[Chains A and B, circulating fractions of insulin]

Experimental hyperthyroidism in the dog, and clinical hyperthyroidism in man, were studied as activation models of insulin catabolism. Plasma insulin and its A and B chains, were estimated by radioimmunoassay in 8 dogs, before and after daily administration of 1 mg of thyroxine per animal. A and B chains rose quite definitely under treatment, whereas the increase in blood insulin was only slight. Blood sugar did not vary. Glucose tolerance tests during treatment, showed that A and B chains did not follow a curve parallel to insulin were situated during the test, at a higher level in dogs treated with thyroxine than in controls. The estimations carried out in man, confirmed the high levels of the A and B chains in hyperthyroidism. The use of certain anti-insulin antisera for radioimmunoassay of insulin demonstrated cross reactions with A or B chains. This phenomenon may be the cause of excessive evaluation of plasma insulin in subjects with levels of circulating chains A and B. Chains A and B in the blood seem to be a sign of the level of metabolic destruction of the insulin and not that of secretion.