Intravascular adenosine: the endothelial mediators of its negative dromotropic effects

Adenosine and its receptors in the heart: regulation, retaliation and adaptation

The purine nucleoside adenosine is an important regulator within the cardiovascular system, and throughout the body. Released in response to perturbations in energy state, among other stimuli, local adenosine interacts with 4 adenosine receptor sub-types on constituent cardiac and vascular cells: A(1), A(2A), A(2B), and A(3)ARs. These G-protein coupled receptors mediate varied responses, from modulation of coronary flow, heart rate and contraction, to cardioprotection, inflammatory regulation, and control of cell growth and tissue remodeling. Research also unveils an increasingly complex interplay between members of the adenosine receptor family, and with other receptor groups. Given generally favorable effects of adenosine receptor activity (e.g. improving the balance between myocardial energy utilization and supply, limiting injury and adverse remodeling, suppressing inflammation), the adenosine receptor system is an attractive target for therapeutic manipulation. Cardiovascular adenosine receptor-based therapies are already in place, and trials of new treatments underway. Although the complex interplay between adenosine receptors and other receptors, and their wide distribution and functions, pose challenges to implementation of site/target specific cardiovascular therapy, the potential of adenosinergic pharmacotherapy can be more fully realized with greater understanding of the roles of adenosine receptors under physiological and pathological conditions. This review addresses some of the major known and proposed actions of adenosine and adenosine receptors in the heart and vessels, focusing on the ability of the adenosine receptor system to regulate cell function, retaliate against injurious stressors, and mediate longer-term adaptive responses.

Ischemic preconditioning improves maximal performance in humans

Repeated episodes of ischemia followed by reperfusion, commonly referred to as ischemic preconditioning (IPC), represent an endogenous protective mechanism that delays cell injury. IPC also increases blood flow and improves endothelial function. We hypothesize that IPC will improve physical exercise performance and maximal oxygen consumption. The purpose of the study was to examine the effect of ischemic preconditioning in leg skeletal muscles on cycling exercise performance in healthy individuals. Fifteen healthy, well-trained subjects performed two incremental maximal exercise tests on a bicycle ergometer. Power output, oxygen consumption, ventilation, respiratory quotient, and heart rate were measured continuously. Blood pressure and blood lactate were measured before and after the test. One exercise test was performed after the application of ischemic preconditioning, using a protocol of three series of 5-min ischemia at both legs with resting periods of 5 min in between. The other maximal cycling test served as a control. Tests were conducted in counterbalanced order, at least 1 week apart, at the same time of the day. The repeated ischemic periods significantly increased maximal oxygen consumption from 56.8 to 58.4 ml/min per kg (P = 0.003). Maximal power output increased significantly from 366 to 372 W (P = 0.05). Ischemic preconditioning had no effect on ventilation, respiratory quotient, maximal heart rate, blood pressure or on blood lactate. Repeated short-term leg ischemia prior to an incremental bicycle exercise test improves maximal oxygen consumption by 3% and power output by 1.6%. This protocol, which is suggested to mimic the effects of ischemic preconditioning, may have important implications for exercise performance.

Intraluminal-restricted 17 beta-estradiol exerts the same myocardial protection against ischemia/reperfusion injury in vivo as free 17 beta-estradiol

Several in vitro studies show that in animals and isolated cells, 17 beta-estradiol induces cardiovascular protective effects and it has also been observed that it reduces coronary heart disease risk. However, the use of estrogens to improve or protect cardiovascular function in humans has been controversial, this might be explained by the wide variety of effects, because estrogen receptors (ER) are expressed ubiquitously. Therefore, a cell-specific targeting therapeutic approach might be necessary. 17 beta-Estradiol was coupled to a large modified dextran through an aminocaproic spacer. For this study we used intact and gonadectomized male Wistar rats, 15 days after surgical procedure. Intravascular administration of 17 beta-estradiol-macromolecular conjugate, prior to coronary reperfusion diminishes the area of damage induced by coronary ischemia reperfusion (I/R) injury on an in vivo model. This effect was observed at 17 beta-estradiol sub-physiological concentrations [0.01 nmol/L], it is mediated by luminal endothelial ER alpha activation. 17 beta-Estradiol-macromolecular conjugate decreases phosphorylation level of PKC alpha and Akt, as part of the process to induce myocardial protection against coronary I/R. We proved that the hormone-macromolecular conjugate labeled with [3H]estradiol remained confined in the intravascular space the conjugate was not internalized into organs like heart, lung or liver. It is noteworthy that the 17 beta-estradiol-macromolecular conjugate has a slow renal elimination, which might increase its pharmacological advantage. We concluded that the stimulus of endothelial estrogen receptors is enough to decrease the myocardial damage induced by coronary reperfusion.

