Reversal of endothelin-induced vasoconstriction by endothelium-dependent and -independent vasodilators in isolated hearts and vascular rings

Pharmacological Tuning of Adenosine Signal Nuances Underlying Heart Failure With Preserved Ejection Fraction

Heart failure with preserved ejection fraction (HFpEF) roughly represents half of the cardiac failure events in developed countries. The proposed 'systemic microvascular paradigm' has been used to explain HFpHF presentation heterogeneity. The lack of effective treatments with few evidence-based therapeutic recommendations makes HFpEF one of the greatest unmet clinical necessities worldwide. The endogenous levels of the purine nucleoside, adenosine, increase significantly following cardiovascular events. Adenosine exerts cardioprotective, neuromodulatory, and immunosuppressive effects by activating plasma membrane-bound P1 receptors that are widely expressed in the cardiovascular system. Its proven benefits have been demonstrated in preclinical animal tests. Here, we provide a comprehensive and up-to-date critical review about the main therapeutic advantages of tuning adenosine signalling pathways in HFpEF, without discounting their side effects and how these can be seized.

The mechanical uncoupler blebbistatin is associated with significant electrophysiological effects in the isolated rabbit heart

Blebbistatin (BS) is a recently discovered inhibitor of the myosin II isoform and has been adopted as the mechanical uncoupler of choice for optical mapping, because previous studies suggest that BS has no significant cardiac electrophysiological effects in a number of species. The aim of this study was to determine whether BS affects cardiac electrophysiology in isolated New Zealand White rabbit hearts. Langendorff-perfused hearts (n= 39) in constant-flow mode had left ventricular monophasic action potential duration (MAPD) measured at apical and basal regions during constant pacing (300 ms cycle length). Standard action potential duration restitution was obtained using the single extrastimulus method with measurement of the maximal restitution slope. Ventricular fibrillation threshold was measured as the minimal current inducing sustained ventricular fibrillation with burst pacing (30 stimuli, at 30 ms intervals). Optical action potentials were recorded using the voltage-sensitive dye di-4-ANEPPS. Measurements were taken at baseline and after 60 min perfusion with BS (5 μm). Blebbistatin significantly prolonged left ventricular apical (mean ± SEM; from 129.9 ± 2.9 to 170.7 ± 4.1 ms, P < 0.001, n= 8) and basal MAPD (from 135.0 ± 2.3 to 163.3 ± 5.6 ms, P < 0.001) and effective refractory period (from 141.3 ± 4.8 to 175.6 ± 3.7 ms, P < 0.001) whilst increasing the maximal slope of restitution (apex, from 0.79 ± 0.09 to 1.57 ± 0.16, P < 0.001; and base, from 0.71 ± 0.06 to 1.44 ± 0.24, P < 0.001) and ventricular fibrillation threshold (from 5.3 ± 1.1 to 17.0 ± 2.9 mA, P < 0.001). In other hearts, blebbistatin significantly prolonged optically recorded action potentials (from 136.5 ± 6.3 to 173.0 ± 7.9 ms, P < 0.05, n= 4). In control experiments, the increase of MAPD with blebbistatin was present whether the hearts were perfused in constant-pressure mode (n= 5) or in unloaded conditions (n= 5). These data show that blebbistatin significantly affects cardiac electrophysiology. Its use in optical mapping studies should be treated with caution.

Stimulation of ANP secretion by 2-Cl-IB-MECA through A(3) receptor and CaMKII

Adenosine is a potent mediator of myocardial protection against hypertrophy via A(1) or A(3) receptors that may be partly related to atrial natriuretic peptide (ANP) release. However, little is known about the possible involvement of the A(3) receptor on ANP release. We studied the effects of the A(3) receptor on atrial functions and its modification in hypertrophied atria. A selective A(3) receptor agonist, 2-chloro-N(6)-(3-iodobenzyl) adenosine-5'-N-methyluronamide (2-CI-IB-MECA), was perfused into isolated, beating rat atria with and without receptor modifiers. 2-CI-IB-MECA dose-dependently increased the ANP secretion, which was blocked by the A(3) receptor antagonist, but the increased atrial contractility and decreased cAMP levels induced by 30muM 2-CI-IB-MECA were not affected. The 100muM 2-(1-hexylnyl)-N-methyladenosine (HEMADO) and N(6)-(3-iodobenzyl) adenosine-5'-N-methyluronamide (IB-MECA), A(3) receptor agonist, also stimulated the ANP secretion without positive inotropy. The potency for the stimulation of ANP secretion was 2-CI-IB-MECA>>IB-MECA=HEMADO. The inhibition of the ryanodine receptor or calcium/calmodulin-dependent kinase II (CaMKII) attenuated 2-CI-IB-MECA-induced ANP release, positive inotropy, and translocation of extracellular fluid. However, the inhibition of L-type Ca(2+) channels, sarcoplasmic reticulum Ca(2+)-reuptake, phospholipase C or inositol 1,4,5-triphosphate receptors did not affect these parameters. 2-CI-IB-MECA decreased cAMP level, which was blocked only with an inhibitor of CaMKII or adenylyl cyclase. These results suggest that 2-CI-IB-MECA increases the ANP secretion mainly via A(3) receptor activation and positive inotropy by intracellular Ca(2+) regulation via the ryanodine receptor and CaMKII.

