Effect of Hypoxia-Reoxygenation on Endothelial Function in Porcine Cardiac Microveins

Reperfusion injury as a target for diminishing infarct size

Therapies for preventing reperfusion injury (RI) have been widely studied. However, the attempts to transfer cardioprotective therapies for reducing RI from experiments into clinical practice have been so far unsuccessful. Pathophysiological mechanisms of RI are complicated and compose of many pathways e.g. hypercontracture-mediated sarcolemma rupture, mitochondrial permeability transition pore persistent opening, reactive oxygen species formation, inflammation and no-reflow phenomenon. Based on research, it cannot be determined which mechanism dominates, probably they cooperate with a domination of one or another in different clinical circumstances. Our hypothesis is, that only intervention that at the same time interferes with different (all?) pathways of RI may turn out to be effective in decreasing the final area of infarction.

Protection of coronary endothelial function during cardiac surgery: potential of targeting endothelial ion channels in cardioprotection

Vascular endothelium plays a critical role in the control of blood flow by producing vasoactive factors to regulate vascular tone. Ion channels, in particular, K⁺ channels and Ca²⁺-permeable channels in endothelial cells, are essential to the production and function of endothelium-derived vasoactive factors. Impairment of coronary endothelial function occurs in open heart surgery that may result in reduction of coronary blood flow and thus in an inadequate myocardial perfusion. Hyperkalemic exposure and concurrent ischemia-reperfusion during cardioplegic intervention compromise NO and EDHF-mediated function and the impairment involves alterations of K⁺ channels, that is, K_ATP and K_Ca, and Ca²⁺-permeable TRP channels in endothelial cells. Pharmacological modulation of these channels during ischemia-reperfusion and hyperkalemic exposure show promising results on the preservation of NO and EDHF-mediated endothelial function, which suggests the potential of targeting endothelial K⁺ and TRP channels for myocardial protection during cardiac surgery.

Intracellular pH in the resistance vasculature: regulation and functional implications

Net acid extrusion from vascular smooth muscle (VSMCs) and endothelial cells (ECs) in the wall of resistance arteries is mediated by the Na(+),HCO(3)(-) cotransporter NBCn1 (SLC4A7) and the Na(+)/H(+) exchanger NHE1 (SLC9A1) and is essential for intracellular pH (pH(i)) control. Experimental evidence suggests that the pH(i) of VSMCs and ECs modulates both vasocontractile and vasodilatory functions in resistance arteries with implications for blood pressure regulation. The connection between disturbed pH(i) and altered cardiovascular function has been substantiated by a genome-wide association study showing a link between NBCn1 and human hypertension. On this basis, we here review the current evidence regarding (a) molecular mechanisms involved in pH(i) control in VSMCs and ECs of resistance arteries at rest and during contractions, (b) implications of disturbed pH(i) for resistance artery function, and (c) involvement of disturbed pH(i) in the pathogenesis of vascular disease. The current evidence clearly implies that pH(i) of VSMCs and ECs modulates vascular function and suggests that disturbed pH(i) either consequent to disturbed regulation or due to metabolic challenges needs to be taken into consideration as a mechanistic component of artery dysfunction and disturbed blood pressure regulation.

Calcium-activated potassium channels in vasculature in response to ischemia-reperfusion

Based on the genetic relationship, single-channel conductance, and gating mechanisms, calcium-activated potassium (KCa) channels identified in vasculature can be divided into 3 groups including large-conductance KCa, small, and intermediate conductance KCa. KCa channels in smooth muscle and endothelial cells are essential for the regulation of vascular tone. Vascular dysfunction under ischemia-reperfusion (I-R) or hypoxia-reoxygenation (H-R) conditions is associated with modulations of KCa channels that are attributable to multiple mechanisms. Most studies in this regard relied on the change of relaxation components sensitive to certain channel blockers to indicate the alteration of KCa channels under I-R conditions, which however provided conflicting results for the effect of I-R. The possible mechanisms involved in KCa channel modulation under I-R/H-R include overproduction of reactive oxygen species such as superoxide anion, hydrogen peroxide, and peroxynitrite, increase of intracellular H ion, and lactate accumulation, etc. However, more studies are necessary to further understand the discrepancies in the sensitivity of KCa channels to I-R injury in different vascular beds.

