Altered functional coupling of coronary K+ channels in diabetic dyslipidemic pigs is prevented by exercise

Specificity of Ca2+-activated K+ channel modulation in atherosclerosis and aerobic exercise training

Vascular smooth muscle cells express several isoforms of a number of classes of K⁺ channels. Potassium channels play critical roles in the regulation of vascular smooth muscle contraction as well as vascular smooth muscle cell proliferation or phenotypic modulation. There is ample evidence that it is Ca²⁺ that enables these two seemingly disparate functions to be tightly coupled both in healthy and disease processes. Because of the central position that potassium channels have in vasocontraction, vasorelaxation, membrane potential, and smooth muscle cell proliferation, these channels continue to possess the potential to serve as novel therapeutic targets in cardiovascular disease. While there are questions that remain regarding the complete interactions between K⁺ channels, vascular regulation, smooth muscle cell proliferation, and phenotypic modulation in physiological and pathophysiological conditions, a broad understanding of the contributions of each class of K⁺ channel to contractile and proliferative states of the vasculature has been reached. This brief review will discuss the current understanding of the role of K⁺ channels in vascular smooth muscle cells in health and disease using the porcine vascular smooth muscle cell model with particular attention to new scientific discoveries contributed by the authors regarding the effect of endurance exercise on the function of the K⁺ channels.

K+ channels in the coronary microvasculature of the ischemic heart

Ischemic heart disease is the leading cause of death and a major public health and economic burden worldwide with expectations of predicted growth in the foreseeable future. It is now recognized clinically that flow-limiting stenosis of the large coronary conduit arteries as well as microvascular dysfunction in the absence of severe stenosis can each contribute to the etiology of ischemic heart disease. The primary site of coronary vascular resistance, and control of subsequent coronary blood flow, is found in the coronary microvasculature, where small changes in radius can have profound impacts on myocardial perfusion. Basal active tone and responses to vasodilators and vasoconstrictors are paramount in the regulation of coronary blood flow and adaptations in signaling associated with ion channels are a major factor in determining alterations in vascular resistance and thereby myocardial blood flow. K+ channels are of particular importance as contributors to all aspects of the regulation of arteriole resistance and control of perfusion into the myocardium because these channels dictate membrane potential, the resultant activity of voltage-gated calcium channels, and thereby, the contractile state of smooth muscle. Evidence also suggests that K+ channels play a significant role in adaptations with cardiovascular disease states. In this review, we highlight our research examining the role of K+ channels in ischemic heart disease and adaptations with exercise training as treatment, as well as how our findings have contributed to this area of study.

Coronary Large Conductance Ca2+-Activated K+ Channel Dysfunction in Diabetes Mellitus

Diabetes mellitus (DM) is an independent risk of macrovascular and microvascular complications, while cardiovascular diseases remain a leading cause of death in both men and women with diabetes. Large conductance Ca2+-activated K+ (BK) channels are abundantly expressed in arteries and are the key ionic determinant of vascular tone and organ perfusion. It is well established that the downregulation of vascular BK channel function with reduced BK channel protein expression and altered intrinsic BK channel biophysical properties is associated with diabetic vasculopathy. Recent efforts also showed that diabetes-associated changes in signaling pathways and transcriptional factors contribute to the downregulation of BK channel expression. This manuscript will review our current understandings on the molecular, physiological, and biophysical mechanisms that underlie coronary BK channelopathy in diabetes mellitus.

Chronic exercise mediates epigenetic suppression of L-type Ca2+ channel and BKCa channel in mesenteric arteries of hypertensive rats

BACKGROUND

Regular exercise is a lifestyle intervention for controlling hypertension and has an improving effect on vascular function. Voltage-gated L-type Ca (LTCC) and large-conductance Ca-activated K (BKCa) channels are two principal mediators of vascular smooth muscle cell contractility and arterial tone. The present study tested the hypothesis that DNA methylation dynamics plays a key role in exercise-induced reprogramming and downregulation of LTCC and BKCa channel in mesenteric arteries from spontaneously hypertensive rats (SHRs).

METHODS

SHRs and Wistar-Kyoto (WKY) rats were subjected to exercise training or kept sedentary, and vascular molecular and functional properties were evaluated.

RESULTS

Exercise inhibited hypertension-induced upregulation of LTCC and BKCa channel function in mesenteric arteries by repressing LTCC α1c and BKCa β1 subunit expression. In accordance, exercise triggered hypermethylation of α1c and β1 gene in SHR, with concomitant decreasing TET1, increasing DNMT1 and DNMT3b expression in mesenteric arteries, as well as altering peripheral α-KG and S-adenosylmethionine/ S-adenosylhomocysteine ratio. Acting synergistically, these exercise-induced functional and molecular amelioration could allow for attenuating hypertension-induced elevation in arterial blood pressure.

CONCLUSION

Our results indicate that exercise suppresses LTCC and BKCa channel function via hypermethylation of α1c and β1 subunits, which contributes to the restoration of mesenteric arterial function and vasodilation during hypertension.

Guidelines for animal exercise and training protocols for cardiovascular studies

Whole body exercise tolerance is the consummate example of integrative physiological function among the metabolic, neuromuscular, cardiovascular, and respiratory systems. Depending on the animal selected, the energetic demands and flux through the oxygen transport system can increase two orders of magnitude from rest to maximal exercise. Thus, animal models in health and disease present the scientist with flexible, powerful, and, in some instances, purpose-built tools to explore the mechanistic bases for physiological function and help unveil the causes for pathological or age-related exercise intolerance. Elegant experimental designs and analyses of kinetic parameters and steady-state responses permit acute and chronic exercise paradigms to identify therapeutic targets for drug development in disease and also present the opportunity to test the efficacy of pharmacological and behavioral countermeasures during aging, for example. However, for this promise to be fully realized, the correct or optimal animal model must be selected in conjunction with reproducible tests of physiological function (e.g., exercise capacity and maximal oxygen uptake) that can be compared equitably across laboratories, clinics, and other proving grounds. Rigorously controlled animal exercise and training studies constitute the foundation of translational research. This review presents the most commonly selected animal models with guidelines for their use and obtaining reproducible results and, crucially, translates state-of-the-art techniques and procedures developed on humans to those animal models.

Swine Disease Models for Optimal Vascular Engineering

Swine disease models are essential for mimicry of human metabolic and vascular pathophysiology, thereby enabling high-fidelity translation to human medicine. The worldwide epidemic of obesity, metabolic disease, and diabetes has prompted the focus on these diseases in this review. We highlight the remarkable similarity between Ossabaw miniature swine and humans with metabolic syndrome and atherosclerosis. Although the evidence is strongest for swine models of coronary artery disease, findings are generally applicable to any vascular bed. We discuss the major strengths and weaknesses of swine models. The development of vascular imaging is an example of optimal vascular engineering in swine. Although challenges regarding infrastructure and training of engineers in the use of swine models exist, opportunities are ripe for gene editing, studies of molecular mechanisms, and use of swine in coronary artery imaging and testing of devices that can move quickly to human clinical studies.

