Objective: Contraction of cardiac myocytes is initiated by Ca(2+) entry through the voltage-dependent L-type Ca(2+) 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 Ca(2+) current (I(Ca,L)) density in myocytes compared with 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)P(3)]. This stimulatory effect was blocked by taxol, suggesting that PI(3,4,5)P(3) 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 versus nondiabetic animals. PI(3,4,5)P(3) infusion eliminated only the difference in voltage dependence of steady-state inactivation of I(Ca,L).
Conclusions: Decreased PI 3-kinase signaling in myocytes from type 1 diabetic mice leads to reduced Ca(2+) entry through the LTCC, which might contribute to the negative effect of diabetes on cardiac contractility.