Effect of age and intake on growth hormone kinetics in dairy heifers

Abstract

The effects of aging and intake on growth hormone (GH) kinetics and GH-releasing factor (GRF)-induced GH concentrations were studied in two groups of 12 Holstein heifers each (80 d, 85 kg: young; and 273 d of age, 246 kg: old). Each group was then equally subdivided into full-fed (FF) and restricted-fed (RF) subgroups. After 11 d of intake treatment, animals were infused for 3 hr with GH (1.5 mg/hr) in order to calculate GH metabolic clearance rate (MCR), secretion rate (SR) and half-life ( t 1 2 ). Two d later, total plasma volume was determined and the following day, all heifers received a GRF challenge (5 μg/kg i.v.). The following values are LSM ± SE for young-FF, young-RF, old-FF and old-RF. Rate of secretion was not affected by any treatment, averaging 1.51, 1.25, 1.34, and 1.40 ± .23 μg/min. Aging increased (P<.01) MCR (186, 159, 382, and 300 ± 21 ml/min) and increased plasma volume (P<.01), which resulted in lower basal GH concentrations. Aging also decreased (P<.01) the area under the GH response curve following GRF injection (AUC: 12442, 21114, 5155, and 6308 ± 1776 ng · min/ml) but did not affect average GH quantity in the plasma after the GRF challenge. Feed restriction decreased (P<.05) MCR, but not enough to affect basal GH concentrations. However, feed restriction increased GRF-induced GH concentrations in young heifers only; since all GH kinetic variables were similarly affected by intake in old and young heifers, this effect of feed restriction in the young heifers probably resulted from a higher pituitary secretion of GH when challenged with GRF. Concentrations of insulin, glucagon, glucose, insulin-like growth factor-I (IGF-I) and free fatty acids (FFA) were determined. There was a correlation (P<.001) between IGF-I and GH MCR (r 2=.65) and AUC (r 2=−.66), and a correlation between FFA and GH secretion rate (r 2=−.41, P<.05). The AUC was related to the MCR (r 2=−.67, P<.001). These data therefore indicate that post-secretory metabolism of GH plays an important role in determining GH concentrations in peripheral circulation. The effect of aging (from 3 to 9 mo) on GH concentrations is mostly due to increased MCR and plasma volume, while feed restriction affects the kinetics of GH and the GRF-induced secretion.