Alterations in cardiac function and subcellular membrane activities after hypervitaminosis D3

Targeting the mitochondrial Ca2+ uniporter complex in cardiovascular disease

Cardiovascular diseases (CVDs), the leading cause of death worldwide, share in common mitochondrial dysfunction, in specific a dysregulation of Ca²⁺ uptake dynamics through the mitochondrial Ca²⁺ uniporter (MCU) complex. In particular, Ca²⁺ uptake regulates the mitochondrial ATP production, mitochondrial dynamics, oxidative stress, and cell death. Therefore, modulating the activity of the MCU complex to regulate Ca²⁺ uptake, has been suggested as a potential therapeutic approach for the treatment of CVDs. Here, the role and implications of the MCU complex in CVDs are presented, followed by a review of the evidence for MCU complex modulation, genetically and pharmacologically. While most approaches have aimed within the MCU complex for the modulation of the Ca²⁺ pore channel, the MCU subunit, its intra- and extra- mitochondrial implications, including Ca²⁺ dynamics, oxidative stress, post-translational modifications, and its repercussions in the cardiac function, highlight that targeting the MCU complex has the translational potential for novel CVDs therapeutics.

Cardioprotection of ginsenoside Rb1 against ischemia/reperfusion injury is associated with mitochondrial permeability transition pore opening inhibition

OBJECTIVE

To investigate the role of ginsenoside Rb1 (Gs-Rb1) in cardioprotection against ischemia/reperfusion (I/R) or hypoxia/reoxygenation (H/R) injury and to explore whether the cardioprotective action is mediated via attenuating the formation of mitochondrial permeability transition pore (mPTP).

METHODS

A Langendorff-perfused model of rat heart was employed. I/R injury was induced by breaking off perfusion for 40 min then reperfusion for 60 min. Gs-Rb1 (100 μmol/L) were administrated for 10 min before I/R. Infarct size was estimated by the 2,3,5-triphenyl tetrazolium chloride (TTC) staining. Lactate dehydrogenase (LDH) and creatine kinase (CK) released from effluents were measured. Transmission electron microscopy was performed to assess morphological difference between cardiac mitochondrial isolated from I/R rats and Gs-Rb1 pretreated rats. Western blot analysis was used to determine phosphorylation of protein kinase B/Akt, and its downstream target glycogen synthase kinase 3β (GSK-3β). Incubation isolated cardiac mitochondria with Gs-Rb1, Ca2+-induced opening of the mPTP was assessed by the reduction of absorbance at 520 nm (A520). Neonatal rat cardiomyocytes were subjected to hypoxia 9 h followed by reoxygenation 4 h to induce H/R injury. After pretreated with different concentration of Gs-Rb1 (6.25, 25, 100 μmol/L ), cell viability was assessed by 3-(4,5-dimethylthiazol-2-yl)-2,5- diphenyltetrazolium bromide (MTT) method. The membrane potential was estimated by Rh123 fluorescence. mPTP opening was measured using the probe calcein-AM.

RESULTS

Gs-Rb1 100 μmol/L significantly reduced the infarct size of hearts (26.39%±11.67% vs. I/R group 56.68%±5.88%, P<0.01). Compared with the I/R group, Gs-Rb1 pretreatment decreased LDH and CK levels in the coronary effluent (P<0.05 or P<0.01) as well as attenuated destructive ultrastructure induced by I/R. The protective effect of Gs-Rb1 involved in phosphorylating protein kinase B/PKB (Akt) and GSK-3β. In mitochondria isolated from rat hearts, significant inhibition of Ca2+-induced swelling was observed in samples that were pretreated with Gs-Rb1 (6.25, 25, 100, 400 μmol/L) for 10 min. When cardiomyocytes were isolated from neonatal rat and subjected to H/R, cell viability was increased with treatment of Gs-Rb1 (6.25, 25, 100 μmol/L ). Gs-Rb1 inhibited mPTP opening and restored subsequent loss of mitochondrial membrane potential.

CONCLUSION

Gs-Rb1 presents cardioprotective effect against I/R or H/R injury which involves in activating Akt, phosphorylating GSK-3β and inhibiting mPTP opening.

