Respiration and Insulin Synthesis in the Islets of Langerhans

Abstract

A new radiorespirometric apparatus is described. It permits a simultaneous estimation of proinsulin and insulin and of carbon dioxide production by the isolated pancreatic islets. The results show that the carbon dioxide production from labeled glucose, mannose, fructose, ribose, and xylitol correlates very well with the amount of proinsulin and insulin newly synthesized by the isolated rat islets. In the presence of glucose, the greatest carbon dioxide production was obtained and the largest quantity of proinsulin and insulin was synthesized. Mannose was metabolized to carbon dioxide at two-thirds the rate of glucose and was also two-thirds as effective as glucose in stimulating insulin synthesis. Very little of fructose, ribose, and xylitol were metabolized to carbon dioxide and very little proinsulin and insulin were produced in their presence. It was also observed that the presence of xylitol did not alter the rate of metabolism of glucose to carbon dioxide by the islets. In contrast, although pyruvate was metabolized to carbon dioxide very well, little insulin was synthesized in its presence.These results confirmed our previous findings and lent further evidence to our suggestion that the biosynthesis of proinsulin and insulin depends not only on the provision of energy and substrate but also on some 'triggering' factor or factors produced during the metabolism of glucose and mannose by the pancreatic islets.