Potassium channel openers prevent potassium-induced calcium loading of cardiac cells: possible implications in cardioplegia

Kir1.1 and SUR1 are not implicated as subunits of an adenosine triphosphate-sensitive potassium channel involved in diazoxide cardioprotection

Objective

The adenosine triphosphate-sensitive potassium channel opener diazoxide mimics ischemic preconditioning and is cardioprotective. Clarification of diazoxide's site and mechanism of action could lead to targeted pharmacologic therapies for patients undergoing cardiac surgery. Several mitochondrial candidate proteins have been investigated as potential adenosine triphosphate-sensitive potassium channel components. Renal outer medullary potassium (Kir1.1) and sulfonylurea sensitive regulatory subunit 1 have been suggested as subunits of a mitochondrial adenosine triphosphate-sensitive potassium channel. We hypothesized that pharmacologic blockade or genetic deletion (knockout) of renal outer medullary potassium and sensitive regulatory subunit 1 would result in loss of diazoxide cardioprotection in models of global ischemia with cardioplegia.

Methods

Myocyte volume and contractility were compared after Tyrode's physiologic solution (20 minutes), stress (hyperkalemic cardioplegia ± diazoxide, ± VU591 (Kir1.1 inhibitor), N = 9 to 23 each, 20 min), and Tyrode's (20 minutes). Isolated mouse (wild-type, sensitive regulatory subunit 1 [-/-], and cardiac knockout renal outer medullary potassium) hearts were given cardioplegia ± diazoxide (N = 9-16 each) before global ischemia (90 minutes) and 30 minutes reperfusion. Left ventricular pressures were compared before and after ischemia.

Results

Stress (cardioplegia) was associated with reduced myocyte contractility that was prevented by diazoxide. Isolated myocytes were not responsive to diazoxide in the presence of VU591. In isolated hearts, diazoxide improved left ventricular function after prolonged ischemia compared with cardioplegia alone in wild-type and knockout (sensitive regulatory subunit 1 [-/-] and cardiac knockout renal outer medullary potassium) mice.

Conclusions

Isolated myocyte and heart models may measure independent and separate actions of diazoxide. By definitive genetic deletion, these data indicate that sensitive regulatory subunit 1 and renal outer medullary potassium are not implicated in cardioprotection by diazoxide.

Comparison of Del Nido and Blood Cardioplegia in Pediatric Patients Undergoing Surgical Repair for Congenital Heart Disease

The aim of the study is to investigate the impact of two different cardioplegia solutions, the del Nido (dN) and blood cardioplegia (BC), on postoperative troponin concentrations, vasoactive-inotrope score, and length of hospital stay in pediatric patients undergoing cardiovascular surgery for CHD. 80 subjects aged between 1 and 120 months who were scheduled for surgical repair for a CHD were prospectively enrolled in this study. Study subjects were allocated to one of the study groups using simple randomization technique as follows: The del Nido cardioplegia group (n = 40, median age 8.5 [5.5-14] months) and conventional blood cardioplegia group (n = 40, median age 11 [5-36] months). Aortic cross-clamp time and cardiopulmonary bypass time were recorded in all subjects. Troponin I and vasoactive-inotropic score, which indicates the amount of cardiovascular support by various inotropes or vasopressors, were recorded following the repair. The difference in troponin I, vasoactive-inotropic score (VIS), length of ICU stay, and length of hospital stay between the two groups was the primary outcome measure of this study. The volume of cardioplegia was significantly lower in dN group than that of the BC group (p < 0.001). Cardiopulmonary bypass time and aortic cross-clamp time were significantly shorter in subjects receiving dN cardioplegia than those receiving BC (p = 0.006, and p = 0.001, respectively). Subjects assigned to BC had higher Troponin I concentrations at postoperative 24th hour compared to subjects receiving dN cardioplegia [1.60 (0.92-2.49) ng/mL vs. 1.03 (0.55-1.83) ng/mL, p = 0.045]. VIS was also significantly higher in BC group at 24th [10 (10-13) vs. 10 (5-10), p = 0.032] and 48th hours [10 (1.5-10) vs. 0 (0-10), p = 0.005] compared to that of the dN cardioplegia group. The median extubation time was 7.5 (3.5-20.5) hours in dN cardioplegia group and 5 (4-10) hours in the BC group (p = 0.384). There were no significant differences between the groups with respect to the length of ICU stay and length of hospital stay. No mortality and no significant arrhythmias requiring medical or electrical cardioversion were noted in any of the groups. In conclusion, dN cardioplegia provides shorter aortic cross-clamp time and cardiopulmonary bypass time, and lower postoperative troponin I concentration and vasoactive-inotrope scores compared to BC in pediatric subjects undergoing surgical repair for CHD. However, lengths of ICU and hospital stay are similar in dN cardioplegia and BC groups.

Safety and Effectiveness of Del Nido Cardioplegia in Comparison to Blood-Based St. Thomas Cardioplegia in Congenital Heart Surgeries: A Prospective Randomized Controlled Study

Background:

To compare the safety and effectiveness of del Nido cardioplegia with blood-based St Thomas Hospital (BSTH) cardioplegia in myocardial protection in congenital heart surgery.

Methods:

It is a prospective, open-labeled, randomized controlled study conducted at National Heart Institute, Kuala Lumpur from July 2018 to July 2019. All patients with simple and complex congenital heart diseases (CHD) with good left ventricular function (left ventricular ejection fraction [LVEF] >50%) were included while those with LVEF <50% were excluded. A total of 100 patients were randomized into two groups of 50 each receiving either del Nido or BSTH cardioplegia. Primary end points were the spontaneous return of activity following aortic cross-clamp release and ventricular function between two groups. Secondary end point was myocardial injury as assessed by troponin T levels.

Results:

Cardiopulmonary bypass and aortic cross-clamp time, return of spontaneous cardiac activity following the aortic cross-clamp release, the duration of mechanical ventilation, and intensive care unit stay were comparable between two groups. Statistically significant difference was seen in the amount and number of cardioplegia doses delivered ( P < .001). The hemodilution was significantly less in the del Nido complex CHD group compared to BSTH cardioplegia ( P = .001) but no difference in blood usage ( P = .36). The myocardial injury was lesser (lower troponin T release) with del Nido compared to BSTH cardioplegia ( P = .6).

Conclusion:

Our study showed that both del Nido and BSTH cardioplegia are comparable in terms of myocardial protection. However, single, less frequent, and lesser volume of del Nido cardioplegia makes it more suitable for complex repair.

Association between cardioplegia and postoperative atrial fibrillation in coronary surgery

OBJECTIVE

The aim of this multicenter study was to evaluated whether cold or warm cardioplegia are associated with postoperative atrial fibrillation (POAF) and the prognostic role of the latter on early stroke and neurological mortality.

METHOD

This was a retrospective analysis of prospective collected data from 9 cardiac centers in Italy and the United States including patients undergoing surgery between 2010 and 2018. From the 9 institutional databases, 17,231 patients underwent isolated CABG on-pump, using either warm cardioplegia (n = 7730) or cold cardioplegia (n = 9501); among the latter group blood and crystalloid cardioplegia were used in 691 and 8810 patients, respectively. After matching, two pairs of 4162 patients (overall cohort 8324) were analyzed.

RESULTS

In matched population, the rate of POAF was 18% (1472 cases), 15% (608) in warm group versus 21% (864) in cold group (p < 0.001). Multivariable analysis confirmed that cold cardioplegia was associated with higher rate of POAF, along with age, hypercholesterolemia, LVEF, reoperation, preoperative IABP, previous stroke, cardiopulmonary and cross-clamp. Moreover, cold cardioplegia as well as POAF increased the rate of postoperative stroke as well as early mortality and neurological mortality Propensity-weighted cohort included 11,830 (70%) patients out of 17,231. After adjustment, both cold blood and cold crystalloid cardioplegia negatively influenced POAF, stroke and neurological mortality.

CONCLUSIONS

Warm cardioplegia may reduce the rate of POAF in CABG patients with respect to cold cardioplegia, either blood or crystalloid. This has a prognostic impact on postoperative stroke and neurological mortality.

Comparing del Nido and Conventional Cardioplegia in Infants and Neonates in Congenital Heart Surgery

BACKGROUND

The aim of this study was to evaluate outcome measures after the use of del Nido (dN) cardioplegia compared with conventional multidose high-potassium (non-dN) cardioplegia in neonates and infants.

METHODS

We retrospectively analyzed data in patients, aged younger than 1 year, undergoing cardiopulmonary bypass (CPB) from January 2012 to August 2015. We changed our cardioplegia protocol from non-dN to dN administered in a single or infrequently dosed strategy in September 2013. The outcomes of the dN group (n = 107) are compared with the non-dN group (n = 118). We analyzed variables for demographic, intraoperative, early postoperative, and discharge variables.

RESULTS

The two groups were similar in age, weight, height, CPB, and cross-clamp time; preoperative and postoperative echocardiographic systolic functions; first 24-hour postoperative urine output and inotropic score; length of stay; and mortality rate. The Society of Thoracic Surgeons/European Association for Cardio-Thoracic Surgery Congenital Heart Surgery (STAT) mortality category was significantly higher in the dN group (p = 0.03). The cardioplegia dosing interval was lower for the non-dN group (p < 0.001). The volume and doses of cardioplegia per patient were significantly higher in the non-dN group (p < 0.001). In a subanalysis, when the Norwood patients were excluded from both groups, the overall STAT mortality category difference was no longer significant. The demographic, early postoperative, and discharge variables still showed no significant difference when the two groups were compared.

CONCLUSIONS

Similar outcomes can be achieved with less frequent interruption of the operation and lower volume of cardioplegia when using dN cardioplegia solution compared with conventional cardioplegia. The dN cardioplegia with extended ischemic interval can be used as an alternative strategy in the neonatal and infant population during cardiac operations.

