Biosynthesis of proinsulin and other islet cell proteins in pancreatic beta cells of the rat: a radioautographic evaluation of glucose effects in vitro.

Abstract

Rat pancreatic islets were incubated in vitro with L-[4,5-3H]leucine or with L-[2,3-3H]tryptophan in Krebs-Ringer bicarbonate buffer, containing 0, 5, or 20 mM glucose. Incorporation of labeled amino acids in islet cells was evaluated quantitatively by a validated radioautographic procedure. Incorporation of labeled leucine into [3H]proinsulin and [3H]insulin was measured by immunoprecipitation and into other islet proteins by trichloroacetic acid precipitation. Incorporation of labeled amino acids in pancreatic beta cells was patchy and not uniform. Up to 20 to 35% of beta cells, mainly in central regions of the islets, showed poor or no incorporation of label. Peripheral nonbeta, endocrine islet cells and mesenchymal islet cells were all uniformly labeled. Incorporation of either amino acid into nonbeta endocrine and mesenchymal cells was not much affected by the absence of glucose in the incubation buffer. In contrast, incorporation of the amino acids into beta cells was strikingly affected. Incorporation of [3H]leucine into proinsulin and insulin at 0 mM glucose measured by specific immunoprecipitation and by silver grain densities over beta cells was 15 to 20 times less than at 20 mM glucose. Incorporation of [3H]tryptophan, an amino acid absent in proinsulin, into nonhormonal, sedentary beta cell proteins studied by radioautography, similarly, was strikingly affected in the absence of glucose. Thus, radioautography revealed a great sensitivity of both hormonal and nonhormonal protein biosynthesis in the beta cell to the concentration of glucose in the medium.