Downregulation of BK channel function and protein expression in coronary arteriolar smooth muscle cells of type 2 diabetic patients

Aims

Type 2 diabetes (T2D) is strongly associated with cardiovascular morbidity and mortality in patients. Vascular large conductance Ca2+-activated potassium (BK) channels, composed of four pore-forming α subunits (BK-α), and four regulatory β1 subunits (BK-β1), are densely expressed in coronary arterial smooth muscle cells (SMCs) and play an important role in regulating vascular tone and myocardial perfusion. However, the role of BK channels in coronary microvascular dysfunction of human subjects with diabetes is unclear. In this study, we examined BK channel function and protein expression, and BK channel-mediated vasodilation in freshly isolated coronary arterioles from T2D patients.

Methods and results

Atrial tissues were obtained from 16 patients with T2D and 25 matched non-diabetic subjects during cardiopulmonary bypass procedure. Microvessel videomicroscopy and immunoblot analysis were performed in freshly dissected coronary arterioles and inside-out single BK channel currents was recorded in enzymatically isolated coronary arteriolar SMCs. We found that BK channel sensitivity to physiological Ca2+ concentration and voltage was downregulated in the coronary arteriolar SMCs of diabetic patients, compared with non-diabetic controls. BK channel kinetics analysis revealed that there was significant shortening of the mean open time and prolongation of the mean closed time in diabetic patients, resulting in a remarkable reduction of the channel open probability. Functional studies showed that BK channel activation by dehydrosoyasaponin-1 was diminished and that BK channel-mediated vasodilation in response to shear stress was impaired in diabetic coronary arterioles. Immunoblot experiments confirmed that the protein expressions of BK-α and BK-β1 subunits were significantly downregulated, but the ratio of BK-α/BK-β1 was unchanged in the coronary arterioles of T2D patients.

Conclusions

Our results demonstrated for the first time that BK channel function and BK channel-mediated vasodilation were abnormal in the coronary microvasculature of diabetic patients, due to decreased protein expression and altered intrinsic properties of BK channels.

Porcine models for studying complications and organ crosstalk in diabetes mellitus

The worldwide prevalence of diabetes mellitus and obesity is rapidly increasing not only in adults but also in children and adolescents. Diabetes is associated with macrovascular complications increasing the risk for cardiovascular disease and stroke, as well as microvascular complications leading to diabetic nephropathy, retinopathy and neuropathy. Animal models are essential for studying disease mechanisms and for developing and testing diagnostic procedures and therapeutic strategies. Rodent models are most widely used but have limitations in translational research. Porcine models have the potential to bridge the gap between basic studies and clinical trials in human patients. This article provides an overview of concepts for the development of porcine models for diabetes and obesity research, with a focus on genetically engineered models. Diabetes-associated ocular, cardiovascular and renal alterations observed in diabetic pig models are summarized and their similarities with complications in diabetic patients are discussed. Systematic multi-organ biobanking of porcine models of diabetes and obesity and molecular profiling of representative tissue samples on different levels, e.g., on the transcriptome, proteome, or metabolome level, is proposed as a strategy for discovering tissue-specific pathomechanisms and their molecular key drivers using systems biology tools. This is exemplified by a recent study providing multi-omics insights into functional changes of the liver in a transgenic pig model for insulin-deficient diabetes mellitus. Collectively, these approaches will provide a better understanding of organ crosstalk in diabetes mellitus and eventually reveal new molecular targets for the prevention, early diagnosis and treatment of diabetes mellitus and its associated complications.

"A Step and a Ceiling": mechanical properties of Ca2+ spark vasoregulation in resistance arteries by pressure-induced oxidative activation of PKG

We investigated the biomechanical relationship between intraluminal pressure within small mesenteric resistance arteries, oxidant activation of PKG, Ca2+ sparks, and BK channel vasoregulation. Mesenteric resistance arteries from wild type (WT) and genetically modified mice with PKG resistance to oxidative activation were studied using wire and pressure myography. Ca2+ sparks and Ca2+ transients within vascular smooth muscle cells of intact arteries were characterized using high-speed confocal microscopy of intact arteries. Arteries were studied under conditions of varying intraluminal pressure and oxidation. Intraluminal pressure specifically, rather than the generic stretch of the artery, was necessary to activate the oxidative pathway. We demonstrated a graded step activation profile for the generation of Ca2+ sparks and also a functional "ceiling" for this pressure --sensitive oxidative pathway. During steady state pressure - induced constriction, any additional Ca2+ sensitive-K+ channel functional availability was independent of oxidant activated PKG. There was an increase in the amplitude, but not the Area under the Curve (AUC) of the caffeine-induced Ca2+ transient in pressurized arteries from mice with oxidant-resistant PKG compared with wild type. Overall, we surmise that intraluminal pressure within resistance arteries controls Ca2+ spark vasoregulation through a tightly controlled pathway with a graded onset switch. The pathway, underpinned by oxidant activation of PKG, cannot be further boosted by additional pressure or oxidation once active. We propose that these restrictive characteristics of pressure-induced Ca2+ spark vasoregulation confer stability for the artery in order to provide a constant flow independent of additional pressure fluctuations or exogenous oxidants.

Chronic interval exercise training prevents BKCa channel-mediated coronary vascular dysfunction in aortic-banded miniswine

Conventional treatments have failed to improve the prognosis of heart failure with preserved ejection fraction (HFpEF) patients. Thus, the purpose of this study was to determine the therapeutic efficacy of chronic interval exercise training (IT) on large-conductance Ca²⁺-activated K⁺ (BK_Ca) channel-mediated coronary vascular function in heart failure. We hypothesized that chronic interval exercise training would attenuate pressure overload-induced impairments to coronary BK_Ca channel-mediated function. A translational large-animal model with cardiac features of HFpEF was used to test this hypothesis. Specifically, male Yucatan miniswine were divided into three groups ( n = 7/group): control (CON), aortic banded (AB)-heart failure (HF), and AB-interval trained (HF-IT). Coronary blood flow, vascular conductance, and vasodilatory capacity were measured after administration of the BK_Ca channel agonist NS-1619 both in vivo and in vitro in the left anterior descending coronary artery and isolated coronary arterioles, respectively. Skeletal muscle citrate synthase activity was decreased and left ventricular brain natriuretic peptide levels increased in HF vs. CON and HF-IT animals. A parallel decrease in NS-1619-dependent coronary vasodilatory reserve in vivo and isolated coronary arteriole vasodilatory responsiveness in vitro were observed in HF animals compared with CON, which was prevented in the HF-IT group. Although exercise training prevented BK_Ca channel-mediated coronary vascular dysfunction, it did not change BK_Ca channel α-subunit mRNA, protein, or cellular location (i.e., membrane vs. cytoplasm). In conclusion, these results demonstrate the viability of chronic interval exercise training as a therapy for central and peripheral adaptations of experimental heart failure, including BK_Ca channel-mediated coronary vascular dysfunction. NEW & NOTEWORTHY Conventional treatments have failed to improve the prognosis of heart failure with preserved ejection fraction (HFpEF) patients. Our findings show that chronic interval exercise training can prevent BK_Ca channel-mediated coronary vascular dysfunction in a translational swine model of chronic pressure overload-induced heart failure with relevance to human HFpEF.

