Biphasic redistribution of muscarinic receptor and the altered receptor phosphorylation and gene transcription are underlying mechanisms in the rat heart during sepsis

Lipopolysaccharide-induced sepsis impairs M2R-GIRK signaling in the mouse sinoatrial node

Sepsis has emerged as a global health burden associated with multiple organ dysfunction and 20% mortality rate in patients. Numerous clinical studies over the past two decades have correlated the disease severity and mortality in septic patients with impaired heart rate variability (HRV), as a consequence of impaired chronotropic response of sinoatrial node (SAN) pacemaker activity to vagal/parasympathetic stimulation. However, the molecular mechanism(s) downstream to parasympathetic inputs have not been investigated yet in sepsis, particularly in the SAN. Based on electrocardiography, fluorescence Ca2+ imaging, electrophysiology, and protein assays from organ to subcellular level, we report that impaired muscarinic receptor subtype 2-G protein-activated inwardly-rectifying potassium channel (M2R-GIRK) signaling in a lipopolysaccharide-induced proxy septic mouse model plays a critical role in SAN pacemaking and HRV. The parasympathetic responses to a muscarinic agonist, namely IKACh activation in SAN cells, reduction in Ca2+ mobilization of SAN tissues, lowering of heart rate and increase in HRV, were profoundly attenuated upon lipopolysaccharide-induced sepsis. These functional alterations manifested as a direct consequence of reduced expression of key ion-channel components (GIRK1, GIRK4, and M2R) in the mouse SAN tissues and cells, which was further evident in the human right atrial appendages of septic patients and likely not mediated by the common proinflammatory cytokines elevated in sepsis.

Non-invasive Early Prediction of Septic Acute Kidney Injury by Doppler-Based Renal Resistive Indexes Combined With Echocardiographic Parameters: An Experimental Study

Non-invasive early prediction of septic acute kidney injury (S-AKI) is still urgent and challenging. Increased Doppler-based renal resistive index (RRI) has been shown to be associated with S-AKI, but its clinical use is limited, which may be explained by the complex effects of systemic circulation. Echocardiogram allows non-invasive assessment of systemic circulation, which may provide an effective supplement to RRI. To find the value of RRI combined with echocardiographic parameters in the non-invasive early prediction of S-AKI, we designed this experiment with repeated measurements of ultrasonographic parameters in the early stage of sepsis (3, 6, 12, and 24 h) in cecum ligation and puncture (CLP) rats (divided into AKI and non-AKI groups at 24 h based on serum creatinine), with sham-operated group serving as controls. Our results found that RRI alone could not effectively predict S-AKI, but when combined with echocardiographic parameters (heart rate, left ventricular end-diastolic internal diameter, and left ventricular end-systolic internal diameter), the predictive value was significantly improved, especially in the early stage of sepsis (3 h, AUC: 0.948, 95% CI 0.839–0.992, P < 0.001), and far earlier than the conventional renal function indicators (serum creatinine and blood urea nitrogen), which only significantly elevated at 24 h. Our method showed novel advances and potential in the early detection of S-AKI.

Right atrium cholinergic deficit in septic rats

Heart rate variability (HRV) is mainly determined by the influence of both branches of the Autonomic Nervous System over the sinus node. Low HRV has been associated with a worse prognosis in patients with sepsis. The objective of this study was to explain the reduction in HRV during experimental sepsis in adult rats. We recorded the heart's electrical activity by telemetry in conscious unrestrained male rats before and 1day after the induction of peritonitis (N=39) or sham peritonitis (N=15). Then, we analyzed the chronotropic responsiveness of the isolated heart to the autonomic neurotransmitters and determined catecholamine concentrations in blood plasma and acetylcholine and choline concentrations in the right atrium. The surviving septic rats (N=33) had increased heart rate (HR) and diminished HRV. Despite the higher HR in situ, the spontaneous basal HR in septic and sham isolated hearts was the same. The isolated septic hearts showed acetylcholine hypersensitivity (log (IC50,M)=-7.2±0.2 vs. -6.0±0.4, P=0.025) and lower concentrations of choline in their right atriums (in nMol/mg protein: 0.6±0.1 vs. 1.6±0.6, P=0.013). Norepinephrine concentration in blood plasma from septic rats was higher (in ng/ml: 29.2±8.4 vs. 5.8±4.1, P=0.019). In conclusion, septic rats present a deregulation of the autonomic nervous system, not only sympathetic overexcitation but also parasympathetic dysfunction.

Changes in group II phospholipase A2 gene expression in rat heart during sepsis

BACKGROUND

This study was undertaken to investigate alterations of group II phospholipase A2 (PLA2) gene expression and its underlying mechanism in rat heart during different phases of sepsis.

