Sanda Despa

Calcium (Ca²⁺) signaling regulates a wide range of processes in the heart, from contractility and excitability to energy supply and cell growth. Consequently, Ca²⁺ signaling plays a critical role in cardiac adaptation to both physiological and pathophysiological stress. This review examines the role of Ca²⁺ signaling in the heart’s physiological adaptation to pregnancy and its pathological maladaptation in diabetes. We focus on Ca²⁺-dependent mechanisms involved in hypertrophy, energy imbalance, and electrical remodeling in these two conditions, highlighting shared signaling pathways, functional outcomes, and key knowledge gaps. A deeper understanding of these mechanisms could reveal novel therapeutic targets to improve cardiac health in pregnancy and diabetes.

Aims/hypothesis Women with prior gestational diabetes mellitus (GDM) have higher incidence of age-associated diseases, including type 2 diabetes, CVD and cognitive impairment. Human studies cannot readily determine whether GDM causes these conditions or the underlying mechanisms. Here we used a well-validated rat model of GDM to address these questions. Methods Rats with beta cell-specific expression of human amylin, a pancreatic hormone, were used as a GDM model. Five-month-old female rats were randomly assigned to no-pregnancy, one-pregnancy and two-pregnancies experimental groups. GTTs and transthoracic echocardiography were performed at baseline and during the postpartum period. At 18 months of age, the novel object recognition test was administered, followed by euthanasia and organ collection. Results All female rats developed glucose intolerance and showed cardiac remodelling and impaired left ventricular relaxation with ageing. Glucose intolerance was exacerbated in rats with prior GDM pregnancies compared with nulliparous rats, with significant differences starting at 9 months of age. However, blood glucose levels were comparable in the three groups during the course of the study. Rats with two GDM-complicated pregnancies had increased left ventricular mass compared with the other groups following the second pregnancy and until the end of the study. At 18 months of age, rats with prior GDM pregnancies presented aggravated demyelination, particularly in the hippocampus and mid-brain region, oxidative stress and neuroinflammation, and had a lower recognition index in the novel object recognition test compared with nulliparous rats. Higher parity exacerbated these effects. Shorter telomeres and reduced mitochondrial DNA content, two hallmarks of biological ageing, were found in the brain, heart and pancreas of rats with prior GDM. Conclusions/interpretation These findings support the concept that GDM is a sex-specific risk factor for ageing-related diseases, and point to accelerated cellular ageing as a contributing mechanism. Data availability Cardiac echocardiography and GTT data are available at Dataverse under the identifier https://doi.org/10.7910/DVN/R2HITG Graphical Abstract

Background: Mitochondrial dysfunction contributes to the cardiac remodeling triggered by type 2 diabetes (T2D). Mitochondrial Ca ²⁺ concentration ([Ca ²⁺ ] _m ) modulates the oxidative state and cytosolic Ca ²⁺ regulation. Thus, we investigated how T2D affects mitochondrial Ca ²⁺ fluxes, the downstream consequences on myocyte function, and the effects of normalizing mitochondrial Ca ²⁺ transport. Methods and Results: We compared myocytes/hearts from transgenic rats with late‐onset T2D (rats that develop late‐onset T2D due to heterozygous expression of human amylin in the pancreatic β‐cells [HIP] model) and their nondiabetic wild‐type (WT) littermates. [Ca ²⁺ ] _m was significantly lower in myocytes from diabetic HIP rats compared with WT cells. Ca ²⁺ extrusion through the mitochondrial Na ⁺ /Ca ²⁺ exchanger (mitoNCX) was elevated in HIP versus WT myocytes, particularly at moderate and high [Ca ²⁺ ] _m , while mitochondrial Ca ²⁺ uptake was diminished. Mitochondrial Na ⁺ concentration was comparable in WT and HIP rat myocytes and remained remarkably stable while manipulating mitoNCX activity. Lower [Ca ²⁺ ] _m was associated with oxidative stress, increased sarcoplasmic reticulum Ca ²⁺ leak in the form of Ca ²⁺ sparks, and mitochondrial dysfunction in T2D hearts. MitoNCX inhibition with CGP‐37157 reduced oxidative stress, Ca ²⁺ spark frequency, and stress‐induced arrhythmias in HIP rat hearts while having no significant effect in WT rats. In contrast, activation of the mitochondrial Ca ²⁺ uniporter with SB‐202190 enhanced spontaneous sarcoplasmic reticulum Ca ²⁺ release and had no significant effect on arrhythmias in both WT and HIP rat hearts. Conclusions: [Ca ²⁺ ] _m is reduced in myocytes from rats with T2D due to a combination of exacerbated mitochondrial Ca ²⁺ extrusion through mitoNCX and impaired mitochondrial Ca ²⁺ uptake. Partial mitoNCX inhibition limits sarcoplasmic reticulum Ca ²⁺ leak and arrhythmias in T2D hearts, whereas mitochondrial Ca ²⁺ uniporter activation does not.

