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Decreased L-type Ca²⁺ Current in Cardiac Myocytes of Type 1 Diabetic Akita Mice Due to Reduced Phosphatidylinositol 3-kinase Signaling

From the ¹Department of Physiology and Biophysics and the Institute of Molecular Cardiology and
the ²Department of Medicine, Stony Brook University, Stony Brook, New York; and
the ³Department of Veterans Affairs Medical Center, Northport, New York; and
the ⁴Department of Cardiac Surgery, 2^nd Hospital Xiang Ya Medical School, Central South University, Changsha, China

Correspondence: richard.lin{at}sunysb.edu

Contraction of cardiac myocytes is initiated by Ca²⁺ entry through the voltage-dependent L-type Ca²⁺ channel (LTCC). Previous studies have shown that phosphatidylinositol (PI) 3-kinase (PI3K) signaling modulates LTCC function. Since PI3Ks are key mediators of insulin action, we examined LTCC function in cardiac myocytes isolated from insulin-deficient diabetic mice heterozygous for the Ins2^Akita mutation. Diabetic mice had a cardiac contractility defect, reduced PI3K signaling in the heart and decreased L-type Ca²⁺ current (I_Ca,L) density in myocytes as compared to control nondiabetic littermates. The lower I_Ca,L density in myocytes from diabetic mice is due at least in part to reduced cell surface expression of the LTCC. I_Ca,L density in myocytes from diabetic mice was increased to control levels by insulin treatment or intracellular infusion of PI 3,4,5-trisphosphate (PI(3,4,5)P₃). This stimulatory effect was blocked by taxol, suggesting that PI(3,4,5)P₃ stimulates microtubule-dependent trafficking of the LTCC to the cell surface. The voltage dependence of steady state activation and inactivation of I_Ca,L was also shifted to more positive potentials in myocytes from diabetic vs. nondiabetic animals. PI(3,4,5)P₃ infusion eliminated only the difference in voltage dependence of steady state inactivation of I_Ca,L. These results indicate that decreased PI3K signaling in myocytes from type 1 diabetic mice leads to reduced Ca²⁺ entry through the LTCC, which might contribute to the negative effect of diabetes on cardiac contractility.

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