Biomedical subjects
David J Chambers
Publications and source records attributed to David J Chambers.
Myocardial protection with intermittent cross-clamp fibrillation: does preconditioning play a role?
OBJECTIVE: Previously, we showed intermittent cross-clamp fibrillation afforded equivalent protection to cardioplegia. This study examined whether protection induced by intermittent cross-clamp fibrillation involves an ischemic preconditioning mechanism. METHODS: Isolated Langendorff-perfused rat hearts were subjected to three different studies to determine: Study 1, whether a single intermittent cross-clamp fibrillation episode (10 min) and reperfusion (10 min) before prolonged ischemia acts as a preconditioning trigger for protection; Study 2, whether cardioprotection induced by intermittent cross-clamp fibrillation alone (no prolonged ischemia) involves a preconditioning mechanism; Study 3, whether intermittent cross-clamp fibrillation cardioprotection can be prevented by targeting putative components of the preconditioning mechanism (protein kinase C or the mitochondrial ATP-sensitive potassium (K(ATP)) channel). Hearts were reperfused (60 min) and recovery of function (left ventricular developed pressure measured using an intraventricular balloon) and myocardial injury (creatine kinase leakage) were measured. RESULTS: In Study 1, recovery of function in the single intermittent cross-clamp fibrillation hearts was 61+/-3% (mean+/-SEM) (p<0.05) compared to 41+/-2% in control group; glibenclamide (a non-specific ATP-sensitive potassium (K(ATP))-channel blocker) prevented this preconditioning protection (37+/-4%). In Study 2, recovery of function in intermittent cross-clamp fibrillation hearts (62+/-3%) was significantly (p<0.05) higher than intermittent cross-clamp fibrillation hearts treated with glibenclamide (33+/-2%) and ischemia hearts (30+/-5%). In Study 3, protection by intermittent cross-clamp fibrillation (60+/-3%; p<0.05) was attenuated by protein kinase C inhibition (chelerythrine, 34+/-3%) and mitochondrial K(ATP)-channel blockade (5-hydroxydecanoate, 27+/-4%) to levels not significantly different from that of ischemia hearts (25+/-4%). CONCLUSIONS: The cardioprotective efficacy of intermittent cross-clamp fibrillation was attenuated by protein kinase C inhibition or K(ATP)-channel blockade. Involvement of these putative preconditioning cascade components in association with cardioprotection induced by intermittent cross-clamp fibrillation, suggests a role for the ischemic preconditioning mechanism.
Changes in P-selectin expression on cardiac microvessels in blood-perfused rat hearts subjected to ischemia-reperfusion.
BACKGROUND: During cardiac surgery involving cardiopulmonary bypass, activation of polymorphonuclear cells is believed to contribute to ischemia-reperfusion injury and subsequent myocardial impairment of function. The early tethering of polymorphonuclear cells to blood vessel walls depends upon recognition of the adhesion molecule P-selectin on endothelium. The purpose of this study was to define the kinetic changes in expression of P-selectin on myocardial vessels in a model of global ischemia-reperfusion injury. METHODS: In a novel recirculating blood-based perfusion system, rat hearts were subjected to 30 minutes of aerobic perfusion, 60 minutes of global ischemia, and 60 minutes of reperfusion, or to 120 minutes of continuous aerobic blood perfusion (with or without leukocyte/platelet depletion). Heart function (left ventricular developed pressure), heart rate, and perfusion pressure were monitored throughout. Hearts were sampled at defined periods for microvascular expression of P-selectin, identified by immunohistochemistry. RESULTS: In control (nonperfused) hearts and in hearts subjected to perfusion and ischemia, few cardiac vessels (8% to 16%) expressed P-selectin. After 15 minutes of reperfusion, P-selectin was present on the majority of vessels (77%; p < 0.05) but expression decreased subsequently throughout the remaining duration of reperfusion. Interestingly, upregulation of P-selectin also occurred when hearts were subjected to continuous perfusion alone (no ischemia), but this upregulation was less rapid. Depletion of leukocytes/platelets from the blood perfusate did not modify P-selectin expression. CONCLUSIONS: The augmented expression of P-selectin on myocardial vessels during reperfusion of ischemic hearts probably reflects changes induced during global ischemia and by the duration of perfusion through the nonbiological tubing of the circuit. That is likely to mimic the effects initiated during cardiopulmonary bypass.
Stereospecific synthesis of 1,2-cis glycosides by vinyl-mediated IAD.