Reversing chronic remodeling in heart failure

Chronic heart failure is a debilitating condition with significant morbidity, mortality and an increasing economic burden. The past 20 years have witnessed great strides in both medical and device-based therapies for heart failure. Central to these developments has been the ability to favorably reverse the chronic processes by which the failing heart remodels. In addition to pharmacotherapies, such as beta-blockade, and inhibition of the renin-angiotensin-aldosterone system, surgical remodeling, containment devices and new methods to restore synchronous contraction have been added to the armamentarium, in some instances, providing clear improvement to both symptoms and mortality. In more advanced stages of heart failure, left ventricular-assist devices provide marked unloading of the failing ventricle and such therapy has provided unique insights into the molecular and cellular mechanisms underlying reverse remodeling, given the immediate access to cardiac tissue. Genetic and cellular approaches, as well as new small molecule targets, may provide future avenues for reverse remodeling of the failing heart, improving symptoms and disease outcome.

Regulated Overexpression of the A 1 -Adenosine Receptor in Mice Results in Adverse but Reversible Changes in Cardiac Morphology and Function

Background— Both the A 1 - and A 3 -adenosine receptors (ARs) have been implicated in mediating the cardioprotective effects of adenosine. Paradoxically, overexpression of both A 1 -AR and A 3 -AR is associated with changes in the cardiac phenotype. To evaluate the temporal relationship between AR signaling and cardiac remodeling, we studied the effects of controlled overexpression of the A 1 -AR using a cardiac-specific and tetracycline-transactivating factor–regulated promoter.

Methods and Results— Constitutive A 1 -AR overexpression caused the development of cardiac dilatation and death within 6 to 12 weeks. These mice developed diminished ventricular function and decreased heart rate. In contrast, when A 1 -AR expression was delayed until 3 weeks of age, mice remained phenotypically normal at 6 weeks, and >90% of the mice survived at 30 weeks. However, late induction of A 1 -AR still caused mild cardiomyopathy at older ages (20 weeks) and accelerated cardiac hypertrophy and the development of dilatation after pressure overload. These changes were accompanied by gene expression changes associated with cardiomyopathy and fibrosis and by decreased Akt phosphorylation. Discontinuation of A 1 -AR induction mitigated cardiac dysfunction and significantly improved survival rate.

Conclusions— These data suggest that robust constitutive myocardial A 1 -AR overexpression induces a dilated cardiomyopathy, whereas delaying A 1 -AR expression until adulthood ameliorated but did not eliminate the development of cardiac pathology. Thus, the inducible A 1 -AR transgenic mouse model provides novel insights into the role of adenosine signaling in heart failure and illustrates the potentially deleterious consequences of selective versus nonselective activation of adenosine-signaling pathways in the heart.

Expression of adenosine receptors in cardiac fibroblasts as a function of insulin and glucose level

Adenosine among other factors is known to regulate the growth and function of cardiac fibroblasts (CFs). Its action is mediated by cell-surface receptors linked to a variety of signaling systems. The goal of present work was to examine the effects of glucose and insulin on adenosine receptors (ARs) mRNA and protein level in primary culture of rat CFs by means of real-time PCR and Western blot. Elevated glucose level increased the expression of A(1)-AR, A(2A)-AR, decreased the expression of A(3)-AR, and had no effect on A(2B)-AR expression. On the other hand insulin suppressed the expression of A(1)-AR, and A(2B)-AR, and had no effect on A(2A)-AR and A(3)-AR expression. Our measurements showed that accumulation of cAMP in response to ARs agonists correlated well with the changes in receptors expression level. These results indicate that changes in glucose and insulin level independently and differentially regulate the ARs expression and functional state in CFs.

