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 f actor (GRF)-induced GH concentrations were studied in 7 young (3 mo, 3 9 kg) and 7 old (30 mo, 156 kg) Yorkshire x Landrace female pigs. Jugu lar catheters were surgically inserted and 60 hr later total serum vol ume 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 cl earance 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 ani mal 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 unalter ed (P>.1). Basal GH concentrations were lower in older pigs (P<.10) bu t 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 to tal GH secretion (AUC x MCR) and average total serum GH (mean GH post- injection x serum volume) were calculated per pig, these variables sig nificantly increased between 3 and 30 mo of age. Basal IGF-I concentra tions 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. Th e present data therefore indicate that age alters both SR and post-sec retory metabolism of GH. Since GRF-induced GH concentrations and post- secretory metabolism of GH are not altered similarly by age, both thes e variables must be considered when describing GH responses of swine.