Population-based studies identified an association between a prior pregnancy complicated by gestational diabetes mellitus (GDM) and cardiac hypertrophy and dysfunction later in life. It is however unclear whether GDM initiates this phenotype and what are the underlying mechanisms. We addressed these questions by using female rats that express human amylin (HIP rats) as a GDM model and their wild-type (WT) littermates as the normal pregnancy model. Pregnant and two months postpartum HIP females had increased left-ventricular mass and wall thickness compared to non-pregnant HIP females, which indicates the presence of concentric hypertrophy. These parameters were unchanged in WT females during both pregnancy and postpartum periods. Hypertrophic Ca²⁺-dependent calcineurin/NFAT signaling was stimulated two months after giving birth in HIP females but not in the WT. In contrast, the CaMKII/HDAC hypertrophy pathway was active immediately after giving birth and returned to the baseline by two months postpartum in both WT and HIP females. Myocytes from two months postpartum HIP females exhibited slower Ca²⁺ transient relaxation and higher diastolic Ca²⁺ levels, which may explain calcineurin activation. No such effects occurred in the WT. These results suggest that a GDM-complicated pregnancy accelerates the development of pathological cardiac remodeling likely through activation of calcineurin/NFAT signaling.

By controlling the function of various sarcolemmal and mitochondrial ion transporters, intracellular Na⁺ concentration ([Na⁺]_i) regulates Ca²⁺ cycling, electrical activity, the matching of energy supply and demand, and oxidative stress in cardiac myocytes. Thus, maintenance of myocyte Na⁺ homeostasis is vital for preserving the electrical and contractile activity of the heart. [Na⁺]_i is set by the balance between the passive Na⁺ entry through numerous pathways and the pumping of Na⁺ out of the cell by the Na⁺/K⁺-ATPase. This equilibrium is perturbed in heart failure, resulting in higher [Na⁺]_i. More recent studies have revealed that [Na⁺]_i is also increased in myocytes from diabetic hearts. Elevated [Na⁺]_i causes oxidative stress and augments the sarcoplasmic reticulum Ca²⁺ leak, thus amplifying the risk for arrhythmias and promoting heart dysfunction. This mini-review compares and contrasts the alterations in Na⁺ extrusion and/or Na⁺ uptake that underlie the [Na⁺]_i increase in heart failure and diabetes, with a particular emphasis on the emerging role of Na⁺ - glucose cotransporters in the diabetic heart.