Mechanisms of oxidative stress and myocardial protection during open-heart surgery

Cold heart protection via cardioplegia administration, limits the amount of oxygen demand. Systemic normothermia with warm cardioplegia was introduced due to the abundance of detrimental effects of hypothermia. A temperature of 32-33°C in combination with tepid blood cardioplegia of the same temperature appears to be protective enough for both; heart and brain. Reduction of nitric oxide (NO) concentration is in part responsible for myocardial injury after the cardioplegic cardiac arrest. Restoration of NO balance with exogenous NO supplementation has been shown useful to prevent inflammation and apoptosis. In this article, we discuss the "deleterious" effects of the oxidative stress of the extracorporeal circulation and the up-to-date theories of "ideal'' myocardial protection.

A cardioprotective preservation strategy employing ex vivo heart perfusion facilitates successful transplant of donor hearts after cardiocirculatory death

BACKGROUND

Ex vivo heart perfusion (EVHP) has been proposed as a means to facilitate the resuscitation of donor hearts after cardiocirculatory death (DCD) and increase the donor pool. However, the current approach to clinical EVHP may exacerbate myocardial injury and impair function after transplant. Therefore, we sought to determine if a cardioprotective EVHP strategy that eliminates myocardial exposure to hypothermic hyperkalemia cardioplegia and minimizes cold ischemia could facilitate successful DCD heart transplantation.

METHODS

Anesthetized pigs sustained a hypoxic cardiac arrest and a 15-minute warm ischemic standoff period. Strategy 1 hearts (S1, n = 9) underwent initial reperfusion with a cold hyperkalemic cardioplegia, normothermic EVHP, and transplantation after a cold hyperkalemic cardioplegic arrest (current EVHP strategy). Strategy 2 hearts (S2, n = 8) underwent initial reperfusion with a tepid adenosine-lidocaine cardioplegia, normothermic EVHP, and transplantation with continuous myocardial perfusion (cardioprotective EVHP strategy).

RESULTS

At completion of EVHP, S2 hearts exhibited less weight gain (9.7 ± 6.7 [S2] vs 21.2 ± 6.7 [S1] g/hour, p = 0.008) and less troponin-I release into the coronary sinus effluent (4.2 ± 1.3 [S2] vs 6.3 ± 1.5 [S1] ng/ml; p = 0.014). Mass spectrometry analysis of oxidized pleural in post-transplant myocardium revealed less oxidative stress in S2 hearts. At 30 minutes after wean from cardiopulmonary bypass, post-transplant systolic (pre-load recruitable stroke work: 33.5 ± 1.3 [S2] vs 19.7 ± 10.9 [S1], p = 0.043) and diastolic (isovolumic relaxation constant: 42.9 ± 6.7 [S2] vs 65.2 ± 21.1 [S1], p = 0.020) function were superior in S2 hearts.

CONCLUSION

In this experimental model of DCD, an EVHP strategy using initial reperfusion with a tepid adenosine-lidocaine cardioplegia and continuous myocardial perfusion minimizes myocardial injury and improves short-term post-transplant function compared with the current EVHP strategy using cold hyperkalemic cardioplegia before organ procurement and transplantation.

Hyperkalemic cardioplegia for adult and pediatric surgery: end of an era?

Despite surgical proficiency and innovation driving low mortality rates in cardiac surgery, the disease severity, comorbidity rate, and operative procedural difficulty have increased. Today's cardiac surgery patient is older, has a "sicker" heart and often presents with multiple comorbidities; a scenario that was relatively rare 20 years ago. The global challenge has been to find new ways to make surgery safer for the patient and more predictable for the surgeon. A confounding factor that may influence clinical outcome is high K(+) cardioplegia. For over 40 years, potassium depolarization has been linked to transmembrane ionic imbalances, arrhythmias and conduction disturbances, vasoconstriction, coronary spasm, contractile stunning, and low output syndrome. Other than inducing rapid electrochemical arrest, high K(+) cardioplegia offers little or no inherent protection to adult or pediatric patients. This review provides a brief history of high K(+) cardioplegia, five areas of increasing concern with prolonged membrane K(+) depolarization, and the basic science and clinical data underpinning a new normokalemic, "polarizing" cardioplegia comprising adenosine and lidocaine (AL) with magnesium (Mg(2+)) (ALM™). We argue that improved cardioprotection, better outcomes, faster recoveries and lower healthcare costs are achievable and, despite the early predictions from the stent industry and cardiology, the "cath lab" may not be the place where the new wave of high-risk morbid patients are best served.

Effects of oral nicorandil therapy on sympathetic nerve activity and cardiac events in patients with chronic heart failure: subanalysis of our previous report using propensity score matching

PURPOSE

Nicorandil, an adenosine triphosphate-sensitive potassium channel opener, improves cardiac sympathetic nerve activity (CSNA) in patients with ischaemic heart disease. However, the long-term effects on both CSNA, as evaluated by (123)I-metaiodobenzylguanidine (MIBG) scintigraphy, and prognosis have not been determined in patients with chronic heart failure (CHF).

METHODS

This study was a subanalysis of our previous results that serial (123)I-MIBG scintigraphic studies are the most useful prognostic indicator in CHF patients. The study group comprised 208 patients with CHF (left ventricular ejection fraction <45 %) but no cardiac events for at least 5 months identified on the basis of a history of decompensated acute heart failure requiring hospitalization. These patients underwent (123)I-MIBG scintigraphy and echocardiography just before leaving the hospital and again 6 months later. We selected 170 patients and used propensity propensity score matching to compare patients who received oral nicorandil (85 patients) and those who did not (85 patients). The patients were followed up for a median of 5.03 years, with the primary and secondary study end-points defined as the occurrence of a fatal cardiac event and a major adverse cardiac event (MACE), respectively.

RESULTS

After treatment, the extent of changes in (123)I-MIBG scintigraphic and echocardiographic parameters in the nicorandil group were more favourable than in those not receiving nicorandil. Of the 170 patients, a fatal cardiac event occurred in 42, and a MACE in 68 during the study. Multivariate Cox regression analyses revealed that no nicorandil treatment was a significant predictor of both cardiac death and MACE in our patients with CHF. On Kaplan-Meier analysis, the rates of freedom from cardiac death or from MACE in the nicorandil group were significantly higher than in those not receiving nicorandil (all p<0.05).

CONCLUSION

Long-term nicorandil treatment improves CSNA and left ventricular parameters in patients with CHF. Furthermore, this agent is potentially effective for reducing the incidence of cardiac events in patients with CHF.

Modulation of mitochondrial bioenergetics in the isolated Guinea pig beating heart by potassium and lidocaine cardioplegia: implications for cardioprotection

Mitochondria are damaged by cardiac ischemia/reperfusion (I/R) injury but can contribute to cardioprotection. We tested if hyperkalemic cardioplegia (CP) and lidocaine (LID) differently modulate mitochondrial (m) bioenergetics and protect hearts against I/R injury. Guinea pig hearts (n = 71) were perfused with Krebs Ringer's solution before perfusion for 1 minute just before ischemia with either CP (16 mM K) or LID (1 mM) or Krebs Ringer's (control, 4 mM K). The 1-minute perfusion period assured treatment during ischemia but not on reperfusion. Cardiac function, NADH, FAD, m[Ca], and superoxide (reactive oxygen species) were assessed at baseline, during the 1-minute perfusion, and continuously during I/R. During the brief perfusion before ischemia, CP and LID decreased reactive oxygen species and increased NADH without changing m[Ca]. Additionally, CP decreased FAD. During ischemia, NADH was higher and reactive oxygen species was lower after CP and LID, whereas m[Ca] was lower only after LID. On reperfusion, NADH and FAD were more normalized, and m[Ca] and reactive oxygen species remained lower after CP and LID. Better functional recovery and smaller infarct size after CP and LID were accompanied by better mitochondrial function. These results suggest that mitochondria may be implicated, directly or indirectly, in protection by CP and LID against I/R injury.

Myocardial protection by nicorandil during open-heart surgery under cardiopulmonary bypass

BACKGROUND

To evaluate the myocardial protective effect of nicorandil when used as an adjuvant to cold hyperkalaemic cardioplegia in open-heart surgery.

METHODS

Patients who underwent surgery under cardiopulmonary bypass (CPB) for mitral valve replacement (MVR, 23 patients) or coronary artery bypass grafting (CABG, 24 patients) were entered in a double-blind study. The patients were randomized to a nicorandil Group (N) or placebo Group (P). Nicorandil 0.1 mg kg-1 (Group N), or normal saline (Group P), were administered at three time points: (1) after aortic cannulation, but prior to going on CPB, (2) 5 min before aortic cross-clamping and (3) 5 min before reperfusion. The following variables were studied: (a) time until electromechanical arrest after cardioplegia administration (Tarrest), (b) time until return of electromechanical activity after aortic cross-clamp removal (Trecovery), (c) incidence of postoperative myocardial infarction or low output syndromes (d) dysrhythmias requiring intervention after aortic cross-clamp removal and (e) haemodynamic changes after nicorandil administration.

RESULTS

The Tarrest after cardioplegia administration was significantly faster in nicorandil group in both MVR and CABG patients (P 75 IU L-1 in MVR patients was significantly lower in the Group N than in placebo patients (P < 0.05). However, in CABG patients there was no such significant difference. The incidence of dysrhythmias requiring intervention after aortic cross-clamp removal was also less in Group N. Administration of 0.1 mg kg-1 boluses of nicorandil did not cause significant haemodynamic changes or precipitate dysrhythmias in any patient.

CONCLUSION

Nicorandil enhances the myocardial protective effect of cold hyperkalaemic cardioplegia in cardiac surgery patients.

Comparative effects of nicorandil with isosorbide mononitrate on cardiac sympathetic nerve activity and left ventricular function in patients with ischemic cardiomyopathy

BACKGROUND

Nicorandil, an adenosine triphosphate-sensitive potassium channel opener, improves left ventricular (LV) remodeling after myocardial infarction in rat models. However, the effects of chronic nicorandil therapy on cardiac sympathetic nerve activity in patients with ischemic cardiomyopathy have not been determined.

