THE EFFECT OF RESTRICTED FEEDING ON PLASMA GROWTH-HORMONE (GH) CONCENTRATIONS IN GROWING AMERICAN KESTRELS

In intact growing American Kestrels (Falco sparverius), plasma growth hormone (GH) concentration was correlated with body mass (r(Pearson) = -0.7504; P = 0.0001; n = 44). High GH concentrations were observed du ring rapid growth periods (11 and 17 days of age) and lower values wer e present during slow growth periods (23 and 30 days of age: age at fl edging). This maturational decline in the plasma GH concentration is d ue to age-related changes in the structure and the sensitivity/respons iveness of the somatotrophs of the pituitary gland to GH releasing and inhibiting factors. Restricted feeding (i.e., 90% and 70% of the cont rol ad libitum intake) increased plasma GH concentrations above those of control birds during rapid growth periods but not during slow growt h periods. Growth delay was observed with kestrels having lower body m ass (16-19 and 24-26 days of age) and shorter antebrachium (10-12 and 16-19 days of age) than control birds according to the severity of the food restriction. As kestrels reached fledging (30 days of age), body mass was reduced only in birds fed the most restricted diet (70% at 3 0-33 days of age) and the antebrachium length was similar in birds fed the control diet and those fed the restricted diet (24-26 and 30-33 d ays of age). Food restriction increases the responsiveness of the pitu itary gland to thyrotropin-releasing hormone (TRH: a GH releasing fact or) and suppresses thyroid function (inhibiting factor of GH secretion ), both resulting in an increased GH secretion. Changes in plasma horm one concentrations are needed to secure an adequate energy supply to v ital organs during growth and food restriction.