The cardiac adrenergic system in ischaemia: differential role of acidosis and energy depletion

Effects of hypoxia-ischemia and inotropes on expression of cardiac adrenoceptors in the preterm fetal sheep

Preterm infants frequently suffer cardiovascular compromise, with hypotension and/or low systemic blood flow, leading to tissue hypoxia-ischemia (HI). Many preterm infants respond inadequately to inotropic treatments using adrenergic agonists such as dobutamine (DB) or dopamine (DA). This may be because of altered cardiac adrenoceptor expression because of tissue HI or prolonged exposure to adrenergic agonists. We assessed the effects of severe HI with and without DB/DA treatment on cardiac adrenoceptor expression in preterm fetal sheep. Fetal sheep (93-95 days) exposed to sham surgery or severe HI induced by umbilical cord occlusion received intravenous DB or saline for 74 h (HI + DB, HI, Sham + DB, Sham). The HI groups were also compared with fetal sheep exposed to HI and DA. Fetal hearts were collected to determine β-adrenoceptor numbers using [¹²⁵I]-cyanopindolol binding and mRNA expression of β₁-, β₂-, α_1A-, α_2A-, or α_2B-adrenoceptors. The HI group had increased β-adrenoceptor numbers compared with all other groups in all four heart chambers ( P < 0.05). This increase in β-adrenoceptor numbers in the HI group was significantly reduced by DB infusion in all four heart chambers, but DA infusion in the HI group only reduced β-adrenoceptor numbers in the left atria and ventricle. DB alone did not affect β-adrenoceptor numbers in the sham animals. Changes in β₁-adrenoceptor mRNA levels trended to parallel the binding results. We conclude that HI upregulates preterm fetal cardiac β-adrenoceptors, but prolonged exposure to adrenergic agonists downregulates adrenoceptors in the preterm heart exposed to HI and may underpin the frequent failure of inotropic therapy in preterm infants. NEW & NOTEWORTHY This is the first study, to our knowledge, on the effects of hypoxia-ischemia and adrenergic agonists on adrenoceptors in the preterm heart. In fetal sheep, we demonstrate that hypoxia-ischemia increases cardiac β-adrenoceptor numbers. However, exposure to both hypoxia-ischemia and adrenergic agonists (dobutamine or dopamine) reduces the increase in β-adrenoceptor numbers, which may underpin the inadequate response in human preterm infants to inotropic therapy using adrenergic agonists. Dobutamine alone does not affect the cardiac adrenoceptors in the sham animals.

Endothelin-1 and norepinephrine overflow from cardiac sympathetic nerve endings in myocardial ischemia

In protracted myocardial ischemia, sympathetic activation with carrier-mediated excessive norepinephrine (NE) release from its nerve endings due to reversal of NE transporter in an outward direction is a prominent cause of arrhythmias and cardiac dysfunction. Endothelin-1 (ET-1) and its receptors are intimately involved in the regulation of this carrier-mediated NE overflow in protracted myocardial ischemia. The ET-1 system is often complex, sometimes involving opposing actions depending on which receptor subtype is activated, which cells are affected, and whether stimuli are endogenously generated or exogenously applied. Therefore, a detailed understanding of the ET-1 system is important for applying drugs acting on this system in clinical settings for the treatment of ischemic cardiac disease. This article provides a detailed analysis of how the ET-1 system is involved in the regulation of carrier-mediated NE release from sympathetic nerve endings in protracted myocardial ischemia.

Role of selective V2-receptor-antagonism in septic shock: a randomized, controlled, experimental study

Introduction

V₂-receptor (V₂R) stimulation potentially aggravates sepsis-induced vasodilation, fluid accumulation and microvascular thrombosis. Therefore, the present study was performed to determine the effects of a first-line therapy with the selective V₂R-antagonist (Propionyl₁-D-Tyr(Et)₂-Val₄-Abu₆-Arg_8,9)-Vasopressin on cardiopulmonary hemodynamics and organ function vs. the mixed V_1aR/V₂R-agonist arginine vasopressin (AVP) or placebo in an established ovine model of septic shock.

Methods

After the onset of septic shock, chronically instrumented sheep were randomly assigned to receive first-line treatment with the selective V₂R-antagonist (1 μg/kg per hour), AVP (0.05 μg/kg per hour), or normal saline (placebo, each n = 7). In all groups, open-label norepinephrine was additionally titrated up to 1 μg/kg per minute to maintain mean arterial pressure at 70 ± 5 mmHg, if necessary.

