Mesenteric Lymph From Rats With Thermal Injury Prolongs the Action Potential and Increases Ca2+ Transient in Rat Ventricular Myocytes

Cellular basis of burn-induced cardiac dysfunction and prevention by mesenteric lymph duct ligation

BACKGROUND

Myocardial contractile depression develops 4 to 24 h after major burn injury. We have reported previously that in a rat burn injury model (≈40% of total body surface area burn), mesenteric lymph duct ligation (LDL) prior to burn prevented myocardial dysfunction. However, the underlying cellular and molecular mechanisms are not well understood.

MATERIALS AND METHODS

Left ventricular myocytes were isolated from sham burn (control), sham burn with LDL (sham + LDL), burn, and burn with LDL (burn + LDL) rats at 4 and 24 h after burn or sham burn. Electrophysiological techniques were used to study myocyte size, contractility and L-type Ca2+ channel current (ICa). Further studies examined changes in the messenger RNA expression levels of pore-forming subunit of the L-type Ca(2+) channel, α1C, and its auxiliary subunits, β1, β2, β3, and α2δ1, which modulate the abundance of the ICa in post-burn hearts.

RESULTS

Depressed myocyte contractility (≈20%) developed during 4 to 24 h post-burn compared with control, sham + LDL, or burn + LDL groups, a pattern of changes consistent with whole heart studies. There was no significant alteration in myocyte size. The ICa density was significantly decreased (≈30%) at 24 h post-burn, whereas the messenger RNA expression levels of Ca(2+) channel gene were not significantly altered at 4 and 24 h after burn injury.

CONCLUSIONS

These results suggest that the post-burn contractile phenotype in vivo was also present in isolated myocytes in vitro, but cellular remodeling was not a major factor. The results also suggest that changes in ICa regulation, but not from Ca(2+) channel gene modification, may be a key element involved in post-burn contractile depression and the beneficial effects of LDL.

Cellular mechanism underlying burn serum-generated bidirectional regulation of excitation-contraction coupling in isolated rat cardiomyocytes

Myocardial depressant factors have long been recognized to be present in burn serum (BS) and contribute to burn-generated cardiac contractile dysfunction. However, much of the cellular and molecular mechanism for its role in the development of the cardiac deficiency remains unknown. In this study, we investigated the effect of BS on myocardial contractility and Ca handling in single rat cardiomyocytes. The results revealed that BS (5% by volume) bidirectionally regulated cardiac excitation-contraction (EC) coupling. The action potential-elicited Ca transient and cell shortening were increased by 28.0% ± 9.7% and 34.7% ± 12.5% within 20 min after BS stimulation (the upregulation phase), but decreased by 20.5% ± 6.8% and 32.3% ± 5.1% at 60 min after BS stimulation (the downregulation phase). There was a 32.0% ± 5.8% reduction in sarcoplasmic reticulum (SR) Ca content at the downregulation phase, whereas no alteration was detected at the upregulation phase. The incidences of spontaneous Ca sparks and Ca waves were significantly increased after BS stimulation, no matter at the upregulation or downregulation phase. The hyperactive Ca sparks and Ca waves could be completely abolished by antioxidative treatment (vitamin A, 0.2 mM; and vitamin E, 1 mM) and partially reversed by NOS inhibitor L-NAME (100 μM), but not by blocking Ca influx with nifedipine (1 μM). With the normalization of Ca sparks, BS-induced alterations of action potential-elicited Ca transient and contractility were prevented by antioxidative therapy. Taken together, we propose that BS-associated bidirectional regulation of EC coupling is attributed largely to oxidative stress-induced hyperactivity of ryanodine receptors, increasing EC coupling through enhancing intracellular Ca release initially, but subsequently decreasing EC coupling by partially depleting SR Ca content through enhancement of Ca spark-mediated SR leak.

Mesenteric lymph from rats with trauma-hemorrhagic shock causes abnormal cardiac myocyte function and induces myocardial contractile dysfunction

