title: The significance of intensity and duration of exercise on natural immunity in rats.
abstract: OBJECTIVE
  The purpose of this study was to examine a) the relationship between running distance (km x d(-1)) and b) the duration of exercise training in weeks on the effects on natural immune function in spontaneously hypertensive rats (SHR).
  
  
  METHODS
  Exercise consisted of voluntary running in wheels for 5 or 11 wk. In vivo cytotoxicity was measured as clearance of injected 51Cr-labeled YAC-1 lymphoma cells from the lungs.
  
  
  RESULTS
  Increased in vivo cytotoxicity was seen after 5 wk of exercise (P < 0.001) but not after voluntary exercise for 11 wk. If the wheels were locked 3 d x wk(-1) during the last 6 wk of running, thus restricting the exercise to 4 d x wk(-1), the exercise-induced immunoenhancement seen after 5 wk of exercise was maintained also after 11 wk of exercise. When compared with the sedentary controls after 5 wk of exercise, all runners regardless of running distance exhibited significantly higher in vivo clearance of tumor cells from the lungs, and no overall significant correlation was seen between running distance and retained radioactivity. However, the lowest activity runners (< 4 km x d(-1)) exhibited significantly lower in vivo clearance of tumor cells from the lungs when compared with animals running more than 4 km x d(-1).
  
  
  CONCLUSIONS
  We conclude that the duration of exercise training, and to some extent the running distance, has significant effects on the training-induced increase in natural immune function in rats. Furthermore, we conclude that a resting or recovery period during long-term exercise training is important to maintain the immunoenhancing effects in response to exercise.
authors.affiliation: Institute of Physiology and Pharmacology, Department of Physiology, Göteborg University, Sweden. inga.jons@fysiologi.gu.se
authors.forename: I H
authors.initials: IH
authors.lastname: Jonsdottir
authors.forename: P
authors.initials: P
authors.lastname: Hoffmann
mesh_terms: D000818:Animals; D003601:Cytotoxicity Tests, Immunologic; D007113:Immunity, Innate; D010805:Physical Conditioning, Animal; D005082:Physical Exertion / Q000502:physiology*; D051381:Rats; D011918:Rats, Inbred SHR; D013997:Time Factors; D014407:Tumor Cells, Cultured / Q000276:immunology