Evaluation of β3-adrenoceptor-mediated relaxation in intact and endotoxin-treated equine digital veins

Abstract

Objective - To investigate the functional expression of β3-adrenoceptors (β3-ARs) in equine digital veins (EDVs) and to examine whether β3-AR relaxation was altered in EDVs incubated with endotoxin. Sample Population - Forelimbs obtained from 30 horses. Procedure - Forelimbs were obtained from horses in an abattoir. Equine digital veins were carefully removed from distal portions of the forelimbs. Rings of dissected EDVs were mounted in 5-mL organ baths to record isometric tension in the presence of various β3-AR agonists (SR 58611A, ZD 2079, and ZM 215001). Results - In intact EDVs, isoprenaline, SR 58611A, ZD 2079, and ZM 215001 induced concentration-dependent relaxation. Isoprenaline and SR 58611A-induced relaxations were reduced or unaffected by nadolol, respectively. In intact EDVs, SR 58611A-induced relaxation was significantly reduced in the presence of 2μM ZM 215001 (used as a β3-AR antagonist). In endothelium-denuded EDVs or intact EDVs in the presence of a nitric oxide synthase inhibitor, isoprenaline and SR 58611A-induced relaxations were significantly decreased. The endothelium-independent relaxation to SR 58611A was significantly inhibited in the presence of ZM 215001. In endotoxin-treated EDV, isoprenaline- and SR 58611A-induced relaxations were significantly reduced. In these conditions, cycloheximide (a protein synthesis inhibitor) and ibuprofen (a cyclooxygenase inhibitor) restored the relaxant response to SR 58611A. Conclusions and Clinical Relevance - β3-Adrenoceptors are functionally expressed in EDVs. Incubation in the presence of endotoxin, used as an in vitro model of laminitis, induced an alteration of β-AR-mediated relaxations in EDVs, which could be the consequence of cyclooxygenase induction and subsequent prostanoid production.