Insulin induces phosphorylation and activation of JAK2 tyrosine, as well as its association with STAT1 and SHP2 in insulin-sensitive tissues of intact rats, thus demonstrating a new pathway in transduction of insulin signals. We investigated this pathway in hearts of rats in three situations of insulin resistance: 72 h of fasting, chronic treatment with dexamethasone, and acute treatment with epinephrine. The acute treatment with epinephrine showed no difference in insulin-induced JAK2 tyrosine phosphorylation or JAK2/STAT1 and JAK2/SHP2 association in comparison with the control. In fasted rats the JAK2 protein concentration decreased, accompanied by a decrease in the stoichiometry of the phosphorylation to 70%, an increase in association of JAK2/STAT1 to 160%, and a decrease in JAK2/SHP2 association to 85%. In the dexamethasone-treated group, the JAK2 protein concentrations increased but the stoichiometry of its phosphorylation decreased to 20%, whereas the JAK2/STAT1 and JAK2/SHP2 associations changed by 70% and 170%, respectively. In fasting and dexamethasone-treated rats, therefore, insulin-induced JAK2 tyrosine phosphorylation decreases, and the JAK2 protein expression is differentially regulated such that the insulin-induced JAK2 association with SHP2 and STAT1 shows opposite interactions with the kinase.49501-9