Endothelins in the microvasculature and heart in diabetes

Involvement of mitochondrial dynamics and mitophagy in diabetic endothelial dysfunction and cardiac microvascular injury

Endothelial cells (ECs), found in the innermost layer of blood vessels, are crucial for maintaining the structure and function of coronary microcirculation. Dysregulated coronary microcirculation poses a fundamental challenge in diabetes-related myocardial microvascular injury, impacting myocardial blood perfusion, thrombogenesis, and inflammation. Extensive research aims to understand the mechanistic connection and functional relationship between cardiac EC dysfunction and the development, diagnosis, and treatment of diabetes-related myocardial microvascular injury. Despite the low mitochondrial content in ECs, mitochondria act as sensors of environmental and cellular stress, influencing EC viability, structure, and function. Mitochondrial dynamics and mitophagy play a vital role in orchestrating mitochondrial responses to various stressors by regulating morphology, localization, and degradation. Impaired mitochondrial dynamics or reduced mitophagy is associated with EC dysfunction, serving as a potential molecular basis and promising therapeutic target for diabetes-related myocardial microvascular injury. This review introduces newly recognized mechanisms of damaged coronary microvasculature in diabetes-related microvascular injury and provides updated insights into the molecular aspects of mitochondrial dynamics and mitophagy. Additionally, novel targeted therapeutic approaches against diabetes-related microvascular injury or endothelial dysfunction, focusing on mitochondrial fission and mitophagy in endothelial cells, are summarized.

Improvement in cardiac function of diabetic rats by bosentan is not associated with changes in the activation of PKC isoforms

We previously demonstrated that chronic treatment with the mixed endothelin A and B (ET(A) and ET(B)) receptor blocker bosentan improved isolated working heart function in streptozotocin (STZ) diabetic rats. Endothelin-1 (ET-1) peptide levels, ET-1 mRNA and ET(A) and ET(B) receptor mRNA were all increased in diabetic hearts, but were unaffected by bosentan treatment, indicating that the beneficial effects of bosentan on heart appear to be on downstream effectors of ET-1 and ET receptors rather than the ET-1 system itself. Stimulation of ET-1 receptors leads to increased activation of protein kinase C (PKC), which is associated with PKC translocation from the cytosol to the membrane. Persistent activation of specific PKC isoforms has been proposed to contribute to diabetic cardiomyopathy. The purpose of this study was to determine whether chronic treatment with bosentan influences the activation of PKC isoforms in hearts from diabetic rats. Male Wistar rats were divided into four groups: control, bosentan-treated control, diabetic, and bosentan-treated diabetic. Diabetes was induced by the intravenous injection of 60 mg/kg streptozotocin. One week later, treatment with bosentan (100 mg/kg/day) by oral gavage was begun and continued for 10 weeks. The heart was then removed, homogenized, separated into soluble (cytosolic) and particulate (membrane) fractions and PKC isoform content in each fraction was determined by Western blotting. PKC alpha, beta2, delta, epsilon and zeta were all detected in hearts from both control and diabetic rats. However, no change in the levels or distribution between the soluble and particulate fractions of any of these isoforms could be detected in chronic diabetic hearts compared to control, whether untreated or treated with bosentan. These observations indicate that bosentan does not improve cardiac performance in STZ diabetic rats by affecting the activation of PKC isoforms.

Effect of chronic endothelin blockade on PKC isoform distribution in mesenteric arteries from diabetic rats

Hemodynamic changes, including increased vasoconstriction and reduced blood flow have been detected in both human diabetic patients and in animal models of diabetes. We previously demonstrated that the endothelin (ET) system was upregulated and involved in mediating the exaggerated vasoconstrictor responses in superior mesenteric artery (SMA) from diabetic rats. Chronic treatment of diabetic rats with the dual endothelin receptor antagonist, bosentan abolished the enhanced contractile responses in diabetic SMA. The biological actions of ET-1 have been shown to be coupled to the hydrolysis of phosphotidylinositol 4,5-biphosphate and phosphotidylcholine and the subsequent production of diacylglycerol (DAG). DAG is an activator of the classical and novel isoforms of PKC. Increases in PKC activity, associated with translocation of specific PKC isoforms from the cytosol to the membrane, have been implicated in the vasoconstrictor effect of ET-1. The goal of the present study was to determine whether chronic treatment of diabetic rats with bosentan influences the activation of specific PKC isoforms in SMA from diabetic rats. Elevated levels of PKCbeta2 in both the cytosol and membrane fractions and PKCepsilon in the membrane fraction were detected in SMA from diabetic rats. However, neither the levels nor the distribution between the cytosol and membrane fractions of any of these PKC isoforms were affected by the treatment of the diabetic rats with bosentan. These observations indicate that bosentan improves vascular reactivity in STZ-diabetic rats by mechanisms other than correction of increased activities of PKC isoforms.