Glucose-induced alterations of cytosolic free calcium in cultured rat tail artery vascular smooth muscle cells

Cellular and molecular effects of hyperglycemia on ion channels in vascular smooth muscle

Diabetes affects millions of people worldwide. This devastating disease dramatically increases the risk of developing cardiovascular disorders. A hallmark metabolic abnormality in diabetes is hyperglycemia, which contributes to the pathogenesis of cardiovascular complications. These cardiovascular complications are, at least in part, related to hyperglycemia-induced molecular and cellular changes in the cells making up blood vessels. Whereas the mechanisms mediating endothelial dysfunction during hyperglycemia have been extensively examined, much less is known about how hyperglycemia impacts vascular smooth muscle function. Vascular smooth muscle function is exquisitely regulated by many ion channels, including several members of the potassium (K+) channel superfamily and voltage-gated L-type Ca2+ channels. Modulation of vascular smooth muscle ion channels function by hyperglycemia is emerging as a key contributor to vascular dysfunction in diabetes. In this review, we summarize the current understanding of how diabetic hyperglycemia modulates the activity of these ion channels in vascular smooth muscle. We examine underlying mechanisms, general properties, and physiological relevance in the context of myogenic tone and vascular reactivity.

AKAP5 complex facilitates purinergic modulation of vascular L-type Ca2+ channel CaV1.2

The L-type Ca2+ channel CaV1.2 is essential for arterial myocyte excitability, gene expression and contraction. Elevations in extracellular glucose (hyperglycemia) potentiate vascular L-type Ca2+ channel via PKA, but the underlying mechanisms are unclear. Here, we find that cAMP synthesis in response to elevated glucose and the selective P2Y11 agonist NF546 is blocked by disruption of A-kinase anchoring protein 5 (AKAP5) function in arterial myocytes. Glucose and NF546-induced potentiation of L-type Ca2+ channels, vasoconstriction and decreased blood flow are prevented in AKAP5 null arterial myocytes/arteries. These responses are nucleated via the AKAP5-dependent clustering of P2Y11/ P2Y11-like receptors, AC5, PKA and CaV1.2 into nanocomplexes at the plasma membrane of human and mouse arterial myocytes. Hence, data reveal an AKAP5 signaling module that regulates L-type Ca2+ channel activity and vascular reactivity upon elevated glucose. This AKAP5-anchored nanocomplex may contribute to vascular complications during diabetic hyperglycemia.

Purinergic Signaling During Hyperglycemia in Vascular Smooth Muscle Cells

The activation of purinergic receptors by nucleotides and/or nucleosides plays an important role in the control of vascular function, including modulation of vascular smooth muscle excitability, and vascular reactivity. Accordingly, purinergic receptor actions, acting as either ion channels (P2X) or G protein-coupled receptors (GCPRs) (P1, P2Y), target diverse downstream effectors, and substrates to regulate vascular smooth muscle function and vascular reactivity. Both vasorelaxant and vasoconstrictive effects have been shown to be mediated by different purinergic receptors in a vascular bed- and species-specific manner. Purinergic signaling has been shown to play a key role in altering vascular smooth muscle excitability and vascular reactivity following acute and short-term elevations in extracellular glucose (e.g., hyperglycemia). Moreover, there is evidence that vascular smooth muscle excitability and vascular reactivity is severely impaired during diabetes and that this is mediated, at least in part, by activation of purinergic receptors. Thus, purinergic receptors present themselves as important candidates mediating vascular reactivity in hyperglycemia, with potentially important clinical and therapeutic potential. In this review, we provide a narrative summarizing our current understanding of the expression, function, and signaling of purinergic receptors specifically in vascular smooth muscle cells and discuss their role in vascular complications following hyperglycemia and diabetes.

Impaired Endothelium-Dependent Hyperpolarization Underlies Endothelial Dysfunction during Early Metabolic Challenge: Increased ROS Generation and Possible Interference with NO Function

Endothelial dysfunction is a hallmark of diabetic vasculopathies. Although hyperglycemia is believed to be the culprit causing endothelial damage, the mechanism underlying early endothelial insult in prediabetes remains obscure. We used a nonobese high-calorie (HC)-fed rat model with hyperinsulinemia, hypercholesterolemia, and delayed development of hyperglycemia to unravel this mechanism. Compared with aortic rings from control rats, HC-fed rat rings displayed attenuated acetylcholine-mediated relaxation. While sensitive to nitric oxide synthase (NOS) inhibition, aortic relaxation in HC-rat tissues was not affected by blocking the inward-rectifier potassium (Kir) channels using BaCl₂ Although Kir channel expression was reduced in HC-rat aorta, Kir expression, endothelium-dependent relaxation, and the BaCl₂-sensitive component improved in HC rats treated with atorvastatin to reduce serum cholesterol. Remarkably, HC tissues demonstrated increased reactive species (ROS) in smooth muscle cells, which was reversed in rats receiving atorvastatin. In vitro ROS reduction, with superoxide dismutase, improved endothelium-dependent relaxation in HC-rat tissues. Significantly, connexin-43 expression increased in HC aortic tissues, possibly allowing ROS movement into the endothelium and reduction of eNOS activity. In this context, gap junction blockade with 18-β-glycyrrhetinic acid reduced vascular tone in HC rat tissues but not in controls. This reduction was sensitive to NOS inhibition and SOD treatment, possibly as an outcome of reduced ROS influence, and emerged in BaCl₂-treated control tissues. In conclusion, our results suggest that early metabolic challenge leads to reduced Kir-mediated endothelium-dependent hyperpolarization, increased vascular ROS potentially impairing NO synthesis and highlight these channels as a possible target for early intervention with vascular dysfunction in metabolic disease. SIGNIFICANCE STATEMENT: The present study examines early endothelial dysfunction in metabolic disease. Our results suggest that reduced inward-rectifier potassium channel function underlies a defective endothelium-mediated relaxation possibly through alteration of nitric oxide synthase activity. This study provides a possible mechanism for the augmentation of relatively small changes in one endothelium-mediated relaxation pathway to affect overall endothelial response and highlights the potential role of inward-rectifier potassium channel function as a therapeutic target to treat vascular dysfunction early in the course of metabolic disease.

Added Sugar Intake is Associated with Blood Pressure in Older Females

Hypertension or high blood pressure (BP) is highly prevalent in the aging population. Notably, diet and lifestyle have a strong influence on BP. We investigated the association between dietary factors and BP in older adults. This cross-sectional study included 128 participants, aged 65–80 years. Multiple linear regressions were conducted to examine the associations between diet, including meats, vegetables, grains, fruits, dairy, fats, and added sugar, and BP. There was a significant association between intake of added sugar and systolic BP and diastolic BP in females after controlling for age, income, body mass index, physical activity levels, daily calorie intake, and BP medication use. The model predicted that a decrease of 2.3 teaspoons (0.5 standard deviation) of added sugar would result in a 8.4 mmHg drop in systolic BP and a 3.7 mmHg drop in diastolic BP. Whole fruit was associated with a reduction in diastolic BP in both males and females, and the model predicted that, for every 0.71 cup increase in whole fruit consumption, there would be a decrease in diastolic BP of 2.8 mmHg. Our findings support the dietary guidelines of limiting daily intake of added sugar and increasing fruit consumption to promote overall cardiovascular health in older adults.

Adenylyl cyclase 5-generated cAMP controls cerebral vascular reactivity during diabetic hyperglycemia

Elevated blood glucose (hyperglycemia) is a hallmark metabolic abnormality in diabetes. Hyperglycemia is associated with protein kinase A (PKA)-mediated stimulation of L-type Ca2+ channels in arterial myocytes resulting in increased vasoconstriction. However, the mechanisms by which glucose activates PKA remain unclear. Here, we showed that elevating extracellular glucose stimulates cAMP production in arterial myocytes, and that this was specifically dependent on adenylyl cyclase 5 (AC5) activity. Super-resolution imaging suggested nanometer proximity between subpopulations of AC5 and the L-type Ca2+ channel pore-forming subunit CaV1.2. In vitro, in silico, ex vivo and in vivo experiments revealed that this close association is critical for stimulation of L-type Ca2+ channels in arterial myocytes and increased myogenic tone upon acute hyperglycemia. This pathway supported the increase in L-type Ca2+ channel activity and myogenic tone in two animal models of diabetes. Our collective findings demonstrate a unique role for AC5 in PKA-dependent modulation of L-type Ca2+ channel activity and vascular reactivity during acute hyperglycemia and diabetes.