METHODS

Thirty-six patients with ischemic cardiomyopathy (LV ejection fraction [LVEF] < 40%) who underwent successful revascularization procedure before 6 months were treated by standard conventional therapy. Eighteen patients were randomized to additionally receive nicorandil (15 mg/d), whereas the other 18 patients received isosorbide mononitrate (40 mg/d). The delayed heart-to-mediastinum count ratio (H/M ratio), delayed total defect score (TDS), and washout rate (WR) were determined from (123)I-meta-iodobenzylguanidine (MIBG) scintigraphy before and 6 months after treatment. Left ventricular end-diastolic volume (LVEDV) and LVEF were determined by echocardiography.

RESULTS

Total defect score, H/M ratio, WR, LVEDV, and LVEF at baseline were similar for both groups. After treatment, in patients receiving nicorandil, TDS decreased from 50 +/- 6 to 40 +/- 11 (P < .005), H/M ratio increased from 1.68 +/- 0.23 to 1.79 +/- 0.26 (P = .005), and WR decreased from 46% +/- 9% to 40% +/- 12% (P < .005). In addition, LVEDV decreased from 178 +/- 31 to 157 +/- 30 mL (P < .0005), and LVEF increased from 33% +/- 6% to 39% +/- 7% (P < .05). In patients receiving isosorbide mononitrate, no significant changes were observed in these parameters. Moreover, there was a significant correlation between the percent change of LVEF and that of TDS from baseline to 6 months in the patients receiving nicorandil (r = -0.569, P < .05).

CONCLUSIONS

The present study demonstrates improvements in cardiac (123)I-MIBG scintigraphic and echocardiographic parameters with nicorandil treatment. These findings indicate that nicorandil can improve cardiac sympathetic nerve activity and LV function in patients with ischemic cardiomyopathy.

Effect of L-carnitine on cardiomyocyte apoptosis and cardiac function in patients undergoing heart valve replacement operation

The effects of L-carnitine, as an ingredient of cardioplegia solution, on cardiac function and cardiomyocyte apoptosis in patients undergoing heart valve replacement operation were investigated. Twenty-three cases undergoing heart valve replacement with cardiopulmonary bypass (CPB) were randomly allocated into two groups: L-carnitine group (n = 12, 12 g/L L-carnitine was put in the ST. Thomas cardioplegia) and control group (n = 11, identical to the L-carnitine group except that normal saline was administered instead of L-carnitine). Serum cardial troponin I (cTnI) levels, the left ventricular ejection fraction (LVEF), and cardiac index (CI) were measured perioperatively. A bit of myocardial tissue obtained from right atria was taken before CPB and by the end of intracardiac procedure to undergo electron microscopy examination and estimate apoptosis by terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL). From the end of CPB to 3 days after operation, the serum levels of cTnI in the L-carnitine group was significantly lower than that in the control group (P < 0.05). Heart color ultrasonogram showed that the CI index and LVEF at 7th day postoperatively in the L-carnitine group were significantly higher than in the control group (P < 0.05). Compared to the control group, L-carnitine significantly alleviated the morphologic changes of cardiac muscle cells (electron microscopy examination) and decreased the amounts of apoptotic cardiac muscle cells (TUNEL). Furthermore, the dosage of vasoactive drugs used after operation was significantly less in the L-carnitine group (P < 0.01). It was concluded that L-carnitine cardioplegia solution could improve cardiac function in patients undergoing heart valve replacement operation and alleviate CPB-mediated apoptosis of cardiac muscle cells.

Organ arrest, protection and preservation: natural hibernation to cardiac surgery

Cardiac surgery continues to be limited by an inability to achieve complete myocardial protection from ischemia-reperfusion injury. This paper considers the following questions: (1) what lessons can be learned from mammalian hibernators to improve current methods of human myocardial arrest, protection and preservation? and (2) can the human heart be pharmacologically manipulated during acute global ischemia to act more like the heart of a hibernating mammal? After reviewing the major entropy-slowing strategies of hibernation, a major player identified in the armortarium is maintenance of the membrane potential. The resting membrane potential of the hibernator's heart appears to be maintained close to its pre-torpid state of around -85 mV. In open-heart surgery, 99% of all surgical heart arrest solutions (cardioplegia) employ high potassium (>16 mM) which depolarises the membrane voltage from -85 to around -50 mV. However, depolarising potassium cardioplegia has been increasingly linked to myocyte and microvascular damage leading to functional loss during reperfusion. Our recent work has been borrowed from hibernation biology and is focused on a very different arrest strategy which 'clamps' the membrane near its resting potential and depresses O2 consumption from baseline by about 90%. The new 'polarising' cardioplegia incorporates adenosine and lidocaine (AL) as the arresting combination, not high potassium. Studies in the isolated rat heart show that AL cardioplegia delivered at 37 degrees C can arrest the heart for up to 4 h with 70-80% recovery of the cardiac output, 85-100% recovery of heart rate, systolic pressure and rate-pressure product and 70-80% of baseline coronary flows. Only 14% of hearts arrested with crystalloid St. Thomas' solution No. 2 cardioplegia survived after 4 h. In conclusion, maintenance of the myocardial membrane potential near or close to its resting state appears to be an important feature of the hibernator's heart that may find great utility in surgical arrest and cellular preservation strategies. Identifying and safely turning 'off' and 'on' the entropy-slowing genes to down-regulate the hibernator's heart and applying this to human organs and tissues remains a major challenge for future genomics and proteomics.

Polarized arrest with warm or cold adenosine/lidocaine blood cardioplegia is equivalent to hypothermic potassium blood cardioplegia

BACKGROUND

Hypothermic depolarizing hyperkalemic (K + 20 mEq/L) blood cardioplegia is the "gold standard" in cardiac surgery. K + has been associated with deleterious consequences, eg, intracellular calcium overload. This study tested the hypothesis that elective arrest in a polarized state with adenosine (400 micromol/L via adenosine triphosphate-sensitive potassium channel opening) and the Na + channel blocker lidocaine (750 micromol/L) as the arresting agents in blood cardioplegia provides cardioprotection comparable to standard hypothermic K + -blood cardioplegia.

METHODS

Anesthetized dogs were placed on cardiopulmonary bypass and assigned to 1 of 3 groups receiving antegrade cardioplegia delivered every 20 minutes for 1 hour of arrest: cold (10 degrees C) K + -blood cardioplegia (n = 6), cold (10 degrees C) adenosine/lidocaine blood cardioplegia (n = 6), or warm (37 degrees C) adenosine/lidocaine blood cardioplegia (n = 6). After an hour of arrest, cardiopulmonary bypass was discontinued, and reperfusion was continued for 120 minutes.

RESULTS

Time to arrest was longer with cold and warm adenosine/lidocaine blood cardioplegia (175 +/- 19 seconds and 143 +/- 19 seconds, respectively) compared with K + -blood cardioplegia (27 +/- 2 seconds; P < .001). Postcardioplegia left ventricular systolic function (slope of the end-systolic pressure/dimension relationship) was comparable among the 3 groups (K + -blood cardioplegia, 15.2 +/- 2.1 mm Hg/mm; cold adenosine/lidocaine blood cardioplegia, 15.9 +/- 3.4 mm Hg/mm; warm adenosine/lidocaine blood cardioplegia, 14.1 +/- 2.8 mm Hg/mm; P = .90). Plasma creatine kinase activity in cold and warm adenosine/lidocaine blood cardioplegia was similar to that in K + -blood cardioplegia at 120 minutes of reperfusion (cold adenosine/lidocaine blood cardioplegia, 11.5 +/- 2.1 IU/g protein; warm adenosine/lidocaine blood cardioplegia, 10.1 +/- 0.9 IU/g protein; K + -blood cardioplegia, 7.6 +/- 0.8 IU/g protein; P = .17). Postcardioplegia coronary artery endothelial function was preserved in all groups.

CONCLUSIONS

Intermittent polarized arrest with warm or cold adenosine/lidocaine blood cardioplegia provided the same degree of myocardial protection as intermittent hypothermic K + -blood cardioplegia in normal hearts.

Pinacidil improves contractile function and intracellular calcium handling in isolated cardiac myocytes exposed to simulated cardioplegic arrest

BACKGROUND

We examined the effects of pinacidil on contractile function and intracellular calcium in isolated rat cardiomyocytes exposed to cardioplegic solution.

METHODS

Rat myocytes were incubated at 24 degrees C for 2 hours in cardioplegic solution with or without pinacidil (50 micromol/L), then they were perfused with Krebs-Henseleit solution with a gas phase of 95% O2/5% CO2 at the same temperature. Contraction and intracellular calcium transients were then measured by video tracking and spectrofluorometry.

RESULTS

During 20 minutes of perfusion after 2 hours in cardioplegic solution with pinacidil, (1) the recovery of contractile function was significantly increased in terms of both amplitude of contraction (98.30% +/- 9.90% versus 81.00% +/- 11.25%; p < 0.05) and peak velocity of cell shortening (100.90% +/- 13.79% versus 76.89% +/- 18.14%; p < 0.01) when compared with myocytes in cardioplegic solution without pinacidil; (2) the amplitudes of the intracellular calcium transients evoked by electrical stimulation and caffeine (10 mmol/L) increased by 23.31% to approximately 40.72% and 61.73%, respectively, compared with those in cardioplegic solution without pinacidil; and (3) the decay time of the caffeine-induced intracellular calcium transient decreased by 36.64% +/- 15.10% relative to that measured in cardioplegic solution without pinacidil. The effects induced by supplementing the cardioplegic solution with pinacidil were diminished in the presence of glibenclamide (10 micromol/L).

CONCLUSIONS

Addition of the adenosine triphosphate-sensitive potassium-channel opener, pinacidil, to a high potassium cardioplegic solution improves recovery of contractile properties and cytosolic calcium in isolated rat cardiac myocytes.

