title: α(1) 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 α(1) adrenergic receptor (α(1) AR) activation with phenylephrine (PE) and relaxation via β(2) adrenergic receptor (β(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 α(1) AR and β(2) AR gene expression using qPCR, whilst 7 day fracture callus was examined via immunohistochemistry for both α(1) AR and β(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 α(1) AR antagonist, prazosin (10(-6)  M; n = 7, mean 5% increase, 95% CI: 2-11%). Terbutaline did not relax callus. Gene expression of α(1) ARs was constant throughout fracture healing; however, β(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 α(1) AR-like immunoreactivity but not β(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 α(1) AR agonists such as noradrenaline may tonically contract callus in vivo to promote osteogenesis.
authors: Musculoskeletal Research Centre, School of Human Biosciences, La Trobe University, Victoria 3086, Australia.
authors: Stuart J
authors: SJ
authors: McDonald
authors: Philip C
authors: PC
authors: Dooley
authors: Aaron C
authors: AC
authors: McDonald
authors: Elvan
authors: E
authors: Djouma
authors: Johannes A
authors: JA
authors: Schuijers
authors: Alex R
authors: AR
authors: Ward
authors: Brian L
authors: BL
authors: Grills
mesh_terms: D058646:Adrenergic alpha-1 Receptor Agonists / Q000494:pharmacology*; D000818:Animals; D001696:Biomechanical Phenomena; D002146:Bony Callus / Q000187:drug effects / Q000502:physiology*; D017102:Fracture Healing / Q000187:drug effects*; D010656:Phenylephrine / Q000494:pharmacology*; D011224:Prazosin / Q000494:pharmacology; D051381:Rats; D018343:Receptors, Adrenergic, beta-2 / Q000187:drug effects; D012253:Rib Fractures / Q000503:physiopathology*; D013726:Terbutaline / Q000494:pharmacology
chemical_list: D058646:Adrenergic alpha-1 Receptor Agonists; D018343:Receptors, Adrenergic, beta-2; D010656:Phenylephrine; D013726:Terbutaline; D011224:Prazosin