EFFECTS OF GLUCOSE OR INSULIN INFUSIONS ON GROWTH-HORMONE SECRETION IN MALE RED DEER

The growth hormone (GH) secretory pattern in male red deer is associat ed with the seasonal growth cycle. During this cycle metabolic state c hanges from weight gain in spring to weight loss in winter. However sh ort-term metabolic changes due to feeding could also alter the GH patt ern. To investigate the effect of such changes on GH secretion, the ac ute feedback of blood glucose level on the GH secretory pattern was ex amined. Six yearling male red deer were infused iv with glucose (G; 15 0 mg/kg/hr) or insulin (I; 30 mU/kg/hr) for a 12-hr period, 1 week apa rt. GH was measured in jugular venous blood every 10 min, for 12 hr be fore, during, and 6 hr after the infusions. Glucose, insulin, IGF-I, a nd haematocrit were also measured. There was no difference (P > 0.05) in glucose levels between G and I prior to infusions (5.8 vs 6.0 mmol/ liter, SED = 0.42). Glucose levels rose to 8.7 mmol/liter during G and fell to 3.4 mmol/liter (SED = 0.72, P < 0.001) during I, then returne d towards normal postinfusion. Insulin levels increased during G and I (P < 0.01) with no difference (P > 0.05) between G and I during prein fusion (163 +/- 7.6 pmol/liter) or infusion (259 vs 264 +/- 16.5 pmol/ liter) periods. There were no differences (P > 0.05) in GH secretory c haracteristics, mean IGF-1, or haematocrit between G and I. However, t here were significant effects of infusion within the treatments. Mean GH declined (P < 0.05) from 1.8 ng/ml (both treatments) preinfusion to 1.13 and 1.31 ng/ml during G and I infusion, respectively. GH pulse a mplitude was lower during I infusion (5.6 ng/ml vs 8.2 ng/ml preinfusi on, P < 0.05, SED = 1.0) and the change in amplitude from preinfusion to infusion differed (P > 0.05) with an increase in G and a decrease i n I (+0.6 and -2.6, SED = 1.1). IGF-1 levels were stable and averaged 555 and 520 ng/ml (SED = 34.9) for G; and I, respectively. Haematocrit declined from 34.3 +/- 1.85% over the first 4 hr of sampling to 25.7 +/- 0.97% for the remainder of the sampling period. The finding that t here were no major alterations in GH secretory patterns during 12 hr o f hypoglycemia and hyperglycemia suggests that GH secretion in the mal e red deer is relatively insensitive to short-term changes in metaboli c state. (C) 1996 Academic Press, Inc.