Lactate activates ATP-sensitive potassium channels in guinea pig ventricular myocytes

Lactate: A key regulator of the immune response

Lactate, the end product of both anaerobic and aerobic glycolysis in proliferating and growing cells-with the latter process known as the Warburg effect-is historically considered a mere waste product of cell and tissue metabolism. However, research over the past ten years has unveiled multifaceted functions of lactate that critically shape and impact cellular biology. Beyond serving as a fuel source, lactate is now known to influence gene expression through histone modification and to function as a signaling molecule that impacts a wide range of cellular activities. These properties have been particularly studied in the context of both adaptive and innate immune responses. Here, we review the diverse roles of lactate in the regulation of the immune system during homeostasis and disease pathogenesis (including cancer, infection, cardiovascular diseases, and autoimmunity). Furthermore, we describe recently proposed therapeutic interventions for manipulating lactate metabolism in human diseases.

Vasoplegic Syndrome in Cardiac Surgery: A Narrative Review of Etiologic Mechanisms and Therapeutic Options

Vasoplegic syndrome, a form of distributive shock that may manifest during or after cardiopulmonary bypass, is a serious complication that increases morbidity and mortality after cardiac surgery. No consensus definition exists, but vasoplegic syndrome is generally described as a state of pathologic vasodilation causing hypotension refractory to fluid resuscitation and vasopressor therapy, and resulting in organ malperfusion despite a normal or increased cardiac output. Diagnosis can be complex as there is a broad differential diagnosis for low systemic vascular resistance in the cardiac surgical patient. Interpretation of hemodynamic data can also be challenging in the setting of mixed shock states and mechanical support. This narrative review summarizes the pathophysiology of vasoplegic syndrome, the literature concerning its incidence and risk factors, the hemodynamic parameters important to the diagnosis of vasoplegic syndrome, a consensus definition of the syndrome, and a proposed goal-directed treatment framework.

CNDP2: An Enzyme Linking Metabolism and Cardiovascular Diseases?

The heart requires a substantial amount of energy to function, utilising various substrates including lipids, glucose and lactate as energy sources. In times of increased stress, lactate becomes the primary energy source of the heart, but persistently elevated lactate levels are linked to poor patient outcomes and increased mortality. Recently, carnosine dipeptidase II (CNDP2) was discovered to catalyse the formation of Lac-Phe, an exercise-induced metabolite derived from lactate, which has been shown to suppress appetite in mice and reduce adipose tissue in humans. This review discusses CNDP2, including its role in lactate clearance, carnosine hydrolysis, oxidative stress regulation, and involvement in metabolite regulation. The association between CNDP2 and cardiometabolic and renal diseases is also explored, and knowledge gaps are highlighted. CNDP2 appears to be a complex participant in human physiological processes and disease, necessitating additional research to unveil its functions and potential therapeutic applications.

Progress in Lactate Metabolism and Its Regulation via Small Molecule Drugs

Lactate, once viewed as a byproduct of glycolysis and a metabolic "waste", is now recognized as an energy-providing substrate and a signaling molecule that modulates cellular functions under pathological conditions. The discovery of histone lactylation in 2019 marked a paradigm shift, with subsequent studies revealing that lactate can undergo lactylation with both histone and non-histone proteins, implicating it in the pathogenesis of various diseases, including cancer, liver fibrosis, sepsis, ischemic stroke, and acute kidney injury. Aberrant lactate metabolism is associated with disease onset, and its levels can predict disease outcomes. Targeting lactate production, transport, and lactylation may offer therapeutic potential for multiple diseases, yet a systematic summary of the small molecules modulating lactate and its metabolism in various diseases is lacking. This review outlines the sources and clearance of lactate, as well as its roles in cancer, liver fibrosis, sepsis, ischemic stroke, myocardial infarction, and acute kidney injury, and summarizes the effects of small molecules on lactate regulation. It aims to provide a reference and direction for future research.

Atrioventricular re-entrant tachycardia and atrioventricular node re-entrant tachycardia in a patient with cancer under chemotherapy: a case report and literature review

Cardio-oncology is a new field of interest in cardiology focusing on the detection and treatment of cardiovascular diseases, such as arrhythmias, myocarditis, and heart failure, as side-effects of chemotherapy and radiotherapy. The association between chemotherapeutic agents and arrhythmias has previously been established. Atrial tachyarrhythmias, particularly atrial fibrillation, are most common, but ventricular arrhythmias, including those related to treatment-induced QT prolongation, and bradyarrhythmias can also occur. However, the association between chemotherapeutic agents and atrioventricular re-entrant tachycardia (AVRT)/atrioventricular node re-entrant tachycardia (AVNRT) remains poorly understood. Here, we report a patient with new-onset AVRT/AVNRT and lung cancer who underwent chemotherapy. We considered that chemotherapy or cancer itself may have been a trigger for the initiation of paroxysmal AVRT/AVNRT, and that radiofrequency catheter ablation was effective in treating this type of tachycardia. Here, possible mechanisms and potential genes (mostly ion channels) involved in AVRT/AVNRT are summarized and the mechanisms underlying the possible regulatory patterns of cancer cells and chemotherapy on ion channels are reviewed. Finally, we considered that ion channel abnormalities may link cancer or chemotherapy to the onset of AVRT/AVNRT. The aim of the present study was to highlight the association between chemotherapeutic agents and AVRT/AVNRT and to provide new insights for future research. Understanding the intermediate mechanisms between chemotherapeutic agents and AVRT/AVNRT may be beneficial in preventing chemotherapy-evoked AVRT/AVNRT (and/or other arrhythmias) in future.

The role of lactate in cardiovascular diseases

Cardiovascular diseases pose a major threat worldwide. Common cardiovascular diseases include acute myocardial infarction (AMI), heart failure, atrial fibrillation (AF) and atherosclerosis. Glycolysis process often has changed during these cardiovascular diseases. Lactate, the end-product of glycolysis, has been overlooked in the past but has gradually been identified to play major biological functions in recent years. Similarly, the role of lactate in cardiovascular disease is gradually being recognized. Targeting lactate production, regulating lactate transport, and modulating circulating lactate levels may serve as potential strategies for the treatment of cardiovascular diseases in the future. The purpose of this review is to integrate relevant clinical and basic research on the role of lactate in the pathophysiological process of cardiovascular disease in recent years to clarify the important role of lactate in cardiovascular disease and to guide further studies exploring the role of lactate in cardiovascular and other diseases. Video Abstract.

Current understanding of the contribution of lactate to the cardiovascular system and its therapeutic relevance

Research during the past decades has yielded numerous insights into the presence and function of lactate in the body. Lactate is primarily produced via glycolysis and plays special roles in the regulation of tissues and organs, particularly in the cardiovascular system. In addition to being a net consumer of lactate, the heart is also the organ in the body with the greatest lactate consumption. Furthermore, lactate maintains cardiovascular homeostasis through energy supply and signal regulation under physiological conditions. Lactate also affects the occurrence, development, and prognosis of various cardiovascular diseases. We will highlight how lactate regulates the cardiovascular system under physiological and pathological conditions based on evidence from recent studies. We aim to provide a better understanding of the relationship between lactate and cardiovascular health and provide new ideas for preventing and treating cardiovascular diseases. Additionally, we will summarize current developments in treatments targeting lactate metabolism, transport, and signaling, including their role in cardiovascular diseases.

