN/mm in raced and control CV arteries, respectively. This is the first report of an effect of region and/or exercise on mechanical characteristics of small pulmonary vessels. These findings may implicate pulmonary vein remodeling in exercise-induced pulmonary hemorrhage pathogenesis.