[reaction: see text] Stereospecific 1,2-cis glycosylation of 2-O-vinyl thioglycosides, synthesized from the corresponding alcohols by Ir-catalyzed transvinylation with vinyl acetate, is achieved by iodine-mediated tethering of a range of primary and secondary carbohydrate acceptors, followed by intramolecular aglycon delivery (IAD). The use of such an intramolecular glycosylation strategy furnishes the desired alpha-gluco and beta-manno disaccharides in an entirely stereoselective manner.
Experimental studies on myocardial protection with intermittent cross-clamp fibrillation: additive effect of the sodium-hydrogen exchanger inhibitor, cariporide.
BACKGROUND: We previously showed that intermittent cross clamping with fibrillation affords myocardial protection equivalent to cardioplegic arrest. In this study, we examined whether cariporide (Aventis Pharma, Frankfurt, Germany), a specific sodium-hydrogen exchanger inhibitor, enhanced the protective effect of intermittent cross-clamp fibrillation (ICCF). METHODS: Isolated rat hearts were Langendorff-perfused (20 mins) with bicarbonate buffer and function (left ventricular developed pressure) measured. In each of three separate protocols that incorporated progressively longer ischemic durations, hearts were randomly allocated to one of three groups: group 1 was the control group with 40, 60, or 80 minutes of continuous global ischemia. Group 2 was the ICCF group with 4, 6, or 8 cycles of 10 minutes ICCF and 10 minutes of reperfusion in sinus rhythm. Group 3 was the ICCF plus cariporide group, which was the same as group 2, but also with 3 micromoles/L cariporide present in perfusate from 10 minutes before the ICCF cycles. Hearts were reperfused for 60 minutes with drug-free buffer and recovery (percentage of initial function) was measured. Hearts were maintained at 37 degrees C throughout the protocols. In protocol 3 (80 minutes ischemia per 8 cycles of ICCF), creatine kinase leakage (myocardial injury) and triphenyl tetrazolium chloride staining (myocardial viability) were also measured. Protocols 1, 2, and 3 had n = 8 hearts, n = 6 hearts, and n = 6 hearts in each group, respectively. RESULTS: In the three protocols, the recoveries of left ventricular developed pressure in the control group, the ICCF group, and the ICCF plus cariporide group, respectively, for protocol 1 were: 26% +/- 3%, 70% +/- 2% (p < 0.05 vs the control group) and 74% +/- 2% (p < 0.05 vs the control group), respectively. For protocol 2 these were: 16% +/- 2%, 55% +/- 1% (p < 0.05 vs the control group), and 70% +/- 3% (p < 0.05 vs the control and ICCF groups), respectively. For protocol 3 these were: 8% +/- 2%, 41% +/- 3% (p < 0.05 vs the control group), and 63% +/- 2% (p < 0.05 vs the control and ICCF groups), respectively. Recovery of left ventricular end-diastolic pressure mirrored that of left ventricular developed pressure in all protocols. In protocol 3, total creatine kinase leakage (international units per gram wet weight) was 88 +/- 12, 47 +/- 4 (p < 0.05 vs the control group), and 17 +/- 1 (p < 0.05 vs the control and ICCF groups), respectively, and triphenyl tetrazolium chloride staining (arbitrary units per gram wet weight) was 0.17 +/- 0.04 in the control group, 0.39 +/- 0.04 (p < 0.05 vs the control group) in the ICCF group, and 0.47 +/- 0.08 (p < 0.05 vs the control group) in the ICCF plus cariporide group, respectively. CONCLUSIONS: Sodium-hydrogen exchanger inhibition with cariporide enhances the myocardial protection afforded by ICCF, with the additive benefit becoming more apparent with increasing severity of the ischemic insult. Sodium-hydrogen exchanger inhibition may provide a significant protective reserve during ICCF, particularly when longer procedures are required.
Stereoselective synthesis of C-glycosides from carboxylic acids: the tandem Tebbe-Claisen approach.
A variety of beta- or alpha-C-glycosides may be readily accessed in an entirely stereoselective fashion from esters derived from the reaction of carboxylic acids and 3-hydroxy glycals, by way of a tandem reaction sequence of Tebbe methylenation and Claisen rearrangement. Though of wide scope, for example allowing the synthesis of 1-6 linked C-disaccharides, the methodology does not currently allow the synthesis of C-glycosyl alpha-amino acids.
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.
Stereoselective 1,2-cis glycosylation of 2-O-allyl protected thioglycosides.
