The eicosanoids are a family of lipid-derived autocoids that are released in response to a variety of physical and hormonal stimuli. In this study, prostaglandin E2 (PGE2) and leukotriene B4 (LTB4) were measured in the digital veins of clinically normal horses and horses with chronic laminitis to determine whether these arachidonic acid metabolites have a role in mediating signs of hoof pain and lesions associated with chronic laminitis. Horses were evaluated at rest and after a brief exercise period, to determine whether eicosanoids are released into the circulation after mild concussion. Digital vein eicosanoid concentrations in horses with signs of hoof pain attributable to chronic laminitis were not different than those in clinically normal horses. There was no difference in resting and postexercise PGE2 or LTB4 concentrations. Mean digital vein PGE2 concentration for the 2 groups was 187.18 pg/ml, whereas mean digital vein LTB4 concentration for the 2 groups was 74.71 pg/ml. These data do not support the hypothesis that PGE2 and LTB4 have a role in mediating the signs of pain and pathologic features of chronic laminitis.

Using radionuclide-labeled 15-micrometer-diameter microspheres injected into the left ventricle, we examined blood flow to the thyroid gland, adrenal glands, kidneys, and various gastrointestinal tract tissues in 9 healthy horses while they were standing quietly (rest) and during exercise at 2 work intensities (8 and 13 m/s). Hemodynamic measurements were made during steady-state conditions, as judged by the stability of heart rate as well as aortic, pulmonary, and right atrial pressures. The similarity of blood flow values for the left and the right kidneys during each of the 3 conditions indicated adequate mixing of microspheres with blood. In standing horses, of all tissues examined, the thyroid gland had the highest blood flow (1,655.2 +/- 338.5 ml/min/100 g)--being about threefold that in the kidneys. Adrenal blood flow, by contrast, was only 25% of that in the kidneys (589.5 +/- 50.4 ml/min/100 g). Among the gastrointestinal tract tissues, glandular stomach and pancreas had the highest blood flows (214.3 +/- 21.6 and 197.6 +/- 23.4 ml/min/100 g, respectively). Small intestinal perfusion was not different from that in the ventral colon and cecum, but their values exceeded those for the dorsal and small colons. Exercise at 8 and 13 m/s caused significant increase in adrenal blood flow as vascular resistance decreased significantly. In the kidneys, blood flow was only insignificantly affected during exercise at 8 m/s, but at 13 m/s there was a profound reduction in renal blood flow as intense renal vasoconstriction occurred. Vasoconstriction also caused thyroid and pancreatic blood flow to decrease significantly at both levels of exertion. Significant vasoconstriction occurring in all gastrointestinal tract tissues at 8 and 13 m/s caused blood flow to be diverted away from these vascular beds. Thus, our data indicated that renal, adrenal, and splanchnic organ/tissue blood flow responses of strenuously exercising horses closely resemble those described for exercising ponies.