Adenosine and lidocaine: a new concept in nondepolarizing surgical myocardial arrest, protection, and preservation

OBJECTIVE

Depolarizing potassium cardioplegia has been increasingly linked to left ventricular dysfunction, arrhythmia, and microvascular damage. We tested a new polarizing normokalemic cardioplegic solution employing adenosine and lidocaine as the arresting, protecting, and preserving cardioprotective combination. Adenosine hyperpolarizes the myocyte by A1 receptor activation, and lidocaine blocks the sodium fast channels.

METHODS

Isolated perfused rat hearts were switched from the working mode to the Langendorff (nonworking) mode and arrested for 30 minutes, 2 hours, or 4 hours with 200 micromol/L adenosine and 500 micromol/L lidocaine in Krebs-Henseleit buffer (10 mmol/L glucose, pH 7.7, at 37 degrees C) or modified St Thomas' Hospital solution no. 2, both delivered at 70 mm Hg and 37 degrees C (arrest temperature 22 degrees C to 35 degrees C).

RESULTS

Adenosine and lidocaine hearts achieved faster mechanical arrest in (25 +/- 2 seconds, n = 23) compared with St Thomas' Hospital solution hearts (70 +/- 5 seconds, n = 24; P=.001). After 30 minutes of arrest, both groups developed comparable aortic flow at approximately 5 minutes of reperfusion. After 2 and 4 hours of arrest (cardioplegic solution delivered every 20 minutes for 2 minutes at 37 degrees C), only 50% (4 of 8) and 14% (1 of 7) of St Thomas' Hospital solution hearts recovered aortic flow, respectively. All adenosine and lidocaine hearts arrested for 2 hours (n = 7) and 4 hours (n = 9) recovered 70% to 80% of their prearrest aortic flows. Similarly, heart rate, systolic pressures, and rate-pressure products recovered to 85% to 100% and coronary flows recovered to 70% to 80% of prearrest values. Coronary vascular resistance during delivery of cardioplegic solution was significantly lower (P <.05) after 2 and 4 hours in hearts arrested with adenosine and lidocaine cardioplegic solution compared with hearts arrested with St Thomas' Hospital solution.

CONCLUSIONS

We conclude that adenosine and lidocaine polarizing cardioplegic solution confers superior cardiac protection during arrest and recovery compared with hyperkalemic depolarizing St Thomas' Hospital cardioplegic solution.

Na+/H+ exchange inhibition with cardioplegia reduces cytosolic [Ca2+] and myocardial damage after cold ischemia

Cold cardioplegia protects against reperfusion damage. Blocking Na+/H+ exchange may be as protective as cardioplegia by improving the left ventricular pressure (LVP)-[Ca2+] relationship after cold ischemia. In guinea pig isolated hearts subjected to cold ischemia (4 h, 17 degrees C) and reperfusion, the cardioprotective effects of a Krebs-Ringer (KR) solution, a cardioplegia solution, a KR solution containing the Na+/H+ exchange inhibitor eniporide (1 microM), and a cardioplegia solution containing eniporide were compared. Treatments were given before and initially after cold ischemia. Systolic and diastolic [Ca2+] were calculated from indo-1 fluorescence transients recorded at the LV free wall. During ischemia, diastolic [Ca2+] increased in each group but more so in the KR group. Peak systolic and diastolic [Ca2+] on initial reperfusion were highest after KR and smallest after cardioplegia + eniporide. After reperfusion, systolic-diastolic LVP (% of baseline) and infarct size (%), respectively, were KR, 47 +/- 3%, 37 +/- 4%; cardioplegia, 71 +/- 5%*, 20 +/- 2.2%*; KR + eniporide, 73 +/- 5%*, 11 +/- 3%* dagger; and cardioplegia + eniporide 77 +/- 3%*, 10 +/- 1.4%* dagger (*P </= 0.05 vs KR; dagger P </= 0.05 vs cardioplegia). Ca2+ overload was reduced in each treated group, and most in the cardioplegia + eniporide group, and was associated with the improved function. Inhibition of Na+/H+ exchange was as effective as cardioplegia in restoring function and better than cardioplegia in reducing infarct size after hypothermic ischemia. The combination of cardioplegia and Na+/H+ exchange inhibition did not produce additive protective effects but caused a larger decrease in Ca2+ loading.

Continuous tepid blood cardioplegia can preserve coronary endothelium and ameliorate the occurrence of cardiomyocyte apoptosis

OBJECTIVE

In modern cardiac surgery, crystalloid or blood cardioplegic solutions have been used widely for myocardial protection; however, ischemia does occur during protection with intermittent infusion of cold crystalloid or blood cardioplegic solutions. The present study was designed to evaluate the effect of different cardioplegic methods on myocardial apoptosis and coronary endothelial injury after global ischemia, cardiopulmonary bypass (CPB), and reperfusion in anesthetized open-chest dogs.

METHODS

The dogs were classified into five groups to identify the injury of myocardium and coronary endothelium: group 1, normothermic CPB without cardiac arrest; group 2, hypothermic CPB with continuous tepid blood cardioplegia, and with cardiac arrest; group 3, hypothermic CPB with intermittent cold blood cardioplegia, and with cardiac arrest; group 4, hypothermic CPB with intermittent cold crystalloid cardioplegia, and with cardiac arrest; and group 5, sham-operated control group. During CPB, cardiac arrest was achieved with different cardioplegia solutions for 60 min, followed by reperfusion for 4 h before the myocardium and coronary arteries were harvested. Coronary arteries were harvested immediately and analyzed by scanning electron microscopy. Cardiomyocytic apoptosis was detected using terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end labeling, Western blot, and DNA ladder methods.

RESULTS

Regardless of the detection method used, significantly higher percentages of apoptotic cardiomyocytes were found in group 3 and group 4 than in other groups. Expression of caspase-3 correlated with increased apoptosis. Scanning electron microscopy revealed severe endothelial injury of coronary arteries in group 3 and group 4.

CONCLUSION

These results point to an important explanation for the difference in cardiac recovery after hypothermic ischemia and arrest with various cardioplegic solutions.

K(ATP) channel opener protects neonatal rabbit heart better than St. Thomas' solution

OBJECTIVES

Myocardial protection with ATP-sensitive potassium channel (K(ATP) channel) openers is as effective as St. Thomas' cardioplegia (StTCP) in adult rabbit hearts. This study compares the effectiveness of the K(ATP) channel opener pinacidil to StTCP in protecting neonatal rabbit hearts exposed to global ischemia.

METHODS

Seventeen neonatal rabbit hearts (7-9 days old) perfused with Krebs-Henseleit buffer (KHB) on a Langendorff apparatus underwent 90 min of normothermic ischemia. Six (ischemia control) received no pretreatment before or during ischemia. Six others (pinacidil) received a 3-min infusion of 50 microM pinacidil in KHB without StTCP at the onset of ischemia. Five others (StTCP) received a 3-min infusion of StTCP at the onset of ischemia. After 60 min of KHB reperfusion, recovery of left ventricular (LV) performance and coronary flow (CF) were measured and compared to preischemia. A paired t test was used for comparison between drug-treated and untreated groups.

RESULTS

Pinacidil-treated hearts had significantly better recovery of left ventricular developed pressure (47 +/- 3.8 mmHg vs 32 +/- 2.5 mmHg, P < 0.05), contractility (+dP/dt(max); 885.4 +/- 74 mmHg vs 643.7 +/- 65 mmHg, P < 0.05), left ventricular end diastolic pressure (10.5 +/- 0.9 mmHg vs 17.4 +/- 1.2 mmHg P < 0.05), compliance (-dP/dt(max); 994.2 +/- 86 mmHg vs 673.6 +/- 69 mmHg, P < 0.05), and CF (5.9 +/- 0.4 ml/min vs 4.2 +/- 0.2 ml/min, P < 0.05) compared to ischemic control. StTCP only improved the recovery of -dP/dt(max) (877.4 +/- 73 mmHg/s vs 673.6 +/- 69 mmHg/s, P < 0.05) and CF (5.7 +/- 0.3 ml/min vs 4.2 +/- 0.2 ml/min, P < 0.05) compared to control.

CONCLUSIONS

Pinacidil pretreatment provided superior recovery of systolic performance compared to St. Thomas' cardioplegia solution in neonatal hearts. Myocardial protection by pretreatment with the K(ATP) channel opener pinacidil may be a new strategy for myocardial protection during pediatric cardiac surgery.

Mechanisms and alternative methods of achieving cardiac arrest

Elective cardiac arrest during surgery can be achieved by inducing depolarization, polarization, or influencing calcium mechanisms. Depolarized arrest, induced by elevating the extracellular potassium concentration, is currently the most commonly used technique. However, injury associated with ionic imbalance involving sodium and calcium overload, together with maintained metabolic processes aimed at correcting these imbalances, have lead to alternatives being sought. "Polarized" arrest, induced by sodium-channel blockers or by agents that activate potassium channels, has been shown to exert equal or superior protection. Similarly, agents that induce calcium desensitization may also prove to enhance protection. These alternative techniques, however, require extensive characterization before introduction into routine clinical use can be recommended.

Left ventricular remodeling after infarction: sequential MR imaging with oral nicorandil therapy in rat model

PURPOSE

To use magnetic resonance (MR) imaging in quantification of the short- and long-term effects of therapy with orally administered nicorandil on left ventricular (LV) geometry and function independent of infarction size.

MATERIALS AND METHODS

Forty-six rats were subjected to reperfused infarction and randomly divided into two groups. Group 1 rats (n = 21) were treated with nicorandil (3 mg/kg/day in drinking water) for 4 days before infarction and 8 weeks after infarction (hereafter, the nicorandil group). Group 2 rats (n = 25) received tap water for the same period and served as the control group. Mesoporphyrin- (as a necrosis-specific agent) enhanced MR imaging was used to define necrotic myocardium on day 2 after infarction in all 46 animals. Contrast material-enhanced MR images showed large but identical infarction size in 11 control and 11 nicorandil rats. Only these 22 rats underwent repeat MR imaging at 8 weeks after infarction. The following variables were measured: LV volumes, ejection fraction, mass, wall thickness, and infarction size. Student t test and analysis of variance for repeated measurements were used for statistical analysis.