Ser1928 phosphorylation by PKA stimulates the L-type Ca2+ channel CaV1.2 and vasoconstriction during acute hyperglycemia and diabetes

Hypercontractility of arterial myocytes and enhanced vascular tone during diabetes are, in part, attributed to the effects of increased glucose (hyperglycemia) on L-type Ca_V1.2 channels. In murine arterial myocytes, kinase-dependent mechanisms mediate the increase in Ca_V1.2 activity in response to increased extracellular glucose. We identified a subpopulation of the Ca_V1.2 channel pore-forming subunit (α1_C) within nanometer proximity of protein kinase A (PKA) at the sarcolemma of murine and human arterial myocytes. This arrangement depended upon scaffolding of PKA by an A-kinase anchoring protein 150 (AKAP150) in mice. Glucose-mediated increases in Ca_V1.2 channel activity were associated with PKA activity, leading to α1_C phosphorylation at Ser¹⁹²⁸ Compared to arteries from low-fat diet (LFD)-fed mice and nondiabetic patients, arteries from high-fat diet (HFD)-fed mice and from diabetic patients had increased Ser¹⁹²⁸ phosphorylation and Ca_V1.2 activity. Arterial myocytes and arteries from mice lacking AKAP150 or expressing mutant AKAP150 unable to bind PKA did not exhibit increased Ser¹⁹²⁸ phosphorylation and Ca_V1.2 current density in response to increased glucose or to HFD. Consistent with a functional role for Ser¹⁹²⁸ phosphorylation, arterial myocytes and arteries from knockin mice expressing a Ca_V1.2 with Ser¹⁹²⁸ mutated to alanine (S1928A) lacked glucose-mediated increases in Ca_V1.2 activity and vasoconstriction. Furthermore, the HFD-induced increases in Ca_V1.2 current density and myogenic tone were prevented in S1928A knockin mice. These findings reveal an essential role for α1_C phosphorylation at Ser¹⁹²⁸ in stimulating Ca_V1.2 channel activity and vasoconstriction by AKAP-targeted PKA upon exposure to increased glucose and in diabetes.

Elevated Glucose Levels Promote Contractile and Cytoskeletal Gene Expression in Vascular Smooth Muscle via Rho/Protein Kinase C and Actin Polymerization

Both type 1 and type 2 diabetes are associated with increased risk of cardiovascular disease. This is in part attributed to the effects of hyperglycemia on vascular endothelial and smooth muscle cells, but the underlying mechanisms are not fully understood. In diabetic animal models, hyperglycemia results in hypercontractility of vascular smooth muscle possibly due to increased activation of Rho-kinase. The aim of the present study was to investigate the regulation of contractile smooth muscle markers by glucose and to determine the signaling pathways that are activated by hyperglycemia in smooth muscle cells. Microarray, quantitative PCR, and Western blot analyses revealed that both mRNA and protein expression of contractile smooth muscle markers were increased in isolated smooth muscle cells cultured under high compared with low glucose conditions. This effect was also observed in hyperglycemic Akita mice and in diabetic patients. Elevated glucose activated the protein kinase C and Rho/Rho-kinase signaling pathways and stimulated actin polymerization. Glucose-induced expression of contractile smooth muscle markers in cultured cells could be partially or completely repressed by inhibitors of advanced glycation end products, L-type calcium channels, protein kinase C, Rho-kinase, actin polymerization, and myocardin-related transcription factors. Furthermore, genetic ablation of the miR-143/145 cluster prevented the effects of glucose on smooth muscle marker expression. In conclusion, these data demonstrate a possible link between hyperglycemia and vascular disease states associated with smooth muscle contractility.

Distinct and complementary roles for α and β isoenzymes of PKC in mediating vasoconstrictor responses to acutely elevated glucose

BACKGROUND AND PURPOSE

We investigated the hypothesis that elevated glucose increases contractile responses in vascular smooth muscle and that this enhanced constriction occurs due to the glucose-induced PKC-dependent inhibition of voltage-gated potassium channels.

EXPERIMENTAL APPROACH

Patch-clamp electrophysiology in rat isolated mesenteric arterial myocytes was performed to investigate the glucose-induced inhibition of voltage-gated potassium (Kv ) current. To determine the effects of glucose in whole vessel, wire myography was performed in rat mesenteric, porcine coronary and human internal mammary arteries.

KEY RESULTS

Glucose-induced inhibition of Kv was PKC-dependent and could be pharmacologically dissected using PKC isoenzyme-specific inhibitors to reveal a PKCβ-dependent component of Kv inhibition dominating between 0 and 10 mM glucose with an additional PKCα-dependent component becoming evident at concentrations greater than 10 mM. These findings were supported using wire myography in all artery types used, where contractile responses to vessel depolarization and vasoconstrictors were enhanced by increasing bathing glucose concentration, again with evidence for distinct and complementary PKCα/PKCβ-mediated components.

CONCLUSIONS AND IMPLICATIONS

Our results provide compelling evidence that glucose-induced PKCα/PKCβ-mediated inhibition of Kv current in vascular smooth muscle causes an enhanced constrictor response. Inhibition of Kv current causes a significant depolarization of vascular myocytes leading to marked vasoconstriction. The PKC dependence of this enhanced constrictor response may present a potential therapeutic target for improving microvascular perfusion following percutaneous coronary intervention after myocardial infarction in hyperglycaemic patients.

Emerging roles of A-kinase anchoring proteins in cardiovascular pathophysiology

Heart and blood vessels ensure adequate perfusion of peripheral organs with blood and nutrients. Alteration of the homeostatic functions of the cardiovascular system can cause hypertension, atherosclerosis, and coronary artery disease leading to heart injury and failure. A-kinase anchoring proteins (AKAPs) constitute a family of scaffolding proteins that are crucially involved in modulating the function of the cardiovascular system both under physiological and pathological conditions. AKAPs assemble multifunctional signaling complexes that ensure correct targeting of the cAMP-dependent protein kinase (PKA) as well as other signaling enzymes to precise subcellular compartments. This allows local regulation of specific effector proteins that control the function of vascular and cardiac cells. This review will focus on recent advances illustrating the role of AKAPs in cardiovascular pathophysiology. The accent will be mainly placed on the molecular events linked to the control of vascular integrity and blood pressure as well as on the cardiac remodeling process associated with heart failure. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

In situ fiber-optical monitoring of cytosolic calcium in tissue explant cultures

We present a fluorescence-lifetime based method for monitoring cell and tissue activity in situ, during cell culturing and in the presence of a strong autofluorescence background. The miniature fiber-optic probes are easily incorporated in the tight space of a cell culture chamber or in an endoscope. As a first application we monitored the cytosolic calcium levels in porcine tracheal explant cultures using the Calcium Green-5N (CG5N) indicator. Despite the simplicity of the optical setup we are able to detect changes of calcium concentration as small as 2.5 nM, with a monitoring time resolution of less than 1 s.

Ca(2+) handling alterations and vascular dysfunction in diabetes

More than 65% of patients with diabetes mellitus die from cardiovascular disease or stroke. Hyperglycemia, due to either reduced insulin secretion or reduced insulin sensitivity, is the hallmark feature of diabetes mellitus. Vascular dysfunction is a distinctive phenotype found in both types of diabetes and could be responsible for the high incidence of stroke, heart attack, and organ damage in diabetic patients. In addition to well-documented endothelial dysfunction, Ca(2+) handling alterations in vascular smooth muscle cells (VSMCs) play a key role in the development and progression of vascular complications in diabetes. VSMCs provide not only structural integrity to the vessels but also control myogenic arterial tone and systemic blood pressure through global and local Ca(2+) signaling. The Ca(2+) signalosome of VSMCs is integrated by an extensive number of Ca(2+) handling proteins (i.e. channels, pumps, exchangers) and related signal transduction components, whose function is modulated by endothelial effectors. This review summarizes recent findings concerning alterations in endothelium and VSMC Ca(2+) signaling proteins that may contribute to the vascular dysfunction found in the diabetic condition.

