α1 Adrenergic Receptor Agonist, Phenylephrine, Actively Contracts Early Rat Rib Fracture Callus Ex Vivo

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 alpha(1) adrenergic receptor (alpha(1)AR) activation with phenylephrine (PE) and relaxation via beta(2) adrenergic receptor (beta(2)AR) 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 alpha(1)AR and beta(2)AR gene expression using qPCR, whilst 7 day fracture callus was examined via immunohistochemistry for both alpha(1)AR and beta(2)AR-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 alpha(1)AR antagonist, prazosin (10(-6) M; n = 7, mean 5% increase, 95% CI: 2-11%). Terbutaline did not relax callus. Gene expression of alpha(1)ARs was constant throughout fracture healing; however, beta(2)AR expression was down-regulated at 7 days compared to unfractured rib (p < 0.01). Furthermore, osteoprogenitor cells of early fibrous callus displayed considerable alpha(1)AR-like immunoreactivity but not beta(2)AR-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 alpha(1)AR agonists such as noradrenaline may tonically contract callus in vivo to promote osteogenesis. (C) 2010 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 29:740-745, 2011