Results

Compared to AVP- and placebo-treated animals, the selective V₂R-antagonist stabilized cardiopulmonary hemodynamics (mean arterial and pulmonary artery pressure, cardiac index) as effectively and increased intravascular volume as suggested by higher cardiac filling pressures. Furthermore, left ventricular stroke work index was higher in the V₂R-antagonist group than in the AVP group. Notably, metabolic (pH, base excess, lactate concentrations), liver (transaminases, bilirubin) and renal (creatinine and blood urea nitrogen plasma levels, urinary output, creatinine clearance) dysfunctions were attenuated by the V₂R-antagonist when compared with AVP and placebo. The onset of septic shock was associated with an increase in AVP plasma levels as compared to baseline in all groups. Whereas AVP plasma levels remained constant in the placebo group, infusion of AVP increased AVP plasma levels up to 149 ± 21 pg/mL. Notably, treatment with the selective V₂R-antagonist led to a significant decrease of AVP plasma levels as compared to shock time (P < 0.001) and to both other groups (P < 0.05 vs. placebo; P < 0.001 vs. AVP). Immunohistochemical analyses of lung tissue revealed higher hemeoxygenase-1 (vs. placebo) and lower 3-nitrotyrosine concentrations (vs. AVP) in the V₂R-antagonist group. In addition, the selective V₂R-antagonist slightly prolonged survival (14 ± 1 hour) when compared to AVP (11 ± 1 hour, P = 0.007) and placebo (11 ± 1 hour, P = 0.025).

Conclusions

Selective V₂R-antagonism may represent an innovative therapeutic approach to attenuate multiple organ dysfunction in early septic shock.

The radical trap 5,5-dimethyl-1-pyrroline N-oxide exerts dose-dependent protection against myocardial ischemia-reperfusion injury through preservation of mitochondrial electron transport

Free radicals are important mediators of myocardial ischemia-reperfusion injury. Nitrone spin traps have been shown to scavenge free radicals. The cardioprotective effect of the spin trap, 5,5-dimethyl-1-pyrroline N-oxide (DMPO), was investigated in an isolated heart model of global ischemia and reperfusion. Rat hearts were perfused and subjected to global ischemia for 30 min followed by reperfusion with four treatment groups of varying DMPO concentration (0.5-10 mM) administered before induction of ischemia. DMPO treatment improved the recovery of left ventricular (LV) function and coronary flow over the 30-min period of reperfusion compared with untreated hearts. Enhanced recovery was observed for all doses studied but was highest with 1 mM treatment with 2.4-fold higher recovery of LV developed pressure and 37% reduction in infarct size. Superoxide was measured by tissue fluorometry using the O(2)* probe hydroethidine. Hearts treated with 1 mM DMPO showed a significant reduction in O(2)* production compared with control hearts both over the first 5 min of ischemia and upon reperfusion after 30 min of global ischemia. Studies of mitochondrial function demonstrated that 1 mM DMPO increased the recovery of function of complexes I, II/III, and IV after 30 min of reperfusion. Immunoblotting with antibodies against complexes I, II, and IV further revealed marked up-regulation of mitochondrial proteins, suggesting that DMPO prevents their ischemic degradation via scavenging oxygen radicals generated during ischemia/reperfusion. Thus, DMPO functions as a protective agent against ischemic and postischemic injury via radical scavenging, conferring robust dose-dependent protection with salvage of mitochondrial function and redox homeostasis.

Time course and mechanisms of phosphorylation of phospholamban residues in ischemia-reperfused rat hearts. Dissociation of phospholamban phosphorylation pathways

Sarcoplasmic reticulum (SR) dysfunction is one of the multiple alterations that occurs in ischemia-reperfused hearts. Because SR function is regulated by phosphorylation of phospholamban (PLB), a SR protein phosphorylated by cAMP-dependent protein kinase (PKA) at Ser(16)and Ca(2+)-calmodulin-dependent protein kinase (CaMKII) at Thr(17), the phosphorylation of these residues during ischemia and reperfusion was examined in Langendorff-perfused rat hearts. Ser(16)phosphorylation increased significantly after 20 min of ischemia from 2.5+/-0.6% to 99.8+/-25.5% of maximal isoproterenol-induced site-specific phosphorylation and decreased to control values immediately after reperfusion. Thr(17)phosphorylation transiently increased at 2-5 min of ischemia and at 1 min of reperfusion (R1, 166.2+/-28.2%). The ischemia-induced increase in Ser(16)phosphorylation was significantly diminished in hearts from catecholamine-depleted animals and/or after beta-blockade and abolished in the presence of the PKA-inhibitor, H-89. Thr(17)phosphorylation at the beginning of ischemia was blunted by nifedipine, whereas at R1 it was significantly diminished by perfusion with 0 m m Ca(2+)in the presence of EGTA and by the Na(+)/Ca(2+)exchanger inhibitor KB-R7943. KN-93, used to specifically inhibit CaMKII, decreased Thr(17)phosphorylation at R1 and significantly prolonged half relaxation time. The results demonstrated a dissociation between the phosphorylation of PLB sites, being phosphorylation of Ser(16)dependent on the beta-adrenergic cascade during ischemia and phosphorylation of Thr(17)on Ca(2+)influx both, at the beginning of ischemia and reperfusion. Phosphorylation of Thr(17)at the onset of reflow may provide the cell a mechanism to cope with Ca(2+)overload, transiently favoring the recovery of relaxation during early reperfusion.