Serum ionized magnesium in diabetic older persons

OBJECTIVE

Several alterations of magnesium metabolism have been associated with type 2 diabetes pathophysiology, a condition particularly frequent in older persons. We aimed to evaluate serum total (Mg-tot) and serum ionized magnesium (Mg-ion) in older persons with type 2 diabetes in order to explore clinically applicable methods for the detection of magnesium deficit.

MATERIAL/METHODS

Mg-tot and Mg-ion were measured in 105 fasting subjects with type 2 diabetes (mean age: 71.1±0.8 years; M/F: 45/60) and in 100 age-matched non-diabetic control persons (mean age: 72.2±0.8 years; M/F: 42/58).

RESULTS

Mg-ion concentrations were significantly lower in diabetic persons compared with controls (0.49±0.05 mmol/L vs. 0.55±0.05 mmol/L; p<0.001). Mg-tot was also slightly but significantly lower in diabetic patients (0.82±0.007 mmol/L vs. 0.84±0.006 mmol/L; p<0.05). There was an almost complete overlap in the values of Mg-tot in older diabetic patients and controls; conversely, 44.8% of diabetic patients had Mg-ion values below 0.47 mmol/L, while none of the controls did. After adjustment for age, sex, BMI, and triglycerides, Mg-tot was significantly associated with FBG in all the participants (p<0.05) and Mg-ion was significantly associated with FBG in all the participants (p<0.01) and with HbA1c in diabetic participants (p<0.001).

CONCLUSIONS

Alterations of magnesium serum concentrations are common in type 2 diabetic older adults; Mg-ion evaluation may help to identify subclinical magnesium depletion (i.e. in patients with normal Mg-tot); the close independent associations of Mg-tot and Mg-ion with FBG and with HbA1c reinforce the possible link between magnesium homeostasis and altered glucose metabolism.

AKAP150 contributes to enhanced vascular tone by facilitating large-conductance Ca2+-activated K+ channel remodeling in hyperglycemia and diabetes mellitus

RATIONALE

Increased contractility of arterial myocytes and enhanced vascular tone during hyperglycemia and diabetes mellitus may arise from impaired large-conductance Ca(2+)-activated K(+) (BKCa) channel function. The scaffolding protein A-kinase anchoring protein 150 (AKAP150) is a key regulator of calcineurin (CaN), a phosphatase known to modulate the expression of the regulatory BKCa β1 subunit. Whether AKAP150 mediates BKCa channel suppression during hyperglycemia and diabetes mellitus is unknown.

OBJECTIVE

To test the hypothesis that AKAP150-dependent CaN signaling mediates BKCa β1 downregulation and impaired vascular BKCa channel function during hyperglycemia and diabetes mellitus.

METHODS AND RESULTS

We found that AKAP150 is an important determinant of BKCa channel remodeling, CaN/nuclear factor of activated T-cells c3 (NFATc3) activation, and resistance artery constriction in hyperglycemic animals on high-fat diet. Genetic ablation of AKAP150 protected against these alterations, including augmented vasoconstriction. d-glucose-dependent suppression of BKCa channel β1 subunits required Ca(2+) influx via voltage-gated L-type Ca(2+) channels and mobilization of a CaN/NFATc3 signaling pathway. Remarkably, high-fat diet mice expressing a mutant AKAP150 unable to anchor CaN resisted activation of NFATc3 and downregulation of BKCa β1 subunits and attenuated high-fat diet-induced elevation in arterial blood pressure.

CONCLUSIONS

Our results support a model whereby subcellular anchoring of CaN by AKAP150 is a key molecular determinant of vascular BKCa channel remodeling, which contributes to vasoconstriction during diabetes mellitus.

Elevated Ca2+ sparklet activity during acute hyperglycemia and diabetes in cerebral arterial smooth muscle cells

Ca(+) sparklets are subcellular Ca(2+) signals produced by the opening of L-type Ca(2+) channels (LTCCs). In cerebral arterial myocytes, Ca(2+) sparklet activity varies regionally, resulting in low and high activity, "persistent" Ca(2+) sparklet sites. Although increased Ca(2+) influx via LTCCs in arterial myocytes has been implicated in the chain of events contributing to vascular dysfunction during acute hyperglycemia and diabetes, the mechanisms underlying these pathological changes remain unclear. Here, we tested the hypothesis that increased Ca(2+) sparklet activity contributes to higher Ca(2+) influx in cerebral artery smooth muscle during acute hyperglycemia and in an animal model of non-insulin-dependent, type 2 diabetes: the dB/dB mouse. Consistent with this hypothesis, acute elevation of extracellular glucose from 10 to 20 mM increased the density of low activity and persistent Ca(2+) sparklet sites as well as the amplitude of LTCC currents in wild-type cerebral arterial myocytes. Furthermore, Ca(2+) sparklet activity and LTCC currents were higher in dB/dB than in control myocytes. We found that activation of PKA contributed to higher Ca(2+) sparklet activity during hyperglycemia and diabetes. In addition, we found that the interaction between PKA and the scaffolding protein A-kinase anchoring protein was critical for the activation of persistent Ca(2+) sparklets by PKA in cerebral arterial myocytes after hyperglycemia. Accordingly, PKA inhibition equalized Ca(2+) sparklet activity between dB/dB and wild-type cells. These findings suggest that hyperglycemia increases Ca(2+) influx by increasing Ca(2+) sparklet activity via a PKA-dependent pathway in cerebral arterial myocytes and contributes to vascular dysfunction during diabetes.

Changes in expression and activity of the secretory pathway Ca2+ ATPase 1 (SPCA1) in A7r5 vascular smooth muscle cells cultured at different glucose concentrations

Diabetes mellitus-related vascular disease is often associated with both a dysregulation of Ca2+ homoeostasis and enhanced secretory activity in VSMCs (vascular smooth muscle cells). Here, we employ a commonly used rat cell line for VSMCs (A7r5 cells) to investigate the effects of glucose on the expression and activity of the SPCA1 (secretory pathway Ca2+-ATPase 1; also known as ATP2C1), which is a P-type Ca2+ pump located in the Golgi apparatus that plays a key role in the secretory pathway. Our results show that mRNA expression levels of SPCA1 are significantly increased in A7r5 cells cultured in high glucose (25.0 mM)-supplemented medium compared with normal glucose (5.55 mM)-supplemented medium. SPCA1 protein expression levels and thapsigargin-insensitive Ca2+-dependent ATPase activity were also consistent with a higher than normal expression level of SPCA1 in high-glucose-cultured A7r5 cells. Analysis of AVP (arginine-vasopressin)-induced cytosolic Ca2+ transients in A7r5 cells (after pre-treatment with thapsigargin) showed faster rise and decay phases in cells grown in high glucose medium compared with cells grown in normal glucose medium, supporting the observation of increased SPCA expression/activity. The significant levels of both Ca2+-ATPase activity and AVP-induced Ca2+ transients, in the presence of thapsigargin, indicate that SPCA must play a significant role in Ca2+ uptake within VSMCs. We therefore propose that, if such increases in SPCA expression and activity also occur in primary VSMCs, this may play a substantial role in the aetiology of diabetes mellitus-associated vascular disease, due to alterations in Ca2+ homoeostasis within the Golgi apparatus.

Relationship between hyperglycemia and coronary vascular resistance in non-diabetic patients

BACKGROUND

Hyperglycemia upon hospital admission in patients with acute myocardial infarction is associated with the no-reflow phenomenon after successful reperfusion, and increased mortality. However, the mechanism underlying this phenomenon remains unclear. Therefore, the aim of this study was to characterize coronary hemodynamics in a homogenous group of non-diabetic patients without coronary artery disease.

