Impairment of glucose metabolism and energy transfer in the rat heart.Mol Cell Biochem. 2003;249:157-165. [PMID: 12956411 DOI: 10.1007/978-1-4419-9236-9_20]

Oxidative stress in early metabolic syndrome impairs cardiac RyR2 and SERCA2a activity and modifies the interplay of these proteins during Ca2+ waves

We investigated how oxidative stress (OS) alters Ca2+ handling in ventricular myocytes in early metabolic syndrome (MetS) in sucrose-fed rats. The effects of N-acetyl cysteine (NAC) or dl-Dithiothreitol (DTT) on systolic Ca2+ transients (SCaTs), diastolic Ca2+ sparks (CaS) and Ca2+ waves (CaW), recorded by confocal techniques, and L-type Ca2+ current (ICa), assessed by whole-cell patch clamp, were evaluated in MetS and Control cells. MetS myocytes exhibited decreased SCaTs and CaS frequency but unaffected CaW propagation. In Control cells, NAC/DTT reduced RyR2/SERCA2a activity blunting SCaTs, CaS frequency and CaW propagation, suggesting that basal ROS optimised Ca2+ signalling by maintaining RyR2/SERCA2a function and that these proteins facilitate CaW propagation. Conversely, NAC/DTT in MetS recovered RyR2/SERCA2a function, improving SCaTs and CaS frequency, but unexpectedly decreasing CaW propagation. We hypothesised that OS decreases RyR2/SERCA2a activity at early MetS, and while decreased SERCA2a favours CaW propagation, diminished RyR2 restrains it.

Tale of two kinases: Protein kinase A and Ca2+/calmodulin-dependent protein kinase II in pre-diabetic cardiomyopathy

Metabolic syndrome is a pre-diabetic state characterized by several biochemical and physiological alterations, including insulin resistance, visceral fat accumulation, and dyslipidemias, which increase the risk for developing cardiovascular disease. Metabolic syndrome is associated with augmented sympathetic tone, which could account for the etiology of pre-diabetic cardiomyopathy. This review summarizes the current knowledge of the pathophysiological consequences of enhanced and sustained β-adrenergic response in pre-diabetes, focusing on cardiac dysfunction reported in diet-induced experimental models of pre-diabetic cardiomyopathy. The research reviewed indicates that both protein kinase A and Ca²⁺/calmodulin-dependent protein kinase II play important roles in functional responses mediated by β₁-adrenoceptors; therefore, alterations in the expression or function of these kinases can be deleterious. This review also outlines recent information on the role of protein kinase A and Ca²⁺/calmodulin-dependent protein kinase II in abnormal Ca²⁺ handling by cardiomyocytes from diet-induced models of pre-diabetic cardiomyopathy.

Impaired Activity of Ryanodine Receptors Contributes to Calcium Mishandling in Cardiomyocytes of Metabolic Syndrome Rats

Metabolic syndrome (MetS) has become a global epidemic. MetS is a serious health problem because of its related cardiovascular complications, which include hypertension and delayed heart rate recovery after exercise. The molecular bases of cardiac dysfunction in MetS are still under scrutiny and may be related to anomalies in the activity and expression of key proteins involved in the cardiac excitation-contraction coupling (ECC). The cardiac Ca²⁺ channel/ryanodine receptor (RyR2) participates in releasing Ca²⁺ from internal stores and plays a key role in the modulation of ECC. We examined alterations in expression, phosphorylation status, Ca²⁺ sensitivity, and in situ function (by measuring Ca²⁺ sparks and Ca²⁺ transients) of RyR2; alterations in these characteristics could help to explain the Ca²⁺ handling disturbances in MetS cardiomyocytes. MetS was induced in rats by adding commercially refined sugar (30% sucrose) to their drinking water for 24 weeks. Cardiomyocytes of MetS rats displayed decreased Ca²⁺ transient amplitude and cell contractility at all stimulation frequencies. Quiescent MetS cardiomyocytes showed a decrease in Ca²⁺ spark frequency, amplitude, and spark-mediated Ca²⁺ leak. The [³H]-ryanodine binding data showed that functionally active RyRs are significantly diminished in MetS heart microsomes; and exhibited rapid Ca²⁺-induced inactivation. The phosphorylation of corresponding Ser2814 (a preferential target for CaMKII) of the hRyR2 was significantly diminished. RyR2 protein expression and Ser2808 phosphorylation level were both unchanged. Further, we demonstrated that cardiomyocyte Ca²⁺ mishandling was associated with reduced SERCA pump activity due to decreased Thr17-PLN phosphorylation, suggesting a downregulation of CaMKII in MetS hearts, though the SR Ca²⁺ load remained unchanged. The reduction in the phosphorylation level of RyR2 at Ser2814 decreases RyR2 availability for activation during ECC. In conclusion, the impaired in situ activity of RyR2 may also account for the poor overall cardiac outcome reported in MetS patients; hence, the SERCA pump and RyR2 are both attractive potential targets for future therapies.

Effects of palmitate on Ca(2+) handling in adult control and ob/ob cardiomyocytes: impact of mitochondrial reactive oxygen species

Obesity and insulin resistance are associated with enhanced fatty acid utilization, which may play a central role in diabetic cardiomyopathy. We now assess the effect of the saturated fatty acid palmitate (1.2 mmol/l) on Ca(2+) handling, cell shortening, and mitochondrial production of reactive oxygen species (ROS) in freshly isolated ventricular cardiomyocytes from normal (wild-type) and obese, insulin-resistant ob/ob mice. Cardiomyocytes were electrically stimulated at 1 Hz, and the signal of fluorescent indicators was measured with confocal microscopy. Palmitate decreased the amplitude of cytosolic Ca(2+) transients (measured with fluo-3), the sarcoplasmic reticulum Ca(2+) load, and cell shortening by approximately 20% in wild-type cardiomyocytes; these decreases were prevented by the general antioxidant N-acetylcysteine. In contrast, palmitate accelerated Ca(2+) transients and increased cell shortening in ob/ob cardiomyocytes. Application of palmitate rapidly dissipated the mitochondrial membrane potential (measured with tetra-methyl rhodamine-ethyl ester) and increased the mitochondrial ROS production (measured with MitoSOX Red) in wild-type but not in ob/ob cardiomyocytes. In conclusion, increased saturated fatty acid levels impair cellular Ca(2+) handling and contraction in a ROS-dependent manner in normal cardiomyocytes. Conversely, high fatty acid levels may be vital to sustain cardiac Ca(2+) handling and contraction in obesity and insulin-resistant conditions.