AGE-RELATED-CHANGES IN SECRETION RATE AND POST-SECRETORY METABOLISM OF GROWTH-HORMONE IN SWINE

The effect of age on growth hormone (GH) metabolism and GH-releasing factor (GRF)-induced GH concentrations were studied in 7 young (3 mo, 39 kg) and 7 old (30 mo, 156 kg) Yorkshire x Landrace female pigs. Jugular catheters were surgically inserted and 60 hr later total serum volume was determined. The following day, all animals were infused for 3 hr with GH (30.3 ng.min/kg B.W.) in order to calculate GH metabolic clearance rate (MCR), secretion rate (SR) and half-life (t1/2). Two days later, 15 mug/kg of GRF was injected i.v. into all pigs. On a per animal basis, aging increased (P<.01) MCR (299 vs 132 ml/min), SR (714 vs 422 ng/min) and serum volume (6.6 vs 2.0 1), whereas t1/2 was unaltered (P>.1). Basal GH concentrations were lower in older pigs (P<.10) but the GRF-induced GH concentrations (measured as GH peak or area under the curve, AUC) were not affected by age (P>.1). Yet, when induced total GH secretion (AUC x MCR) and average total serum GH (mean GH post-injection x serum volume) were calculated per pig, these variables significantly increased between 3 and 30 mo of age. Basal IGF-I concentrations were lower in older pigs (P<.01), yet, there was a tendency (P=.10) for these pigs to show a greater IGF-I response to GH infusion. The present data therefore indicate that age alters both SR and post-secretory metabolism of GH. Since GRF-induced GH concentrations and post-secretory metabolism of GH are not altered similarly by age, both these variables must be considered when describing GH responses of swine.