Alloxan-induced diabetes exacerbates coronary atherosclerosis and calcification in Ossabaw miniature swine with metabolic syndrome

Background

There is a preponderance of evidence implicating diabetes with increased coronary artery disease (CAD) and calcification (CAC) in human patients with metabolic syndrome (MetS), but the effect of diabetes on CAD severity in animal models remains controversial. We investigated whether diabetes exacerbates CAD/CAC and intracellular free calcium ([Ca²⁺]_i) dysregulation in the clinically relevant Ossabaw miniature swine model of MetS.

Methods

Sixteen swine, eight with alloxan-induced diabetes, were fed a hypercaloric, atherogenic diet for 6 months. Alloxan-induced pancreatic beta cell damage was examined by immunohistochemical staining of insulin. The metabolic profile was confirmed by body weight, complete blood panel, intravenous glucose tolerance test (IVGTT), and meal tolerance test. CAD severity was assessed with intravascular ultrasound and histology. [Ca²⁺]_i handling in coronary smooth muscle (CSM) cells was assessed with fura-2 ratiometric imaging.

Results

Fasting and post-prandial blood glucose, total cholesterol, and serum triglycerides were elevated in MetS-diabetic swine. This group also exhibited hypoinsulinemia during IVGTT and less pancreatic beta cell mass when compared to lean and MetS-nondiabetic swine. IVUS analysis revealed that MetS-diabetic swine had greater percent wall coverage, percent plaque burden, and calcium index when compared to lean and MetS-nondiabetic swine. Fura-2 imaging of CSM [Ca²⁺]_i revealed that MetS-nondiabetic swine exhibited increased sarcoplasmic reticulum Ca²⁺ store release and Ca²⁺ influx through voltage-gated Ca²⁺ channels compared to lean swine. MetS-diabetic swine exhibited impaired Ca²⁺ efflux.

Conclusions

Diabetes exacerbates coronary atherosclerosis and calcification in Ossabaw miniature swine with MetS, accompanied by progression of [Ca²⁺]_i dysregulation in advanced CAD/CAC. These results recapitulate increased CAD in humans with diabetes and establish Ossabaw miniature swine as an animal model for future MetS/diabetes comorbidity studies.

Critical contribution of KV1 channels to the regulation of coronary blood flow

Ion channels in smooth muscle control coronary vascular tone, but the identity of the potassium channels involved requires further investigation. The purpose of this study was to evaluate the functional role of KV1 channels on porcine coronary blood flow using the selective antagonist correolide. KV1 channel gene transcripts were found in porcine coronary arteries, with KCNA5 (encoding KV1.5) being most abundant (P < 0.001). Immunohistochemical staining demonstrated KV1.5 protein in the vascular smooth muscle layer of both porcine and human coronary arteries, including microvessels. Whole-cell patch-clamp experiments demonstrated significant correolide-sensitive (1-10 µM) current in coronary smooth muscle. In vivo studies included direct intracoronary infusion of vehicle or correolide into a pressure-clamped left anterior descending artery of healthy swine (n = 5 in each group) with simultaneous measurement of coronary blood flow. Intracoronary correolide (~0.3-3 µM targeted plasma concentration) had no effect on heart rate or systemic pressure, but reduced coronary blood flow in a dose-dependent manner (P < 0.05). Dobutamine (0.3-10 µg/kg/min) elicited coronary metabolic vasodilation and intracoronary correolide (3 µM) significantly reduced coronary blood flow at any given level of myocardial oxygen consumption (P < 0.001). Coronary artery occlusions (15 s) elicited reactive hyperemia and correolide (3 µM) reduced the flow volume repayment by approximately 30 % (P < 0.05). Taken together, these data support a major role for KV1 channels in modulating baseline coronary vascular tone and, perhaps, vasodilation in response to increased metabolism and transient ischemia.

Exercise Prevents Upregulation of RyRs-BKCa Coupling in Cerebral Arterial Smooth Muscle Cells From Spontaneously Hypertensive Rats

OBJECTIVE

Regular exercise is an effective nonpharmacological means of preventing and controlling hypertension. However, the molecular mechanisms underlying its effects remain undetermined. The hypothesis that hypertension increases the functional coupling of large-conductance Ca(2+)-activated K(+) (BKCa) channels with ryanodine receptors in spontaneously hypertensive rats (SHR) as a compensatory response to an increase in intracellular Ca(2+) concentration in cerebral artery smooth muscle cells was assessed here. It was further hypothesized that exercise training would prevent this increase in functional coupling.

APPROACH AND RESULTS

SHR and Wistar-Kyoto (WKY) rats were randomly assigned to sedentary groups (SHR-SED and WKY-SED) and exercise training groups (SHR-EX and WKY-EX). Cerebral artery smooth muscle cells displayed spontaneous transient outward currents at membrane potentials more positive than -40 mV. The amplitude of spontaneous transient outward currents together with the spontaneous Ca(2+) sparks in isolated cerebral artery smooth muscle cells was significantly higher in SHR-SED than in WKY-SED. Moreover, hypertension displayed increased whole-cell BKCa, voltage-gated Ca(2+) channel, but decreased KV currents in cerebral artery smooth muscle cells. In SHRs, the activity of the single BKCa channel increased markedly, and the protein expression of BKCa (β1, but not α-subunit) also increased, but KV1.2 decreased significantly. Exercise training ameliorated all of these functional and molecular alterations in hypertensive rats.

CONCLUSIONS

These data indicate that hypertension leads to enhanced functional coupling of ryanodine receptors-BKCa to buffer pressure-induced constriction of cerebral arteries, which attributes not only to an upregulation of BKCa β1-subunit function but also to an increase of Ca(2+) release from ryanodine receptors. However, regular aerobic exercise efficiently prevents augmented coupling and so alleviates the pathological compensation and restores cerebral arterial function.