Vasoplegic Syndrome after Cardiopulmonary Bypass in Cardiovascular Surgery: Pathophysiology and Management in Critical Care

Vasoplegic syndrome (VS) is a common complication following cardiovascular surgery with cardiopulmonary bypass (CPB), and its incidence varies from 5 to 44%. It is defined as a distributive form of shock due to a significant drop in vascular resistance after CPB. Risk factors of VS include heart failure with low ejection fraction, renal failure, pre-operative use of angiotensin-converting enzyme inhibitors, prolonged aortic cross-clamp and left ventricular assist device surgery. The pathophysiology of VS after CPB is multi-factorial. Surgical trauma, exposure to the elements of the CPB circuit and ischemia-reperfusion promote a systemic inflammatory response with the release of cytokines (IL-1β, IL-6, IL-8, and TNF-α) with vasodilating properties, both direct and indirect through the expression of inducible nitric oxide (NO) synthase. The resulting increase in NO production fosters a decrease in vascular resistance and a reduced responsiveness to vasopressor agents. Further mechanisms of vasodilation include the lowering of plasma vasopressin, the desensitization of adrenergic receptors, and the activation of ATP-dependent potassium (KATP) channels. Patients developing VS experience more complications and have increased mortality. Management includes primarily fluid resuscitation and conventional vasopressors (catecholamines and vasopressin), while alternative vasopressors (angiotensin 2, methylene blue, hydroxocobalamin) and anti-inflammatory strategies (corticosteroids) may be used as a rescue therapy in deteriorating patients, albeit with insufficient evidence to provide any strong recommendation. In this review, we present an update of the pathophysiological mechanisms of vasoplegic syndrome complicating CPB and discuss available therapeutic options.

Definitions and pathophysiology of vasoplegic shock

Vasoplegia is the syndrome of pathological low systemic vascular resistance, the dominant clinical feature of which is reduced blood pressure in the presence of a normal or raised cardiac output. The vasoplegic syndrome is encountered in many clinical scenarios, including septic shock, post-cardiac bypass and after surgery, burns and trauma, but despite this, uniform clinical definitions are lacking, which renders translational research in this area challenging. We discuss the role of vasoplegia in these contexts and the criteria that are used to describe it are discussed. Intrinsic processes which may drive vasoplegia, such as nitric oxide, prostanoids, endothelin-1, hydrogen sulphide and reactive oxygen species production, are reviewed and potential for therapeutic intervention explored. Extrinsic drivers, including those mediated by glucocorticoid, catecholamine and vasopressin responsiveness of the blood vessels, are also discussed. The optimum balance between maintaining adequate systemic vascular resistance against the potentially deleterious effects of treatment with catecholamines is as yet unclear, but development of novel vasoactive agents may facilitate greater understanding of the role of the differing pathways in the development of vasoplegia. In turn, this may provide insights into the best way to care for patients with this common, multifactorial condition.

Blood Lactate Accumulation: Hemodynamics and Acid Base Status

The authors evaluated the relationship between blood lactate accumulation and hemodynamics in a prospective controlled animal study. Seven healthy pigs were anesthetized and mechanically ventilated with a 0.6 fraction of inspired oxygen to achieve a normal PaCO2. Catheters for blood sampling, lactate infusion, and hemodynamic assessment were inserted into the femoral vein and femoral artery. Bolus infusions (466 µmol • kg— 1 over 1 minute) of a neutral solution of sodium lactate were given at 0, 30, 60, and 90 minutes. Each bolus was followed by continuous infusion of the same lactate solution (31.7 µmol • kg— 1 • min—1 after the first, with the rate doubled after each subsequent bolus). Arterial blood samples were drawn at baseline and at 30, 60, 90, and 120 minutes, with simultaneous hemodynamic assessment. Significant increases were seen in blood lactate (from 1.6 ± 0.4 mmol/L at baseline to 4.4 ± 0.8 at 30 minutes, 8.0 ± 1.0 at 60 minutes, and 14.4 ± 2.1 at 90 minutes), pH (from 7.40 ± 0.03 to 7.44 ± 0.05 at 30 minutes, 7.50 ± 0.05 at 60 minutes, and 7.56 ± 0.04 at 90 minutes), and HCO3 — (from 28 ± 1.4 mmol/L to 30 ± 0.7 at 30 minutes, 34 ± 0.9 at 60 minutes, and 37 ± 1.2 at 90 minutes). PaCO2 decreased significantly from 44 ± 3 torr at baseline to 40 ± 4.7 torr at 90 minutes. Blood pressure decreased after each lactate bolus, recovering within 2 minutes initially but not after later injections. Cardiac output, heart rate, and glucose increased significantly, mean arterial pressure and systemic vascular resistance decreased, and stroke volume did not change. Thus, the accumulation of blood lactate due to exogenous administration of a neutral solution of sodium lactate was associated with significant systemic vasodilation, increased cardiac output, increased heart rate, and metabolic alkalosis.

Effects of pH on nifekalant-induced electrophysiological change assessed in the Langendorff heart model of guinea pigs

Since information regarding the effects of pH on the extent of nifekalant-induced repolarization delay and torsades de pointes remains limited, we assessed it with a Langendorff heart model of guinea pigs. First, we investigated the effects of pH change from 7.4 to 6.4 on the bipolar electrogram simulating surface lead II ECG, monophasic action potential (MAP), effective refractory period (ERP), and terminal repolarization period (TRP) and found that acidic condition transiently enhanced the ventricular repolarization. Next, we investigated the effects of pH change from 6.4 to 7.4 in the presence of nifekalant (10 μM) on the ECG, MAP, ERP, TRP, and short-term variability (STV) of MAP90 and found that the normalization of pH prolonged the MAP90 and ERP while the TRP remained unchanged, suggesting the increase in electrical vulnerability of the ventricle. Meanwhile, the STV of MAP90 was the largest at pH 6.4 in the presence of nifekalant, indicating the increase in temporal dispersion of repolarization, which gradually decreased with the return of pH to 7.4.Thus, a recovery period from acidosis might be more dangerous than during the acidosis, because electrical vulnerability may significantly increase for this period while temporal dispersion of repolarization remained increased.

Metabolic determinants of electrical failure in ex-vivo canine model of cardiac arrest: evidence for the protective role of inorganic pyrophosphate

RATIONALE

Deterioration of ventricular fibrillation (VF) into asystole or severe bradycardia (electrical failure) heralds a fatal outcome of cardiac arrest. The role of metabolism in the timing of electrical failure remains unknown.

OBJECTIVE

To determine metabolic factors of early electrical failure in an ex-vivo canine model of cardiac arrest (VF+global ischemia).

