α1 adrenergic receptor agonist, phenylephrine, actively contracts early rat rib fracture callus ex vivo

Abstract

Early, soft fracture callus that links fracture ends together is smooth muscle-like in nature. We aimed to determine if early fracture callus could be induced to contract and relax ex vivo by similar pathways to smooth muscle, that is, contraction via α1 adrenergic receptor (α1AR) activation with phenylephrine (PE) and relaxation via β2 adrenergic receptor (β2AR) stimulation with terbutaline. A sensitive force transducer quantified 7 day rat rib fracture callus responses in modified Krebs-Henseliet (KH) solutions. Unfractured ribs along with 7, 14, and 21 day fracture calluses were analyzed for both α1AR and β2AR gene expression using qPCR, whilst 7 day fracture callus was examined via immunohistochemistry for both α1AR and β2AR- immunoreactivity. In 7 day callus, PE (10-6 M) significantly induced an increase in force that was greater than passive force generated in calcium-free KH (n = 8, mean 51% increase, 95% CI: 26-76%). PE-induced contractions in calluses were attenuated by the α1AR antagonist, prazosin (10-6 M; n = 7, mean 5% increase, 95% CI: 2-11%). Terbutaline did not relax callus. Gene expression of α1ARs was constant throughout fracture healing; however, β2AR expression was down-regulated at 7 days compared to unfractured rib (p < 0.01). Furthermore, osteoprogenitor cells of early fibrous callus displayed considerable α1AR-like immunoreactivity but not β2AR-like immunoreactivity. Here, we demonstrate for the first time that early fracture callus can be pharmacologically induced to contract. We propose that increased concentrations of α1AR agonists such as noradrenaline may tonically contract callus in vivo to promote osteogenesis. Copyright © 2010 Orthopaedic Research Society.