Activation of adenosine A1 receptor attenuates cardiac hypertrophy and prevents heart failure in murine left ventricular pressure-overload model

Sympathomimetic stimulation, angiotensin II, or endothelin-1 is considered to be an essential stimulus mediating ventricular hypertrophy. Adenosine is known to protect the heart from excessive catecholamine exposure, reduce production of endothelin-1, and attenuate the activation of the renin-angiotensin system. These findings suggest that adenosine may also attenuate myocardial hypertrophy. To verify this hypothesis, we examined whether activation of adenosine receptors can attenuate cardiac hypertrophy and reduce the risk of heart failure. Our in vitro study of neonatal rat cardiomyocytes showed that 2-chloroadenosine (CADO), a stable adenosine analogue, inhibits protein synthesis of cardiomyocytes induced by phenylephrine, endothelin-1, angiotensin II, or isoproterenol, which were mimicked by the stimulation of adenosine A1 receptors. For our in vivo study, cardiac hypertrophy was induced by transverse aortic constriction (TAC) in C57BL/6 male mice. Four weeks after TAC, both heart to body weight ratio (6.80+/-0.18 versus 8.34+/-0.33 mg/g, P<0.0001) as well as lung to body weight ratio (6.23+/-0.27 versus 10.03+/-0.85 mg/g, P<0.0001) became significantly lower in CADO-treated mice than in the TAC group. Left ventricular fractional shortening and left ventricular dP/dtmax were improved significantly by CADO treatment. Similar results were obtained using the selective adenosine A1 agonist N6-cyclopentyladenosine (CPA). A nonselective adenosine antagonist, 8-(p-sulfophenyl)-theophylline, and a selective adenosine A1 antagonist, 8-cyclopentyl-1,3-dipropylxanthine, eliminated the antihypertrophic effect of CADO and CPA, respectively. The plasma norepinephrine level was decreased and myocardial expression of regulator of G protein signaling 4 was upregulated in CADO-treated mice. These results indicate that the stimulation of adenosine receptors attenuates both the cardiac hypertrophy and myocardial dysfunction via adenosine A1 receptor-mediated mechanisms.

Role of ET-1 in the regulation of coronary circulation

Given that circulating ET levels in heart failure, in particular, may reach physiological threshold for coronary constrictor responses, the primary objective of the present review is to consider coronary vessels as an important target for circulating and locally produced endothelin(s). In healthy vessels, ET-1 causes biphasic coronary responses characterized by a transient dilation of large and small arteries followed by a sustained constriction. ETB receptors are pivotal in the early dilation of resistance vessels, whereas dilation of conductance vessels may be a secondary phenomenon triggered by flow increases. Exogenous ET-1 causes coronary constriction almost exclusively through ETA receptor activation. Human and canine large epicardial coronary vessels display significant baseline ET-1 dependent tone in vitro and in vivo, an ETA-dependent process. In contrast, ETB receptors located on smooth muscle cells are apparently less important for producing constrictor responses. NO production may serve as an important counter-regulatory mechanism to limit ET-dependent effects on coronary vessels. Conversely, in a dysfunctional endothelium, the loss of NO may augment ET-1 production and activity. By lifting the ET-dependent burden from coronary vessels, ET receptor blockade should help to ensure a closer match between cardiac metabolic demand and coronary perfusion.

Phosphodiesterase type 5 inhibition enhances vasorelaxation caused by nitroprusside in guinea pig intact heart and isolated aorta

Increased vascular smooth muscle cyclic guanine monophosphate (cGMP) results in vascular relaxation. The vascular effects of stimulating cGMP production with 10(-8)-10(-4) M nitroprusside (NP) and inhibiting cGMP hydrolysis with 10(-8)-10(-4) M zaprinast (ZAP), a selective type V inhibitor of cGMP phosphodiesterase (PDE), were assessed in isolated guinea pig hearts and aortic rings. Coronary flow (CF) IC50 values for NP and ZAP, respectively, were 0.8+/-0.1 x 10(-6) M and 3.6+/-0.1 x 10(-6) M; for coronary sinus pO2 IC50 values were 0.7+/-0.1 x 10(-6) M and 3.7+/-0.1 x 10(-6) M. CF increased by 13+/-2% with 10(-6) NP, and by 12+/-2% with 10(-5) M ZAP; percentage O2 extraction (%O2E) decreased by 17+/-3% with NP and 28+/-4% with ZAP. Together, 10(-6) M NP + 10(-5) M ZAP augmented the increased in CF to 23+/-3% of control, and the decrease in percentage O2 extraction (%O2E) to 40+/-4% of control. Other cardiac effects of NP and ZAP were minimal. In norepinephrine preconstricted aortic rings, the IC50 for relaxation was elicited at 0.4+/-0.1 x 10(-6) M NP and 6.1+/-0.1 x 10(-6) M ZAP. NP given with ZAP gave a logarithmic relation so that IC50 [NP] = -(57 log10 [ZAP]) + 416; R2 = 0.95. NP, 3 x 10(-7) M; ZAP, 3 x 10(-6) M; and NP + ZAP combined increased aortic tissue cGMP by eight-, nine-, and 15-fold, respectively. Inhibiting cGMP hydrolysis may be an effective approach to augment vasorelaxation elicited by cGMP synthesis in the heart.