Vascular effects of aerobic exercise training in rat adult offspring exposed to hypoxia-induced intrauterine growth restriction

KEY POINTS

Prenatal hypoxia, one of the most common consequences of complicated pregnancies, leads to intrauterine growth restriction (IUGR) and impairs later-life endothelium-dependent vascular function. Early interventions are needed to ultimately reduce later-life risk for cardiovascular disease. Aerobic exercise training has been shown to prevent cardiovascular diseases. Whether exercise can be used as an intervention to reverse the vascular phenotype of this susceptible population is unknown. Aerobic exercise training enhanced endothelium-derived hyperpolarization-mediated vasodilatation in gastrocnemius muscle arteries in male IUGR offspring, and did not improve nitric oxide-mediated vasodilatation in IUGR offspring. Understanding the mechanisms by which exercise impacts the cardiovascular system in a susceptible population and the consideration of sexual dimorphism is essential to define whether exercise could be used as a preventive strategy in this population.

ABSTRACT

Hypoxia in utero is a critical insult causing intrauterine growth restriction (IUGR). Adult offspring born with hypoxia-induced IUGR have impaired endothelium-dependent vascular function. We tested whether aerobic exercise improves IUGR-induced endothelial dysfunction. Pregnant Sprague-Dawley rats were exposed to control (21% oxygen) or hypoxic (11% oxygen) conditions from gestational day 15 to 21. Male and female offspring from normoxic and hypoxic (IUGR) pregnancies were randomized at 10 weeks of age to either an exercise-trained or sedentary group. Exercise-trained rats ran on a treadmill for 30 min at 20 m min(-1) , 5 deg gradient, 5 days week(-1) , for 6 weeks. Concentration-response curves to phenylephrine and methylcholine were performed in second order mesenteric and gastrocnemius muscle arteries, in the presence or absence of l-NAME (100 μm), MnTBAP (peroxynitrite scavenger; 10 μm), apamin (0.1 μm) and TRAM-34 (an intermediate-conductance calcium-activated potassium channel blocker; 10 μm), or indomethacin (5 μm). In adult male IUGR offspring, prenatal hypoxia had no effect on total vasodilator responses in either vascular bed. Aerobic exercise training in IUGR males, however, improved endothelium-derived hyperpolarization (EDH)-mediated vasodilatation in gastrocnemius muscle arteries. Female IUGR offspring had reduced NO-mediated vasodilatation in both vascular beds, along with decreased total vasodilator responses and increased prostaglandin-mediated vasoconstriction in gastrocnemius muscle arteries. In contrast to males, aerobic exercise training in IUGR female offspring had no effect on either vascular bed. Exercise may not prove to be a beneficial therapy for specific vascular pathways affected by prenatal hypoxia, particularly in female offspring.

The diabetic vasculature: physiological mechanisms of dysfunction and influence of aerobic exercise training in animal models

Diabetes mellitus (DM) is associated with a number of complications of which chronic vascular complications are undoubtedly the most complex and significant consequence. With a significant impact on health care, 50-80% of people with diabetes die of cardiovascular disease (including coronary artery disease, stroke, peripheral vascular disease and other vascular disease), making it the major cause of morbidity and mortality in diabetic patients. A healthy lifestyle is essential in the management of DM, especially the inclusion of aerobic exercise, which has been shown effective in reducing the deleterious effects in vasculature. Interest in exercise studies has increased significantly with promising results that demonstrate a future for investigation. Considering the importance of this emerging field, the aim of this mini-review is to summarize and integrate animal studies investigating physiological mechanisms of vascular dysfunction and remodeling in type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM) and how these are influenced by chronic aerobic exercise training.

Exercise training-enhanced, endothelium-dependent dilation mediated by altered regulation of BK(Ca) channels in collateral-dependent porcine coronary arterioles

OBJECTIVE

Test the hypothesis that exercise training increases the contribution of BK(Ca) channels to endothelium-mediated dilation in coronary arterioles from collateral-dependent myocardial regions of chronically occluded pig hearts and may function downstream of H2O2.

METHODS

An ameroid constrictor was placed around the proximal left circumflex coronary artery to induce gradual occlusion in Yucatan miniature swine. Eight weeks postoperatively, pigs were randomly assigned to sedentary or exercise training (treadmill; 14 week) regimens.

RESULTS

Exercise training significantly enhanced bradykinin-mediated dilation in collateral-dependent arterioles (~125 μm diameter) compared with sedentary pigs. The BK(Ca) -channel blocker, iberiotoxin alone or in combination with the H2O2 scavenger, polyethylene glycol catalase, reversed exercise training-enhanced dilation in collateral-dependent arterioles. Iberiotoxin-sensitive whole-cell K+ currents (i.e., BK(Ca)-channel currents) were not different between smooth muscle cells of nonoccluded and collateral-dependent arterioles of sedentary and exercise trained groups.

CONCLUSIONS

These data provide evidence that BK(Ca)-channel activity contributes to exercise training-enhanced endothelium-dependent dilation in collateral-dependent coronary arterioles despite no change in smooth muscle BK(Ca)-channel current. Taken together, our findings suggest that a component of the bradykinin signaling pathway, which stimulates BK(Ca) channels, is enhanced by exercise training in collateral-dependent arterioles and suggest a potential role for H2O2 as the mediator.

Heart of the matter: coronary dysfunction in metabolic syndrome

Metabolic syndrome (MetS) is a collection of risk factors including obesity, dyslipidemia, insulin resistance/impaired glucose tolerance, and/or hypertension. The incidence of obesity has reached pandemic levels, as ~20-30% of adults in most developed countries can be classified as having MetS. This increased prevalence of MetS is critical as it is associated with a two-fold elevated risk for cardiovascular disease. Although the pathophysiology underlying this increase in disease has not been clearly defined, recent evidence indicates that alterations in the control of coronary blood flow could play an important role. The purpose of this review is to highlight current understanding of the effects of MetS on regulation of coronary blood flow and to outline the potential mechanisms involved. In particular, the role of neurohumoral modulation via sympathetic α-adrenoceptors and the renin-angiotensin-aldosterone system (RAAS) are explored. Alterations in the contribution of end-effector K(+), Ca(2+), and transient receptor potential (TRP) channels are also addressed. Finally, future perspectives and potential therapeutic targeting of the microcirculation in MetS are discussed. This article is part of a Special Issue entitled "Coronary Blood Flow".

