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Heterogeneity of insulin receptors in rat tissues as detected with the partial agonist B29,B29'-suberoyl-insulin.

Using the insulin receptor partial agonist B29,B29'-suberoyl-insulin, a covalently dimerized insulin derivative, we previously demonstrated a heterogeneity of signal transduction by insulin receptors in two cell systems. The present study was designed to characterize the heterogeneity of insulin receptors in different rat tissues with this agent. Binding of 125I-insulin to insulin receptors and its inhibition by B29,B29'-suberoyl-insulin or by unlabeled insulin were assayed in plasma membranes from brain, spleen, adipocytes, and liver. IC50 values of B29,B29'-suberoyl-insulin were different in all tissues investigated (brain < spleen < adipocytes < liver). In contrast, IC50 values of insulin were identical, with the exception of spleen membranes (spleen < brain = adipocytes = liver). Furthermore, the IC50 ratios (B29 dimer/insulin) were significantly different, ranging from 0.7 (brain) to 12.8 (liver). Solubilization and partial purification of insulin receptors failed to abolish the marked difference between brain and liver (IC50 ratios of 1.8 and 7.1, respectively). The apparent molecular masses of the alpha subunits of insulin receptors, as labeled with a photoreactive insulin derivative, appeared identical in liver and spleen but were significantly lower in adipocytes and brain (liver = spleen > adipocytes > brain). The tissue-specific expression of the known insulin receptor isoforms generated by alternative splicing (insulin receptor types A and B), as assessed by polymerase chain reaction amplification with oligonucleotide primers flanking exon 11, was not correlated with the differences in the IC50 values and ratios for insulin and B29,B29'-suberoyl-insulin. Furthermore, IC50 values of both insulin and the B29 dimer were 3-fold lower in membranes from Rat1 cells overexpressing insulin receptor type A, compared with membranes with insulin receptor type B; the IC50 ratios were identical. No additional alternative splicing of insulin receptor mRNA was found by polymerase chain reaction amplification and digestion with HaeIII and AluI of seven overlapping domains of the receptor alpha subunit. These data suggest a heterogeneity of insulin receptors in rat tissues that is unrelated to alternative splicing of the insulin receptor gene.