IS THE GROWTH-HORMONE RESPONSE TO INSULIN DUE TO HYPOGLYCAEMIA, HYPERINSULINAEMIA OR A FALL IN PLASMA FREE FATTY ACIDS?

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Abstract
Seven normal male volunteers were given a 2 h i.v. infusion of insulin (2 mu [milliunits] kg-1 min-1). During the first hour, in which normoglycemia was maintained by glucose infusion, no detectable increase in growth hormone (GH) secretion occurred. When controlled symptomatic hypoglycemia was produced during the 2nd hour of the insulin infusion a brisk rise in plasma GH concentration was observed in all the subjects studied. Since it was possible that the falls in the concentration of plasma free fatty acids (FFA) occurring during infusion of insulin also acted as a stimulus to GH release the study was repeated in 4 subjects but normoglycemia was maintained throughout the entire 2 h period of insulin infusion. In these experiments there was no rise in plasma GH concentration although a similar fall in plasma FFA was produced. Hypoglycemia per se was the important stimulus to GH secretion and not hyperinsulinemia or a lowering of plasma FFA. Furthersome there appeared to be a threshold hypoglycemic stimulus to GH secretion. In no subject was a rise in plasma GH seen without a fall in plasma glucose greater than 1.4 mmol/l. Prolonged mild hypoglycemia did not stimulate GH secretion.