METHODS AND RESULTS

Metabolomic screening was performed in left ventricular biopsies collected before and after 0.3, 2, 5, 10 and 20 min of VF and global ischemia. Electrical activity was monitored via plunge needle electrodes and pseudo-ECG. Four out of nine hearts exhibited electrical failure at 10.1±0.9 min (early-asys), while 5/9 hearts maintained VF for at least 19.7 min (late-asys). As compared to late-asys, early-asys hearts had more ADP, less phosphocreatine, and higher levels of lactate at some time points during VF/ischemia (all comparisons p<0.05). Pre-ischemic samples from late-asys hearts contained ∼25 times more inorganic pyrophosphate (PPi) than early-asys hearts. A mechanistic role of PPi in cardioprotection was then tested by monitoring mitochondrial membrane potential (ΔΨ) during 20 min of simulated-demand ischemia using potentiometric probe TMRM in rabbit adult ventricular myocytes incubated with PPi versus control group. Untreated myocytes experienced significant loss of ΔΨ while in the PPi-treated myocytes ΔΨ was relatively maintained throughout 20 min of simulated-demand ischemia as compared to control (p<0.05).

CONCLUSIONS

High tissue level of PPi may prevent ΔΨm loss and electrical failure at the early phase of ischemic stress. The link between the two protective effects may involve decreased rates of mitochondrial ATP hydrolysis and lactate accumulation.

Nitric oxide-mediated relaxation to lactate of coronary circulation in the isolated perfused rat heart

The objective of this study was to analyze the effects of lactate on coronary circulation. Rat hearts were perfused in a Langendorff preparation, and the coronary response to lactate (3-30 mM) was recorded after precontracting coronary vasculature with 11-dideoxy-1a,9a-epoxymethanoprostaglandin F2α (U46619), in the presence or the absence of the inhibitor of nitric oxide synthesis, N-omega-nitro-l-arginine methyl ester (l-NAME, 10 M), the blocker of Ca-dependent potassium channels, tetraethylammonium (TEA, 10 M), or the blocker of adenosine triphosphate-sensitive potassium channels, glybenclamide (10 M). The effects of lactate were also studied in isolated segments of rat coronary arteries that were precontracted with U46619, with or without endothelium. In perfused hearts, lactate induced concentration-dependent coronary vasodilatation and a reduction in myocardial contractility (left ventricular developed pressure and dP/dt) without altering the heart rate. Coronary vasodilatation in response to lactate was reduced by l-NAME but unaffected by TEA or glybenclamide. The effects of lactate on myocardial contractility were unchanged by l-NAME, TEA, or glybenclamide. In isolated coronary artery segments, lactate also produced relaxation, an effect attenuated by removing the endothelium. Together these findings suggest that lactate exerts coronary vasodilatory effects through the release of endothelial nitric oxide, independently of potassium channels. These findings may be relevant for the regulation of coronary circulation when lactate levels are elevated.

Physiologic responses to severe hemorrhagic shock and the genesis of cardiovascular collapse: can irreversibility be anticipated?

BACKGROUND

The causes of cardiovascular collapse (CC) during hemorrhagic shock (HS) are unknown. We hypothesized that vascular tone loss characterizes CC, and that arterial pulse pressure/stroke volume index ratio or vascular tone index (VTI) would identify CC.

METHODS

Fourteen Yorkshire-Durock pigs were bled to 30 mmHg mean arterial pressure and held there by repetitive bleeding until rendered unable to compensate (CC) or for 90 min (NoCC). They were then resuscitated in equal parts to shed volume and observed for 2 h. CC was defined as a MAP < 30 mmHg for 10 min or <20 mmHg for 10 s. Study variables were recorded at baseline (B0), 30, 60, 90 min after bleeding and at resuscitation (R0), 30, and 60 min afterward.

RESULTS

Swine were bled to 32% ± 9% of total blood volume. Epinephrine (Epi) and VTI were low and did not change in NoCC after bleeding compared with CC swine, in which both increased (0.97 ± 0.22 to 2.57 ± 1.42 mcg/dL, and 173 ± 181 to 939 ± 474 mmHg/mL, respectively), despite no differences in bled volume. Lactate increase rate (LIR) increased with hemorrhage and was higher at R0 for CC, but did not vary in NoCC. VTI identified CC from NoCC and survivors from non-survivors before CC. A large increase in LIR was coincident with VTI decrement before CC occurred.

CONCLUSIONS

Vasodilatation immediately prior to CC in severe HS occurs at the same time as an increase in LIR, suggesting loss of tone as the mechanism causing CC, and energy failure as its probable cause.

Early on-cardiopulmonary bypass hypotension and other factors associated with vasoplegic syndrome

BACKGROUND

Vasoplegic syndrome is a form of vasodilatory shock that can occur after cardiopulmonary bypass (CPB). We hypothesized that the severity and duration of the decline in mean arterial pressure immediately after CPB is begun can be used as a predictor of patients will develop vasoplegia in the immediate post-CPB period and of poor clinical outcome. We quantified the decline in mean arterial pressure by calculating an area above the mean arterial blood pressure curve.

METHODS AND RESULTS

We retrospectively analyzed 2823 adult cardiac surgery cases performed between July 2002 and December 2006. Of these 2823, 577 (20.4%) were vasoplegic after separation from CPB. We found that 1645 patients (58.3%) had a clinically significant decline in mean arterial pressure after starting CPB (area above the mean arterial blood pressure curve >0) and were significantly more likely to become vasoplegic (23.0% versus 16.9%; odds ratio, 1.26; 95% confidence interval, 1.12 to 1.43; P<0.001). These patients were also far more likely either to die in hospital or to have a length of stay >10 days (odds ratio, 3.30; 95% confidence interval, 1.44 to 7.57; P=0.005). Additional risk factors for developing vasoplegia that were identified included the additive euroSCORE, procedure type, prebypass mean arterial pressure, length of bypass, administration of pre-CPB vasopressors, core temperature on CPB, pre- and post-CPB hematocrit, the preoperative use of beta-blockers or angiotensin-converting enzyme inhibitors, and the intraoperative use of aprotinin.

CONCLUSIONS

The results of this investigation suggest that it is possible to predict vasoplegia intraoperatively before separation from CPB and that the presence of a clinically significant area above the mean arterial blood pressure curve serves as a predictor of poor clinical outcome.

Hemodynamic resuscitation with arginine vasopressin reduces lung injury after brain death in the transplant donor

BACKGROUND

The autonomic storm accompanying brain death leads to neurogenic pulmonary edema and triggers development of systemic and pulmonary inflammatory responses. Neurogenic vasoplegia exacerbates the pulmonary injury caused by brain death and primes the lung for ischemia reperfusion injury and primary graft dysfunction in the recipient. Donor resuscitation with norepinephrine ameliorates the inflammatory response to brain death, however norepinephrine has deleterious effects, particularly on the heart. We tested the hypothesis that arginine vasopressin is a suitable alternative to norepinephrine in managing the hypotensive brain dead donor.

METHODS

Brain death was induced in Wistar rats by intracranial balloon inflation. Pulmonary capillary leak was estimated using radioiodinated albumin. Development of pulmonary edema was assessed by measurement of wet and dry lung weights. Cell surface expression of CD11b/CD18 by neutrophils was determined using flow cytometry. Enzyme-linked immunosorbent assays were used to measure the levels of TNFalpha, IL-1beta, CINC-1, and CINC-3 in serum and bronchoalveolar lavage. Quantitative reverse-transcription polymerase chain reaction was used to determine the expression of cytokine mRNA (IL-1beta, CINC-1 and CINC-3) in lung tissue.

