Plasma Met-enkephalin and catecholamine responses to insulin-induced hypoglycaemia in greyhounds

The involvement of endogenous opioid peptides in the stress response was investigated by measuring plasma concentrations of Met-enkephalin-like immunoreactivity (MLI), adrenaline and noradrenaline during insulin-induced hypoglycaemia in conscious greyhounds. Moreover, the molecular forms of circulating MLI were characterized using gel filtration chromatography. In the first group of animals, i.v. administration of insulin (0.3 units/kg) provoked marked hypoglycaemia (blood glucose concentrations fell from 4.4 +/- 0.1 to 1.5 +/- 0.2 mmol/l; mean +/- S.E.M.) which was associated with significant (P less than 0.001) rises in plasma MLI concentrations from a basal concentration of 45 +/- 8 to a peak of 189 +/- 39 ng/l. A within-subject study comparing five different insulin doses ranging from 0.004 to 0.3 units/kg showed dose-related effects on blood glucose with nadir concentrations of 4.1 +/- 0.6 mmol/l (after the smallest dose of insulin) and 0.8 +/- 0.1 mmol/l (after the largest dose of insulin). This was associated with dose-related rises in plasma MLI with peak concentrations of 56 +/- 17 and 558 +/- 35 ng/l, plasma adrenaline with peak concentrations of 0.45 +/- 0.06 and 15.76 +/- 1.33 nmol/l and plasma noradrenaline with peak concentrations of 0.49 +/- 0.07 and 2.27 +/- 0.45 nmol/l following the smallest and largest doses of insulin respectively. These results are the first demonstration of raised plasma MLI concentrations following hypoglycaemia. Moreover, they show that the hormonal responses vary with the degree of hypoglycaemia achieved. Together with reports by other investigators these findings might suggest opioid modulation of the responses of the sympathoadrenal system to hypoglycaemia.(ABSTRACT TRUNCATED AT 250 WORDS)