RESULTS

The size of the necrotic region on mesoporphyrin-enhanced MR images was 39% +/- 3 of the size of the left ventricle in the control group and 41% +/- 2 in the nicorandil group (difference not significant, unpaired Student t test). Pretreatment with nicorandil for 6 days before imaging did not reduce LV dilation or improve function compared with those in control animals with identical infarction size. Eight weeks after infarction, control animals showed deterioration in LV function, wall thinning, and gradient in regional dysfunction (analysis of variance test). Nicorandil produced significant salutary effects on LV ejection fraction (37% +/- 3 in the nicorandil group vs 24% +/- 3 in the control group), end-diastolic volume (0.53 mL +/- 0.03 vs 0.65 mL +/- 0.04), end-systolic volume (0.36 mL +/- 0.03 vs 0.49 mL +/- 0.05), LV wall thickening in remote noninfarcted myocardium (28% +/- 2 vs 19% +/- 1), and a rim of infarction (16% +/- 2 vs 8% +/- 1) (P <.05 for all parameters). The increase in LV mass was reduced in the nicorandil group (0.73 g +/- 0.03) compared with that in the control group (0.89 g +/- 0.04) (P <.05).

CONCLUSION

In animals studied longitudinally, MR imaging demonstrated the deleterious changes in LV geometry and function in the period after infarction and the salutary effects of medical therapy.

Diazoxide protects the rabbit heart following cardioplegic ischemia

K(ATP) channels are present in sarcolemmal and mitochondrial membranes. This study tests the hypothesis that opening mitochondrial K(ATP) channels with Diazoxide (DZ) improves tolerance to cardioplegic ischemia during surgery. Twenty-two rabbit hearts were perfused with Krebs-Henseleit buffer (KHB) on a Langendorff apparatus and underwent 50 min of 37 degrees C global ischemia with St Thomas' cardioplegia (STCP). Hearts were divided into three groups. Ten (control) received no pretreatment. Seven (DZ) received 10 min of 30 microM DZ, a selective mitochondrial K(ATP) opener, in KHB before arrest with STCP containing 30 microM DZ. Five (5-HD + DZ) received 10 min of 100 microM sodium 5-hydroxydecanoate (5-HD), a selective mitochondrial K(ATP) channel blocker, followed by 10 min of 30 microM DZ and 100 microM 5-HD in KHB before arrest with STCP + 30 microM DZ + 100 microM 5-HD. LV developed pressure (LVDP), dP/dt and coronary flow (CF) were measured after 60 min of reperfusion. Diazoxide pretreatment significantly improved the recovery of LV function and coronary flow compared to control (LVDP: 49 +/- 5* vs. 31 +/- 4; +dP/dtmax 927 +/- 93 vs. 507 +/- 85 mmHg/sec*; CF 33 +/- 4 vs. 22 +/- 2 ml/min, *p < 0.05). Mitochondria K(ATP) channel blockade with 5-HD prevented DZ's salutary effect on the recovery of LV and vascular function. Diazoxide pretreatment protects the rabbit heart during cardioplegic ischemia by opening mitochondrial K(ATP) channels. Opening mitochondrial K(ATP) channels may be a new strategy for improving myocardial protection during cardiac surgery.

Aprikalim reduces the Na+-Ca2+ exchange outward current enhanced by hyperkalemia in rat ventricular myocytes

BACKGROUND

[corrected] Aprikalim, an adenosine triphosphate (ATP) sensitive K+ (K(ATP)) channel opener, attenuates the elevation of intracellular Ca2+ concentration ([Ca2+]i) and improves the contractile functions after hyperkalemic and hypothermic cardioplegia. There is evidence that cardioplegia increases the Na+-Ca2+ exchange activity without affecting Ca2+ influx through L-type Ca2+ channels or Ca2+ content in the sarcoplasmic reticulum, the intracellular Ca2+ store.

METHODS

We measured the Na+-Ca2+ exchange outward current with the patch-clamp technique in single rat ventricular myocytes exposed to hyperkalemia and hypothermia in the presence of aprikalim. The intracellular calcium concentration ([Ca2+]i) during cardioplegia, and the contractile function and [Ca2+]i transients induced by electrical stimulation or caffeine during rewarming and reperfusion in single ventricular myocytes were also determined. Contraction and [Ca2+]i were determined with video tracking and spectrofluorometry, respectively.

RESULTS

Aprikalim, 100 micromol/L, the effect of which was blocked by glibamclamide, a K(ATP) inhibitor, significantly attenuated the hyperkalemia-elevated Na+-Ca2+ exchange current by 26% and 11% at 22 degrees C and 4 degrees C, respectively. Aprikalim also attenuated significantly the [Ca2+]i elevated during cardioplegia. Furthermore aprikalim significantly attenuated the reduction in amplitude and prolongation in duration of contraction of myocytes after cardioplegia. The effects of aprikalim mimicked those of nickle (Ni2+), a Na+-Ca2+ exchange blocker. The electrically or caffeine-induced [Ca2+]i transients were unaltered by cardioplegia or aprikalim.

CONCLUSIONS

Aprikalim attenuates the Na+-Ca2+ exchange outward current elevated by hyperkalemia, which may attenuate the [Ca2+]i elevation during hyperkalemia and improve the contractile function after cardioplegia in the ventricular myocyte. The study provides further support that addition of a K(ATP) channel opener to the cardioplegic solution may produce beneficial effects in open heart surgery.

Potassium channel openers: therapeutic potential in cardiology and medicine

Potassium (K(+)) channel openers (KCOs) define a class of chemically diverse agents that share a common molecular target, the metabolism-regulated ATP-sensitive K(+) (K(ATP)) channel. In view of the unique function that K(ATP) channels play in the maintenance of cellular homeostasis, this novel class of ion channel modulators adds to existent pharmacotherapy with potential in promoting cellular protection under conditions of metabolic stress. Indeed, experimental studies have demonstrated broad therapeutic potential for KCOs, including roles as cardioprotective agents, vasodilators, bronchodilators, bladder relaxants, anti-epileptics, insulin secretagogues and promoters of hair growth. However, clinical experience with these drugs is limited and their place in patient management needs to be fully established.

Controlled nicorandil administration for myocardial protection during coronary artery bypass grafting under cardiopulmonary bypass

Nicorandil is a hybrid potassium channel opener, and recent experimental studies have demonstrated its efficacy in myocardial protection against ischemia-reperfusion. This clinical study was designed to examine the myocardial protective effect of nicorandil administered during cardiopulmonary bypass. Seventy adult patients, 53 men and 17 women, undergoing elective coronary artery bypass grafting were randomly assigned to two groups, one receiving nicorandil during cardiopulmonary bypass (group N, n = 35) and the other receiving no nicorandil for control (group C, n = 35). Nicorandil was administered at each dose of 0.1 mg/kg into the cardiopulmonary bypass circuit according to the following schedule: (1) bolus injection at the initiation of cardiopulmonary bypass, (2) continuous infusion for 5 min before aortic cross-clamping, (3) bolus administration at 5 min before reperfusion, and (4) continuous infusion for 5 min before reperfusion. The time required for achieving cardiac arrest after the initial cardioplegia was significantly reduced in group N in comparison with that in group C. After aortic unclamping, the number of patients showing a significant ST segment change on the electrocardiogram was significantly fewer in group N, whereas the number of patients showing spontaneous recovery of heart beat was significantly greater. As for the myocardial protective effect, group N showed lower plasma levels of malondialdehyde, human-heart fatty acid-binding protein, and peak creatine kinase-MB, and required lower doses of catecholamine. Our results suggest that nicorandil administration during cardiopulmonary bypass provides enhanced myocardial protective effects against ischemia-reperfusion in patients undergoing coronary artery bypass grafting.

Diazoxide protects mitochondria from anoxic injury: implications for myopreservation

BACKGROUND

Heart muscle primarily relies on adenosine triphosphate produced by oxidative phosphorylation and is highly vulnerable to anoxic insult. Although a number of strategies aimed at improving myopreservation are available, no effective means of preserving mitochondrial energetics under conditions of anoxic injury have been developed. Openers of mitochondrial adenosine triphosphate-sensitive potassium channels have emerged as powerful cardioprotective agents presumably capable of maintaining mitochondrial function under metabolic stress. Here, we evaluated the ability of a prototype mitochondrial adenosine triphosphate-sensitive potassium channel opener, diazoxide, to preserve oxidative phosphorylation in mitochondria subjected to anoxia and reoxygenation.

METHODS

Mitochondria were isolated from rat hearts and subjected to 20 minutes of anoxia, followed by reoxygenation. Mitochondrial respiration and oxidative phosphorylation, as well as mitochondrial integrity, were assessed by means of ion-selective minielectrodes, high-performance liquid chromatography, fluorometry, and electron microscopy.

RESULTS

Anoxia-reoxygenation decreased the rate of adenosine diphosphate-stimulated oxygen consumption, inhibited adenosine triphosphate production, and disrupted mitochondrial integrity. On average, anoxic stress reduced adenosine diphosphate-stimulated respiration from 291 +/- 14 to 141 +/- 15 ng-atoms O(2). min(-1). mg(-1) protein and decreased the rate of adenosine triphosphate production from 752 +/- 14 to 414 +/- 34 nmol adenosine triphosphate. min(-1). mg(-1) protein. After anoxia, the majority (88%) of mitochondria was damaged or swollen and released adenylate kinase, a marker of mitochondrial integrity. Diazoxide (100 micromol/L), present throughout anoxia, preserved adenosine diphosphate-stimulated respiration at 255 +/- 7 ng-atoms O(2). min(-1). mg(-1) protein and adenosine triphosphate production at 640 +/- 39 nmol adenosine triphosphate. min(-1). mg(-1) protein. Diazoxide also protected mitochondrial structure from anoxia-mediated damage, so that after anoxic stress, 67% of mitochondria remained intact and adenylate kinase was confined to the mitochondria.

