Evidence for linkage between a region on chromosome 1p and the acute insulin response in Pima Indians.

A low acute insulin response (AIR) is a predictor of non-insulin-dependent diabetes mellitus (NIDDM) in insulin-resistant Pima Indians. We have initiated a search for regions of the genome linked with the AIR using sib-pair linkage analysis as a first step in identifying genes that are determinants of this phenotype. Eighteen short tandem-repeat polymorphisms from chromosome 1 were genotyped in over 900 Pima Indians and tested for linkage with NIDDM and in a subset of Pima Indians for linkage with AIR. The anonymous DNA marker D1S198 on chromosome 1p was linked with AIR (P = 0.000056) in 175 sib pairs from 60 families, all with normal glucose tolerance, but no linkage was observed between D1S198 and NIDDM (P = 0.44, 996 sib pairs). Additional markers genotyped on chromosome 1 did not show linkage with AIR or NIDDM. This study indicates that a locus on chromosome 1p may be a determinant of the phenotypic variation seen in the AIR.

Insulin-inducible changes in insulin receptor mRNA splice variants.

Alternative splicing of insulin receptor pre-mRNA has been shown to be regulated in a tissue-specific and developmental manner. We investigated whether the receptor ligand might regulate the relative distribution of alternatively spliced mRNA in insulin-sensitive cells and found that changes in the relative expression of the two alternatively spliced insulin receptor RNA isotypes expressed in hepatocytes are regulated by insulin. In addition, we observed significant differences (p < or = 0.001) in insulin receptor isotype expression in subjects who were hyperinsulinemic and insulin-resistant versus subjects who were insulin-sensitive. These results support a role for insulin in the regulation of the relative expression of alternatively spliced mRNA expressed in insulin-responsive cells and tissues.

Tyrosine kinase activity of skeletal muscle during insulin infusion in humans.

Insulin receptor tyrosine kinase is stimulated by insulin in vivo, and this provides a mechanism by which the signal from insulin is transmitted into target cells. This study examined the time course of the in vivo activation of the kinase. Five nondiabetic Pima Indians had a euglycemic clamp at an insulin dose of 600 mU/min.m2, resulting in plasma insulin concentrations of about 15 nM by 30 min. Percutaneous muscle biopsies were taken from the vastus lateralis before and at regular intervals during insulin infusion, and the in vivo and in vitro tyrosine kinase activities were measured. There was a rapid in vivo activation of the kinase, detectable at less than 10 min and reaching a maximum within 30 min of insulin infusion. The time course of in vivo kinase activity, plasma insulin concentrations, and insulin-mediated glucose disposal rates displayed parallel patterns, indicating close interrelationships among these variables. The insulin concentration at which the kinase activity was maximal was about 10 nM both in vivo and in vitro. In vitro, however, this maximum increased with the degree of the kinase activation in vivo, indicating that the kinase potential in vitro is dependent on previous insulin exposure in vivo. We conclude that in vivo activation of the insulin receptor tyrosine kinase in human skeletal muscle is a rapid process, related to insulin action on glucose disposal, and that circulating insulin primes inactive insulin receptor molecules for subsequent tyrosine kinase activation by a mechanism that remains to be elucidated.

Insulin-sensitive tyrosine kinase activity changes in parallel with plasma insulin and glucose concentrations in humans.

Insulin receptor tyrosine kinase is an important step in insulin action. We examined the relationship between diet-induced changes in glucose metabolism and changes in skeletal muscle insulin-sensitive tyrosine kinase activity in 12 nondiabetic subjects. Subjects were fed a traditional, high carbohydrate Pima Indian diet and a modern, high fat western diet for 2 weeks in a randomized cross-over design. At the end of each dietary period, glucose tolerance was assessed, insulin sensitivity (SI) was estimated by Bergman's minimal model method, and insulin receptor concentration and tyrosine kinase activity were determined on lectin-purified extracts from quadriceps femoris muscle. Compared to the traditional diet, the modern diet was associated with a deterioration of glucose tolerance and an increase in glucose-induced plasma insulin levels. As expected, SI changes were associated with opposite changes in plasma insulin levels. However, the changes in maximal tyrosine kinase activity were negatively correlated with changes in SI (r = -0.69; P less than 0.01) and positively correlated with changes in plasma glucose (r = 0.70; P less than 0.01) and insulin response to glucose (r = 0.57; P less than 0.025). These results suggest that the site of diet-induced changes in insulin action is beyond the insulin-sensitive tyrosine kinase. The results further suggest that the kinase activity is modulated by prevailing plasma insulin levels.

Insulin-sensitive tyrosine kinase: relationship with in vivo insulin action in humans.

To investigate the relationship of insulin receptor kinase with insulin resistance in humans, we studied insulin-sensitive tyrosine kinase activity in muscle biopsies taken from 20 Pima Indians [14 nondiabetics, 6 with non-insulin-dependent mellitus (NIDDM)] during euglycemic clamps, at insulin concentrations of approximately 68 microU/ml (low dose) and approximately 1,170 microU/ml (high dose). In the nondiabetics, the low dose, insulin-induced kinase activation in vivo was 1.5-fold the activity in the fasting state (P less than 0.05), whereas in the diabetics, the kinase activity actually decreased by 40% relative to fasting (P less than 0.05). The difference in delta-kinase in vivo was significant (P less than 0.01) between the two groups. Similarly, the kinase activation in vitro in response to 1 nM insulin was lower in diabetic subjects compared with nondiabetics (P less than 0.01). These data indicate that, in NIDDM, both in vitro and in vivo insulin-stimulated tyrosine kinase activity is impaired. Among nondiabetics, the kinase sensitivity to insulin, calculated as the ratio of the kinase activity at 1 nM insulin in vitro to the kinase activity at 100 nM insulin, was positively correlated with plasma insulin concentrations 2 h after an oral glucose load (r = 0.69, P less than 0.01). Thus, in nondiabetic subjects with insulin resistance, insulin activation of the kinase is not reduced, but the kinase sensitivity to insulin increases with increasing plasma insulin levels. Therefore, the site of insulin resistance in nondiabetic subjects is distal to the insulin receptor kinase. Furthermore, it is possible that circulating insulin, by increasing the kinase sensitivity to insulin, is a determinant of the receptor kinase activity.