MATERIALS AND METHODS

Sepsis was induced by cecal ligation and puncture (CLP). Experiments were divided into three groups, control, early sepsis, and late sepsis. Early and late sepsis refers to those animals sacrificed at 9 and 18 h, respectively, after CLP. PLA2 enzyme activity, group II PLA2 protein level, messenger RNA (mRNA) abundance, transcription rate, and half-life were measured.

RESULTS

PLA2 activity was decreased by 29% during early sepsis but it was increased by 49% during late sepsis. Group II PLA2 protein level was decreased by 27% during early sepsis but it was increased by 35.3% during late sepsis. Group II PLA2 mRNA was decreased by 21% during early sepsis but it was increased by 141% during late sepsis. The transcription rate of group II PLA2 mRNA was reduced by 25% during early sepsis but it was elevated by 67% during late sepsis. The half-life of group II PLA2 mRNA remained unaltered during early and late phases of sepsis.

CONCLUSIONS

These results demonstrate that PLA2 activity, group II PLA2 protein level, the mRNA abundance, and transcription rate were concurrently underexpressed during early sepsis, while they were overexpressed during late sepsis, with no change in the degradation of gene transcript. These data indicate that the biphasic changes in group II PLA2 gene expression are regulated transcriptionally during sepsis.

Intracellular Redistribution of Dihydropyridine Receptor in the Rat Heart During the Progression of Sepsis

BACKGROUND

Dihydropyridine receptor (DHPR) regulates the rate and force of cardiac muscle contraction. This study examined the alteration in the intracellular redistribution of DHPR and its association with the development of the two distinct cardiodynamic states in the rat heart during the progression of sepsis.

MATERIAL AND METHODS

Sepsis was induced by cecal ligation and puncture (CLP). DHPRs were assayed using [(3)H]PN200-100 binding and photoaffinity labeling with [(3)H]azidopine followed by polyacrylamide gel electrophoresis.

RESULTS

[(3)H]PN200-110 binding shows that during the early hyperdynamic phase of sepsis (9 h post-CLP), the Bmax was increased by 27% in sarcolemma while decreased by 24% in light vesicle. During the late hypodynamic phase of sepsis (18 h post-CLP), the Bmax was decreased by 39% in sarcolemma but increased by 59% in light vesicle. The sum of the Bmax for both membrane fractions was increased by 16% during early sepsis while decreased by 17% during late sepsis. Photoaffinity labeling shows that the incorporation of [(3)H]azidopine into 165 kDa peptides during early sepsis was increased by 28% in sarcolemma whereas decreased by 23% in light vesicle. During late sepsis, the incorporation was decreased by 38% in sarcolemma but increased by 46% in light vesicle. The sum of the 165 kDa peptides for both membrane fractions was increased by 13% during early while decreased by 13% during late sepsis.

CONCLUSIONS

These data indicate that DHPRs in the rat heart were externalized from light vesicles to sarcolemma during the early hyperdynamic phase whereas they were internalized from surface membranes to intracellular sites during the late hypodynamic phase of sepsis. Furthermore, DHPRs were overexpressed during early sepsis while they were underexpressed during late sepsis. Alterations in the expression and intracellular redistribution of DHPRs may contribute to the development of the biphasic cardiodynamic states during the progression of sepsis.

Altered phosphorylation and calcium sensitivity of cardiac myofibrillar proteins during sepsis

Altered phosphorylation and Ca(2+) sensitivity of cardiac myofibrillar proteins during different phases of sepsis were investigated. Sepsis was induced by cecal ligation and puncture (CLP). The results show that phosphorylation of troponin I (TnI) was increased by 268% during the early phase (9 h after CLP) but decreased by 46% during the late phase (18 h after CLP) of sepsis. Phosphorylation of C protein was increased by 76% during the early phase but decreased by 41% during the late phase of sepsis. Phosphorylation of myosin light chain-2 (MLC-2) remained unaltered during the early phase but was decreased by 38% during the late phase of sepsis. Phosphorylation of TnT was unaffected during the progression of sepsis. The increases in the phosphorylation of TnI and C protein during early sepsis were associated with the decrease in the Ca(2+) sensitivity of myofilaments and the increases in myocardial changes in tension development (+dP/dt(max)) and cAMP level. The decreases in the phosphorylation of TnI and C protein during late sepsis coincided with the declines in the activities of myofibrillar ATPase, Ca(2+) sensitivity of myofilaments, myocardial +/-dP/dt(max), and cAMP content. The increases and the decreases in the phosphorylation of TnI and C protein, +/-dP/dt(max), and the tissue cAMP level were sensitive to isoproterenol stimulation and propranolol inhibition. These findings suggest that alterations in the phosphorylation of myofibrillar proteins, such as TnI, C protein, and MLC-2, and changes in the activities and the Ca(2+) sensitivity of myofibrillar ATPase may contribute to the altered cardiac function during the progression of sepsis. Furthermore, the sepsis-induced alterations in the phosphorylation and Ca(2+) sensitivity of cardiac myofibrillar proteins were mediated via a beta-adrenergic receptor pathway.