Heterogeneity of autonomic regulation of small pulmonary vessels in horse lungs

Blood flow distribution and pressure gradients in the pulmonary circulation are determined by vascular anatomy and vascular tone of pulmonary arteries and veins. Little is known about the reactivity of small arteries and veins in the equine pulmonary circulation to autonomic neurotransmitters, and if there is regional heterogeneity in the reactivity of these important resistance vessels that might contribute to preferential distribution of blood flow to the caudodorsal lung. We used wire myography to assess the reactivity of small pulmonary arteries and veins isolated from caudodorsal and cranioventral lung regions from horses with no racing history, to the sympathomimetics, phenylephrine and isoproterenol, and the parasympathomimetic, methacholine. We found that: 1) phenylephrine, was without effect on arteries and veins indicating a lack of functional -adrenergic receptors in these vessels. 2) In precontracted vessels, the -receptor agonist, isoproterenol, relaxed caudodorsal and cranioventral arteries, but was more efficacious in caudodorsal arteries (caudodorsal 49% relaxation vs. cranioventral 28% relaxation); isoproterenol was without significant effect on small pulmonary veins from either region. 3) The muscarinic receptor agonist, methacholine, contracted caudodorsal arteries, but relaxed cranioventral arteries, and all veins. Regional heterogeneity in autonomic reactivity in small arteries suggests that during exercise, when sympathetic outflow is increased and parasymathetic outflow is reduced, caudodorsal arteries would preferentially dilate, transmitting a greater proportion of pulmonary artery flow and pressure to the caudodorsal lung, potentially contributing to the pathogenesis of exercise-induced pulmonary hemorrhage.