Ca2+ regulatory mechanisms of exercise protection against coronary artery disease in metabolic syndrome and diabetes

Chronic exercise attenuates coronary artery disease (CAD) in humans largely independent of reductions in risk factors; thus major protective mechanisms of exercise are directly within the coronary vasculature. Further, tight control of diabetes, e.g., blood glucose, can be detrimental. Accordingly, knowledge of mechanisms by which exercise attenuates diabetic CAD could catalyze development of molecular therapies. Exercise attenuates CAD (atherosclerosis) and restenosis in miniature swine models, which enable precise control of exercise parameters (intensity, duration, and frequency) and characterization of the metabolic syndrome (MetS) and diabetic milieu. Intracellular Ca(2+) is a pivotal second messenger for coronary smooth muscle (CSM) excitation-contraction and excitation-transcription coupling that modulates CSM proliferation, migration, and calcification. CSM of diabetic dyslipidemic Yucatan swine have impaired Ca(2+) extrusion via the plasmalemma Ca(2+) ATPase (PMCA), downregulation of L-type voltage-gated Ca(2+) channels (VGCC), increased Ca(2+) sequestration by the sarcoplasmic reticulum (SR) Ca(2+) ATPase (SERCA), increased nuclear Ca(2+) localization, and greater activation of K channels by Ca(2+) release from the SR. Endurance exercise training prevents Ca(2+) transport changes with virtually no effect on the diabetic milieu (glucose, lipids). In MetS Ossabaw swine transient receptor potential canonical (TRPC) channels are upregulated and exercise training reverses expression and TRPC-mediated Ca(2+) influx with almost no change in the MetS milieu. Overall, exercise effects on Ca(2+) signaling modulate CSM phenotype. Future studies should 1) selectively target key Ca(2+) transporters to determine definitively their causal role in atherosclerosis and 2) combine mechanistic studies with clinical outcomes, e.g., reduction of myocardial infarction.

Tuning in to the ‘right’ calcium channel regulation in experimental models of diabetes

Elucidation of cellular and molecular mechanisms underlying vascular disease is of fundamental importance to the development of pharmacological agents to target these pathways. Pinho et al. in this issue of the BJP provide highly compelling evidence that the δ isoform of phosphatidyl inositol 3-kinase (PI3K δ) was upregulated and accounted for the increase in L-type, voltage-gated, Ca channel current in aortic vascular smooth muscle (VSM) cells of a mouse model of type 1 diabetes. There are several key issues of broad fundamental significance to this work. Firstly, what is the ‘right’ answer about calcium channel regulation in diabetes? Conflicting reports of increased and decreased Ca channel current may be due to specificity of the vascular bed and species. Then, the time course of diabetic vasculopathy may influence the expression of contractile versus proliferative phenotypes of VSM. Also the metabolic characterization of diabetes may enlighten or confound any study of diabetic vascular disease. These issues need attention to move forward work in this area.

Differential effects of diet-induced obesity on BKCa {beta}1-subunit expression and function in rat skeletal muscle arterioles and small cerebral arteries

Mechanisms underlying obesity-related vascular dysfunction are unclear. This study examined the effect of diet-induced obesity on expression and function of large conductance Ca(2+)-activated potassium channel (BK(Ca)) in rat pressurized small resistance vessels with myogenic tone. Male Sprague-Dawley rats fed a cafeteria-style high fat diet (HFD; ∼30% energy from fat) for 16-20 wk were ∼30% heavier than controls fed standard chow (∼13% fat). Obesity did not alter BK(Ca) α-subunit function or α-subunit protein or mRNA expression in vessels isolated from the cremaster muscle or middle-cerebral circulations. In contrast, BK(Ca) β(1)-subunit protein expression and function were significantly reduced in cremaster muscle arterioles but increased in middle-cerebral arteries from obese animals. Immunohistochemistry showed α- and β(1)-subunits were present exclusively in the smooth muscle of both vessels. Cremaster muscle arterioles from obese animals showed significantly increased medial thickness, and media-to-lumen ratio and pressurized arterioles showed increased myogenic tone at 30 mmHg, but not at 50-120 mmHg. Myogenic tone was not affected by obesity in middle-cerebral arteries. The BK(Ca) antagonist iberiotoxin constricted both cremaster muscle and middle-cerebral arterioles from control rats; this effect of iberiotoxin was abolished in cremaster muscle arteries only from obese rats. Diet-induced obesity has contrasting effects on BK(Ca) function in different vascular beds, through differential effects on β(1)-subunit expression. However, these alterations in BK(Ca) function had little effect on overall myogenic tone, suggesting that the mechanisms controlling myogenic tone can be altered and compensate for altered BK(Ca) expression and function.

Endothelial dysfunction in diabetes: multiple targets for treatment

Robert Furchgott's discovery of the obligatory role that the endothelium plays in the regulation of vascular tone has proved to be a major advance in terms of our understanding of the cellular basis of diabetic vascular disease. Endothelial dysfunction, as defined by a reduction in the vasodilatation response to an endothelium-dependent vasodilator (such as acetylcholine) or to flow-mediated vasodilatation, is an early indicator for the development of the micro- and macroangipathy that is associated with diabetes. In diabetes, hyperglycaemia plays a key role in the initiation and development of endothelial dysfunction; however, the cellular mechanisms involved as well as the importance of dyslipidaemia and co-morbidities such as hypertension and obesity remain incompletely understood. In this review, we discuss the mechanisms whereby hyperglycaemia, oxidative stress and dyslipidaemia can alter endothelial function and highlight their effects on endothelial nitric oxide synthase (eNOS), the endothelium-dependent hyperpolarising factor (EDHF) pathway(s), as well as on the role of endothelium-derived contracting factors (EDCFs) and adipocyte-derived vasoactive factors such as adipose-derived relaxing factor (ADRF).

Role of potassium channels in coronary vasodilation

K+ channels in coronary arterial smooth muscle cells (CASMC) determine the resting membrane potential ( Em) and serve as targets of endogenous and therapeutic vasodilators. Em in CASMC is in the voltage range for activation of L-type Ca2+ channels; therefore, when K+ channel activity changes, Ca2+ influx and arterial tone change. This is why both Ca2+ channel blockers and K+ channel openers have such profound effects on coronary blood flow; the former directly inhibits Ca2+ influx through L-type Ca2+ channels, while the latter indirectly inhibits Ca2+ influx by hyperpolarizing Em and reducing Ca2+ channel activity. K+ channels in CASMC play important roles in vasodilation to endothelial, ischemic and metabolic stimuli. The purpose of this article is to review the types of K+ channels expressed in CASMC, discuss the regulation of their activity by physiological mechanisms and examine impairments related to cardiovascular disease.

