Insulin Biosynthesis: II. EFFECT OF GLUCOSE ON RIBONUCLEIC ACID SYNTHESIS IN ISOLATED RAT ISLETS

Abstract

Abstract Studies were designed to determine the effect of glucose on RNA metabolism in isolated Islets of Langerhans. [3H]Uridine incorporation into RNA by isolated islets was linear for 2 hours and was increased 2-fold by raising glucose in the incubation media from 2.8 to 15.3 mm. Previously incubating islets with [3H]uridine at 2.8 mm glucose and then transferring them into 15.3 mm glucose in the absence of [3H]uridine showed that [3H]uridine incorporation into RNA was stimulated by high glucose independent of effects on the intracellular transport of uridine. Glucose also stimulated the uptake and phosphorylation of uridine and increased the islet UTP pool size, but under the conditions of these studies the relative specific activity of the UTP pool was the same at 2.8 or 16.6 mm glucose. Since high glucose did not affect the rate of degradation of islet RNA, it was concluded that increasing the glucose concentration in the incubation medium increased the over-all rate of islet RNA synthesis. Base composition analysis of islet RNA labeled for 1 hour in 32PO4 revealed a guanosine + cytosine content of 29.6% for islets incubated in 2.8 mm glucose and a guanosine + cytosine content of 45.6% for islet RNA synthesized in 16.6 mm glucose. Sucrose gradient analysis indicated that the [3H]RNA synthesized after 1 hour of incubation was heterodisperse (i.e. from 4 S to g45 S), and that glucose stimulated the greater production of large molecular weight RNA. Islets pulsed with [3H]uridine for 1 hour in 16.6 mm glucose and then chased with cold uridine for 3 hours showed that much of the large molecular weight RNA was ribosomal precursor. Fractionation of islets into crude nuclear, soluble, and polysomal fractions revealed that glucose increased RNA synthesis in each fraction. After 1 hour of incubation, 65% of the labeled RNA was still in the nucleus. Glucose had no effect on distribution of RNA within these fractions. Sucrose gradient analysis of the RNA extracted from these subcellular fractions revealed a predominant 4 S peak for the soluble fraction, partially degraded RNA in the nuclear fraction, and a prominent 8 to 10 S peak for the polysomal fraction. DNA-RNA hybridization studies showed that RNA synthesized in 16.6 mm glucose hybridized to DNA in greater amounts than RNA labeled in 2.8 mm glucose. Thermal elution profiles showed that the RNA labeled in 16.6 mm glucose dissociated from the DNA at lower temperatures. These results indicate that glucose stimulates the over-all rate of islet RNA synthesis and alters the nature of the RNA species synthesized.