RESULTS

There was a significant increase in pulmonary capillary permeability, wet/dry lung weight ratios, neutrophil integrin expression and pro-inflammatory cytokines in serum (TNFalpha, IL-1beta, CINC-1 and CINC-3), bronchoalveolar lavage (TNFalpha and IL-1beta) and lung tissue (IL-1beta and CINC-1) in braindead animals compared to controls. Correction of neurogenic hypotension with either arginine vasopressin or norepinephrine limits edema, reduces pulmonary capillary leak, and modulates systemic and pulmonary inflammatory responses to brain death.

CONCLUSIONS

Arginine vasopressin and norepinephrine are equally effective in treating the hypotensive pulmonary donor in this rodent model.

Inhibition of potassium channels in critical illness

PURPOSE OF REVIEW

The development of refractory arterial hypotension represents a significant problem in the treatment of critically ill patients, especially during sepsis. Increased activation of ATP-sensitive potassium channels in vascular smooth muscle cells is critically implicated in the pathophysiology of sepsis-induced vasodilation and vascular hyporesponsiveness to catecholamines. Pharmacological blockade of ATP-sensitive potassium channels has been proposed as a goal-directed therapeutic approach to stabilize hemodynamics in septic patients.

RECENT FINDINGS

In different animal models of sepsis, ATP-sensitive potassium channel inhibition with intravenously infused sulfonylureas effectively reversed sepsis-induced systemic vasodilation and hypotension. Two recent clinical trials, however, failed to demonstrate beneficial effects of enterally administered glibenclamide on norepinephrine requirements and blood pressure in septic shock patients. Relevant problems related to ATP-sensitive potassium channel blockade with sulfonylureas in human septic shock include the route of administration (enteral versus intravenous) and the dose itself (benefit-risk relationship). In addition, significant adverse events may result from unspecific inhibition of nonvascular ATP-sensitive potassium channels.

SUMMARY

Inhibition of ATP-sensitive potassium channels remains an attractive option to treat excessive vasodilation in the presence of systemic inflammation. Before this knowledge can be translated into clinical practice, however, future research is needed to define the role of ATP-sensitive potassium channels in critical illness and their specific inhibition in different tissues in more detail.

Role of adenosine triphosphate-sensitive potassium channel inhibition in shock states: physiology and clinical implications

Shock states are associated with an impaired tissue oxygen supply-demand relationship and perturbations within the microcirculation, leading to global tissue hypoxia, finally resulting in multiple-organ failure or even death. Two of the most frequent causes of shock are acute hemorrhage and sepsis. Although the origin and the pathophysiology of hemorrhagic and septic shock are basically different, the involvement of adenosine triphosphate-sensitive potassium (KATP) channels, as an important regulator of vascular smooth muscles tone, plays a pivotal role under both conditions. Because the excessive activation of vascular KATP channels is a major cause of arterial hypotension and vascular hyporesponsiveness to catecholamines, the pharmacological inhibition of KATP channels may represent a goal-directed therapeutic option to stabilize the hemodynamic situation in shock states. Despite promising results of preclinical studies, the efficacy of this innovative therapeutic approach remains to be confirmed in the clinical setting. The differences in the species, the comorbidity, and the difficulty in determining the exact onset of shock in clinical practice and, thus, any duration-related alterations in vascular responses and KATP channel activation may explain the discrepancy between the results obtained from experimental and clinical studies. Currently, two of the most relevant problems related to effective KATP blockade in shock states are represented by (1) the dose itself (benefit-risk ratio) and (2) the route of administration (oral vs. i.v.). This review article critically elucidates the published in vivo studies on the role of KATP channel inhibition in both described shock forms and discusses the advantages and the potential pitfalls related to the treatment of human shock states.

Effect of adenosine triphosphate-sensitive potassium channel inhibitors on coronary metabolic vasodilation

The ATP-sensitive potassium (K(ATP)) channel is a distinct type of potassium ion channel that is found in the vascular smooth muscle cells of a variety of mammalian species, including humans. The activity of K(ATP) channels is determined by many factors including cellular ATP and ADP levels, thus providing a link between cellular metabolism and vascular tone through its effects on membrane potential. Experimental studies using inhibitors of K(ATP) channels, such as the sulfonuylurea class of drugs, indicate that these channels modulate coronary vascular tone including the hyperaemia induced by increased myocardial metabolism. This review examines the evidence linking K(ATP) channels to the regulation of coronary vascular tone and the potential clinical implications of pharmacologic therapies that act on K(ATP) channels.

Continuously infused glipizide reverses the hyperdynamic circulation in ovine endotoxemia

In advanced sepsis, hemodynamic support is often complicated by a tachyphylaxis against exogenous catecholamines. Although activation of adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channels plays a pivotal role in the pathogenesis of hyperdynamic vasodilatory shock, previous studies demonstrated only a transient increase in mean arterial pressure (MAP) after bolus administration of K(ATP) channel inhibitors. We hypothesized that a continuous infusion of the sulfonylurea glipizide, a K(ATP) channel inhibitor, may reverse cardiovascular dysfunctions in sepsis permanently. Eighteen adult sheep were instrumented for chronic study. After a baseline measurement in healthy ewes, endotoxin (Salmonella typhosa, 10 ng kg(-1) min(-1)) was continuously infused for 19 h. After 16 h of endotoxemia, the surviving sheep (n = 14) were randomly assigned to be treated with either glipizide (5 mg/kg, followed by a continuous infusion of 8 mg kg(-1) h(-1)) or placebo (normal saline; each n = 7). Measurements of cardiopulmonary hemodynamics, global oxygen transport, acid-base status, and urine output were performed in the healthy state, after 16 h of endotoxemia, and during 3 h of glipizide infusion. Continuous infusion of glipizide reversed the endotoxin-induced hyperdynamic circulation, as indicated by significant increases in MAP and systemic vascular resistance index, as well as decreases in cardiac index and heart rate (P < 0.001 each). In addition, glipizide increased urine output as compared with untreated controls (P < 0.001). The anticipated decrease in glucose plasma levels was prevented by infusion of glucose 5%. From these results, we conclude that continuous glipizide infusion may represent a useful therapeutic option in the treatment of arterial hypotension related to sepsis and systemic inflammatory response syndrome.

The prognostic value of muscle StO2 in septic patients

OBJECTIVE

To quantify sepsis-induced alterations in changes in muscle tissue oxygenation (StO(2)) after an ischemic challenge using near-infrared spectroscopy (NIRS), and to test the hypothesis that these alterations are related to outcome.

DESIGN

Prospective study.

SETTING

Thirty-one-bed, university hospital Department of Intensive Care.

PATIENTS

Seventy-two patients with severe sepsis or septic shock, 18 hemodynamically stable, acutely ill patients without infection, and 18 healthy volunteers.

INTERVENTIONS

Three-minute occlusion of the brachial artery using a cuff inflated 50[Symbol: see text]mmHg above systolic arterial pressure.