CONCLUSIONS

The present study demonstrates that diazoxide diminishes anoxia-induced functional and structural deterioration of cardiac mitochondria. By protecting mitochondria and preserving myocardial energetics, diazoxide may be useful under conditions of reduced oxygen availability, including global surgical ischemia or storage of donor heart.

Protein kinase C isoform-dependent myocardial protection by ischemic preconditioning and potassium cardioplegia

OBJECTIVE

Ischemic preconditioning combined with potassium cardioplegia does not always confer additive myocardial protection. This study tested the hypothesis that the efficacy of ischemic preconditioning under potassium cardioplegia is dependent on protein kinase C isoform.

METHODS

Isolated and crystalloid-perfused rat hearts underwent 5 cycles of 1 minute of ischemia and 5 minutes of reperfusion (low-grade ischemic preconditioning) or 3 cycles of 5 minutes of ischemia and 5 minutes of reperfusion (high-grade ischemic preconditioning) or time-matched continuous perfusion. These hearts received a further 5 minutes of infusion of normal buffer or oxygenated potassium cardioplegic solution. The isoform nonselective protein kinase C inhibitor chelerythrine (5 micromol/L) was administered throughout the preischemic period. All hearts underwent 35 minutes of normothermic global ischemia followed by 30 minutes of reperfusion. Isovolumic left ventricular function and creatine kinase release were measured as the end points of myocardial protection. Distribution of protein kinase C alpha, delta, and epsilon in the cytosol and the membrane fractions were analyzed by Western blotting and quantified by a densitometric assay.

RESULTS

Low-grade ischemic preconditioning was almost as beneficial as potassium cardioplegia in improving functional recovery; left ventricular developed pressure 30 minutes after reperfusion was 70 +/- 15 mm Hg (P <.01) in low-grade ischemic preconditioning and 77 +/- 14 mm Hg (P <.001) in potassium cardioplegia compared with values found in unprotected control hearts (39 +/- 12 mm Hg). Creatine kinase release during reperfusion was also equally inhibited by low-grade ischemic preconditioning (18.2 +/- 10.6 IU/g dry weight, P <.05) and potassium cardioplegia (17.6 +/- 6.7 IU/g, P <.01) compared with control values. However, low-grade ischemic preconditioning in combination with potassium cardioplegia conferred no significant additional myocardial protection; left ventricular developed pressure was 80 +/- 17 mm Hg, and creatine kinase release was 14.8 +/- 11.0 IU/g. In contrast, high-grade ischemic preconditioning with potassium cardioplegia conferred better myocardial protection than potassium cardioplegia alone; left ventricular developed pressure was 121 +/- 16 mm Hg (P <.001), and creatine kinase release was 8.3 +/- 5.8 IU/g (P <.05). Chelerythrine itself had no significant effect on functional recovery and creatine kinase release in the control hearts, but it did inhibit the salutary effects not only of low-grade and high-grade ischemic preconditioning but also those of potassium cardioplegia. Low-grade ischemic preconditioning and potassium cardioplegia enhanced translocation of protein kinase C alpha to the membrane, whereas high-grade ischemic preconditioning also enhanced translocation of protein kinase C delta and epsilon. Chelerythrine inhibited translocation of all 3 protein kinase C isoforms.

CONCLUSIONS

These results suggest that myocardial protection by low-grade ischemic preconditioning and potassium cardioplegia are mediated through enhanced translocation of protein kinase C alpha to the membrane. It is therefore suggested that activation of the novel protein kinase C isoforms is necessary to potentiate myocardial protection under potassium cardioplegia.

Effects of pinacidil on coronary Ca(2+)-myosin phosphorylation in cold potassium cardioplegia model

The effects of the potassium (K(+)) channel opener pinacidil (Pin) on the coronary smooth muscle Ca(2+)-myosin light chain (MLC) phosphorylation pathway under hypothermic K(+) cardioplegia were determined by use of an in vitro microvessel model. Rat coronary arterioles (100-260 microm in diameter) were subjected to 60 min of simulated hypothermic (20 degrees C) K(+) cardioplegic solutions (K(+) = 25 mM). We first characterized the time course of changes in intracellular Ca(2+) concentration, MLC phosphorylation, and diameter and observed that the K(+) cardioplegia-related vasoconstriction was associated with an activation of the Ca(2+)-MLC phosphorylation pathway. Supplementation with Pin effectively suppressed the Ca(2+) accumulation and MLC phosphorylation in a dose-dependent manner and subsequently maintained a small decrease in vasomotor tone. The ATP-sensitive K(+) (K(ATP))-channel blocker glibenclamide, but not the nitric oxide (NO) synthase inhibitor N(omega)-nitro-L-arginine methyl ester, significantly inhibited the effect of Pin. K(+) cardioplegia augments the coronary Ca(2+)-MLC pathway and results in vasoconstriction. Pin effectively prevents the activation of this pathway and maintains adequate vasorelaxation during K(+) cardioplegia through a K(ATP)-channel mechanism not coupled with the endothelium-derived NO signaling cascade.

Pinacidil pretreatment extends ischemia tolerance of neonatal rabbit hearts

OBJECTIVES

Activating ATP-sensitive potassium (K(ATP)) channels improves ischemia tolerance of adult rabbit hearts. We hypothesize that (a) endogenous activation of the K(ATP) channel accounts for better ischemia tolerance of neonatal hearts and (b) exogenous K(ATP) channel activation with pinacidil further improves the neonatal heart's tolerance to cardioplegic ischemia.

METHODS

Study 1: Seven (control) neonatal rabbits received intraperitoneal saline, whereas five others (Glib) received 0.3 mg/kg glibenclamide 10 min before sacrifice. They were perfused on Langendorff with Krebs-Henseleit buffer (KHB). Baseline left ventricle (LV) performance and coronary flow (CF) were measured. After 20 min of 37 degrees C ischemia and 10 min of reperfusion, recovery was measured. Study 2: Ten (control) neonatal hearts underwent 90 min of normothermic ischemia with St. Thomas' cardioplegia (STCP) solution administered every 30 min. Ten others were pretreated with a 10-min infusion of 1 microM pinacidil in KHB and received 1 microM pinacidil-enriched STCP. Recovery of LV performance and CF were measured after 60 min of reperfusion.

RESULTS

Study 1: Glib significantly reduced preischemia LV performance by 28%* compared to control hearts. Recovery of Glib-treated hearts was significantly less (67%*) than controls (81%*). Study 2: Pinacidil-treated hearts had significantly better recovery of LV performance (39%*) and CF (78%*) compared to 23 and 52%, respectively, in untreated controls (*P < 0.05 vs control hearts).

CONCLUSIONS

Endogenous K(ATP) channel activation in neonatal hearts contributes to their better tolerance to ischemia. Exogenous K(ATP) channel activation by pinacidil pretreatment and cardioplegic enrichment significantly improved the neonatal rabbit heart's tolerance to cardioplegic ischemia. This may be an important addition to myocardial protection during pediatric cardiac surgery.

Potassium channel openers: are they effective as pretreatment or additives to cardioplegia?

BACKGROUND

This study was designed to test the hypothesis that the potassium channel opener pinacidil (Pin) as a pretreatment (PT) agent or additive to St. Thomas' solution (StT) could enhance myocardial protection.

METHODS

In a parabiotic rabbit Langendorff model, 36 hearts underwent global normothermic ischemia (1 hour) followed by reperfusion (30 minutes). Cardioplegia (50 mL, every 20 minutes) consisted of: StT; PinPT/StT, where Pin PT preceded StT arrest; Pin alone; Pin in StT (Pin/StT); and Pin in low potassium StT. Systolic function after reperfusion (percent recovery of developed pressure) and compliance (diastolic slope from pressure-volume relationship) were measured.

RESULTS

There was no significant difference between StT and PinPT/StT in percent recovery of developed pressure (51.54% +/- 3.5%, 42.17% +/- 4.0%, respectively) or compliance. Likewise, no significant differences occurred between Pin, StT, Pin/StT, and Pin in low potassium StT in percent recovery of developed pressure (58.99% +/- 4.8%, 51.54% +/- 3.5%, 53.09% +/- 3.2%, 66.43% +/- 7.3%, respectively) or compliance.

CONCLUSIONS

Pin is as effective a cardioplegic agent as StT; however, its use as a pretreatment or additive to traditional and Pin in low potassium StT provided no additional benefit in functional recovery.

Donor heart preservation with the potassium channel opener pinacidil: comparison with University of Wisconsin and St. Thomas' solution

BACKGROUND

Hyperpolarized arrest with the potassium channel opener pinacidil has been shown to provide effective myocardial protection during short-term global ischemia. This study tested the hypothesis that pinacidil may provide effective long-term protection for heart transplant preservation.

METHODS

Four concentrations of pinacidil (50 microM, 100 microM, 0.5 mM, 1.0 mM) mixed in Krebs-Henseleit solution were compared with University of Wisconsin and St. Thomas' Hospital solutions in a Krebs-Henseleit perfused rabbit Langendorff model (n = 6 for each group). Hearts underwent 4 hours of hypothermic (4 degrees C) storage. Over a wide range of volumes, left ventricular systolic function, diastolic compliance, and coronary flow were measured prior to and following storage. Time to mechanical and electrical arrest, and post-ischemic percent tissue water were also measured.

RESULTS

Pinacidil 0.5 mM provided the best preservation of post-ischemic systolic function and coronary flow compared with the other pinacidil concentrations and was statistically equivalent to St. Thomas' solution in terms of post-ischemic systolic, diastolic, and flow properties. However, hearts protected with University of Wisconsin solution had significantly better preservation of systolic function and coronary flow.