METHODS AND RESULTS

A total of 104 consecutive non-diabetic patients (mean age, 62+/-14 years) without coronary artery disease underwent Doppler flow study of the left anterior descending coronary artery. Vascular reactivity was examined by intra-coronary administration of papaverine, acetylcholine (Ach), and nitroglycerin using a Doppler guidewire. Coronary vascular resistance (CVR) was calculated as the mean arterial pressure divided by coronary blood flow (CBF). Baseline CVR was shown as CVR at control and minimal CVR was shown as CVR with papaverine administration. Fasting plasma glucose (FPG) level had a significant, positive correlation with baseline CVR and minimal CVR (r=0.24, p<0.02 and r=0.21, p<0.05, respectively). Hemoglobin A1c (HbA1c) also had a significant, positive correlation with baseline CVR and minimal CVR (r=0.31, p<0.01 and r=0.32, p<0.01, respectively). The percent change in CBF induced by Ach was inversely correlated with HbA1c but not with FPG (r=0.22, p<0.05 and r=0.06, p=0.57, respectively). By contrast, neither FPG nor HbA1c had significant correlation with coronary flow reserve to papaverine.

CONCLUSION

These data demonstrate that elevated glucose levels are associated with increases in baseline and minimal coronary vascular resistance. These changes may contribute to unfavorable coronary hemodynamics in non-diabetic patients without coronary heart disease.

Electron paramagnetic resonance investigation on modulatory effect of benidipine on membrane fluidity of erythrocytes in essential hypertension

It has been shown that benidipine, a long-lasting calcium (Ca) channel blocker, may exert its protective effect against vascular disorders by increasing nitric oxide (NO) production. The purpose of the present study was to investigate whether orally administered benidipine might influence the membrane function in patients with essential hypertension. We measured the membrane fluidity of erythrocytes by using an electron paramagnetic resonance (EPR) and spin-labeling method. In the preliminary study using erythrocytes obtained from healthy volunteers, benidipine decreased the order parameter (S) for 5-nitroxide stearate (5-NS) and the peak height ratio (ho/h-1) for 16-NS in the EPR spectra in vitro. The finding indicated that benidipine increased the membrane fluidity and improved the microviscosity of erythrocytes. In addition, it was demonstrated that the effect of benidipine on membrane fluidity of erythrocytes was significantly potentiated by the NO-substrate, L-arginine. In the separate series of the study, we observed that orally administered benidipine for 4 weeks significantly increased the membrane fluidity of erythrocytes with a concomitant increase in plasma NO metabolite levels in hypertensive subjects. The results of the present study demonstrated that benidipine might increase the membrane fluidity and improve the microviscosity of erythrocytes both in vitro and in vivo, to some extent, by the NO-dependent mechanism. Furthermore, it is strongly suggested that orally administered benidipine might have a beneficial effect on the rheologic behavior of erythrocytes and the improvement of the microcirculation in hypertensive subjects.

Magnesium metabolism in hypertension and type 2 diabetes mellitus

The increasing evidence for the clinical relevance of altered magnesium metabolism to states of altered insulin resistance confirms the role of magnesium deficit as a possible underlying common mechanism of the "insulin resistance" of hypertension and altered glucose tolerance. The pioneer work of Lawrence M. Resnick and his group using the cellular ion-based approach that we are only partially presenting here has consistently contributed to the progress of the field, demonstrating (a) the critical importance of magnesium metabolism in regulating insulin sensitivity as well as vascular tone, and blood-pressure homeostasis; (b) that magnesium deficiency, defined on the basis of intracellular free magnesium levels, and or serum ionized magnesium is a common feature of both diabetic and hypertensive states as well as various other cardiovascular and metabolic processes and aging; (c) the ability of environmental factors such as dietary nutrient-sugar and mineral content to alter the set point of steady-state cell ion activity; and (d) that magnesium supplementation is indicated in conditions associated with magnesium deficit although well-designed therapeutic trials of magnesium in essential hypertension and type 2 diabetes mellitus are needed in the near future.

Magnesium metabolism in type 2 diabetes mellitus, metabolic syndrome and insulin resistance

Type 2 diabetes is characterized by cellular and extracellular Mg depletion. Epidemiologic studies showed a high prevalence of hypomagnesaemia and lower intracellular Mg concentrations in diabetic subjects. Insulin and glucose are important regulators of Mg metabolism. Intracellular Mg plays a key role in regulating insulin action, insulin-mediated-glucose uptake and vascular tone. Reduced intracellular Mg concentrations result in a defective tyrosine-kinase activity, post-receptorial impairment in insulin action, and worsening of insulin resistance in diabetic patients. Mg deficit has been proposed as a possible underlying common mechanism of the "insulin resistance" of different metabolic conditions. Low dietary Mg intake is also related to the development of type 2 diabetes. Benefits of Mg supplementation on metabolic profile in diabetic subjects have been found in most, but not all clinical studies, and larger prospective studies are needed to support the potential role of dietary Mg supplementation as a possible public health strategy in diabetes risk.

High glucose activates nuclear factor of activated T cells in native vascular smooth muscle

OBJECTIVE

Hyperglycemia has been suggested to play a role in the development of vascular disease associated with diabetes. Atypical Ca2+ signaling and gene expression are characteristic of vascular dysfunction; however, little is known regarding the effects of high glucose on Ca2+-dependent transcription in the vascular wall.

METHODS AND RESULTS

Using confocal immunofluorescence, we show that modest elevation of extracellular glucose (ie, from 2 to 11.5 mmol/L) increased [Ca2+]i, leading to nuclear accumulation of nuclear factor of activated T cells (NFAT) in intact cerebral arteries from mouse. This was accompanied by increased NFAT-dependent transcriptional activity. Both the increase in Ca2+ and NFAT activation were prevented by the ectonucleotidase apyrase, suggesting a mechanism involving the release of extracellular nucleotides. We provide evidence that the potent vasoconstrictors and growth stimulators UTP and UDP mediate glucose-induced NFAT activation via P2Y receptors. NFAT nuclear accumulation was inhibited by the voltage-dependent Ca2+ channel blockers verapamil and nifedipine, the calcineurin inhibitor cyclosporine A, and the novel NFAT blocker A-285222. High glucose also regulated glycogen synthase kinase 3beta and c-Jun N-terminal kinase activity, yielding decreased kinase activity and reduced export of NFAT from the nucleus, providing additional mechanisms underlying the glucose-induced NFAT activation.

CONCLUSIONS

Our results identify the calcineurin/NFAT signaling pathway as a potential metabolic sensor for the arterial smooth muscle response to high glucose.

Streptozotocin-induced diabetes impairs Mg2+ homeostasis and uptake in rat liver cells

Male Sprague-Dawley rats rendered diabetic by streptozotocin injection presented 10 and 20% decreases in total hepatic Mg2+ content at 4 and 8 wk, respectively, following diabetes onset. This decrease was associated with a parallel decrease in K+ and ATP content and an increase in Na+ level. In diabetic liver cells, the Mg2+ extrusion elicited by alpha1-adrenoceptor stimulation was markedly reduced compared with nondiabetic livers, whereas that induced by beta-adrenoceptor stimulation was unaffected. In addition, diabetic hepatocytes did not accumulate Mg2+ following stimulation of protein kinase C pathway by vasopressin, diacylglycerol analogs, or phorbol 12-myristate 13-acetate derivates despite the reduced basal content in cellular Mg2+. Experiments performed in purified plasma membrane from diabetic livers located the defect at the level of the bidirectional Na+/Mg2+ exchanger operating in the basolateral domain of the hepatocyte cell membrane, which could extrude but not accumulate Mg2+ in exchange for Na+. The impairment of Mg2+ uptake mechanism, in addition to the decrease in cellular ATP level, can contribute to explaining the decrease in liver Mg2+ content observed under diabetic conditions.