MEASUREMENTS AND MAIN RESULTS

Thenar eminence StO(2) was measured continuously by NIRS before (StO(2)baseline), during, and after the 3-min occlusion. Changes in StO(2) were assessed by the slope of increase in StO(2) during the first 14 s following the ischemic period and by the difference between the maximum StO(2) and StO(2)baseline (Delta). The slope was lower in septic patients than in controls and volunteers [2.3 (1.3-3.6), 4.8 (3.5-6.0), and 4.7 (3.2-6.3) %/s, p < 0.001]. Delta was also significantly lower in septic patients than in the other groups. Slopes were lower in septic patients with than without shock [2.0 (1.2-2.9) vs 3.2 (1.8-4.5) %/s, p < 0.05]. In 52 septic patients, in whom the slope was obtained every 24 h for 48 h, slopes were higher in survivors than in non-survivors and tended to increase in survivors but not in non-survivors.

CONCLUSIONS

Altered recovery in StO(2) after an ischemic challenge is frequent in septic patients and more pronounced in the presence of shock. The presence and persistence of these alterations in the first 24[Symbol: see text]h of sepsis are associated with worse outcome.

Short-term effects of glipizide (an adenosine triphosphate-sensitive potassium channel inhibitor) on cardiopulmonary hemodynamics and global oxygen transport in healthy and endotoxemic sheep

In severe sepsis and septic shock, hemodynamic support is often complicated by a tachyphylaxis against exogenous catecholamines. Because activation of adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channels plays a pivotal role in the pathogenesis of hyperdynamic vasodilatory shock, we hypothesized that it may be beneficial to administer a specific K(ATP) channel inhibitor to prevent, or at least attenuate, hemodynamic dysfunction in sepsis. The present study was designed as a prospective and controlled laboratory experiment to elucidate the short-term effects of glipizide, a specific K(ATP) channel inhibitor, on cardiopulmonary hemodynamics and global oxygen transport in healthy sheep and sheep with endotoxemia. Ten adult ewes were anesthetized and operatively instrumented with a pulmonary artery, a femoral artery, and a foley catheter. After 24 h of recovery, healthy sheep received glipizide as a bolus infusion (4 mg/kg over 15 min). After 24 h of recovery, a continuous infusion of endotoxin (Salmonella typhosa, 10 ng.kg.(-1)min) was started in the same sheep and administered for the next 17 h. After 16 h of endotoxemia, glipizide was given as described above. Administration of glipizide was followed by a transient, but significant, increase in mean arterial pressure in both healthy controls (95 +/- 3 mmHg vs. 101 +/- 2 mmHg, P < 0.05) and sheep with endotoxemia (86 +/- 3 mmHg vs. 93 +/- 3 mmHg, P < 0.05). However, the increase in mean arterial pressure was longer lasting in ewes with endotoxemia. Cardiac index, oxygen delivery index, arterial lactate concentrations, and arterial pH were not significantly affected by glipizide. Therefore, administration of glipizide may represent a beneficial therapeutic option to treat arterial hypotension resulting from sepsis and systemic inflammatory response syndrome. Additional studies are required to determine the effects of continuous infusion of glipizide in the presence of systemic inflammation.

ACTIVATION OF THE ATP-DEPENDENT POTASSIUM CHANNEL ATTENUATES NOREPINEPHRINE-INDUCED VASOCONSTRICTION IN THE HUMAN FOREARM

Sepsis-induced vasodilation is characterized by an attenuated sensitivity to vasoconstrictor substances such as norepinephrine, possibly mediated by activation of vascular potassium channels. We determined whether vasodilation associated with potassium channel activation resulted in an attenuated vasoconstrictive response to norepinephrine in humans and whether the vasodilation associated with potassium channel activation could be inhibited by pharmacological potassium channel blockers. In 30 volunteers, the brachial artery was cannulated for infusion of drugs. Forearm blood flow (FBF) was measured in both arms using strain-gauge venous occlusion plethysmography. Forearm vascular resistance (FVR, mean arterial pressure/FBF) was calculated. The effects of vasodilation induced by sodium nitroprusside (SNP, nitric oxide donor) or diazoxide (activator of the ATP-dependent potassium channel) on norepinephrine-mediated vasoconstriction were examined. Also, the effects of potassium channel blockers on vasodilation associated with potassium channel activation were determined. Intraarterial SNP infusion (2 microg/min/dL) increased forearm blood flow by 235%, from (mean +/- SEM) 2.8 +/- 0.7 to 9.4 +/- 1.5 mL/min/dL (P < 0.0001). Subsequent norepinephrine infusion (10, 30, 100, 300, 1000 ng/min/dL) increased FVR dose-dependently from 13 +/- 4 AU to 249 +/- 45 AU at the highest norepinephrine infusion. Intraarterial diazoxide infusion (1 mg/min/dL) increased FBF by 209% from 2.2 +/- 0.3 to 6.8 +/- 1.0 mL/min/dL (P < 0.001). Subsequent norepinephrine infusion increased FVR from 18 +/- 5 to 51 +/- 6 AU at the highest norepinephrine infusion rate (n = 10), significantly different from the norepinephrine-induced effects during SNP coinfusion (P < 0.001). Diazoxide-induced fall in FVR in the infused forearm was inhibited by potassium channel blockers tetraethyl ammonium (1 mg/min/dL, n = 10, P = 0.004) and quinine (50 microg/min/dL, n = 10, P = 0.016). Vasodilation induced by vascular potassium channel activation is associated with an impressive reduction in the vasoconstrictor response to norepinephrine in humans. In accordance with animal experiments, this indicates that potassium channel activation could account for the diminished norepinephrine sensitivity in septic patients. Vasodilation associated with potassium channel activation can be inhibited by pharmacological potassium channel blockade. The possible role of potassium channel blockers during sepsis-induced potassium channel activation and vasodilation in humans needs further elucidation.

Inotropes and vasopressors in adults and foals

Successful treatment with inotropes and vasopressors depends on an understanding of the interplay of flow, pressure, and resistance in the cardiovascular system and an appreciation of the pathophysiologic mechanisms leading to inadequate tissue perfusion. Any treatment strategy is necessarily a compromise between the requirements of different vascular beds.Furthermore. the underlying hemodynamic derangements can change rapidly. Therefore. inotropes and vasopressors should be titrated to measures of improved hemodynamic status, and the treatments should be frequently reviewed.

Parenteral administration of glipizide sodium salt, an inhibitor of adenosine triphosphate-sensitive potassium channels, prolongs short-term survival after severe controlled hemorrhage in rats

OBJECTIVE

Recent experimental evidence suggests that activation of adenosine triphosphate (ATP)-sensitive potassium channels contributes to vascular failure and early mortality after hemorrhagic shock. The present investigation evaluated the effects of the water-soluble sodium salt of glipizide, an inhibitor of ATP-sensitive potassium channels, in anesthetized and awake rats subjected to severe controlled hemorrhage.

DESIGN

Prospective, randomized, controlled study.

SETTING

Animal research laboratory.

SUBJECTS

Male Wistar rats.

INTERVENTIONS

Anesthetized rats were subjected to bleeding to reduce mean arterial pressure to either 40 or 35 mm Hg, which was maintained constant for 60 mins. In addition, awake rats underwent blood withdrawal of 4.25 mL/100 g over 20 mins. At the end of the hemorrhage period and 30 mins later, the animals received intravenous (5 and 20 mg/kg) or intramuscular (10 and 40 mg/kg) injections of glipizide sodium salt or vehicle.

