title: The effect of hypoxic myocardial preconditioning is highly dependent on the light-dark cycle in Wistar rats.
abstract: BACKGROUND
  Cardioprotective effects by hypoxic preconditioning are well known mainly through in vitro experiments. However, the chronobiological aspects of phenomenon of cardioprotection by hypoxic preconditioning have not been studied in in vitro and in vivo experiments, and there are no consistent data regarding the daytime dependence of preconditioning effects.
  
  
  OBJECTIVE
  To determine the protective effects of myocardial preconditioning applied during the dark (active) part of the day regimen, and to obtain an understanding concerning chronophysiological aspects of this phenomenon in in vivo rat experiments.
  
  
  METHODS
  The present study was designed to evaluate the effects of cardiac preconditioning induced by one to three cycles (1PC to 3PC) of asphyxia (5 min of artificial hypoventilation [tidal volume 0.5 mL/100 g of body weight, 20 breaths/min]) and reoxygenation (5 min of artificial ventilation [tidal volume 1 mL/100 g of body weight, 40 breaths/min]) on ventricular arrhythmia threshold (VAT) in the dark (active) part of the day regimen in anesthetized Wistarrats (ketamine/xylazine100 mg/15 mg/kg, intramuscularly, open chest experiments) after the adaptation to the light-dark cycle (12 h:12 h).
  
  
  RESULTS
  In the dark part of the day, hypoventilation-induced asphyxia caused a decrease of the VAT in the control animals, as well as in the 1PC and 2PC groups (P<0.001), which showed only incomplete recovery of the VAT after reoxygenation. In the 3PC group, the VAT values remained close to the preasphyxic level during both types of ventilation.
  
  
  CONCLUSIONS
  Preconditioning by hypoventilation-induced asphyxia had little effect on the changes of the electrical stability of the heart in the dark (active) part of the day regimen in in vivo rat experiments.
authors.affiliation: Department of Physiology, Medical Faculty, Pavol Jozef Safárik University, Kosice, Slovak Republic.
authors.forename: Pavol
authors.initials: P
authors.lastname: Svorc
authors.forename: Roman
authors.initials: R
authors.lastname: Benacka