Role of nitric oxide, tetrahydrobiopterin and peroxynitrite in glucose toxicity-associated contractile dysfunction in ventricular myocytes

AIMS/HYPOTHESIS

Local overproduction of nitric oxide is seen in early stages of diabetes, which can react with superoxide (O(2)(-)) to form peroxynitrite (ONOO(-)). The aim of this study was to examine the effect of scavengers for nitric oxide, O(2)(-), ONOO(-) and NOS cofactor tetrahydrobiopterin (BH(4)) on high glucose-induced cardiac contractile dysfunction.

METHODS

Ventricular myocytes were cultured for 24 h with either normal (N, 5.5 mmol/l) or high (25.5 mmol/l) glucose, with or without the nitric oxide scavengers haemoglobin (100 nmol/l), PTIO (100 micromol/l), the NOS inhibitor L-NMMA (100 micromol/l), superoxide dismutase (SOD, 500 U/ml), the ONOO(-) scavengers urate (100 micromol/l), MnTABP (100 micromol/l), BH(4) (10 micromol/l) and its inactive analogue NH(4) (10 micromol/l), and the GTP cyclohydrolase I inhibitor DAHP (1 mmol/l). Myocyte mechanics, NOS protein expression and activity were evaluated.

RESULTS

High glucose myocytes showed reduced peak shortening, decreased maximal velocity of shortening/relengthening (+/- dL/dt), prolonged relengthening (TR(90)) and normal shortening duration (TPS) associated with reduced cytosolic Ca(2+) rise compared to normal myocytes. The high glucose-induced abnormalities were abrogated or attenuated by urate, MnTBAP, L-NMMA, BH(4), and SOD, whereas unaffected by haemoglobin, PTIO and NH(4). L-NMMA reduced peak shortening while PTIO and DAHP depressed +/- dL/dt and prolonged TPS or TR(90) in normal myocytes. High glucose increased NOS activity, protein expression of eNOS but not iNOS, which were attenuated by L-NMMA and BH(4), respectively.

CONCLUSION/INTERPRETATION

These results suggested that NOS cofactor, NO and ONOO(-) play a role in glucose-induced cardiomyocyte contractile dysfunction and in the pathogenesis of diabetic cardiomyopathy.

Glucose-induced alterations of intracellular ionized magnesium in human lymphocytes

The intracellular ionic content of human erythrocytes may be altered by hyperglycaemia. Despite this, very little is known about the cellular mechanisms linking glucose and cellular magnesium homeostasis. We measured intracellular ionized magnesium in human lymphocytes, by means of a fluorimetric technique, total intracellular magnesium by means of atomic absorption spectrophotometry and intracellular ATP by means of HPLC. The incubation of lymphocytes with D-glucose in the absence of insulin was followed by a significant decrease in intracellular ionized magnesium; this effect did not occur when the cells were incubated with L-glucose. The effect of glucose on intracellular ionized magnesium was blocked by amphotericin B and the EC(50) of the effect of glucose on intracellular ionized magnesium was about 5 mmol/l of glucose. The increase of intracellular ionized magnesium in cells incubated in the absence of glucose was followed by a decrease in intracellular ATP. In a Na(+)-free medium the decrease of intracellular ionized magnesium in the presence of glucose was still present and the incubation of lymphocytes with glucose did not modify total intralymphocyte magnesium. By selective permeabilization of cell membranes, we established that glucose could not increase compartmentalized intracellular ionized magnesium. Our data supports the hypothesis that glucose per se induces a substantial decrease in intracellular ionized magnesium, which is probably due to an augmented binding of intracellular ionized magnesium to cellular ATP.

alpha2-Heremans Schmid glycoprotein, a putative inhibitor of tyrosine kinase, prevents glucose toxicity associated with cardiomyocyte dysfunction

BACKGROUND

Diabetes leads to impaired glucose metabolism and insulin signaling in the heart, which may contribute to the development of diabetic cardiomyopathy. Insulin stimulates tyrosine phosphorylation of the insulin receptor and insulin receptor substrates. A two-fold increase in insulin-stimulated tyrosine phosphorylation has been reported in diabetic myocardium. The aim of the present study was to examine the effect of a putative inhibitor of tyrosine kinase phosphorylation, alpha(2)-Heremans Schmid glycoprotein (AHSG), on the mechanical dysfunction under a simulated diabetic environment.

METHODS

Isolated ventricular myocytes from adult rats were maintained for 24 h in either normal glucose (NG, 5.5 mM) or high glucose (HG, 25.5 mM) medium with 10(-7) M insulin, and in the absence or presence of AHSG (50 micro g/ml). Contractile indices analyzed included: peak shortening (PS), time-to-PS (TPS), time-to-90% relengthening (TR(90)) and area underneath shortening and relengthening (Area/PS).

RESULTS

Myocytes maintained in HG medium displayed reduced PS and prolonged TPS/TR(90), with enhanced area, compared to the NG myocytes. Interestingly, these HG-induced mechanical dysfunctions were abolished by AHSG. Removal of insulin from the culture medium did not affect the basal myocyte mechanics, but prevented AHSG from completely protecting against the HG-induced mechanical defects (i.e. HG-induced prolongation of TR(90) and area were only partially attenuated by AHSG in the absence of insulin).

CONCLUSIONS

The present data support the notion of tyrosine phosphorylation in the pathogenesis of diabetic cardiomyopathy, and implicate the therapeutic value of tyrosine kinase phosphorylation inhibitors.

Altered cellular magnesium responsiveness to hyperglycemia in hypertensive subjects

Previous studies by our group have identified ionic aspects of insulin resistance in hypertension, in which cellular responses to insulin were influenced by the basal intracellular ionic environment-the lower the cytosolic free magnesium (Mg(i)), the less Mg(i) increased following insulin stimulation. To investigate whether this ionic insulin resistance represents a more general abnormality of cellular responsiveness in hypertension, we studied Mg(i) responses to nonhormonal signals such as hyperglycemia (15 mmol/L) and used (31)P-nuclear magnetic resonance (NMR) spectroscopy to measure Mg(i) in erythrocytes from normal (NL, n=14) and hypertensive (HTN, n=12) subjects before and 30, 60, 120, and 180 minutes after in vitro glucose incubations. Basal Mg(i) levels were significantly lower in HTN subjects than in NL subjects (169+/-10 versus 205+/-8 micromol.L(-1), P<0.01). In NL cells, hyperglycemia significantly lowered Mg(i), from 205+/-8 micromol.L(-1) (basal, T=0) to 181+/-8, 162+/-6, 152+/-7, and 175+/-9 micromol.L(-1) (T=30, 60, 120, and 180, respectively; P<0.005 versus T=0 at all times). In HTN cells, maximal Mg(i) responses to hyperglycemia were blunted, from 169+/-10 micromol.L(-1) (basal, T=0) to 170+/-11, 179+/-12, 181+/-14, and 173+/-15 micromol.L(-1) (T=30, 60, 120, and 180, respectively; P=NS versus T=0 at all times). For all subjects, Mg(i) responses to hyperglycemia were closely related to basal Mg(i) levels: the higher the Mg(i), the greater the response (n=26, r=0.620, P<0.001). Thus, (1) erythrocytes from hypertensive vis-à-vis normotensive subjects are resistant to the ionic effects of extracellular hyperglycemia on Mg(i) levels, and (2) cellular ionic responses to glucose depend on the basal Mg(i) environment. Altogether, these data support a role for altered extracellular glucose levels in regulating cellular magnesium metabolism and also suggest the importance of ionic factors in determining cellular responsiveness to nonhormonal as well as hormonal signals.

