Effects of age and recombinant equine somatotropin (eST) administration onimmune function in female horses

Aging has been associated with declines in somatotropin and IGF-I levels as well as declines in immune function. To determine the effects of age and w hether ST administration could reverse immunosenescence in horses, eight yo ung and eight aged female standardbred horses were given 10 mg/d recombinan t equine somatotropin (eST) or vehicle buffer for 49 d. Plasma IGF-I concen trations in both age groups were higher in eST-treated animals (P < 0.001), and higher in young eST-treated mares than in aged eST-treated mares durin g wk 4 to 7 (P < 0.001). There was a trend toward lower monocyte and granul ocyte numbers (P = 0.07) in mares treated with eST. Aged mares treated with eST had lower lymphocyte numbers (P < 0.005). The percentage of CD4(+) lym phocytes was higher in aged mares (P < 0.001), and the percentage of CD8(+) lymphocytes was higher in young mares (P < 0.01). Lymphocyte proliferation in response to concanavalin A, phytohemagglutinin, and pokeweed mitogen wa s not lower in aged mares (P = 0.17, 0.17, and 0.13 respectively). Aged mar es treated with eST showed a lower peak primary antibody response to keyhol e limpet hemocyanin (P < 0.05). Young mares treated with eST showed a highe r peak primary antibody response to keyhole limpet hemocyanin (P < 0.05). L ike other species, horses exhibit similar signs of age-related declines in various immune parameters, but those of aging were not reversed with eST tr eatment.