Differences in the Ratio of RNA Encoding Two Isoforms of the Insulin Receptor Between Control and NIDDM Patients: The RNA Variant Without Exon 11 Predominates in Both Groups

Two alternative forms of the insulin receptor with different affinities for insulin are expressed as a result of alternative splicing of RNA corresponding to exon 11 of the IR gene. The percentage of IR-RNA molecules without exon 11, encoding the high-affinity isoform, was determined by cDNA-mediated PCR amplification of RNA extracts from the quadriceps femoris muscle of healthy control subjects (n = 9) and NIDDM patients (n = 7). In both patients and control individuals, a majority of the IR-RNA molecules contained exon 11. In addition, the proportion of IR-RNA molecules without exon 11 was decreased in patients (21 ± 1%) compared with control subjects (31 ± 3%) (P = 0.018). Careful investigation of the kinetics of the PCR-based assay system, as well as the conditions for separation of the PCR products, allowed us to suggest a possible explanation of the discrepant results concerning the alternative splicing presented in previous reports. The diabetic subjects as a group had higher fasting insulin levels and lower insulin-mediated glucose uptake during a euglycemic-hyperinsulinemic clamp (P = 0.042). However, identification of the regulatory pathways leading to the splicing alteration in NIDDM patients requires further investigation.