MEASUREMENTS AND MAIN RESULTS

In anesthetized rats subjected to pressure-controlled hemorrhage, glipizide sodium salt improved mean arterial pressure in a dose-dependent manner. Compared with the vehicle-treated animals, mean arterial pressure increased from 41.6 +/- 4.6 to 63.1 +/- 3.1 mm Hg in the 20 mg/kg intravenous group and from 33.2 +/- 4.9 to 54.0 +/- 4.7 mm Hg in the 40 mg/kg intramuscular group 60 mins after a 40-mm Hg shock. Furthermore, the drug did not affect the hemorrhage-induced changes in blood glucose concentrations. However, the higher doses of glipizide sodium salt attenuated the increments in plasma concentrations of lactate, alanine aminotransferase, creatinine, and amylase. Moreover, the higher doses markedly improved short-term survival after pressure- and volume-controlled bleeding. Overall, the intramuscular injections of the drug exerted salutary effects that were comparable to the intravenous administration.

CONCLUSIONS

In rats, parenteral administration of the water-soluble glipizide sodium salt attenuates vascular and end-organ dysfunction associated with severe hemorrhagic shock and prolongs short-term survival. The intramuscular administration provides comparable benefits as obtained by the intravenous injection.

Cariporide enhances lactate clearance upon reperfusion but does not alter lactate accumulation during global ischaemia

Cariporide (HOE 642) inhibits the Na+/H+ exchanger and would be expected to reduce lactate accumulation during ischaemia and stimulate lactate/H+ co-transporter upon reperfusion. The aim of this study was to determine the effect of cariporide on lactate production during global ischaemia and release during reperfusion. Guinea-pig hearts perfused in the Langendorff mode were exposed to 45 min global ischaemia and 30 min reperfusion with or without cariporide (5 or 10 micromol/l). Cardiac function was assessed by measurement of left ventricular developed pressure (LVDP). Lactate and pH were measured in coronary effluent before ischaemia and throughout reperfusion. Tissue metabolites (lactate, adenine nucleotides, guanine nucleotides and purine) were measured in ventricular biopsy samples collected at the beginning and end of ischaemia. Cariporide significantly improved recovery of LVDP (from 66% for control to 88% and 93% for 5 and 10 micromol/l cariporide, respectively). During ischaemia, only 10 micromol/l cariporide produced a small (10%) but significant preservation of ATP and GTP compared to control. This was associated with significant reduction (25%) in ischaemic contracture. Cariporide did not influence lactate accumulation during ischaemia but significantly increased lactate efflux (18%) during the first 60 s of reperfusion. In conclusion, cariporide does not alter lactate accumulation during ischaemia but enhances lactate efflux upon reperfusion, which may have implications for its cardioprotective action.

Science Review: Vasopressin and the cardiovascular system part 2 - clinical physiology

Vasopressin is emerging as a rational therapy for vasodilatory shock states. In part 1 of the review we discussed the structure and function of the various vasopressin receptors. In part 2 we discuss vascular smooth muscle contraction pathways with an emphasis on the effects of vasopressin on ATP-sensitive K+ channels, nitric oxide pathways, and interaction with adrenergic agents. We explore the complex and contradictory studies of vasopressin on cardiac inotropy and coronary vascular tone. Finally, we summarize the clinical studies of vasopressin in shock states, which to date have been relatively small and have focused on physiologic outcomes. Because of potential adverse effects of vasopressin, clinical use of vasopressin in vasodilatory shock should await a randomized controlled trial of the effect of vasopressin's effect on outcomes such as organ failure and mortality.

Effects of sulfonylurea hypoglycemic agents and adenosine triphosphate dependent potassium channel antagonists on ventricular arrhythmias in patients with decompensated heart failure

Hypoglycemic sulfonylureas block cardiac ATP-sensitive potassium channels (K(ATP)). The opening of these channels in cardiomyocytes can induce arrhythmias. In animal studies, sulfonylureas exert an antiarrhythmic effect on the ischemic myocardium, but data on human arrhythmic events are lacking. The study population included 207 patients (age 61 +/- 14 years) admitted for decompensated CHF. The severity of ventricular arrhythmias was assessed by 24-hour Holter monitoring. None of the patients were on parenteral vasoactive therapy or antiarrhythmics during Holter recording. Diabetic patients comprised 48% of the study population, and 34% of diabetic patients were prescribed sulfonylureas. The mean hourly ventricular pairs (3.6 +/- 0.5 vs 1.8 +/- 0.3, P = 0.03), the mean hourly repetitive ventricular beats (5.7 +/- 1.0 vs 2.6 +/- 0.1, P = 0.03), and the frequency of ventricular tachycardia episodes per 24 hours (4.7 +/- 0.8 vs 2.2 +/- 0.4, P = 0.03) were significantly lower in patients with diabetes who were receiving sulfonylureas compared with nondiabetics. No significant difference occurred between patients with diabetes who were not receiving sulfonylureas and nondiabetic patients. Multivariate regression revealed a negative independent relationship between sulfonylurea therapy and hourly ventricular pairs (P = 0.03), the mean hourly repetitive ventricular beats (P = 0.03), and ventricular tachycardia episodes (P = 0.04). In a multiple logistic regression, sulfonylurea therapy was a negative predictor of repetitive ventricular beats (P = 0.01, adjusted OR, 0.31; 95% CI, 0.12-0.78). Concomitant sulfonylurea therapy may reduce the occurrence of complex ventricular ectopy in the setting of severe CHF. These results suggest that cardiac K(ATP) channel activation may be involved in the genesis of ventricular arrhythmias in CHF.

Management of vasodilatory shock: defining the role of arginine vasopressin

The rationale for an arginine vasopressin (argipressin) infusion was put forward after it was discovered that patients in shock states might have an endogenous arginine vasopressin deficiency. Subsequently, several investigations impressively demonstrated that arginine vasopressin can successfully stabilise haemodynamics even in advanced vasodilatory shock. We report on physiological and pharmacological aspects of arginine vasopressin, and summarise current clinical knowledge on employing a continuous arginine vasopressin infusion in critically ill patients with catecholamine-resistant vasodilatory shock of different aetiologies. In view of presented experimental evidence and current clinical experience, a continuous arginine vasopressin infusion of approximately 2 to approximately 6 IU/h can be considered as a supplemental strategy to vasopressor catecholamines in order to preserve cardiocirculatory homeostasis in patients with advanced vasodilatory shock. Because data on adverse effects are still limited, arginine vasopressin should be reserved for patients in whom adequate haemodynamic stabilisation cannot be achieved with conventional vasopressor therapy or who have obvious adverse effects of catecholamines that result in further significant haemodynamic deterioration. For the same reasons, arginine vasopressin should not be used as a single, alternative vasopressor agent instead of catecholamine vasopressors. Future prospective studies will be necessary to define the exact role of arginine vasopressin in the therapy of vasodilatory shock.

Activation of ATP-sensitive potassium channels in hypoxic cardiac failure is not mediated by adenosine-1 receptors in the isolated rat heart

BACKGROUND

Hypoxic cardiac failure is accompanied by action potential shortening, which in part might be a consequence of opening of cardiac ATP-sensitive potassium channels (K(ATP) channels). Coupling of the adenosine-1 receptor (A-1 receptor) to these channels has been described; however, the interaction of A-1-receptors and K(ATP) channels in different models of ischemia is still under debate. The hypothesis as to whether A-1 receptors are involved in hypoxic K(ATP) channel-activation in the saline-perfused rat heart was tested.

