Growth hormone treatment stimulates thymulin production in aged dogs

Diminished thymic morphology and function and a decreased provocative growth hormone (GH) response are characteristic of normal ageing. We have previously demonstrated that bovine growth hormone (bGH) treatment of adult dogs tends to result in a rejuvenation of thymic morphology. The purpose of this study was twofold: (1) to determine concentrations of thymulin (a thymic hormone) in plasma from dogs of various ages, and (2) to evaluate the effect of bGH administration on plasma thymulin concentration and thymus morphology in 'middle-aged' and 'old-aged' dogs. 'Young' (4 months), 'middle-aged' (33-55 months) and 'old-aged' (63-83 months) female beagles served as subjects for this study. Middle- and old-aged dogs were injected with either bGH or bovine serum albumin (control) for 1 month. Thymulin concentration in plasma was determined by the rosette-inhibition bioassay. An age-associated decline in the plasma concentration of thymulin was detected. Plasma thymulin concentration increased in every bGH-treated dog. In middle-aged but not old-aged dogs, bGH treatment resulted in rejuvenation of thymic morphological features as determined by stereological and histomorphological procedures. The results suggest that exogenous GH may be useful for restoration of some immune functions in aged individuals.