Adrenergic receptor control mechanism for growth hormone secretion

The influence of catecholamines on growth hormone secretion has been difficult to establish previously, possibly because of the suppressive effect of the induced hyperglycemia on growth hormone concentrations. In this study, an adrenergic receptor control mechanism for human growth hormone (HGH) secretion was uncovered by studying the effects of alpha and beta receptor blockade on insulin-induced growth hormone elevations in volunteer subjects. Alpha adrenergic blockade with phentolamine during insulin hypoglycemia, 0.1 U/kg, inhibited growth hormon elevations to 30-50% of values in the same subjects during insulin hypoglycemia without adrenergic blockade. More complete inhibition by phentolamine could not be demonstrated at a lower dose of insulin (0.05 U/kg). Beta adrenergic blockade with propranolol during insulin hypoglycemia significantly enhanced HGH concentrations in paired experiments. The inhibiting effect of alpha adrenergic receptor blockade on HGH concentrations could not be attributed to differences in blood glucose or free fatty acid values; however, more prolonged hypoglycemia and lower plasma free fatty acid values may have been a factor in the greater HGH concentrations observed during beta blockade. In the absence of insulin induced hypoglycemia, neither alpha nor beta adrenergic receptor blockade had a detectable effect on HGH concentrations. Theophylline, an inhibitor of cyclic 3'5'-AMP phosphodiesterase activity, also failed to alter plasma HGH concentrations. These studies demonstrate a stimulatory effect of alpha receptors and a possible inhibitory effect of beta receptors on growth hormone secretion.

Radioimmunoassay of the A-chain of insulin

A radioimmunoassay for isolated A-chain is described. The immunospecificity of A-chain is shown to vary depending on the variation in structure about the cysteine residues. The presence of one or two intrachain disulfide bonds alters the immunoreactivity greatly in a system in which A-chain with all four cysteine residues carboxymethylated is the homologous antigen. Species specificity is also suggested by differences in the immunoreactivity of the cleaved products of bovine and porcine insulin. Further attempts to assay A-chain in human plasma by immunological methods must take these two factors into consideration.