The Effect of Restricted Feeding on Plasma Growth Hormone (GH) Concentrations in Growing American Kestrels

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In intact growing American Kestrels (Falco sparverius), plasma growth hormone (GH) concentration was correlated with body mass (rpe.so, = -0.7504; P = 0.0001; n = 44). High GH concentrations were observed during rapid growth periods (11 and 17 days of age) and lower values were present during slow growth periods (23 and 30 days of age: age at fledging). This maturational decline in the plasma GH concentration is due to age-related changes in the structure and the sensitivity/responsiveness of the somatotrophs of the pituitary gland to GH releasing and inhibiting factors. Restricted feeding (i.e., 90% and 70% of the control ad libitum intake) increased plasma GH concentrations above those of control birds during rapid growth periods but not during slow growth periods. Growth delay was observed with kestrels having lower body mass (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 30-33 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 days of age). Food restriction increases the responsiveness of the pituitary gland to thyrotropin-releasing hormone (TRH: a GH releasing factor) and suppresses thyroid function (inhibiting factor of GH secretion), both resulting in an increased GH secretion. Changes in plasma hormone concentrations are needed to secure an adequate energy supply to vital organs during growth and food restriction.