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 associated with the seasonal growth cycle. During this cycle metabolic state changes from weight gain in spring to weight loss in winter. However, short-term metabolic changes due to feeding could also alter the GH pattern. To investigate the effect of such changes on GH secretion, the acute feedback of blood glucose level on the GH secretory pattern was examined. Six yearling male red deer were infused iv with glucose (G; 150 mg/kg/hr) or insulin (I; 30 mU/kg/hr) for a 12-hr period, 1 week apart. GH was measured in jugular venous blood every 10 min, for 12 hr before, during, and 6 hr after the infusions. Glucose, insulin, IGF-1, and 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 returned towards normal postinfusion. Insulin levels increased during G and I (P < 0.01) with no difference (P > 0.05) between G and I during preinfusion (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 characteristics, mean IGF-1, or haematocrit between G and I. However, there 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 amplitude was lower during I infusion (5.6 ng/ml vs 8.2 ng/ml preinfusion, 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 in 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 there were no major alterations in GH secretory patterns during 12 hr of hypoglycemia and hyperglycemia suggests that GH secretion in the male red deer is relatively insensitive to short-term changes in metabolic state.