CONCLUSIONS

This investigation demonstrated that pinacidil in Krebs-Henseleit solution possesses efficacy in long-term donor heart preservation. Pinacidil was equivalent to St. Thomas' solution but inferior to University of Wisconsin solution. Hyperpolarized arrest with potassium channel openers may be a novel strategy to improve donor heart preservation.

Polarization and myocardial protection

Heart surgery or transplantation generally involve global ischemia, and techniques have been developed to protect the myocardium from ischemic and reperfusion injury. Hyperkalemic cardioplegia has been the gold standard for myocardial protection for years, but patients undergoing surgery almost invariably have some postoperative dysfunction. One factor may be the depolarizing nature of hyperkalemia, which results in continuing transmembrane fluxes and metabolism, even during hypothermic ischemia. A potentially beneficial alternative to hyperkalemic (depolarizing) cardioplegia is arrest in a "hyperpolarized" or "polarized" state, which maintains the myocardial membrane potential at or near the resting potential. This should minimize transmembrane fluxes and metabolic demand and improve myocardial protection. These alternative concepts have recently been investigated by using adenosine and potassium-channel openers (which are thought to induce hyperpolarized arrest) or the sodium-channel blocker tetrodotoxin (which induces polarized arrest), and results have been beneficial compared with the results of hyperkalemia. Additional studies are needed before experimental promise can become clinical reality.

Developments in cardioprotection: "polarized" arrest as an alternative to "depolarized" arrest

During cardiac surgery or cardiac transplantation, the heart is subjected to varying periods of global ischemia. The heart must be protected during this ischemic period to avoid additional injury, and techniques have been developed that delay ischemic injury and minimize reperfusion injury. Almost universally, this involves using a hyperkalemic cardioplegic solution and these solutions have become the gold standard for myocardial protection for more than 20 years. Despite the extensive and continued research aimed at improving these basic hyperkalemic cardioplegic solutions, patients undergoing surgery almost invariably experience some degree of postoperative dysfunction. It is likely that this relates to the depolarizing nature of hyperkalemic solutions, which results in ionic imbalance caused by continuing transmembrane fluxes and the consequent maintenance of high energy phosphate metabolism, even during hypothermic ischemia. A potentially beneficial alternative to hyperkalemic cardioplegia is to arrest the heart in a "hyperpolarized" or "polarized" state, which maintains the membrane potential of the arrested myocardium at or near to the resting membrane potential. At these potentials, transmembrane fluxes will be minimized and there should be little metabolic demand, resulting in improved myocardial protection. Recent studies have explored these alternative concepts for myocardial protection. The use of compounds such as adenosine or potassium channel openers, which are thought to induce hyperpolarized arrest, have demonstrated improved protection after normothermic, or short periods of hypothermic, ischemia when compared to hyperkalemic (depolarized) arrest. Similarly, studies from our own laboratory, in which the sodium channel blocker, tetrodotoxin, was used to induce polarized arrest (demonstrated by direct measurement of membrane potential during ischemia) was also shown to provide better recovery of function after 5 hours of long-term hypothermic (7.5 degrees C) storage. These promising initial studies need to be consolidated before experimental promise becomes clinical reality.

Protection of cardiomyocytes by pinacidil during metabolic inhibition and hyperkalemia

The objective of this study is to understand the mechanism underlying the cardioprotective effects of pinacidil, an ATP-sensitive K+ channel (K(ATP)) opener. We examined the effects of 10 microM pinacidil in cultured chicken cardiomyocytes. Pinacidil caused a concentration-dependent delay in metabolic inhibition-induced increase in intracellular calcium concentration ([Ca2+]i) and creatine phosphokinase release, and this action was antagonized by glyburide, a K(ATP) blocker. Neither verapamil, an L-type Ca2+ channel blocker, nor bepridil, a Na+-Ca2+ exchange inhibitor, affected the time course of increase in [Ca2+]i induced by metabolic inhibition. Pinacidil did not have an effect on the amplitude of K+-induced increase in [Ca2+]i, but accelerated the rate of decline following peak stimulation. In contrast, glyburide reduced the amplitude of K+-induced increase in [Ca2+]i and prolonged the rate of decline. These results provide direct evidence that pinacidil protects cardiomyocytes from metabolic inhibition-induced injury by cyanide (CN) through a delay in the onset of increase in [Ca2+]i, rather than by inhibition of the L-type Ca2+-channels or by alteration of Na+-Ca2+ exchange.

Hyperkalemia and ionized hypocalcemia during cardiac arrest and resuscitation: possible culprits for postcountershock arrhythmias?

STUDY OBJECTIVE

Early countershock of ventricular fibrillation (VF) has been shown to improve immediate and long-term outcome of out-of-hospital cardiac arrest. However, studies indicate that countershock of prolonged VF most commonly results in asystole or a nonperfusing bradyarrhythmia (pulseless electrical activity [PEA]), which rarely respond to current therapy. The cause of these postcountershock rhythm disturbances is not well understood but may be related to electrical injury of the globally ischemic myocardium or to local metabolic abnormalities that impair impulse formation and cardiac contraction. The purpose of this study was to evaluate changes in serum potassium and free calcium homeostasis during cardiac arrest and advanced cardiac life support (ACLS) interventions.

METHODS

After sedation, intubation, anesthesia, and instrumentation, VF was induced in 13 dogs. After 7.5 minutes of VF, animals were immediately countershocked, standard closed-chest CPR was initiated, and epinephrine was administered (1 mg in repeated doses if necessary).

RESULTS

Ten animals could not be resuscitated despite 20 minutes of ACLS interventions. In these animals, a progressive increase in serum potassium was observed from the onset of ACLS to the termination of resuscitation efforts (4.3+/-.6 to 6.0+/-.8 mEq/L, P<.01). A significant increase was observed within 10 minutes of beginning ACLS measures. This was accompanied by a decrease in ionized calcium concentration over the same period (4.95+/-.40 to 3.44 mg/dL, P<.01). The decrease in ionized calcium was significant within 5 minutes of ACLS interventions. Nine of these 10 animals had either postcountershock asystole or PEA at the termination of resuscitative efforts. The increase in potassium was not related to acidemia. Successfully resuscitated animals did not demonstrate these electrolyte changes.

CONCLUSION

Ionized hypocalcemia and hyperkalemia occur during prolonged resuscitative efforts and may be related to dysfunctional transcellular ionic transport mechanisms. These cations play important roles in cardiac electrical and contractile activity and may play a role in refractory postcountershock rhythm disturbances.

Long-term myocardial preservation: effects of hyperkalemia, sodium channel, and Na/K/2Cl cotransport inhibition on extracellular potassium accumulation during hypothermic storage

OBJECTIVES

We previously demonstrated improved myocardial preservation with polarized (tetrodotoxin-induced), compared with depolarized (hyperkalemia-induced), arrest and hypothermic storage. This study was undertaken to determine whether polarized arrest reduced ionic imbalance during ischemic storage and whether this was influenced by Na+/K +/2Cl- cotransport inhibition.

METHODS

We used the isolated crystalloid perfused working rat heart preparation (1) to measure extracellular K+ accumulation (using a K+-sensitive intramyocardial electrode) during ischemic (control), depolarized (K+ 16 mmol/L), and polarized (tetrodotoxin, 22 micromol/L) arrest and hypothermic (7.5 degrees C) storage (5 hours), (2) to determine dose-dependent (0.1, 1.0, 10 and 100 micromol/L) effects of the Na +/K+/2Cl- cotransport inhibitor, furosemide, on extracellular K+ accumulation during polarized arrest and 7.5 degrees C storage, and (3) to correlate extracellular K+ accumulation to postischemic recovery of cardiac function.

RESULTS

Characteristic triphasic profiles of extracellular K+ accumulation were observed in control and depolarized arrested hearts; a significantly attenuated profile with polarized arrested hearts demonstrated reduced extracellular K+ accumulation, correlating with higher postischemic function (recovery of aortic flow was 54% +/-4% [P =.01] compared with 39% +/-3% and 32% +/-3% in depolarized and control hearts, respectively). Furosemide (0.1, 1.0, 10, and 100 micromol/L) modified extracellular K+ accumulation by -18%, -38%, -0.2%, and +9%, respectively, after 30 minutes and by -4%, -27%, +31%, and +42%, respectively, after 5 hours of polarized storage. Recovery of aortic flow was 53% +/-4% (polarized arrest alone), 56% +/-8%, 70% +/-2% (P =.04 vs control), 69% +/-4% (P =.04 vs control), and 65% +/-3% ( P =. 04 vs control), respectively.

CONCLUSIONS

Polarized arrest was associated with a reduced ionic imbalance (demonstrated by reduced extracellular K+ accumulation) and improved recovery of cardiac function. Further attenuation of extracellular K + accumulation (by furosemide) resulted in additional recovery.

Gene delivery of Kir6.2/SUR2A in conjunction with pinacidil handles intracellular Ca2+ homeostasis under metabolic stress

Metabolic injury is a complex process affecting various tissues, with intracellular Ca2+ loading recognized as a common precipitating event leading to cell death. We have recently observed that cells overexpressing recombinant ATP-sensitive K+ (KATP) channel subunits may acquire resistance against metabolic stress. To examine whether, under metabolic challenge, intracellular Ca2+ homeostasis can be maintained by an activator of channel proteins, we delivered Kir6.2 and SUR2A genes, which encode KATP channel subunits, into a somatic cell line lacking native KATP channels. Hypoxia-reoxygenation was simulated by application and removal of the mitochondrial poison 2,4 dinitrophenol. Under such metabolic stress, Ca2+ loading was induced by Ca2+ influx during hypoxia and release of Ca2+ from intracellular stores during reoxygenation. Delivery of Kir6.2/SUR2A genes, in conjunction with the KATP channel activator pinacidil, prevented intracellular Ca2+ loading irrespective of whether the channel opener was applied throughout the duration of hypoxia-reoxygenation or transiently during the hypoxic or reoxygenation stage. In all stages of injury, the effect of pinacidil was inhibited by the selective antagonist of KATP channel, 5-hydroxydecanoate. The present study provides evidence that combined use of gene delivery and pharmacological targeting of recombinant proteins can handle intracellular Ca2+ homeostasis under hypoxia-reoxygenation irrespective of the stage of the metabolic insult.

