Regulation of insulin biosynthesis in carp islets (Cyprinus carpio)

The biosynthesis of proinsulin was measured by the incorporation of radiolabelled leucine and phenylalanine into carp islet protein. Even in the absence of glucose or amino acids in the medium the share of proinsulin in the total protein biosynthesis amounted to 40% of the total incorporated [3H]-leucine during 2 h of incubation. Glucose increased the rate of total protein biosynthesis in carp islets by approximately 20%, but no preferential increase of proinsulin synthesis occurred in the presence of glucose. Furthermore leucine did not increase [3H]-phenylalanine incorporation into proinsulin. Cyproheptadine, a specific inhibitor of proinsulin biosynthesis in mammals, did not inhibit in carp islets. The total protein synthesis was strongly dependent on the temperature. The share of proinsulin synthesis decreased from 40% at 17 degrees C to 20% at 4 degrees C. Even more drastically, the conversion of proinsulin to insulin became slower at lower temperatures and was undetectable at 4 degrees C. The results indicate that in contrast to mammals, in islets of the poikilotherm carp, metabolic substrates may play a less important role that the temperature in the regulation of insulin biosynthesis.