Myocardial contractile dysfunction develops following trauma-hemorrhagic shock (T/HS). We have previously shown that, in a rat fixed pressure model of T/HS (mean arterial pressure of 30-35 mmHg for 90 min), mesenteric lymph duct ligation before T/HS prevented T/HS-induced myocardial contractile depression. To determine whether T/HS lymph directly alters myocardial contractility, we examined the functional effects of physiologically relevant concentrations of mesenteric lymph collected from rats undergoing trauma-sham shock (T/SS) or T/HS on both isolated cardiac myocytes and Langendorff-perfused whole hearts. Acute application of T/HS lymph (0.1-2%), but not T/SS lymph, induced dual inotropic effects on myocytes with an immediate increase in the amplitude of cell shortening (1.4 ± 0.1-fold) followed by a complete block of contraction. Similarly, T/HS lymph caused dual, positive and negative effects on cellular Ca²⁺ transients. These effects were associated with changes in the electrophysiological properties of cardiac myocytes; T/HS lymph initially prolonged the action potential duration (action potential duration at 90% repolarization, 3.3 ± 0.4-fold), and this was followed by a decrease in the plateau potential and membrane depolarization. Furthermore, intravenous infusion of T/HS lymph, but not T/SS lymph, caused myocardial contractile dysfunction at 24 h after injection, which mimicked actual T/HS-induced changes; left ventricular developed pressure (LVDP) and the maximal rate of LVDP rise and fall (±dP/dt(max)) were decreased and inotropic response to Ca²⁺ was blunted. However, the contractile responsiveness to β-adrenergic receptor stimulation in the T/HS lymph-infused hearts remained unchanged. These results suggest that T/HS lymph directly causes negative inotropic effects on the myocardium and that T/HS lymph-induced changes in myocyte function are likely to contribute to the development of T/HS-induced myocardial dysfunction.

Cellular mechanisms of burn-related changes in contractility and its prevention by mesenteric lymph ligation

Major burn injury results in impairment of left ventricular (LV) contractile function. There is strong evidence to support the involvement of gut-derived factor(s) transported in mesenteric lymph in the development of burn-related contractile dysfunction; i.e., mesenteric lymph duct ligation (LDL) prevents burn-related contractile depression. However, the cellular mechanisms for altered myocardial contractility of postburn hearts are largely unknown, and the cellular basis for the salutary effects of LDL on cardiac function have not been investigated. We examined contractility, Ca2+ transients, and L-type Ca2+ currents ( ICa) in LV myocytes isolated from four groups of rats: 1) sham burn, 2) sham burn with LDL (sham + LDL), 3) burn (≈40% of total body surface area burn), and 4) burn with LDL (burn + LDL). Myocytes isolated from hearts at 24 h postburn had a depressed contractility (≈20%) at baseline and blunted responsiveness to elevation of bath Ca2+. Myocyte contractility was comparable in sham + LDL and sham burn hearts. LDL completely prevented burn-related changes in myocyte contractility. Mechanistically, the decrease in contractility in myocytes from postburn hearts occurred with a decrease in the amplitude of Ca2+ transients (≈20%) without changes in resting Ca2+ or Ca2+ content of the sarcoplasmic reticulum. On the other hand, ICa density was decreased (≈30%) in myocytes from postburn hearts, with unaltered voltage-dependent properties. Thus burn-related myocardial contractile dysfunction is linked with depressed myocyte contractility associated with a decrease in ICa density. These findings also provide strong evidence that mesenteric lymph is involved in the onset of burn-related cardiomyocyte dysfunction.

Dual effects of mesenteric lymph isolated from rats with burn injury on contractile function in rat ventricular myocytes

Gut-derived factors in intestinal lymph have been shown to trigger myocardial contractile dysfunction. However, the underlying cellular mechanisms remain unclear. We examined the effects of physiologically relevant concentrations of mesenteric lymph collected from rats with 40% burn injury (burn lymph) on excitation-contraction coupling in rat ventricular myocytes. Burn lymph (0.1-5%), but not control mesenteric lymph from sham-burn animals, induced dual positive and negative inotropic effects depending on the concentrations used. At lower concentrations (<0.5%), burn lymph increased the amplitude of myocyte contraction (1.6 +/- 0.3-fold; n = 12). At higher concentrations (>0.5%), burn lymph initially enhanced myocyte contraction, which was followed by a block of contraction. These effects were partially reversible on washout. The initial positive inotropic effect was associated with a prolongation of action potential duration (measured at 90% repolarization, 2.5 +/- 0.6-fold; n = 10), leading to significant increases in the net Ca2+ influx (1.7 +/- 0.1-fold; n = 8). There were no significant changes in the resting membrane potential. The negative inotropic effect was accompanied by a decrease in the action potential plateau (overshoot decrease by 69 +/- 10%; n = 4) and membrane depolarization. Voltage-clamp experiments revealed that the positive inotropic effects of burn lymph were due to an inhibition of the transient outward K+ currents that prolong action potential duration, and the inhibitory effects were due to a concentration-dependent inhibition of Ca2+ currents that lead to a reduction of action potential plateau. These burn lymph-induced changes in cardiac myocyte Ca2+ handling can contribute to burn-induced contractile dysfunction and ultimately to heart failure.