Na+ overload-induced mitochondrial damage in the ischemic heart

Ischemia induces a decrease in myocardial contractility that may lead more or less to contractile dysfunction in the heart. When the duration of ischemia is relatively short, myocardial contractility is immediately reversed to control levels upon reperfusion. In contrast, reperfusion induces myocardial cell death when the heart is exposed to a prolonged period of ischemia. This phenomenon is the so-called "reperfusion injury". Numerous investigators have reported the mechanisms underlying myocardial reperfusion injury such as generation of free radicals, disturbance in the intracellular ion homeostasis, and lack of energy for contraction. Despite a variety of investigations concerning the mechanisms for ischemia and ischemia–reperfusion injury, ionic disturbances have been proposed to play an important role in the genesis of the ischemia–reperfusion injury. In this present study, we focused on the contribution of Na+ overload and mitochondrial dysfunction during ischemia to the genesis of this ischemia–reperfusion injury.Key words: mitochondria, myocardial ischemia, Na+ channels, Na+/H+ exchanger, Na+ overload.

Mitochondrial damage during ischemia determines post-ischemic contractile dysfunction in perfused rat heart

Possible mechanisms underlying sodium overload-induced ischemia/reperfusion injury in perfused rat hearts were examined. Massive accumulation of myocardial Na(+) occurred during ischemia, suggesting cytosolic sodium overload in cardiac cells. Treatment of the pre-ischemic heart with 0.3 micromol/l tetrodotoxin or 3 micromol/l ethyl-isopropyl amiloride enhanced post-ischemic contractile recovery (72 or 82% of initial vs 24% for untreated group), which was associated with suppression of tissue Na(+) accumulation (138 or 141% of initial vs 270% for untreated group), restoration of tissue high-energy phosphates, and preservation of the ability of mitochondria to produce ATP in the ischemic/reperfused heart. The release of cytochrome c from the ischemic heart was observed, which was blocked by treatment of the pre-ischemic heart with these agents. The improvement of post-ischemic contractile recovery by these agents was closely correlated with the ability of mitochondria to produce ATP during ischemia. To examine the effects of sodium overload on mitochondrial function, isolated mitochondria were incubated in the presence of various concentrations of Na(+). Na(+) induced mitochondrial membrane perturbations such as depolarization of the membrane potential, mitochondrial swelling, cytochrome c release from isolated mitochondria, and a reduction in oxidative phosphorylation. These events in the isolated mitochondria were not blocked by the presence of the above agents. The results suggest that cytosolic sodium overload in cardiac cells may induce deterioration of the mitochondrial function during ischemia and that this mitochondrial damage may determine post-ischemic contractile dysfunction in perfused rat hearts.

Protective effect of propranolol on mitochondrial function in the ischaemic heart

1. The present study was aimed to determine whether propranolol improves contractile function of the ischaemic/reperfused heart through protection of the mitochondrial function during ischaemia. 2. Isolated perfused rat hearts were subjected to 35-min ischaemia followed by 60-min reperfusion. Pre-treatment with propranolol at the concentrations of 10 to 100 microM for the final 3 min of pre-ischaemia resulted in the improvement of ischaemia/reperfusion-induced contractile dysfunction, release of creatine kinase (CK) into perfusate, and decrease in myocardial high-energy phosphates. Propranolol also attenuated ischaemia-induced accumulation in Na+, suggesting that cytosolic sodium overload during ischaemia was prevented by propranolol. 3. The mitochondrial oxygen consumption rate of skinned bundles from the perfused heart decreased at the end of ischaemia and it further decreased at the end of reperfusion. These decreases were cancelled by treatment with propranolol. A release of cytochrome c from the perfused heart was observed during ischaemia, and this release was suppressed by treatment with propranolol. 4. To elucidate the direct effect of propranolol on mitochondria, the mitochondria were isolated from normal hearts and their activities were determined in the presence of various concentrations of Na+ and propranolol. The addition of sodium lactate, which mimicked sodium overload in the ischaemic heart, reduced the state 3 respiration, whereas this reduction was not attenuated by the presence of propranolol. 5. These results suggest that cardioprotection of propranolol may be exerted via attenuating Na+ influx into cardiac cells followed by prevention of the mitochondrial dysfunction in the ischaemic heart, leading to improvement of energy production of the heart during reperfusion.