METHODS AND RESULTS

Pharmacologic modulation of the K(ATP) channel by Glibenclamide (inhibitor) and Rimalkalim (activator) and of the A-1 receptor by R(-)-N6-(1-methyl-2-phenylethyl)-adenosine (R(-)-PIA, agonist) and 1,3-diethyl-3,7-dihydro-8-phenyl-purine-2,6-dione (DPX, antagonist) at different oxygen tensions (95% O2 and 20% O2) was performed in isolated Langendorff-rat hearts. Peak systolic pressure (PSP, intraventricular balloon), duration of monophasic action potential (epicardial suction electrode, time to 67% of repolarization: MAP(67%)), coronary flow, and heart rate (HR) were registered. Hypoxic perfusion resulted in a significant reduction of PSP (from 106 +/-11 to 56 +/-8 mmHg, P < 0.005) and shortening of MAP(67%) (from 37 +/-3 to 25 +/-4 ms, P < 0.005). With application of 1 microM Glibenclamide, MAP(67%) returned to normoxic values and PSP increased to 78 +/-9 mmHg (P < 0.005 vs hypoxia). In normoxia, 2 microM Rimalkalin resulted in reduction of MAP(67%) and PSP, which was reversed by Glibenclamide. Application of 0.1 microM R(-)-PIA in normoxia resulted in a decrease of HR (from 235 +/-36/min to 75 +/-41/min, P < 0.005), which was accompanied by an increase of PSP from 96 +/-7 to 126 +/-9 mmHg (P < 0.05) without changes in MAP(67%). These effects were reversible by 1 microM DPX and remained unaffected by application of 1 microM Glibenclamide. Application of 1 microM DPX in hypoxia had no effect on the measured parameters.

CONCLUSION

In isolated rat hearts, the K(ATP) channel-system is activated in hypoxic cardiac failure and contributes to action potential shortening and reduced contractile performance. These effects seem to be independent of the A-1 receptor in this model.

Differential inhibition of functional dilation of small arterioles by indomethacin and glibenclamide

Indomethacin or glibenclamide treatments attenuate functional dilation of larger-diameter "feed" arterioles paired with venules in hamster cremaster muscle. We tested the hypothesis that release of cyclooxygenase products from venules is important for functional dilation of third- and fourth-order arterioles. We also tested whether ATP-sensitive potassium channels are important during functional dilation of smaller arterioles. The microcirculation of hamster cremaster muscle was visualized with in vivo video microscopy. We measured diameter responses of third- and fourth-order arterioles paired and unpaired with venules in response to 2 minutes of muscle field stimulation (40 microseconds, 10 V, 1 Hz). Control diameters of vessels were 31+/-2 (n=19), 13+/-1 (n=12), 12+/-2 (n=12), and 10+/-1 (n=12) for paired and unpaired third-order and paired and unpaired fourth-order arterioles, respectively. In all groups, field stimulation resulted in increases in mean control diameter of >80%. Indomethacin (28 micromol/L) superfused on the preparation was used to inhibit cyclooxygenase metabolism, or glibenclamide (10 micromol/L) was used to block ATP-sensitive potassium channels. Indomethacin attenuated arteriolar vasodilations to electrical stimulation in paired third-order vessels only, whereas glibenclamide attenuated this vasodilation in all 4 groups. These results support a role for ATP-sensitive potassium channels in functional dilation of arterioles of all sizes regardless of whether or not they are paired with venules. Conversely, a role for cyclooxygenase products is limited to larger "feed arterioles" paired with venules. This study provides further evidence that venules may be the source of prostaglandin release during functional hyperemia.

Developmental downregulation of ATP-sensitive potassium conductance in astrocytes in situ

In many neural and non-neural cells, ATP-sensitive potassium (K(ATP)) channels couple the membrane potential to energy metabolism. We investigated the activation of K(ATP) currents in astrocytes of different brain regions (hippocampus, cerebellum, dorsal vagal nucleus) by recording whole-cell currents with the patch-clamp technique in acute rat brain slices. Pharmacological tools, hypoglycemia and specific compounds in the pipette solution (cAMP, UDP), were used to modulate putative K(ATP) currents. The highest rate of K(ATP) specific currents was observed with a pipette solution containing cAMP and external stimulation with diazoxide (0.3 mM). The diazoxide-activated current had a reversal potential negative to -80 mV and was inhibited by tolbutamide (0.2 mM). We found that not all cells activated a K(ATP) current, and that the portion of cells with functional K(ATP) channel expression was developmentally downregulated. Whereas diazoxide activated K(ATP) currents in 57% of the astrocytes in rats aged 8-11 days (n = 21), the rate decreased to 38% at 12-15 days (n = 29) and to 8% at 16-19 days (n = 12). No significant difference was observed for the three brain regions. In recordings without cAMP in the internal solution, only 21% (12-15 days; n = 19) or none (16-19 days; n = 7), respectively, showed a potassium current upon diazoxide application. This metabolically regulated potassium conductance may be of importance, particularly in immature astrocytes with a complex current pattern, which have a relatively high input resistance: K(ATP) currents activated by energy depletion may hyperpolarize the cells, or stabilize a negative resting potential during depolarizing stimuli mediated, e.g., by glutamate receptors and/or uptake carriers.

The effects of potassium-ATP channel modulation on ventricular fibrillation and defibrillation in the pig heart

BACKGROUND

Drugs acting on the cardiac ATP-sensitive potassium (K-ATP) channels may modulate responses to ischaemia and arrhythmogenesis. We investigated the effects of K-ATP channel modulation on frequency patterns of ventricular fibrillation (VF) and on defibrillation threshold (DFT).

METHODS AND RESULTS

Each group of 24 pigs randomly received intravenous levcromakalim (LKM) 40 microgram/kg (K-ATP agonist), glibenclamide (Glib) 20 mg/kg (K-ATP antagonist), saline or vehicle. Firstly, QTc interval was measured before and after drug. VF was then induced by endocardial stimulation and its power spectra and dominant frequencies over 15 min determined by fast Fourier transformation. Secondly, transthoracic DFT was determined (step-up/step-down protocol) before and after each drug. LKM reduced QTc interval (e.g., lead II, 354-321 ms, P<0.05) and increased the dominant VF frequency between 6 and 8 min (9.5+/-0.5 Hz at 6.5 min compared with 7.2+/-0.6 Hz (saline), 7.4+/-0.8 Hz (vehicle), 6.8+/-0.5 Hz (Glib), P=0.03). LKM reduced (to 57.2+/-2.1 mmHg) and Glib increased (to 107.8+/-6.1) mean arterial BP compared with saline (80.3+/-5.6) and vehicle (87. 6+/-7.1; P<0.01). There was no significant difference in defibrillation threshold energy, current or voltage, after any drug.

CONCLUSIONS

Activation of K-ATP channels reduced blood pressure and QTc interval. The lack of major effect on VF dominant frequency and DFT of either LKM or Glib suggests that prior administration of similar drugs to patients should not prejudice outcome from VF cardiac arrest.