A THEORETICAL SIMULATION OF HEMATOPOIETIC STEM CELLS DURING OXYGEN FLUCTUATIONS: PREDICTION OF BONE MARROW RESPONSES DURING HEMORRHAGIC SHOCK

The bone marrow (BM) responds to various diseases, including infections and hemorrhagic shock, by generating immune and blood cells. These cells are derived from a finite number of lymphohematopoietic stem cells (LHSC) close to the endosteal region of the BM. This study presumes that studies on LHSC involving proteomics, computational biology, and genomics could be aided by mathematical models. A theoretical model is developed to predict the responses of proliferating (P) nonproliferating (N) BM cells during acute blood loss when the Po2 in the BM is decreased. Hematopoietic responses were simulated for otherwise healthy individuals who have been subjected to various degrees of blood loss, as represented by 3%, 5%, and 20% O2. The model is robust and could predict hematopoietic activity in the area close to the endosteum during low Po2 as for acute blood loss. Steady-state hematopoiesis at oxygen saturation (80%) in healthy individuals could not be simulated with the equations. Functional assays tested the model with an in vitro assay of the most primitive LHSC (modified long-term culture-initiating cell assay, LTC-IC). The LTC-IC assay showed that 1%, 3% - 5%, and 20% O2 mediate significant increases in the proliferation of the most primitive BM progenitors, as compared with 80% O2. Thus, the functional studies show that the theoretical model is robust and could be used to gain insights into the biology of LHSC during different degrees of blood loss. The utility of such a model in surgical trauma is discussed.

Selective decontamination of the digestive tract attenuated the myocardial inflammation and dysfunction that occur with burn injury

This study examined the effects of oral antibiotics to selectively decontaminate the digestive tract (SDD) on postburn myocardial signaling, inflammation, and function. We hypothesized that antibiotic therapy to eliminate pathogens from the gastrointestinal (GI) tract would reduce myocardial inflammatory responses and improve postburn myocardial performance. Sprague-Dawley rats received polymyxin E (15 mg), tobramycin (6 mg), and 5-flucytosin (100 mg) by oral gavage twice daily for 3 days preburn and 24 h postburn. Experimental groups included 1) sham burn given vehicle (3 ml water), 2) sham plus SDD, 3) burn over 40% total body surface area (TBSA) plus SDD, and 4) burn over 40% TBSA given vehicle. All burns received lactated Ringer solution (4 mg·kg−1·%burn−1); myocardial signaling (PKCε/p38 MAPK/NF-κB) was studied 2, 4, and 24 h postburn; and cytokine secretion (systemic and myocyte secreted cytokines, ELISA) and cardiac function were examined 24 h postburn. Vehicle-treated burn injury increased myocardial PKCε/p38 MAPK expression, promoted NF-κB nuclear translocation, promoted TNF-α, IL-1β, IL-6, and IL-10 secretion, and impaired myocardial function. SDD attenuated burn-related proinflammatory myocardial signaling, cytokine secretion, and myocardial contractile defects. Our data suggest that burn-related loss of GI barrier function and translocation of microbial products serve as upstream mediators of postburn myocardial inflammatory signaling and dysfunction.

RhoA GTPase regulates L-type Ca2+ currents in cardiac myocytes

Regulation of ionic channels plays a pivotal role in controlling cardiac function. Here we show that the Rho family of small G proteins regulates L-type Ca2+ currents in ventricular cardiomyocytes. Ventricular myocytes isolated from transgenic (TG) mice that overexpress the specific GDP dissociation inhibitor Rho GDI-alpha exhibited significantly decreased basal L-type Ca2+ current density (approximately 40%) compared with myocytes from nontransgenic (NTG) mice. The Ca2+ channel agonist BAY K 8644 and the beta-adrenergic agonist isoproterenol increased Ca2+ currents in both NTG and TG myocytes to a similar maximal level, and no changes in mRNA or protein levels were observed in the Ca2+ channel alpha1-subunits. These results suggest that the channel activity but not the expression level was altered in TG myocytes. In addition, the densities of inward rectifier and transient outward K+ currents were unchanged in TG myocytes. The amplitudes and rates of basal twitches and Ca2+ transients were also similar between the two groups. When the protein was delivered directly into adult ventricular myocytes via TAT-mediated protein transduction, Rho GDI-alpha significantly decreased Ca2+ current density, which supports the idea that the defective Ca2+ channel activity in TG myocytes was a primary effect of the transgene. In addition, expression of a dominant-negative RhoA but not a dominant-negative Rac-1 or Cdc42 also significantly decreased Ca2+ current density, which indicates that inhibition of Ca2+ channel activity by overexpression of Rho GDI-alpha is mediated by inhibition of RhoA. This study points to the L-type Ca2+ channel activity as a novel downstream target of the RhoA signaling pathway.