1,25-Dihydroxyvitamin D-stimulated calmodulin binding proteins: a sustained effect on distal tubules

The tubular localization of 1,25-dihydroxyvitamin D[1,25(OH)(2)D(3)]-stimulated calmodulin binding proteins (CaMBP-Ds) in the rat kidney and the specificity of their induction were characterized to better understand renal responses to protracted 1,25(OH)(2)D(3) treatment in vivo. None of the other hormones tested (parathyroid hormone, calcitonin, estradiol-17beta, testosterone, progesterone, hydrocortisone, or dexamethasone) stimulated the CaMBP-Ds, whereas maximal 1,25(OH)(2)D(3) stimulation occurred after a 5- to 7-day treatment with 100 ng/day 1,25(OH)(2)D(3). With the exception of the more ubiquitously distributed CaMBP-D150, the CaMBP-Ds were localized in distal, but not proximal, tubule preparations. 1,25(OH)(2)D(3) induction of vitamin D receptors and the CaMBP-Ds was similar with respect to dose-response and time course. Finally, the CaMBP-Ds remained elevated for at least 4 wk after 1,25(OH)(2)D(3) withdrawal. Because the vitamin D-stimulated renal CaMBP-Ds are principally proteins of the distal tubule, they may be associated with renal regulation of Ca(2+) homeostasis. The sustained induction of CaMBP-Ds is important in addressing the question of whether their induction is a function of normal Ca(2+) homeostasis or a pathophysiological consequence of hypervitaminosis D and hypercalcemia.

The amino bisphosphonate ibandronate prevents vitamin D toxicity and inhibits vitamin D-induced calcification of arteries, cartilage, lungs and kidneys in rats

Experiments were carried out to determine whether the doses of the amino bisphosphonate ibandronate that inhibit bone resorption inhibit soft tissue calcification and death in rats treated with a toxic dose of vitamin D. These studies were prompted by the recent discovery that ibandronate doses that inhibit bone resorption potently inhibit artery calcification induced by treatment with the vitamin K antagonist warfarin. All 16 rats treated with the toxic dose of vitamin D (12.5 mg cholecalciferol x kg(-1)) died by d 6 after the first vitamin D injection (median survival: 4.5 d), whereas the 12 rats treated with vitamin D plus ibandronate (0.25 mg x kg(-1) x d(-1)) were alive and in good health at d 10. Rats treated with vitamin D alone and examined at d 4 had extensive Alizarin red staining for calcification in the aorta, the carotid, hepatic, mesenteric, renal and femoral arteries, kidneys and lungs, whereas rats treated with vitamin D plus ibandronate had no evidence for calcification at any of these tissues when examined at d 7 and 10. Ibandronate treatment also inhibited the dramatic increase in the levels of calcium and phosphate seen in the abdominal aorta, kidneys, lungs and trachea of the vitamin D-treated rats (P < 0.001). Serum calcium levels were, however, not different in rats treated with vitamin D alone (3.4 +/- 0.2 mmol x L(-1)) and in rats treated with vitamin D plus ibandronate (3.5 +/- 0.2 mmol x L(-1)). Treatment with vitamin D alone increased levels of matrix Gla protein, an inhibitor of soft tissue calcification, in the arteries, kidneys, lungs and trachea by 10- to 100-fold, and ibandronate treatment prevented this increase. The importance of these studies in the rat model is that they identify a class of drugs in current clinical use that can be used to treat patients with vitamin D toxicity and that they identify the dose of the drug that is predicted to be effective, namely the dose that inhibits bone resorption. Because there is no other known treatment for vitamin D toxicity, there would seem to be good reason to try bisphosphonates such as ibandronate in future studies aimed at treating patients who have been exposed to toxic levels of vitamin D.