The technique of intramolecular aglycon delivery (IAD), whereby a glycosyl acceptor is temporarily appended to a hydroxyl group of a glycosyl donor is an attractive method that can allow the synthesis of 1,2-cis glycosides in an entirely stereoselective fashion. 2-O-Allyl protected thioglycoside donors are excellent substrates for IAD, and may be glycosylated stereoselectively through a three-step reaction sequence. This sequence consists of quantitative yielding allyl bond isomerisation, to produce vinyl ethers that can then undergo N-iodosuccinimide mediated tethering of the desired glycosyl acceptor, and subsequent intramolecular glycosylation, to yield either alpha-glucosides or beta-mannosides accordingly. Although attempted one-pot tethering and glycosylation is hampered by competitive intermolecular reaction with excess glycosyl acceptor, this problem can be simply overcome by the use of excess glycosyl donor. Allyl mediated IAD is a widely applicable practical alternative to other IAD approaches for the synthesis of beta-mannosides, that is equally applicable for alpha-gluco linkages. It is advantageous in terms of both simplicity of application and yield, and in addition has no requirement for cyclic 4,6-protection of the glycosyl donor.
Myocardial protection with oxygenated esmolol cardioplegia during prolonged normothermic ischemia in the rat.
OBJECTIVE: We previously showed that arrest with multidose infusions of high-dose (1 mmol/L) esmolol (an ultra-short-acting beta-blocker) in oxygenated Krebs-Henseleit buffer (esmolol cardioplegia) provided complete myocardial protection after 40 minutes of normothermic (37 degrees C) global ischemia in isolated rat hearts. In this study we investigated the importance of oxygenation for protection with esmolol cardioplegia, compared it with that of St Thomas' Hospital cardioplegia, and determined the protective efficacy of multidose esmolol cardioplegia for extended ischemic durations. METHODS: Isolated rat hearts (n = 6/group) were perfused in the Langendorff mode at constant pressure (75 mm Hg) with oxygenated Krebs-Henseleit bicarbonate buffer at 37 degrees C. The first part of the first study had four groups: (i) multidose (every 15 minutes) oxygenated (95% oxygen/5% carbon dioxide) Krebs-Henseleit buffer during 60 minutes of global ischemia, (ii) multidose deoxygenated (95% nitrogen/5% carbon dioxide) Krebs-Henseleit buffer during 60 minutes of global ischemia, (iii) multidose oxygenated esmolol cardioplegia during 60 minutes of global ischemia, and (iv) multidose deoxygenated esmolol cardioplegia during 60 minutes of global ischemia. The second part of the first study had three groups: (v) multidose St Thomas' Hospital solution during 60 minutes of global ischemia, (vi) multidose oxygenated St Thomas' Hospital solution during 60 minutes of global ischemia, and (vii) multidose oxygenated esmolol cardioplegia during 60 minutes of global ischemia. In the second study, hearts were randomly assigned to 60, 75, 90, or 120 minutes of global ischemia and at each ischemic duration were subjected to multidose oxygenated constant flow or constant pressure infusion of (i) Krebs-Henseleit buffer (constant flow), (ii) Krebs-Henseleit buffer (constant pressure), (iii) esmolol cardioplegia (constant flow), or (iv) esmolol cardioplegia (constant pressure). All hearts were reperfused for 60 minutes, and recovery of function was measured. RESULTS: Multidose infusion of oxygenated esmolol cardioplegia completely protected the hearts (97% +/- 5%) after 60 minutes of 37 degrees C global ischemia. Deoxygenated esmolol cardioplegia was significantly less protective (45% +/- 8%). Oxygenation of St Thomas' Hospital solution did not alter its protective efficacy in this study (70% +/- 4% vs 69% +/- 7%). Infusion of esmolol cardioplegia at constant pressure provided complete protection for 60, 75, and 90 minutes (104% +/- 5%, 95% +/- 5%, and 95% +/- 3%, respectively), whereas protection with constant-flow esmolol cardioplegic infusion was significantly decreased at ischemic durations longer than 60 minutes. This decrease in efficacy of constant-flow esmolol cardioplegia was associated with increasing coronary perfusion pressure leading to myocardial injury. CONCLUSIONS: Oxygenation of esmolol cardioplegia (Krebs-Henseleit buffer plus 1.0 mmol/L esmolol) was essential for optimal myocardial protection. Multidose infusion of oxygenated esmolol cardioplegia provided good myocardial protection during extended periods of normothermic ischemia. Esmolol cardioplegia may provide an efficacious alternative to hyperkalemia.