Contribution of BK(Ca) channels to local metabolic coronary vasodilation: Effects of metabolic syndrome

This investigation was designed to examine the hypothesis that impaired function of coronary microvascular large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels in metabolic syndrome (MetS) significantly attenuates the balance between myocardial oxygen delivery and metabolism at rest and during exercise-induced increases in myocardial oxygen consumption (MVo(2)). Studies were conducted in conscious, chronically instrumented Ossabaw swine fed a normal maintenance diet (11% kcal from fat) or an excess calorie atherogenic diet (43% kcal from fat, 2% cholesterol, 20% kcal from fructose) that induces many common features of MetS. Data were collected under baseline/resting conditions and during graded treadmill exercise before and after selective blockade of BK(Ca) channels with penitrem A (10 microg/kg iv). We found that the exercise-induced increases in blood pressure were significantly elevated in MetS swine. No differences in baseline cardiac function or heart rate were noted. Induction of MetS produced a parallel downward shift in the relationship between coronary venous Po(2) and MVo(2) (P < 0.001) that was accompanied by a marked release of lactate (negative lactate uptake) as MVo(2) was increased with exercise (P < 0.005). Inhibition of BK(Ca) channels with penitrem A did not significantly affect blood pressure, heart rate, or the relationship between coronary venous Po(2) and MVo(2) in lean or MetS swine. These data indicate that BK(Ca) channels are not required for local metabolic control of coronary blood flow under physiological (lean) or pathophysiological (MetS) conditions. Therefore, diminished function of BK(Ca) channels does not contribute to the impairment of myocardial oxygen-supply demand balance in MetS.

Impaired function of coronary BK(Ca) channels in metabolic syndrome

The role of large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels in regulation of coronary microvascular function is widely appreciated, but molecular and functional changes underlying the deleterious influence of metabolic syndrome (MetS) have not been determined. Male Ossabaw miniature swine consumed for 3-6 mo a normal diet (11% kcal from fat) or an excess-calorie atherogenic diet that induces MetS (45% kcal from fat, 2% cholesterol, 20% kcal from fructose). MetS significantly impaired coronary vasodilation to the BK(Ca) opener NS-1619 in vivo (30-100 microg) and reduced the contribution of these channels to adenosine-induced microvascular vasodilation in vitro (1-100 microM). MetS reduced whole cell penitrem A (1 microM)-sensitive K(+) current and NS-1619-activated (10 microM) current in isolated coronary vascular smooth muscle cells. MetS increased the concentration of free intracellular Ca(2+) and augmented coronary vasoconstriction to the L-type Ca(2+) channel agonist BAY K 8644 (10 pM-10 nM). BK(Ca) channel alpha and beta(1) protein expression was increased in coronary arteries from MetS swine. Coronary vascular dysfunction in MetS is related to impaired BK(Ca) channel function and is accompanied by significant increases in L-type Ca(2+) channel-mediated coronary vasoconstriction.

Diabetic dyslipidemia and exercise alter the plasma low-density lipoproteome in Yucatan pigs

Although low-density lipoprotein (LDL) plays a predominant role in atherogenesis, the low-density lipoproteome has not been fully characterized. Moreover, alterations from a Western diet, diabetes, and physical inactivity on this proteome have yet to be determined. Accordingly, relative quantification was determined in LDL proteins from male Yucatan diabetic dyslipidemic (DD) swine in the early stages of atherosclerosis compared to healthy control (C) and non-diabetic hyperlipidemic (H) swine. Importantly, coronary vascular dysfunction was prevented by aerobic exercise training in these animals (DDX) without altering total LDL concentration. Using 2-DE, Western blot, label-free quantitative MS, and selected reaction monitoring, alterations in the abundance of apolipoproteins A-I, B, C-III, D, E, and J and noncovalently associated proteins were determined in LDL isolated using fast protein liquid chromatography. At least 28 unique proteins, many of which were novel, were identified with high confidence. An apolipoprotein E isoform demonstrated stronger correlation to disease (percent of coronary artery segments with intimal thickening) than some traditional risk factors (total cholesterol, LDL cholesterol, and LDL/HDL cholesterol). Taken together, this work identifies new possible biomarkers, potential therapeutic targets for atherosclerosis, and generates new hypotheses regarding the role of LDL in atherogenesis.

Leukemia inhibitory factor is upregulated in coronary arteries of Ossabaw miniature swine after stent placement

Leukemia inhibitory factor (LIF), an IL-6 class cytokine, is reported to be antiatherosclerotic. Thus, we hypothesized that LIF expression might be altered during in-stent neointimal hyperplasia. Ossabaw miniature swine, a unique large-animal model of metabolic syndrome and cardiovascular disease, were used for these studies. Bare-metal stents were deployed in the left anterior descending and left circumflex coronary arteries. Stents were expanded to either 1.0 x luminal diameter (in accordance with current clinical practice) or 1.3 x (overexpansion). The development of in-stent neointimal hyperplasia was assessed 28-day postimplantation using intravascular ultrasound. The atherosclerotic coverage of the vessel wall was approximately five-fold higher in 1.0 x stents and approximately nine-fold higher in 1.3 x stents 4 weeks after deployment, compared with the same segments before stenting. LIF mRNA was elevated approximately 11-fold in stented segments, relative to unstented epicardial coronary arteries. LIF expression and the intima : media ratio were strongly correlated in 1.0 x stented vessels. Further studies to investigate the nature of the association between LIF and neointimal hyperplasia revealed that vascular smooth muscle cell proliferation was inhibited by LIF treatment in an in-vitro model of atherosclerosis (coronary artery organ culture). These novel and clinically relevant studies show that elevated LIF gene expression is predictive for in-stent neointimal hyperplasia, and suggest that LIF upregulation may be a compensatory mechanism in this setting.

Effects of exercise training and hypercholesterolemia on adenosine activation of voltage-dependent K+ channels in coronary arterioles

Coronary arterioles from hypercholesterolemic swine display attenuated adenosine-mediated vasodilatation that is attributable to the elimination of voltage-dependent K(+) (Kv) channel stimulation. For the present study, we tested the hypotheses that exercise training would correct impaired adenosine-induced dilatation in coronary arterioles from hypercholesterolemic pigs through restoration of adenosine activation of Kv channels and that vasodilatation to the receptor-independent adenylyl cyclase activator, forskolin, would also be attenuated in arterioles from hypercholesterolemic pigs. Pigs were randomly assigned to a control (NC) or high-fat, high-cholesterol (HC) diet for 20 wk. Four weeks after the diet was initiated, pigs from both groups were assigned to exercise training (Ex; 5 days/wk for 16 wk) or sedentary (Sed) protocols, resulting in four groups of pigs: NC-Sed, NC-Ex, HC-Sed, and HC-Ex. Arterioles ( approximately 150 mum) from both HC-Sed and HC-Ex pigs displayed impaired adenosine-mediated dilatation that was attributable to the elimination of 4-aminopyridine (4-AP; 1 mM)-sensitive Kv channel activation compared with NC counterparts. Arteriolar smooth muscle whole cell Kv currents were significantly reduced in HC-Sed compared with NC-Sed, although HC-Ex and NC-Ex did not differ. Forskolin-mediated dilatation was attenuated by 4-AP (1 mM) and in a concentration-dependent manner by tetraethylammonium (TEA; 0.1-1 mM) in NC-Sed but not HC-Sed. Further, TEA-sensitive Kv currents were diminished in cells of HC-Sed compared with NC-Sed pigs. Quantitative RT-PCR revealed similar expression levels of Kv3.1 and 3.3 in arterioles of NC-Sed and HC-Sed swine with undetectable expression of Kv1.1, 3.2, and 3.4. Taken together, these results suggest that hypercholesterolemia-mediated attenuation of adenosine-induced vasodilatation in coronary arterioles is not corrected by exercise training and is likely attributable to an impairment in the pathway coupling adenylyl cyclase with a highly TEA-sensitive Kv channel isoform(s).