New strategy of endothelial protection in cardiac surgery: use of enhancer of endothelial nitric oxide synthase

BACKGROUND

Endothelial dysfunction related to the loss of nitric oxide (NO) production remains an important issue in cardiac surgery. We examined the hypothesis that AVE3085, a novel compound that enhances eNOS transcription, may protect coronary endothelium against hypoxia-reoxygenation (H-R) injury during cardioplegic arrest and the possible mechanism by which this occurs.

METHODS

Porcine coronary small arteries (600-800-microm diameter) were subjected to hypoxia (PO(2) <5 mmHg) in St. Thomas cardioplegic (ST) solution with or without AVE3085 (10 microM) or L-arginine (10 mM) at either 37 or 4 degrees C for 60 min, followed by 30-min reoxygenation. Bradykinin (-10 to -6.5 LogM)-induced, endothelium-dependent relaxation was studied in a myograph in U(46619) precontraction before and after H-R. Protein expressions of eNOS and phosphorylated eNOS at Ser-1177 (p-eNOS(Ser1177)) were also determined.

RESULTS

Exposure to ST solution with H-R at both 37 and 4 degrees C markedly reduced bradykinin-induced relaxation in coronary small arteries. Addition of AVE3085 in ST solution at 37 degrees C preserved the vasorelaxant response to bradykinin (95.7 +/- 2.1% vs. 69.2 +/- 6.6%, p < 0.01), with the protective effect comparable to that of L-arginine (96.1 +/- 3.3% vs. 70.6 +/- 8.7%, p < 0.05). eNOS and p-eNOS(Ser1177) expressions in coronary endothelial cells were significantly increased by the addition of AVE3085 in ST solution during hypoxia (p < 0.05). Protection of endothelium-dependent relaxation from H-R by AVE3085 (70.3 +/- 7.2% vs. 90.5 +/- 2.4%, p < 0.05) also reached a level similar to that by L-arginine (69.9 +/- 9.0% vs. 94.7 +/- 3.9%, p < 0.05) at 4 degrees C.

CONCLUSIONS

We have demonstrated a new mechanism to protect coronary endothelium from H-R injury by using eNOS enhancers. This may form a new strategy in the future development of cardioplegic/preservation solutions with direct targeting of eNOS expression in coronary vasculature.

Normothermic and hypothermic models for studying the deleterious effects of hypoxia-reoxygenation on EDHF-mediated relaxation in isolated porcine coronary arteries

INTRODUCTION

The vasomotor response of the coronary artery is altered by hypoxia-reoxygenation (H-R) induced damage. The aim of our study was to compare and evaluate normothermic and hypothermic models which are suitable for future drug studies of vasoprotective action against H-R injury.

METHODS

Porcine coronary arterial rings were isolated and placed in Krebs-Henseleit (K-H) solution. Rings were exposed to normoxic conditions (control group) and two different H-R conditions: the first induced by a 95% N(2)-5% CO(2) gas mixture (40- and 60-min hypoxia) in a normothermic protocol, and the second induced by hypothermic (4 degrees C) hypoxia-reoxygenation in an air-tight beaker filled with K-H solution (24- and 48-hours hypoxia). Reoxygenation was applied by introducing K-H solution aerated with a 95% O(2)-5% CO(2) mixture under normothermic (37 degrees C) conditions. To test the EDHF-mediated relaxation by substance P, rings were first incubated in L-NNA, nitric oxide synthase inhibitor, and indomethacin, cyclooxygenase inhibitor, and then pre-contracted with thromboxane analogue U-46619. Analysis of the maximum relaxation of the arterial rings was performed by one-way ANOVA, followed by Bonferroni's post-test.

RESULTS

Distal segments of the coronary artery responded faster to contraction induced by U-46619 and were relaxed by substance P to a greater extent than proximal segments. Maximal relaxations of arterial rings induced by a 10 nM solution of substance P were significantly reduced (p<0.001) from the values for normoxic rings (81.0+/-1.0%, n=30) after 40-min H-R (50.5+/-5.3%, n=30), 60-min H-R (32.1+/-3.5%, n=30), 24-hours hypothermic H-R (56.0+/-2.3%, n=30) and after 48-hours hypothermic H-R (38.5+/-5.1%, n=30).

CONCLUSIONS

The model employing 40-min normothermic H-R is as effective as 24-hours hypothermic H-R, and 60-min normothermic H-R as 48-hours hypothermic H-R for studying the deleterious effects of H-R on EDHF-mediated relaxation.