Effects of Ca2+ channel blockers on Ca2+ loading induced by metabolic inhibition and hyperkalemia in cardiomyocytes

The effects of the L-type (nifedipine and verapamil) and the T-type (mibefradil) Ca2+ channel blockers on the increase in intracellular Ca2+ concentration ([Ca2+]i) induced by NaCN metabolic inhibition and hyperkalemia were examined in chicken cardiomyocytes using fluorescence imaging with Fura-2. NaCN induced a slow and sustained rise in [Ca2+]i, which was not affected by pretreating the cells for 5 min with nifedipine, verapamil, or mibefradil at 100 nM or 10 microM. Pretreatment of the cells with 10 microM nifedipine, verapamil, or mibefradil for 5 min remarkably inhibited the K+-induced increase in [Ca2+]i. These inhibitory effects diminished after 48-h pretreatment with nifedipine or verapamil but not with mibefradil. Ryanodine also induces an increase in [Ca2+]i, and this effect was enhanced by 48-h pretreatment of the cells with 10 microM verapamil but not with 10 microM mibefradil. We conclude that the NaCN-induced increase in [Ca2+]i is independent of the Ca2+ influx though the L-type or T-type Ca2+ channels. Chronic inhibition of the L-type Ca2+ channels but not T-type channels may enhance the ryanodine receptor-mediated Ca2+ release, which may be responsible for the development of tolerance to their inhibitory effects on K+-induced increase in [Ca2+]i.

Protective action of 17beta-estradiol in cardiac cells: implications for hyperkalemic cardioplegia

BACKGROUND

Hyperkalemic cardioplegic solutions effectively arrest the heart, but may also induce intracellular Ca2+ loading and cellular hypercontracture, which could contribute to ventricular dysfunction associated with global surgical ischemia. Recently, it has been proposed that 17beta-estradiol may possess protective properties in the ischemic myocardium. The purpose of the present study was to examine the action of 17beta-estradiol on cardiac cells exposed to hyperkalemic stress.

METHODS

Single ventricular cardiomyocytes, a preparation devoid of vascular and neuronal elements, were isolated from guinea pig hearts, loaded with a Ca2+-sensitive fluorescent probe, and imaged by digital epifluorescent microscopy. The emitted fluorescence of the probe, a measure of intracellular Ca2+ concentration, and cell length were simultaneously recorded during hyperkalemic challenge, in the absence or presence of 17beta-estradiol.

RESULTS

In control cardiomyocytes, the cytosolic concentration of Ca2+ was 138+/-11 nmol/L and cell length 93+/-11 microm. Exposure to high K+ (+16 mmol/L KCl) significantly increased cytosolic Ca2+ to 2,191+/-87 nmol/L (p < 0.001), and produced cell shortening (length at 39+/-5 microm; p < 0.001). 17beta-Estradiol (10 micromol/L) acutely prevented high K+ to induce either intracellular Ca2+ loading (144+/-13 nmol/L, p < 0.001) or hypercontracture (91+/-10 microm, p < 0.001). Tamoxifen (10 micromol/L), an antiestrogen, abolished the protective effect of 17beta-estradiol.

CONCLUSIONS

We conclude that 17beta-estradiol prevents hyperkalemia-induced Ca2+ loading and hypercontracture through a direct and tamoxifen-sensitive action in cardiomyocytes. This study raises the possibility that 17beta-estradiol should be considered as a cardioprotective adjunct toward a safer hyperkalemic cardioplegia.

Cardioprotective effects of pinacidil pretreatment and lazaroid (U74500A) preservation in isolated rat hearts after 12-hour hypothermic storage

BACKGROUND

Two important processes in the preservation of the function of donor hearts are the maintenance of ATP-sensitive potassium channel activity during myocardial ischemia and the scavenging of reactive oxygen species formed during reperfusion. The aim of this study was to compare the effect of three protocols on the preservation of hemodynamic function in isolated rat hearts after hypothermic storage. These protocols were: (1) pretreatment of the heart with a potassium channel opener (200 microM pinacidil); (2) storage of the heart in an aspartate-enriched extracellular cardioplegic solution containing the lazaroid antioxidant, U74500A (30 microM); and (3) a combination of protocols 1 and 2.

METHODS

Hearts from Wistar rats were perfused on a Langendorff apparatus. After stabilization in working mode, baseline measurements of heart rate, coronary and aortic flow, and cardiac output were performed. Hearts (n=6 in each group) were then randomized to protocols 1-3, untreated controls, or vehicle-treated controls. Hearts were stored in extracellular-based preservation solution for 12 hr at 2-3 degrees C, remounted on the perfusion apparatus, and stabilized as before; hemodynamic measurements were then repeated.

RESULTS

Recovery of hemodynamic function was enhanced by pinacidil pretreatment or incorporation of lazaroid in the storage solution, but the combination of these two treatments produced the best results.

CONCLUSIONS

Combined pharmacological activation of ATP-sensitive potassium channels before cardioplegia and the addition of U74500A to the preservation solution is associated with significantly enhanced hemodynamic function in the isolated rat heart after 12 hr of hypothermic storage. These data suggest a novel use for these agents in the transplantation context.

The sulfonylurea controversy: more questions from the heart

Myocardial ischemia and infarction are associated with substantially increased morbidity and mortality among patients with diabetes mellitus. Although many factors contribute to the increased morbidity and mortality, in patients with non-insulin-dependent (type II) diabetes mellitus, one contributor may be the use of sulfonylurea drugs, the most widely used oral hypoglycemic agents. Such a possibility, which first arose over a 25 years ago when it was observed that patients taking sulfonylurea drugs had increased cardiovascular mortality, has recently resurfaced after the discovery that sulfonylureas act by inhibiting adenosine triphosphate (ATP)-sensitive potassium channels. In the pancreas, inhibition of ATP-sensitive potassium channels induces release of insulin; but in the heart, inhibition of these channels prevents ischemic preconditioning, an endogenous cardioprotective mechanism that protects the heart from lethal injury. This review outlines the current understanding of the molecular and cellular pharmacodynamics of sulfonylurea drugs and discusses the potential clinical consequences of inhibition of ATP-sensitive potassium channels in the heart of diabetic patients with cardiac disease in whom the use of sulfonylureas may be harmful.

Adenosine prevents K+-induced Ca2+ loading: insight into cardioprotection during cardioplegia

In clinical practice, hyperkalemic cardioplegia induces sarcolemmic depolarization, and therefore is used to arrest the heart during open heart operations. However, the elevated concentration of K+ that is present in cardioplegic solutions promotes intracellular Ca2+ loading, which could aggravate ventricular dysfunction after cardiac operations. This review highlights recent findings that have established, at the single cell level, the protective action of adenosine against hyperkalemia-induced Ca2+ loading. When it was added to hyperkalemic cardioplegic solutions, adenosine, at millimolar concentrations and through a direct action on ventricular cardiomyocytes, prevented K+-induced Ca2+ loading. This action of adenosine required the activation of protein kinase C, and it was effective only in cardiomyocytes with low diastolic Ca2+ levels. Of importance, adenosine did not diminish the magnitude of K+-induced membrane depolarization, allowing unimpeded cardiac arrest. Taken together, these findings provide direct support for the idea that adenosine is valuable when used as an adjunct to hyperkalemic cardioplegia. This idea has emerged from previous clinical studies that have shown improvement of the clinical outcome after cardiac operations when adenosine or related substances were used to supplement cardioplegic solutions. Further studies are required to define more precisely the mechanism of action of adenosine, and the conditions that may determine the efficacy of adenosine as a cytoprotective supplement to cardioplegia.

ATP-sensitive K+ channels in pancreatic, cardiac, and vascular smooth muscle cells

ATP-sensitive K+ (KATP) channels are therapeutic targets for several diseases, including angina, hypertension, and diabetes. This is because stimulation of KATP channels is thought to produce vasorelaxation and myocardial protection against ischemia, whereas inhibition facilitates insulin secretion. It is well known that native KATP channels are inhibited by ATP and sulfonylurea (SU) compounds and stimulated by nucleotide diphosphates and K+ channel-opening drugs (KCOs). Although these characteristics can be shared with KATP channels in different tissues, differences in properties among pancreatic, cardiac, and vascular smooth muscle (VSM) cells do exist in terms of the actions produced by such regulators. Recent molecular biology and electrophysiological studies have provided useful information toward the better understanding of KATP channels. For example, native KATP channels appear to be a complex of a regulatory protein containing the SU-binding site [sulfonylurea receptor (SUR)] and an inward-rectifying K+ channel (Kir) serving as a pore-forming subunit. Three isoforms of SUR (SUR1, SUR2A, and SUR2B) have been cloned and found to have two nucleotide-binding folds (NBFs). It seems that these NBFs play an essential role in conferring the MgADP and KCO sensitivity to the channel, whereas the Kir channel subunit itself possesses the ATP-sensing mechanism as an intrinsic property. The molecular structure of KATP channels is thought to be a heteromultimeric (tetrameric) assembly of these complexes: Kir6.2 with SUR1 (SUR1/Kir6.2, pancreatic type), Kir6.2 with SUR2A (SUR2A/ Kir6.2, cardiac type), and Kir6.1 with SUR2B (SUR2B/Kir6.1, VSM type) [i.e., (SUR/Kir6.x)4]. It remains to be determined what are the molecular connections between the SUR and Kir subunits that enable this unique complex to work as a functional KATP channel.