Platelet hyperactivity and abnormal Ca(2+) homeostasis in diabetes mellitus

We sought to determine the mechanisms for hyperactivity and abnormal platelet Ca(2+) homeostasis in diabetes. The glycosylated Hb (HbA(1c)) level was used as an index of glycemic control. Human platelets were loaded with Ca- green-fura red, and cytosolic Ca(2+) ([Ca(2+)](i)) and aggregation were simultaneously measured. In the first series of experiments, the platelets from diabetic and normal subjects were compared for the ability to release Ca(2+) or to promote Ca(2+) influx. A potent and relatively specific inhibitor of Na(+)/Ca(2+) exchange, 5-(4-chlorobenzyl)-2',4'-dimethylbenzamil (CB-DMB), increased the second phase of thrombin-induced Ca(2+) response, suggesting that the Na(+)/Ca(2+) exchanger works in the forward mode to mediate Ca(2+) efflux. In contrast, in the platelets from diabetics, CB-DMB decreased the Ca(2+) response, indicating that the Na(+)/Ca(2+) exchanger works in the reverse mode to mediate Ca(2+) influx. In the second series of experiments we evaluated the direct effect of hyperglycemia on platelets in vitro. We found that thrombin- and collagen-induced increases in [Ca(2+)](i) and aggregation were not acutely affected by high glucose concentrations of 45 mM. However, when the platelet-rich plasma was incubated with a high glucose concentration at 37 degrees C for 24 h, the second phase after thrombin activation was inhibited by CB-DMB. In addition, collagen-stimulated [Ca(2+)](i) response and aggregation were also increased. Thus in diabetes the direction and activity of the Na(+)/Ca(2+) exchanger is changed, which may be one of the mechanisms for the increased platelet [Ca(2+)](i) and hyperactivity. Prolonged hyperglycemia in vitro can induce similar changes, suggesting hyperglycemia per se may be the factor responsible for the platelet hyperactivity in diabetes.

Regulation of B(2)-kinin receptors by glucose in vascular smooth muscle cells

The development of vascular disease is accelerated in hyperglycemic states. Vascular injury plays a pivotal role in the progression of atherosclerotic vascular disease in diabetes, which is characterized by increased vascular smooth muscle cell (VSMC) proliferation and extracellular matrix accumulation. We previously reported that diabetes alters the activity of the kallikrein-kinin system and results in the upregulation of kinin receptors in the vessel wall. To determine whether glucose can directly influence the regulation of kinin receptors, the independent effect of high glucose (25 mM) on B(2)-kinin receptors (B2KR) in VSMC was examined. A threefold increase in B2KR protein levels and a 40% increase in B2KR surface receptors were observed after treatment with high glucose after 24 h. The mRNA levels of B2KR were also significantly increased by high glucose as early as 4 h later. To elucidate the cellular mechanisms by which glucose regulates B2KR, we examined the role of protein kinase C (PKC). High glucose increased total PKC activity and resulted in the translocation of conventional PKC isoforms (beta(1) and beta(2)), novel (epsilon), and atypical (zeta) PKC isoforms into the membrane. Inhibition of PKC activity prevented the increase in B2KR levels induced by ambient high glucose. These findings provide the first evidence that glucose regulates the expression of B(2) receptors in VSMC and provide a rationale to further study the interaction between glucose and kinins on the pathogenesis of atherosclerotic vascular disease in diabetes.

Vascular Effects of Progesterone : Role of Cellular Calcium Regulation

-Vascular actions of progesterone have been reported, independently of estrogen, affecting both blood pressure and other aspects of the cardiovascular system. To study possible mechanisms underlying these effects, we examined the effects of P in vivo in intact rats and in vitro in isolated artery and vascular smooth muscle cell preparations. In anesthetized Sprague-Dawley rats, bolus intravenous injections of P (100 µg/kg) significantly decreased pressor responses to norepinephrine (0.3 µg/kg). In vitro, progesterone (10(-8) to 10(-5) mmol/L) produced a significant, dose-dependent relaxation of isolated helical strips, both of rat tail artery precontracted with KCl (60 mmol/L) or arginine vasopressin (3 nmol/L), and of rat aorta precontracted with KCl (60 mmol/L) or norepinephrine (0.1 µmol/L). In isolated vascular smooth muscle cells, progesterone (5x10(-)(7) mol/L) reversibly inhibited KCl (30 mmol/L) -induced elevation of cytosolic-free calcium by 64.1+/-5.5% (P:<0.05), and in whole-cell patch-clamp experiments, progesterone (5x10(-6) mol/L) reversibly and significantly blunted L-type calcium channel inward current, decreasing peak inward current to 65.7+/-4.3% of the control value (P:<0.05). Our results provide evidence that progesterone is a vasoactive hormone, inhibiting agonist-induced vasoconstriction. The data further suggest that progesterone effects on vascular tissue may, at least in part, be mediated by modulation of the L-type calcium channel current activity and, consequently, of cytosolic-free calcium content.

Effects of vitamin E and glutathione on glucose metabolism: role of magnesium

Vitamin E is an antioxidant that has been demonstrated to improve insulin action. Glutathione, another natural antioxidant, may also be important in blood pressure and glucose homeostasis, consistent with the involvement of free radicals in both essential hypertension and diabetes mellitus. Our group has recently suggested that the effects of reduced glutathione on glucose metabolism may be mediated, at least in part, by intracellular magnesium levels (Mg([i])). Recent evidence suggests that vitamin E enhances glutathione levels and may play a protective role in magnesium deficiency-induced cardiac lesions. To directly investigate the effects of vitamin E supplementation on insulin sensitivity in hypertension, in relation to the effects on circulating levels of reduced (GSH) and oxidized (GSSG) glutathione and on Mg([i]), we performed a 4-week, double-blind, randomized study of vitamin E administration (600 mg/d) versus placebo in 24 hypertensive patients and measured whole-body glucose disposal (WBGD) by euglycemic glucose clamp, GSH/GSSG ratios, and Mg([i]) before and after intervention. The relationships among WBGD, GSH/GSSG, and Mg([i]) in both groups were evaluated. In hypertensive subjects, vitamin E administration significantly increased WBGD (25.56+/-0.61 to 31.75+/-0.53 micromol/kg of fat-free mass per minute; P<0.01), GSH/GSSG ratio (1.10+/-0.07 to 1.65+/-0.11; P<0.01), and Mg([i]) (1.71+/-0.042 to 1.99+/-0.049 mmol/L; P<0.01). In basal conditions, WBGD was significantly related to both GSH/GSSG ratios (r=0.58, P=0.047) and Mg([i]) (r=0.78, P=0.003). These data show a clinical link between vitamin E administration, cellular magnesium, GSH/GSSG ratio, and tissue glucose metabolism. Further studies are needed to explore the cellular mechanism(s) of this association.

Effects of age, gender and metabolic factors on endothelium-dependent vasodilation: a population-based study

OBJECTIVES

A progressive decline in endothelium-dependent vasodilation (EDV) in the human forearm with age has previously been reported. The aim of this study was to evaluate the interplay between age, gender and metabolic factors on EDV in healthy subjects in a population-based study.

SETTING

Tertiary university hospital.

SUBJECTS AND DESIGN

Thirty-six healthy men and 30 women, aged 20-69 years, underwent measurements of forearm blood flow (FBF) at rest and during local infusions of 2 and 4 microg min-1 of metacholine (evaluating EDV) and 5 and 10 microg min-1 of sodium nitroprusside (evaluating endothelium-independent vasodilation, EIDV) and during reactive hyperaemia by venous occlusion plethysmography.

RESULTS

Age was inversely related to EDV (r = - 0.41, P < 0.05 in men; r = - 0.61, P < 0.01 in women) and maximal FBF during reactive hyperaemia in both men and women. EIDV was significantly related to age in an inverse way in women only. EDV was more pronounced in females than in males before menopause (48 +/- 3 SD years, 635 +/- 186 vs. 502 +/- 269% in males, P < 0.05), but similar in women and men thereafter (374 +/- 141 vs. 370 +/- 185% in men). The slope of the regression line for the relationship between age and EDV was flatter in premenopausal than in postmenopausal women (- 2.3 vs. - 6.4), whilst this slope was similar in younger and older men (- 5.5 vs. - 5.3). In multiple regression analysis, fasting blood glucose levels and the waist/hip ratio remained the only significant predictors of EDV in men (P < 0.01 for both), whilst age was the only significant independent predictor of EDV in women (P < 0.01).

CONCLUSION

The interplay between age and metabolic factors as determinants of endothelial function is different in healthy men and women.

