Decreased L-type Ca2+ current in cardiac myocytes of type 1 diabetic Akita mice due to reduced phosphatidylinositol 3-kinase signaling

OBJECTIVE-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 signaling modulates LTCC function. Because PI 3-kinases are key mediators of insulin action, we investigated whether LTCC function is affected in diabetic animals due to reduced PI 3-kinase signaling. RESEARCH DESIGN AND METHODS-We used whole-cell patch clamping and biochemical assays to compare cardiac LTCC function and PI 3-kinase signaling in insulin-deficient diabetic mice heterozygous for the Ins2(Akita) mutation versus nondiabetic littermates. RESULTS-Diabetic mice had a cardiac contractility defect, reduced PI 3-kinase signaling in the heart, and decreased L-type Ca2+ current (I-Ca,I-L) density in myocytes compared with control nondiabetic littermates. The lower I-Ca,I-L density in myocytes from diabetic mice is due at least in part to reduced cell surface expression of the LTCC. I-Ca,I-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)P-3]. 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,I-L was also shifted to more positive potentials in myocytes from diabetic versus nondiabetic animals. PI(3,4,5)P-3 infusion eliminated only the difference in voltage dependence of steady-state inactivation of I-Ca,I-L. CONCLUSIONS-Decreased PI 3-kinase 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.