The combined use of esophageal electrocardiogram and multiple right parasternal chest leads in the diagnosis of PSVT and determination of accessory pathways involved: A new simple noninvasive approach

INTRODUCTION

Paroxysmal Supraventricular Tachycardia (PSVT) resulting from any cause can at times lead to diagnostic difficulty mainly when due to AVNRT and AVRT. In this study we used a non-invasive approach using esophageal ECG (ECGe) and ECG derived from the various right parasternal chest leads (RPCL) to correctly diagnose the type of PSVT as well the accessory pathways involved in AVRT.

METHODS AND RESULTS

We studied a total of 161 patients (89 male) all having a history of palpitation. All the patients underwent routine 12 lead ECG, ECGe, RPCL and electrophysiologic study. With use of ECGe and RPCL, 71 (44.1%) were diagnosed as having AVNRT and 90 (55.9%) having AVRT with various accessory pathways compared to only 49 (30.4%) and 22 (13.7%) respectively with routine 12 lead ECG. With combined ECGe and RPCL, 80 (49.7%) showed accessory pathway in the left free wall, 4 (2.5%) showed in the right free wall, 6 (3.7%) septal accessory pathway, 60 (37.3%) in the anterior wall and 30 (18.6%) in the posterior wall. In comparison, EPS could show accessory pathways in the left free wall in 81 (50.3%), in the right free wall in 5 (3.1%), in the septum in 4 (2.5%), in the anterior wall in 53 (32.9%) and in the posterior wall in 37 (23.0%) out of the 161 patients.

CONCLUSIONS

ECGe+RPCL can be used as a reliable noninvasive diagnostic tool to identify the nature of tachycardia and the pathways involved in the reentrant ring of PSVT especially in those with multiple atrioventricular accessory pathways or a combination of atrioventricular accessory pathways and dual atrioventricular nodal pathway.

Intracoronary Angiotensin II causes inotropic and vascular effects via different paracrine mechanisms

We hypothesized that Angiotensin II (Ang II), like other circulating hormones, acts exclusively intravascularly. To activate or block solely intravascular Ang II receptors, Ang II and its peptide receptor blocker saralasin (Sar) were covalently coupled to a inert polymer (POL, MW >4000 kD) forming Ang II-POL and Sar-POL. These two nonpermeable polymers, Ang II and Sar, were intracoronarily administered into the isolated, saline-perfused rat hearts. Ang II-POL and Ang II caused a dose-dependent ventricular positive inotropic (+I) and vasoconstrictor effects (+V) which were blocked by Sar. Sar-POL blocked their +I but not their +V. Thus, Ang II and Ang II-POL act on endothelial luminal receptors through paracrine mechanisms. +I were blocked solely by purinoceptor antagonists and paralleled by augmented venous release of ATP degradation products (adenosine, inosine and hypoxanthine). In contrast, +V were blocked solely by aspirin, indomethacin or a thromboxane A2 receptor antagonist. Intracoronary administration of ATP-gamma-S and U46169, a purinergic, and TXA2 agonists, respectively, mimicked +I and +V. The results indicate that ATP is the paracrine inotropic mediator while thromboxane A2 is the vasoconstrictor mediator. Thus, the +I and +V distinct effects by intracoronary Ang II indicate that its diverse mechanism of action along the coronary vascular tree may be due to a functionally heterogeneous endothelium.

Sole activation of three luminal adenosine receptor subtypes in different parts of coronary vasculature

In isolated guinea pig hearts saline perfused at constant flow, adenosine A(1), A(2A), and A(3) (A(x)) agonists covalently bound to a large polymer (Pol; 2,000 kDa) were intracoronarily administered, and three effects were studied: dromotropic, vascular and inotropic. The rank order of potencies were the following: dromotropic (Pol-A(2A)Pol-A(1)>Pol-A(3)) and vascular and inotropic (Pol-A(2A)> or =Pol-A(1)Pol-A(3)), where the rank order of potency for Pol-A(x) depends on the part of the coronary vascular network involved; i.e., there is a vascular heterogeneity. The large size of Pol-A(x) prevents extravascular diffusion and causes it to act solely in the endothelial luminal surface. This implies their cardiac effects are due to endothelial mediators. Inhibition of nitric oxide (NO) and prostaglandin (PG) synthesis with N(G)-nitro-l-arginine methyl ester and indomethacin, respectively, show that the three cardiac effects of Pol-A(1) were mediated by NO and PG, whereas for Pol-A(2A) and Pol-A(3) the mediator was mainly NO but not PG. These results suggest that if Pol-A(x) activated the corresponding endothelial A(x)-receptor subtype, a different mediator would be produced.