Effects of glutathione on red blood cell intracellular magnesium: relation to glucose metabolism

Recent evidence suggests that the endogenous antioxidant glutathione may play a protective role in cardiovascular disease. To directly investigate the role of glutathione in the regulation of glucose metabolism in hypertension, we studied the acute effects of in vivo infusions of this antioxidant (alone or in combination with insulin) on whole body glucose disposal (WBGD) using euglycemic glucose clamp and the effects on total red blood cell intracellular magnesium (RBC-Mg) in hypertensive (n=20) and normotensive (n=30) subjects. The relationships among WBGD, circulating reduced/oxidized glutathione (GSH/GSSG) levels, and RBC-Mg in both groups were evaluated. The in vitro effects of glutathione (100 micromol/L) on RBC free cytosolic magnesium (Mg(i)) were also studied. In vivo infusions of glutathione (15 mg/minx120 minutes) increased RBC-Mg in both normotensives and hypertensives (1.99+/-0.02 to 2.13+/-0.03 mmol/L, P<0.01, and 1.69+/-0.03 to 1.81+/-0.03 mmol/L, P<0.01, respectively). In vitro GSH but not GSSG increased Mg(i) (179+/-3 to 214+/-5 micromol/L, P<0.01). In basal conditions, RBC-Mg values were related to GSH/GSSG ratios (r=0.84, P<0.0001), and WBGD was directly, significantly, and independently related to both GSH/GSSG ratios (r=0.79, P<0.0001) and RBC-Mg (r=0.89, P<0.0001). This was also true when hypertensive and control groups were analyzed separately. On multivariate analysis, basal RBC-Mg (t=6.81, P<0.001), GSH/GSSG (t=3. 67, P<0.02), and blood pressure (t=2.89, P<0.05) were each independent determinants of WBGD, with RBC-Mg explaining 31% of the variability of WBGD. These data demonstrate a direct action of glutathione both in vivo and in vitro to enhance intracellular magnesium and a clinical linkage between cellular magnesium, GSH/GSSG ratios, and tissue glucose metabolism.

The cellular ionic basis of hypertension and allied clinical conditions

Two central concepts of human hypertensive disease remain poorly understood: (1) elevated blood pressure as merely one component of an underlying systemic condition, characterized by multiple defects in diverse tissues (eg, "Syndrome X"), and (2) the heterogeneity of hypertension, in which different and even opposite clinical responses to different dietary and drug therapies are routinely observed among equally hypertensive subjects. To help explain these clinical phenomena, a unifying "ionic hypothesis" is proposed, in which steady-state elevations of cytosolic free calcium and suppressed intracellular free magnesium levels, characteristic features of all hypertension, concomitantly alter the function of many tissues. In blood vessels this causes vasoconstriction, arterial stiffness, and/or hypertension; in the heart, cardiac hypertrophy; in platelets, increased aggregation and thrombosis; in fat and skeletal muscle, insulin resistance; in pancreatic beta cells, other endocrine tissues, and sympathetic neurons, potentiated stimulus-secretion coupling resulting in hyperinsulinemia, increased sympathetic nerve activity, and so on. Furthermore, an analysis of cellular biochemical, dietary-nutrient, and hormonal factors that normally regulate steady-state levels of these intracellular ions suggests an ionic equivalent to Laragh's volume-vasoconstriction analysis of hypertension. This provides a cellular-based explanation for the heterogeneity of hypertension and a rational basis for individualizing dietary and drug recommendations among different hypertensive subjects.

Mechanisms through which high glucose concentration raises [Ca2+]i in renal proximal tubular cells

BACKGROUND

The basal levels of cytosolic calcium ([Ca2+]i) of renal proximal tubular cells of rats with streptozotocin-induced diabetes are elevated. It is possible that this phenomenon is mediated by the hyperglycemia, which may cause both increased calcium influx into and/or decreased calcium efflux out of these cells.

METHODS

We examined whether high glucose concentration in vitro causes acute rise in [Ca2+]i of freshly isolated renal proximal tubular cells and explored the pathways that are involved in such an event.

RESULTS

There were dose and time dependent increments in [Ca2+]i of renal proximal tubular cells exposed to high concentrations of glucose. A similar effect was observed with equimolar concentrations of mannitol or choline chloride but not urea. A substantial part of the rise in [Ca2+]i was inhibited when the media contained verapamil, nifedipine, amlodipine or ryanodine and when the cells were placed in a calcium free media. Inhibitors of G protein(s) (GDPbetaS or pertussis toxin), inhibitors of cAMP-protein kinase A pathway (RpcAMP or H-89), inhibitors of protein kinase C (staurosporine or calphostin) and inhibitor of Na+-H+ exchanger (HOE 694) blocked the rise in a dose dependent manner. High glucose concentration also caused a decrease in ATP content of these cells and a reduction in the Vmax of their Ca2+ATPase.

CONCLUSIONS

The results are consistent with the formulation that the osmotic activity (cell shrinkage) of the high glucose concentration may activate a stretch receptor with subsequent stimulation of various cellular pathways including G protein(s), cAMP-protein kinase A and phospholipase C systems and calcium channels. Activation of these cellular pathways permits both calcium influx into renal tubular cells and mobilization of calcium from their intracellular stores. Further, a decrease in calcium efflux secondary to the reduction in the Vmax of Ca2+ ATPase may occur. It is possible that the rise in [Ca2+]i is critical for the stimulation of the events that lead to restoration of cell volume to normal.

High glucose concentration causes a rise in [Ca2+]i of cardiac myocytes

Diabetes mellitus is associated with an elevation in the basal levels of cytosolic calcium ([Ca2+]i) of cardiac myocytes. This may be due in part to a glucose-induced elevation in [Ca2+]i. The present study examined this issue and explored the cellular pathways responsible for such a phenomenon. A total of 30 mM glucose, mannitol or choline chloride, but not urea, induced a time- and dose-dependent rise in the [Ca2+]i of cardiac myocytes. G protein inhibition by GDP beta S or pertussis toxin produced significant inhibition (> or = 80%) in the rise in [Ca2+]i. Incubation of cardiac myocytes in a calcium free medium or in media containing verapamil, nifedipine or amlodipine almost completely abolished the rise in [CA2+], while ryanodine produced only small reduction (10%) in the glucose-induced rise in [Ca2+]i. Rp-cAMP or H-89, inhibitors of the cAMP-protein kinase A pathway, produced a modest decrease in the rise in [Ca2+]i, while staurosporine (an inhibitor of PKC) and HOE 694 (an inhibitor of the Na(+)-H+ exchanger) had no effect on the rise in [Ca2+]i. The results indicate that the osmotic activity of glucose (cell shrinkage) activates G protein(s), most likely through a stretch receptor, which in turn stimulates calcium channels inhibitable by verapamil, nifedipine and amlodipine, thus permitting a calcium influx into the cardiac myocytes. The increased calcium entry may stimulate a calcium release from intracellular stores by a calcium-induced calcium release process. Thus, in cardiac myocytes direct activation of calcium channels, and to a small extent activation of the cAMP-protein kinase A, and calcium-induced calcium release mediate the high glucose-induced acute rise in their [Ca2+]i.

