Additive effects of growth hormone releasing factor and insulin hypoglycaemia on growth hormone release in man

We have measured GH and PRL changes following separate and combined administration of insulin and GH releasing factor (GRF) in six normal males. Peak GH responses to separate administration of insulin and GRF were comparable (71.4 +/- 10.2 vs 70.1 +/- 27.7 mU/l; mean +/- SEM). However, the peak GH response following combined administration was significantly higher (120.8 +/- 29.7, P less than 0.05) as was the total GH released as calculated by measuring the area under the curve (P less than 0.05). In contrast the PRL response to hypoglycaemia was not altered by the combined administration of insulin and GRF. This effect was not due to any direct action of hypoglycaemia or insulin at pituitary level since basal and 10(-8) M GRF stimulated GH release from rat anterior pituitary cells in vitro was not influenced by varying glucose and insulin levels. Our findings support the hypothesis that GRF and insulin-induced hypoglycaemia release GH via different pathways which are, at least in part, additive.