Endocrine Abstracts

Background: Precise mechanism(s) responsible for stimulation of growth hormone (GH) and cortisol during Glucagon Stimulation Test (GST) are not clear, but seem to be related to glucagon-induced fluctuations in glucose and/or insulin with an initial increase followed by a subsequent fall in glucose and insulin concentrations. We have endeavoured to assess whether elimination of fall in glucose concentrations during GST would obliterate any significant increase in GH and cortisol.

Subjects & Methods: We designed a cross-over study, where glucose, GH, cortisol and insulin were measured in six healthy subjects (age range 28-54, one male) during GST (0, 30, 60, 90, 120, 150 and 180 minutes), and subsequently in the same individuals, when a fall in glucose and insulin was prevented by infusion of 10% glucose that was started at 60 minutes after administration of intramuscular glucagon.

Results: As expected, administration of glucose prevented a fall in glucose and insulin concentrations observed during later time-points of GST (e.g. mean glucose at 180 minutes of GST 176.7 mg/dl vs 71.6 mg/dl, P=0.001, mean insulin 53.6 µU/ml vs 4.52 µU/ml, P=0.003). Despite that there was still an increase in GH concentrations though less pronounced than in cross-over controls (GH_{glucose infusion}: median lowest value 0.17ng/ml vs median maximal value 8.12 ng/ml, P=0.04, GH_{cross-over controls}: median lowest value 0.24 ng/ml vs median maximal value 15.12 ng/ml, P=0.004). The same applied to cortisol concentrations (Cortisol_{glucose infusion}: median lowest value 6.89 µg/dl vs median maximal value 15.15 µg/dl, P=0.05, Cortisol_{cross-over controls}: median lowest value 8.48 µg/dl vs median maximal value 19.75 µg/dl, P=0.01).

Conclusions: Fall in glucose concentrations during the second phase of GST contributes to stimulation of GH and cortisol secretion, but is not the only factor responsible for this phenomenon. Precise mechanisms involved in stimulation of GH and cortisol secretion during glucagon stimulation test still remain to be elucidated.