Potassium channels sensitive to combination of charybdotoxin and apamin regulate the tone of diabetic isolated canine coronary arteries

AIMS

Functional roles of calcium-activated potassium channels on the mechanical activity of epicardial coronary arteries obtained from a canine model of diabetes were investigated.

METHODS

Coronary arteries were isolated from healthy, alloxan-diabetic and insulin-treated diabetic dogs. Basal tensions, contractions induced by the prostaglandin (PG) analogue, U46619, and endothelium-dependent relaxations to acetylcholine (ACh) were modified with charybdotoxin (CHTX) + apamin (APA), inhibitors of calcium-activated potassium channels, as well as with N(omega)-nitro-l-arginine (LNA) + indomethacin (INDO) to suppress the synthesis of nitric oxide (NO) and PGs. The relaxing effect of nitroprusside-sodium (SNP), an NO donor, was also determined.

RESULTS

In diabetic coronary arteries, CHTX + APA did not change while LNA + INDO elevated the basal tension. PG-induced contractions were enhanced by CHTX + APA and by LNA + INDO in all the three groups of animals. CHTX + APA decreased the maximal relaxations to ACh in a partly insulin-dependent manner. LNA + INDO abolished the endothelium-dependent relaxations to ACh. In diabetic coronary arteries, the sensitivity to SNP-induced relaxation was enhanced, insulin independently, suggesting that NO could be partly responsible for maintaining intact ACh-induced vasorelaxation.

CONCLUSION

In diabetic canine coronary artery, the vasomotor responses reflect up-regulation of calcium-activated potassium channels. This endothelial mechanism of the canine epicardial coronary artery may oppose vasoconstrictions in diabetic vascular tissue.

Exercise training prevents Ca2+ dysregulation in coronary smooth muscle from diabetic dyslipidemic yucatan swine

Aerobic exercise training is known to have profound cardioprotective effects in disease, yet cellular mechanisms remain largely undefined. We tested the hypothesis that increased sarcoplasmic reticulum Ca(2+) buffering and increased voltage-gated Ca(2+) channel density underlie coronary smooth muscle intracellular Ca(2+) (Ca(2+)(i)) dysregulation in diabetic dyslipidemia and that exercise training would prevent these increases. Yucatan swine were maintained in 1) control, 2) alloxan-induced hyperglycemic, 3) high fat/cholesterol fed, 4) hyperglycemic plus high fat/cholesterol fed (diabetic dyslipidemic), and 5) diabetic dyslipidemic plus exercise-trained (treadmill running) conditions. After 20 wk, the heart was removed and smooth muscle cells isolated from the right coronary artery. We utilized fura-2 imaging of Ca(2+)(i) levels to separate the functional role of the sarcoendoplasmic reticulum Ca(2+)-ATPase (SERCA) from the Na(+)-Ca(2+) exchanger and the plasmalemmal Ca(2+)-ATPase, and whole-cell patch clamp to examine voltage-gated Ca(2+) channel current density (i.e., Ca(2+) influx). Results indicated that diabetic dyslipidemia impaired plasmalemmal Ca(2+) efflux, increased basal Ca(2+)(i) levels, increased SERCA protein and sarcoplasmic reticulum Ca(2+)(i) buffering, and elicited an approximately 50% decrease in voltage-gated Ca(2+) channel current density. Exercise training concurrent with the diabetic dyslipidemic state restored plasmalemmal Ca(2+) efflux, SERCA protein, sarcoplasmic reticulum Ca(2+)(i) buffering, and voltage-gated Ca(2+) channel current density to control levels. Interestingly, basal Ca(2+)(i) levels were significantly lower in the exercise-trained group compared with control. Collectively, these results demonstrate a crucial role for exercise in the prevention of diabetic dyslipidemia-induced Ca(2+)(i) dysregulation.

Hypercholesterolemia abolishes voltage-dependent K+ channel contribution to adenosine-mediated relaxation in porcine coronary arterioles

Hypercholesterolemic patients display reduced coronary flow reserve in response to adenosine infusion. We previously reported that voltage-dependent K+ (Kv) channels contribute to adenosine-mediated relaxation of coronary arterioles isolated from male miniature swine. For this study, we hypothesized that hypercholesterolemia attenuates Kv channel contribution to adenosine-induced vasodilatation. Pigs were randomly assigned to a control or high fat/high cholesterol diet for 20–24 wk, and then killed. After completion of the experimental treatment, arterioles (∼150 μm luminal diameter) were isolated from the left-ventricular free wall near the apical region of the heart, cannulated, and pressurized at 40 mmHg. Adenosine-mediated relaxation was significantly attenuated in both endothelium-intact and -denuded arterioles from hypercholesterolemic compared with control animals. The classic Kv channel blocker, 4-aminopyridine (1 mM), significantly attenuated adenosine-mediated relaxation in arterioles isolated from control but not hypercholesterolemic animals. Furthermore, the nonselective K+ channel blocker, tetraethylammonium (TEA; 1 mM) significantly attenuated adenosine-mediated relaxation in arterioles from control but not hypercholesterolemic animals. In additional experiments, coronary arteriolar smooth muscle cells were isolated, and whole cell K v currents were measured. Kv currents were significantly reduced (∼15%) in smooth muscle cells from hypercholesterolemic compared with control animals. Furthermore, Kv current sensitive to low concentrations of TEA was reduced (∼45%) in smooth muscle cells from hypercholesterolemic compared with control animals. Our data indicate that hypercholesterolemia abolishes Kv channel contribution to adenosine-mediated relaxation in coronary arterioles, which may be attributable to a reduced contribution of TEA-sensitive K v channels in smooth muscle of hypercholesterolemic animals.