Coronary vasomotor and cardiac electrophysiologic effects of diadenosine polyphosphates and nonhydrolyzable analogs in the guinea pig

Platelet activation in heart disease is important owing to the effects of platelet-derived compounds on myocardial perfusion and cardiac electrophysiology. Diadenosine polyphosphates are secreted from platelets and present in the myocardium, but their electrophysiologic and vasomotor effects are incompletely understood. We used isolated guinea-pig hearts to study the effects of diadenosine triphosphate (Ap3A), tetraphosphate (Ap4A), pentaphosphate (Ap5A), and hexaphosphate (Ap6A) (10 pM-0.1 mM), comparing their actions to those of adenosine, adenosine triphosphate, and non-hydrolyzable Ap4A and Ap5A analogs. Diadenosine polyphosphates (0.1 nM-0.1 microM) transiently reduced coronary perfusion pressure, which recovered during the continued presence of the compounds. At concentrations greater than 0.1 microM effects were maximal and sustained (perfusion pressure decreased from 36.5+/-3.4 to 18.6+/-2.5 mm Hg, p < 0.001, with 1 microM Ap4A). The changes in action potential duration and refractory period developed slowly but were maintained (0.1 nM-1 microM). With 1 nM Ap4A, action potential duration increased from 170.6+/-2.6 to 187.3+/-3.8 ms, p < 0.05, and refractory period increased from 138.5+/-1.6 to 147.9+/-2.0 ms, p < 0.05. Ap4A and its analog reduced QRS duration (from 24.7+/-1.1 to 13.9+/-1.6 ms with 1 microM Ap4A, p < 0.05). P2-purinergic (adenosine triphosphate) receptor antagonism (suramin) reduced perfusion pressure but was without electrophysiologic effect. Other changes in coronary perfusion pressure and electrophysiologic variables associated with Ap4A were not seen in the presence of suramin. P1-(adenosine) antagonism (8-[p-sulfophenyl]theophylline) attenuated the electrophysiologic effects only. Diadenosine polyphosphates have potent cardiac electrophysiologic and coronary vasomotor effects via purinergic receptors, suggesting an important role during platelet activation in acute coronary syndromes.

Effects of arginine vasopressin in the heart are mediated by specific intravascular endothelial receptors

Arginine vasopressin induces vascular, inotropic and arrhythmogenic effects in the heart. Existing evidence, obtained indirectly, suggests that these effects occur through paracrine endothelial mechanisms. To demonstrate this, vasopressin was confined to the intravascular space by covalent coupling to high molecular weight (2x10(6) Da, vasopresin-dextran) dextran. Isolated guinea pig hearts were infused with equivalent concentrations of vasopressin and vasopressin-dextran. The negative inotropic and coronary vasopressor effects of vasopressin-dextran were similar to those evoked by vasopressin; in both cases effects were reversible. Free dextran had no effect on vascular resistance nor in ventricular developed pressure. The inotropic and vascular effects of both vasopressin and vasopressin-dextran were blocked by the vasopressin receptor antagonist [Adamantaneacetyl(1), o-Et-D-Tyr(2), Val(4), Aminobutyryl(6), Arg(8,9)]vasopressin (Adam-vasopressin), indicating that the effects of the two agonists were vasopressin receptor-mediated. To elucidate possible endothelial intermediaries of these effects, isolated guinea pig hearts were infused simultaneously with vasopressin or vasopressin-dextran and several inhibitors either of synthesis or blockers of receptors of possible endothelial mediators. Only reactive blue 2, a P(2y) purinoceptor antagonist, and suramin, a P(2y) and a P(2x) purinoceptor antagonist, caused a total reversal of vascular and inotropic effects of vasopressin and vasopressin-dextran. Pyridoxalphosphate-6-Azophenyl-2'-4'disulphonic acid, a P(2x) purinoceptor antagonist, was without effect. Our results provide direct evidence that the short-term cardiac effects of vasopressin are due to selective activation of intravascular purinoceptors and suggest that an intermediary of these effects is ATP.