Calcium and protein kinase C mediate high-glucose-induced inhibition of inducible nitric oxide synthase in vascular smooth muscle cells

Abnormal vascular smooth muscle (VSMC) proliferation is a key feature in diabetes-associated atherosclerotic disease. Since nitric oxide inhibits VSMC tone, migration, adhesion, and proliferation, we examined the effects of high glucose on IL-1beta-induced NO release from VSMCs in culture. Confluent smooth muscle cells, preincubated with either 5 mmol/L (mM) or 20 mmol/L (mM) glucose for 48 hours, were stimulated with IL-1beta. Nitrite was measured in the culture medium after 24 hours. IL-1beta-induced a 15-fold increase in NO production in normal glucose medium. Glucose (10 to 30 mmol/L (mM)) significantly reduced the response to IL-1beta. High glucose (20 mmol/L (mM)) inhibited IL-1beta-evoked NO production by approximately 50%. IL-1beta-stimulated [3H] citrulline-forming activity of the nitric oxide synthase (NOS) was also significantly lower in high-glucose-exposed cells, and this was reflected in diminished cellular levels of NOS protein. To assess the role of protein kinase C (PKC), membrane PKC activity was measured, and glucose (20 mmol/L (mM)) significantly increased it. Immunoblotting of the membranes revealed a glucose-induced increase in the PKC betaII isoform. 1,2-Dioctanoyl-glycerol, a PKC activator, mimicked the high-glucose effect on IL-1beta-induced NO release, while staurosporine, a PKC inhibitor, reversed it. The role of calcium in the glucose-mediated inhibition of cytokine-induced NO release was determined by treatment with BAPTA, an intracellular chelator of calcium. BAPTA partially reversed the inhibitory effects of glucose. Increasing intracellular calcium by A23187, an ionophore or thapsigargin, an inhibitor of endoplasmic reticulum Ca2+-ATPase, significantly decreased IL-1beta-induced NO release and NOS expression. These results indicate that glucose-induced inhibition of IL-1beta-stimulated NO release and NOS expression may be mediated by PKC activation and increased intracellular calcium.

High extracellular glucose impairs cardiac E-C coupling in a glycosylation-dependent manner

Hyperglycemia is a major manifestation of all forms of diabetes mellitus and is associated with increased risk of cardiovascular disease. It is well established that cardiac excitation-contraction (E-C) coupling is adversely affected in diabetic animals such that ventricular myocyte action potential duration is prolonged and intracellular Ca2+ clearing and mechanical relaxation are slowed. We now report that ventricular myocytes incubated in a culture medium containing high extracellular glucose (25.5 mM) also exhibit these same changes in E-C coupling. These effects are not manifested for approximately 24 h after exposure. Furthermore, in the presence of normal glucose (5.5 mM), relaxation is also prolonged by fructose (20 mM), yet is unaffected by equimolar concentrations of nonmetabolizable sugars such as L-glucose and mannitol, implying that the high glucose effects require glucose entry into the cell and metabolic processing. The prolonged relaxation can also be produced by 5 mM glucosamine (an intermediate of glycosylation) and is blocked by 0.5 microgram/ml tunicamycin (an inhibitor of N-linked glycoprotein synthesis). Culturing myocytes with an inhibitor of glycation (10 mM aminoguanidine) does not prevent the high extracellular glucose concentration effects. Thus our data indicate that high extracellular glucose impairs cellular mechanisms contributing to myocardial relaxation and that this impairment may involve glycosylation of nascent proteins.

Direct magnetic resonance determination of aortic distensibility in essential hypertension: relation to age, abdominal visceral fat, and in situ intracellular free magnesium

To investigate the contribution of vascular compliance to essential hypertension (EH), we developed magnetic resonance imaging (MRI) techniques to directly measure aortic distensibility (AD) in the ascending and descending thoracic and abdominal aorta of fasting normal (n= 10) and EH (n=20) subjects. These results were compared with concurrent MR-based measurements of left ventricular mass index (LVMI) and abdominal subcutaneous and visceral fat and with 31P-MR spectroscopic measurement of in situ intracellular free magnesium levels (Mgi) in brain and skeletal muscle. Aortic distensibility in EH was consistently and significantly reduced at all measured sites (2.5+/-0.4, 2.2+/-0.4, 2.3+/-0.4 versus 7.0+/-1.6, 5.1+/-0.3, 7.3+/-0.8 mm Hg(-1) x 10(-3), P<.05), as was Mgi in the brain (284+/-22 versus 383+/-34 micromol/L, P<.05) and skeletal muscle (397+/-10 versus 527+/-36 micromol/L, P<.05). For all subjects, systolic blood pressure (r=-.662, P<.0001) and LVMI (r=-.484, P<.01) were inversely related to AD. AD and brain Mgi were inversely related to age (AD, r=-.792, P<.0001; brain Mgi: r=-.673, P<.05). AD was inversely related to fasting blood glucose (r=-.413, P<.05) and to abdominal visceral fat (r=-.416, P<.05) but not to body mass index (BMI: r=-.328, P=NS) or subcutaneous fat (r=-.157, P=NS). AD was also significantly and positively related to in situ Mgi, both in the brain and skeletal muscle (brain: r=.712, P<.01; skeletal muscle: r=.632, P<.01). We conclude that (1) MR techniques can be used to coordinately and noninvasively assess cardiac, vascular, metabolic, and ionic aspects of hypertensive disease in humans; (2) increased systolic blood pressure and LVMI in EH may at least in part result from decreased AD; (3) decreased Mgi contributes to arterial stiffness in hypertension and may help to explain the characteristic age-related decreases in AD; and (4) decreased AD may be one mechanism by which abdominal visceral fat contributes to cardiovascular risk.

Elevated cytosolic calcium and impaired proliferation of B lymphocytes in type II diabetes mellitus

Patients with diabetes mellitus have increased susceptibility to infection attributable, at least in part, to defective function of polymorphonuclear leukocytes (PMNLs) and B cells. Certain data suggest that cytosolic calcium ([Ca2+]i) is elevated in various cells in diabetes, and high [Ca2+]i adversely affects cell function. Indeed, the [Ca2+]i of PMNLs of diabetic patients is elevated, and phagocytosis of the PMNLs is impaired. The current study examines whether the [Ca2+]i of B cells is also elevated in diabetes and whether this derangement impairs B cell function. We studied 32 patients with non-insulin-dependent diabetes mellitus (NIDDM) and eight normal subjects. All patients had hyperglycemia (11.6 +/- 0.80 mmol/L) and elevated HbA1c (13.2% +/- 0.99%). The basal levels of [Ca2+]i of the B cells (113 +/- 3.3 nmol/L) were significantly (P < 0.01) higher than the values in normal subjects (85 +/- 1.7 nmol/L). There was a direct and significant correlation (r = 0.88; P < 0.01) between the [Ca2+]i of B cells and the blood levels of glucose. Proliferation of B cells in response to Staphylococcus aureus Cowan I (SAC) was significantly impaired in these patients (7.3 +/- 0.48 x 10(3) cpm v 12.5 +/- 0.61 x 10(3) cpm in normal subjects). Normalization of blood glucose with the hypoglycemic agents, glyburide, was associated with the return of both [Ca2+]i of B cells and their proliferation in response to SAC to normal. The results show that hyperglycemia in type II diabetes mellitus is associated with a significant increase in [Ca2+]i of B cells and with a decrease in their proliferation in response to mitogen. These derangements are reversed after the correction of the hyperglycemia. The data of the current study and those previously reported in PMNLs provide for a new pathogenetic process underlying the dysfunction of these cells in diabetes mellitus.

HPLC fractions of human uremic plasma inhibit the RBC membrane calcium pump

We have reported that uremic plasma filtrates (UF) inhibit the red blood cell (RBC) membrane calcium pump. The inhibitor was dialyzable, smaller than 3,000 molecular weight, heat-stable, and protease-resistant. In the present study, we used reverse-phase preparative HPLC, analytical HPLC, and Sephadex G-25 elution to identify inhibitory fractions. Inhibition was confirmed in three different bioassays: (1) Sr2+ efflux in intact RBC, the primary bio-assay; (2) 45Ca efflux in intact RBC; and (3) calcium ATPase activity in isolated RBC membranes. Active fractions were analyzed by mass spectrometry, capillary electrophoresis, enzymatic analysis, gas chromatography-mass spectrometry, and nuclear magnetic resonance spectroscopy. These demonstrated a number of compounds, including: sugars, polyols, osmolytes like betaine and myoinositol, amino acids, and other metabolites, such as 3-D-hydroxybutyrate, dimethylglycine, trimethylamine-N-oxide, guanidinoacetic acid and glycine. Many individual compounds were then tested for an effect on the calcium pump. Thus, HPLC was able to separate a substantial number of compounds in inhibitory fractions. Efforts are under way for precise identification of the inhibitor, to advance our understanding of uremic toxicity and/or hypertension in CRF.