Changes in T-Tubules and Sarcoplasmic Reticulum in Ventricular Myocytes in Early Cardiac Hypertrophy in a Pressure Overload Rat Model.

BACKGROUND/AIMS: Pressure-overload (PO) causes cardiac hypertrophy (CH), and eventually leads to heart failure (HF). HF ventricular myocytes present transverse-tubules (TT) loss or disarrangement and decreased sarcoplasmic reticulum (SR) density, and both contribute to altered Ca2+ signaling and heart dysfunction. It has been shown that TT remodeling precedes HF, however, it is unknown whether SR structural and functional remodeling also starts early in CH. METHODS: Using confocal microscopy, we assessed TT (with Di-8-ANNEPS) and SR (with SR-trapped Mag-Fluo-4) densities, as well as SR fluorophore diffusion (fluorescence recovery after photobleach; FRAP), cytosolic Ca2+ signaling and ex vivo cardiac performance in a PO rat hypertrophy model induced by abdominal aortic constriction (at 6 weeks). RESULTS: Rats developed CH, while cardiac performance, basal and upon ß-adrenergic stimulation, remained unaltered. TT density decreased by ∼14%, without spatial disarrangement, while SR density decreased by ∼7%. More important, FRAP was ∼30% slower, but with similar maximum recovery, suggesting decreased SR interconnectivity. Systolic and diastolic Ca2+ signaling and SR Ca2+ content were unaltered. CONCLUSION: SR remodeling is an early CH event, similar to TT remodeling, appearing during compensated hypertrophy. Nevertheless, myocytes can withstand those moderate structural changes in SR and TT, preserving normal Ca2+ signaling and contractility.

Enhanced oxidative stress sensitizes the mitochondrial permeability transition pore to opening in heart from Zucker Fa/fa rats with type 2 diabetes.

AIMS: Obesity and diabetes mellitus type 2 (DM2) frequently coexist and increase the propensity of cardiovascular dysfunction by numerous mechanisms. Chief among them are oxidative stress and Ca(2+) dysregulation, and both are inducers of the mitochondrial permeability transition pore (MPTP). Nevertheless, it is unknown whether MPTP formation is triggered in DM2 animals, and thereby contributing to cardiac dysfunction. We assessed MPTP sensitivity and reactive oxygen species production in cardiac mitochondria, as well as cytosolic Ca(2+) handling in ventricular myocytes from rats with DM2. MAIN METHODS: Male Zucker Fa/fa rats (Fa/fa) 32weeks old presenting DM2, concentric hypertrophy, and diastolic dysfunction were used. MPTP formation was evaluated in isolated mitochondria and Ca(2+) handling in ventricular myocytes, by spectrophotometric and confocal microscope techniques, respectively. KEY FINDINGS: We found that the systolic Ca(2+) transient relaxation was ~40% slower, while mitochondrial H2O2 production increased by ~6-fold. MPTP opening in isolated mitochondria from Fa/fa (mFa/fa) was more sensitive to Ca(2+) than in mitochondria from lean rats (mLean), and correlated with increased thiol group exposure. The mFa/fa showed decreased oxidative phosphorylation capacity. The ATP content decreased in myocytes, while the PCr/ATP ratio remained unchanged and caspase 9 activity largely increased in myocytes from Fa/fa animals. SIGNIFICANCE: Our results showed that oxidative stress and diastolic Ca(2+) dysregulation increased MPTP sensitivity leading to mitochondrial dysfunction and apoptosis. Mitochondrial dysfunction could compromise ATP synthesis, and lower ATP could be linked to decreased SERCA2 activity, which might underlie diastolic dysfunction. Prolonged Ca(2+) transients might further exacerbate mitochondrial dysfunction.