INSULIN-RECEPTOR MESSENGER-RNA SPLICING AND ALTERED METABOLIC CONTROLIN AGED AND MILDLY INSULIN-DEFICIENT RATS

Using reverse transcription-competitive polymerase chain reaction, we measured the abundance of the mRNAs encoding the two spliced isoforms of insulin receptor in aged and mildly insulin-deficient rats. Twelve- month-old rats were characterized by peripheral insulin resistance and decreased glucose tolerance. Mild insulin deficiency, obtained by neo natal streptozotocin treatment, was associated with glucose intoleranc e due to reduced glucose-stimulated insulin response. Both models were associated with a decrease in the relative abundance of the mRNA with exon 11 in liver, heart, adipose tissue, and tibialis muscle, whereas a slight increase was seen in the extensor digitorum longus and no ch ange in the soleus muscle. In the three muscles, the expression of the form without exon 11 largely predominated (>90%). In heart and adipos e tissue, the two isoforms were expressed at a similar level in contro l rats. In both tissues, the form without exon 11 increased in strepto zotocin-treated rats, whereas the absolute level of the form with exon 11 decreased in old rats. Although a decreased level of the variant w ith exon 11 correlated with insulin resistance of whole body glucose u ptake, our results indicated that changes in the expression of the ins ulin receptor variants were secondary events and thus not the cause of the insulin resistance in old and mildly insulin-deficient rats.