Fat Cell Assay Research Articles

Guinea Pig Insulin: II. BIOLOGICAL ACTIVITY

Abstract The biological activity of guinea pig insulin was determined to be between 1 and 3 i.u. per mg when measured in vitro using isolated rat tissues and in vivo by mouse convulsion. However, when assayed by blood glucose lowering in the guinea pig, guinea pig insulin has a potency of 8.5 to 9.0 i.u. per mg. The low activity in vitro was not due to a more rapid degradation of guinea pig insulin by the rat tissues. The dose-response relationship in the isolated epididymal fat cell assay showed that guinea pig insulin had an intrinsic biological activity comparable to that of bovine insulin, but a reduced affinity for the tissue receptor sites.

Biosynthesis of glucagon in isolated pancreatic islets of guinea pigs.

1. The biosynthesis of glucagon in guinea-pig A(2) cells was investigated by incubation of isolated islets of Langerhans in the presence of [(3)H]tryptophan for periods of up to 14 days. Proteins were extracted from islets and incubation media and analysed by gel filtration. 2. In addition to very-high-molecular-weight (100000) proteins, the principal tryptophan-containing biosynthetic product after incubation for up to 17h was a protein of minimum mol.wt. 9000, which co-eluted on gel filtration with a peak of glucagon-like immunoreactivity, but was apparently devoid of biological activity in a fat-cell assay. A discrete peak of labelled glucagon was only recovered after incubation for at least 6 days. Losses of glucagon during the extraction and rapid secretion of newly synthesized glucagon into incubation media were excluded as reasons for the lack of recovery of labelled hormone from islets after shorter incubations. 3. The 9000-mol.wt. protein was localized to A(2) cells in experiments using B-cell-depleted islets, and to A(2)-cell granules by subcellular fractionation and electron-microscopic radioautography. Only glucagon was secreted into the incubation medium. 4. Possible relationships between the 9000-mol.wt. protein and glucagon are discussed in the light of postulated mechanisms of glucagon biosynthesis.