title: Effect of CO₂ laser irradiation on hormesis induction in human pulp and periodontal ligament fibroblasts.
abstract: BACKGROUND
  We have recently reported that a low level of CO₂ laser irradiation induced growth stimulation (hormesis) of both human gingival fibroblast (HGF) and oral squamous cell carcinoma cell line (HSC-2), but the extent of hormetic response was much smaller than that previously reported for toxicants and radiation in other experimental systems. Here we investigated the extent of hormetic response induced by CO₂ laser irradiation in human pulp cells (HPCs) and periodontal ligament fibroblast (HPLF).
  
  
  METHODS
  HPC and HPLF cells were established from the periodontal tissues of the first premolar extracted tooth. Cells were cultured for 24, 48 or 72 hours after exposure to various irradiation powers, and the viable cell number was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) method.
  
  
  RESULTS
  CO₂ laser irradiation induced biphasic effects on the growth of both HPC and HPLF cells. The maximum hormetic response was less than 50%. The hormetic response was found within the energy density of 7.98-79.77 J/cm², and cytotoxicity emerged at powers over 132.96 J/cm². Combining with our previous report, HPCs showed the highest hormetic response, followed by HPLFs and then HGFs. Both HPLFs and HGFs showed similar time-course of hormesis response, increasing response with incubation time.
  
  
  CONCLUSIONS
  The hormetic response may be the common survival mechanism by which cells escape from radiation-induced injury. Higher hormetic response of HPCs may reflect their potential for differentiation into one of the components in dentin.
authors.affiliation: Division of Operative Dentistry, Department of Restorative and Biomaterials Sciences, Meikai University School of Dentistry, Sakado, Saitama 350-0283, Japan. iwasaka@dent.meikai.ac.jp
authors.forename: Kensuke
authors.initials: K
authors.lastname: Iwasaka
authors.forename: Eri
authors.initials: E
authors.lastname: Hemmi
authors.forename: Kayoko
authors.initials: K
authors.lastname: Tomita
authors.forename: Sachiyo
authors.initials: S
authors.lastname: Ishihara
authors.forename: Tadashi
authors.initials: T
authors.lastname: Katayama
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Sakagami
mesh_terms: D045744:Cell Line, Tumor; D002470:Cell Survival / Q000528:radiation effects; D002648:Child; D003782:Dental Pulp / Q000528:radiation effects*; D005260:Female; D005347:Fibroblasts / Q000528:radiation effects*; D006801:Humans; D054020:Lasers, Gas; D010513:Periodontal Ligament / Q000166:cytology* / Q000528:radiation effects