A K(ATP) channel deficiency affects resting tension, not contractile force, during fatigue in skeletal muscle

The objective of this study was to determine how an ATP-sensitive K(+) (K(ATP)) channel deficiency affects the contractile and fatigue characteristics of extensor digitorum longus (EDL) and soleus muscle of 2- to 3-mo-old and 1-yr-old mice. K(ATP) channel-deficient mice were obtained by disrupting the Kir6.2 gene that encodes for the protein forming the pore of the channel. At 2-3 mo of age, the force-frequency curve, the twitch, and the tetanic force of EDL and soleus muscle of K(ATP) channel-deficient mice were not significantly different from those in wild-type mice. However, the tetanic force and maximum rate of force development decreased with aging to a greater extent in EDL and soleus muscle of K(ATP) channel-deficient mice (24-40%) than in muscle of wild-type mice (7-17%). During fatigue, the K(ATP) channel deficiency had no effect on the decrease in tetanic force in EDL and soleus muscle, whereas it caused a significantly greater increase in resting tension when compared with muscle of wild-type mice. The recovery of tetanic force after fatigue was not affected by the deficiency in 2- to 3-mo-old mice, whereas in 1-yr-old mice, force recovery was significantly less in muscle of K(ATP) channel-deficient than wild-type mice. It is suggested that the major function of the K(ATP) channel during fatigue is to reduce the development of a resting tension and not to contribute to the decrease in force. It is also suggested that the K(ATP) channel plays an important role in protecting muscle function in older mice.

Coronary vascular K+ATP channels contribute to the maintenance of myocardial perfusion in dogs with pacing-induced heart failure

The functional role of coronary vascular ATP-sensitive potassium (K+ATP) channels in the regulation of coronary blood flow (CBF) has not been determined in chronic heart failure (CHF). To test the hypothesis that K+ATP channels contribute to myocardial perfusion in HF, we examined the effects of intracoronary infusion of glibenclamide, an inhibitor of K+ATP channels, on basal CBF in control and CHF dogs. CHF was produced in mongrel dogs by pacing the right ventricle for 4 weeks. Under anesthesia, CBF in the left anterior descending coronary artery, other hemodynamic and metabolic parameters, or regional myocardial blood flow were measured. Basal CBF was less in CHF dogs than in controls. Glibenclamide at the graded doses (5, 15 and 50 microg x kg(-1) x min(-1) decreased CBF in both control and CHF dogs. The percentage decrease in CBF with glibenclamide at 50 microg x kg(-1) x min(-1) was greater (p<0.01) in CHF dogs than in controls. The greater decrease in CBF with glibenclamide at 50microg x kg(-1) x min(-1) was associated with myocardial ischemia. Glibenclamide decreased myocardial blood flow in each sublayer of the myocardium in the 2 groups. These results suggest that the basal activity of coronary vascular K+ATP channels is increased in CHF dogs but not in controls. This may contribute to the maintenance of myocardial perfusion in CHF.

Coronary microcirculation: physiology and pharmacology

Coronary microvessels play a pivotal role in determining the supply of oxygen and nutrients to the myocardium by regulating the coronary flow conductance and substance transport. Direct approaches analyzing the coronary microvessels have provided a large body of knowledge concerning the physiological and pharmacological characteristics of the coronary circulation, as has the rapid accumulation of biochemical findings about the substances that mediate vascular functions. Myogenic and flow-induced intrinsic vascular controls that determine basal tone have been observed in coronary microvessels in vitro. Coronary microvascular responses during metabolic stimulation, autoregulation, and reactive hyperemia have been analyzed in vivo, and are known to be largely mediated by metabolic factors, although the involvement of other factors should also be taken into account. The importance of ATP-sensitive K(+) channels in the metabolic control has been increasingly recognized. Furthermore, many neurohumoral mediators significantly affect coronary microvascular control in endothelium-dependent and -independent manners. The striking size-dependent heterogeneity of microvascular responses to all of these intrinsic, metabolic, and neurohumoral factors is orchestrated for optimal perfusion of the myocardium by synergistic and competitive interactions. The regulation of coronary microvascular permeability is another important factor for the nutrient supply and for edema formation. Analyses of collateral microvessels and subendocardial microvessels are important for understanding the pathophysiology of ischemic hearts and hypertrophied hearts. Studies of the microvascular responses to drugs and of the impairment of coronary microvessels in diseased conditions provide useful information for treating microvascular dysfunctions. In this article, the endogenous regulatory system and pharmacological responses of the coronary circulation are reviewed from the microvascular point of view.

Pinacidil suppresses contractility and preserves energy but glibenclamide has no effect during muscle fatigue

The effects of 10 microM glibenclamide, an ATP-sensitive K(+) (K(ATP)) channel blocker, and 100 microM pinacidil, a channel opener, were studied to determine how the K(ATP) channel affects mouse extensor digitorum longus (EDL) and soleus muscle during fatigue. Fatigue was elicited with 200-ms-long tetanic contractions every second. Glibenclamide did not affect rate and extent of fatigue, force recovery, or (86)Rb(+) fractional loss. The only effects of glibenclamide during fatigue were: an increase in resting tension (EDL and soleus), a depolarization of the cell membrane, a prolongation of the repolarization phase of action potential, and a greater ATP depletion in soleus. Pinacidil, on the other hand, increased the rate but not the extent of fatigue, abolished the normal increase in resting tension during fatigue, enhanced force recovery, and increased (86)Rb(+) fractional loss in both the EDL and soleus. During fatigue, the decreases in ATP and phosphocreatine of soleus muscle were less in the presence of pinacidil. The glibenclamide effects suggest that fatigue, elicited with intermittent contractions, activates few K(ATP) channels that affect resting tension and membrane potentials but not tetanic force, whereas opening the channel with pinacidil causes a faster decrease in tetanic force, improves force recovery, and helps in preserving energy.

Glimepiride (Hoe490) inhibits the rilmakalim induced decrease in intracellular free calcium and contraction of isolated heart muscle cells from guinea pigs to a lesser extent than glibenclamide

Glibenclamide is a potent inhibitor of the ATP-dependent potassium channel. Opening of the ATP-dependent potassium channel is regarded as a mechanism of ischemic preconditioning. This in vitro study examines the influence of glibenclamide and glimepiride, a new sulfonylurea, on the negative inotropic action of the potassium channel opener rilmakalim in isolated ventricular myocytes. Cardiac myocytes were isolated from adult guinea pig hearts by collagenase perfusion and incubated with rilmakalim (concentration range 0.1-12.0 microM), glibenclamide (concentration range 0.03-3.0 microM) plus rilmakalim (3.0 or 7.5 microM), and glimepiride (0.03-9.0 microM) plus rilmakalim (3.0 or 7.5 microM) and paced by electrical field stimulation. Contractility of the myocytes was evaluated by digital image analysis, intracellular free calcium was determined by means of fura-2 fluorescence measurements, and cell viability was assessed morphologically as well as by measurement of lactate dehydrogenase activity. Rilmakalim reduced the systolic intracellular free calcium and contractility of ventricular myocytes in a concentration dependent manner. This effect was antagonized by glibenclamide at lower concentrations (0.3 microM) than glimepiride (3.0 microM). The smaller antagonistic action of glimepiride on the negative inotropic effect of rilmakalim as compared with glibenclamide most likely reflects a less potent inhibition of ATP-dependent potassium channels by glimepiride.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.