Diabetic dyslipidemia and exercise affect coronary tone and differential regulation of conduit and microvessel K+ current

Spontaneous transient outward K(+) currents (STOCs) elicited by Ca(2+) sparks and steady-state K(+) currents modulate vascular reactivity, but effects of artery size, diabetic dyslipidemia, and exercise on these differentially regulated K(+) currents are unclear. We studied the conduit arteries and microvessels of male Yucatan swine assigned to one of three groups for 20 wk: control (C, n = 7), diabetic dyslipidemic (DD, n = 6), or treadmill-trained DD animals (DDX, n = 7). Circumflex artery blood flow velocity obtained with intracoronary Doppler and lumen diameters obtained by intravascular ultrasound enabled calculation of absolute coronary blood flow (CBF). Ca(2+) sparks were determined in pressurized microvessels, and perforated patch clamp assessed K(+) current in smooth muscle cells isolated from conduits and microvessels. Baseline CBF in DD was decreased versus C. In pressurized microvessels, Ca(2+) spark activity was significantly lower in DD versus C and DDX (P < 0.05 vs. DDX). STOCs were pronounced in microvessel (approximately 35 STOCs/min) in sharp contrast to conduit cells ( approximately 2 STOCs/min). STOCs were decreased by 86% in DD versus C and DDX in microvessels; in contrast, there was no difference in STOCs across groups in conduit cells. Steady-state K(+) current in microvessels was decreased in DD and DDX versus C; in contrast, steady-state K(+) current in conduit cells was decreased in DDX versus DD and C. We conclude that steady-state K(+) current and STOCs are differentially regulated in conduit versus microvessels in health and diabetic dyslipidemia. Exercise prevented diabetic dyslipidemia-induced decreases in baseline CBF, possibly via STOC-regulated basal microvascular tone.

Exercise improves impaired ventricular function and alterations of cardiac myofibrillar proteins in diabetic dyslipidemic pigs

Chronic diabetes is often associated with cardiomyopathy, which may result, in part, from defects in cardiac muscle proteins. We investigated whether a 20-wk porcine model of diabetic dyslipidemia (DD) would impair in vivo myocardial function and yield alterations in cardiac myofibrillar proteins and whether endurance exercise training would improve these changes. Myocardial function was depressed in anesthetized DD pigs (n = 12) compared with sedentary controls (C; n = 13) as evidenced by an approximately 30% decrease in left ventricular fractional shortening and an approximately 35% decrease in +dP/dt measured by noninvasive echocardiography and direct cardiac catheterization, respectively. This depression in myocardial function was improved with chronic exercise as treadmill-trained DD pigs (DDX) (n = 13) had significantly greater fractional shortening and +dP/dt than DD animals. Interestingly, the isoform expression pattern of the myofibrillar regulatory protein, cardiac troponin T (cTnT), was significantly shifted from cTnT1 toward cTnT2 and cTnT3 in DD pigs. Furthermore, this change in cTnT isoform expression pattern was prevented in DDX pigs. Finally, there was a decrease in baseline levels of cAMP-dependent protein kinase-induced phosphorylation of the myofibrillar proteins troponin I and myosin-binding protein-C in DD animals. Overall, these results indicate that 20 wk of DD lead to myocardial dysfunction coincident with significant alterations in myofibrillar proteins, both of which are prevented with endurance exercise training, implying that changes in myofibrillar proteins may contribute, at least in part, to cardiac dysfunction associated with diabetic cardiomyopathy.

Short-term aerobic exercise improves arterial stiffness in type 2 diabetes

OBJECTIVE

To investigate whether: (1) aerobic exercise decreases arterial stiffness and (2) reduction in arterial stiffness is associated with improvement in insulin resistance in type 2 diabetes.

METHODS

Common carotid and femoral arterial stiffness was ultrasonographically evaluated using stiffness index beta in 23 type 2 diabetic subjects before and after a 3-week exercise protocol including ergometer and walking. Insulin sensitivity (Clamp-IR) was assessed using euglycemic-hyperinsulinemic clamp before and after the protocol. Arterial stiffness was also examined in steady hyperinsulinemic state during clamp.

RESULTS

Anthropometrical factors did not change following exercise. Clamp-IR tended to increase after exercise protocol (P = 0.061). Stiffness index beta decreased following exercise in both common carotid and femoral arteries (P = 0.020 and P < 0.001, respectively). DeltaClamp-IR was significantly correlated with the changes in stiffness index beta of both common carotid (P = 0.040) and femoral artery (P = 0.016). Divided into tertiles according to DeltaClamp-IR, decreases in stiffness index beta for both common carotid (P = 0.009) and femoral (P = 0.037) arteries was greater in tertile group with a higher DeltaClamp-IR. Hyperinsulinemia during clamp decreased stiffness index beta in both common carotid (P = 0.031) and femoral (P = 0.025) arteries before exercise, but these effects disappeared after the exercise protocol.

CONCLUSIONS

Short-term aerobic exercise significantly decreased arterial stiffness in both common carotid and femoral arteries, and the reduction of stiffness was associated with improvement of insulin resistance in type 2 diabetes.

Mechanisms of tension changes of the sphincter of Oddi from hypercholesterolemic rabbits

AIM: To observe the effects of hypercholesterolemia (HC) on tension of sphincter of Oddi (SO) in rabbits and to study the mechanisms of the sphincter of Oddi dysfunction (SOD).

METHODS: Twenty-four New Zealand female rabbits were divided randomly into control group and HC group (12 rabbits each). Sphincter of Oddi muscle rings were dissociated from both groups in vitro. Automatic contractility was observed firstly. Then the contraction responses evoked by KCl and CaCl₂ and relaxation responses caused by sodium nitroprusside (SNP) and nifedipine (Nif) were measured.

RESULTS: Compared with control group, the automatic contractile frequency of HC group was increased (P < 0.05), and the automatic contractile amplitude of HC group was decreased (P < 0.05). The tension of SO rings evoked by KCl at low and moderate concentrations (10-40 mmol/L) was significantly higher in HC group than in control group (P < 0.01). The maximum tension was not found difference between two groups, and both could be completely relaxed by Nif (3 mmol/L). Compared with the control group, relaxation responses of SO rings in HC group to SNP (0.1 nmoL/L-1 mmoL/L) were markedly decreased after the administration of KCl (60 mmoL/L) in HC group (P < 0.01). The minimum solution value (0.1 mmoL/L) of Ca²⁺ evoking contraction in HC group was evidently lower than that of the control group (1.0 mmoL/L) (P < 0.001). Tensions of SO rings evoked by Ca²⁺ (2.5 mmoL/L) were entirely relaxed by Nif (3 mmoL/L) both in HC group and control group. After the administration of KCl (60 mmoL/L) relaxation responses of rabbit SO rings to Nif (0.1 nmoL/L-3 mmoL/L) were not found difference between in the two groups. Nif (3 mmoL/L) could completely inhibit the contraction responses evoked by KCl or CaCl₂in both groups.

CONCLUSION: Hypercholesterolemia can lead to SO dysfunction and increase the sensitivity of SO to KCl at low and moderate concentrations in vitro. The overloading of intracellular Ca²⁺ is an important reason for these phenomena, which has no direct relation with L-type voltage-dependent calcium channels of smooth muscle cells.