Loss-of-function mutations in the cytoskeletal protein ankyrin-B (AnkB) cause ventricular tachyarrhythmias in humans. Previously, we found that a larger fraction of the sarcoplasmic reticulum (SR) Ca ²⁺ leak occurs through Ca ²⁺ sparks in AnkB-deficient (AnkB ^+/− ) mice, which may contribute to arrhythmogenicity via Ca ²⁺ waves. Here, we investigated the mechanisms responsible for increased Ca ²⁺ spark frequency in AnkB ^+/− hearts. Using immunoblots and phospho-specific antibodies, we found that phosphorylation of ryanodine receptors (RyRs) by CaMKII is enhanced in AnkB ^+/− hearts. In contrast, the PKA-mediated RyR phosphorylation was comparable in AnkB ^+/− and wild-type (WT) mice. CaMKII inhibition greatly reduced Ca ²⁺ spark frequency in myocytes from AnkB ^+/− mice but had little effect in the WT. Global activities of the major phosphatases PP1 and PP2A were similar in AnkB ^+/− and WT hearts, while CaMKII autophosphorylation, a marker of CaMKII activation, was increased in AnkB ^+/− hearts. Thus, CaMKII-dependent RyR hyperphosphorylation in AnkB ^+/− hearts is caused by augmented CaMKII activity. Intriguingly, CaMKII activation is limited to the sarcolemma–SR junctions since non-junctional CaMKII targets (phospholamban, HDAC4) are not hyperphosphorylated in AnkB ^+/− myocytes. This local CaMKII activation may be the consequence of elevated [Ca ²⁺ ] in the junctional cleft caused by reduced Na ⁺ /Ca ²⁺ exchange activity. Indeed, using the RyR-targeted Ca ²⁺ sensor GCaMP2.2-FBKP12.6, we found that local junctional [Ca ²⁺ ] is significantly elevated in AnkB ^+/− myocytes. The increased incidence of pro-arrhythmogenic Ca ²⁺ sparks and waves in AnkB ^+/− hearts is due to enhanced CaMKII-mediated RyR phosphorylation, which is caused by higher junctional [Ca ²⁺ ] and consequent local CaMKII activation.

Background: Intracellular Na ⁺ concentration ([Na ⁺ ] _i ) regulates Ca ²⁺ cycling, contractility, metabolism, and electrical stability of the heart. [Na ⁺ ] _i is elevated in heart failure, leading to arrhythmias and oxidative stress. We hypothesized that myocyte [Na ⁺ ] _i is also increased in type 2 diabetes (T2D) due to enhanced activity of the Na ⁺ –glucose cotransporter. Methods and Results: To test this hypothesis, we used myocardial tissue from humans with T2D and a rat model of late‐onset T2D ( HIP rat). Western blot analysis showed increased Na ⁺ –glucose cotransporter expression in failing hearts from T2D patients compared with nondiabetic persons (by 73±13%) and in HIP rat hearts versus wild‐type ( WT ) littermates (by 61±8%). [Na ⁺ ] _i was elevated in HIP rat myocytes both at rest (14.7±0.9 versus 11.4±0.7 mmol/L in WT ) and during electrical stimulation (17.3±0.8 versus 15.0±0.7 mmol/L); however, the Na ⁺ /K ⁺ ‐pump function was similar in HIP and WT cells, suggesting that higher [Na ⁺ ] _i is due to enhanced Na ⁺ entry in diabetic hearts. Indeed, Na ⁺ influx was significantly larger in myocytes from HIP versus WT rats (1.77±0.11 versus 1.29±0.06 mmol/L per minute). Na ⁺ –glucose cotransporter inhibition with phlorizin or glucose‐free solution greatly reduced Na ⁺ influx in HIP myocytes (to 1.20±0.16 mmol/L per minute), whereas it had no effect in WT cells. Phlorizin also significantly decreased glucose uptake in HIP myocytes (by 33±9%) but not in WT , indicating an increased reliance on the Na ⁺ –glucose cotransporter for glucose uptake in T2D hearts. Conclusions: Myocyte Na ⁺ –glucose cotransport is enhanced in T2D, which increases Na ⁺ influx and causes Na ⁺ overload. Higher [Na ⁺ ] _i may contribute to arrhythmogenesis and oxidative stress in diabetic hearts.