Lipopolysaccharide-induced Cardiovascular Dysfunction And Its Mechanism

BackgroundThe critical event of septic shock, is the major killer in intensive care units worldwide. Up to now, there is still lack of effective therapeutic strategy. It has now been widely accepted that septic shock is a systemic inflammatory process, resulting from bacterial infection, usually lipopolysaccharide (LPS) stimulation and the subsequent production of proinflammatory cytokines and vasoactive mediators.The predominant features of septic shock are hypotension, vascular hyporeactivity to vasoconstrictors, myocardial depression, tissue hypoperfusion and finally multiple organ failure. In order to explore the underlying mechanism of this syndrome-induced cardiovascular dysfunction, experimental animals given LPS proves to be an ideal approachto establish the septic experimental models. LPS and LPS-induced endogenous mediators have been suggested to participate in the cardiovascular dysfunction during LPS challenge. For many years, there have been many documents focused on the involvement of proinflammatory cytokines. However, clinical trials aimed to downregulate these mediators have proved to be uniformly disappointing. Thus, further studies are transferred to the secondary mediators-modulating therapies.Both nitric oxide (NO) and endothelin-1 (ET-1) are two important vasoactive mediators. Under physiological condition, these two mediators interact with each other and co-regulate the cardiovascular functions. There have been many evidences to indicate that LPS stimulate over production of both ones and thus influence the function of cardiovascular system. But it is still not clear the exact roles of NO and/or ET-1 in LPS-induced cardiovascular dysfunction.AimExperimental septic model is established by intraperitoneal injection of LPS. The study was carried out on the isolated rat aorta ring, isolated rat heart and isolated rat ventricular myocytes to determine the effect of LPS on the vascular response and cardiac contractility and explore the related underlying mechanisms. The main point of this study is to make clear the exact roles of NO and ET-1 in the LPS-induced disturbance in cardiovascular system.Methods1. Animal model of septic shockMale Sprague-Dawley rats were injected with 10 mg/kg LPS intraperitoneally.2. Measurement of serum nitrites/nitrates (NO2-/NO3-)3. Plasma ET-1 radioimmunoassay4. Organ bath experiment5. Isolated rat heart constant flow perfusion6. Isolation of adult rat ventricular myocytes7. Analysis of caffeine-induced [Ca2+]j transients8. Cardiac sarcoplasmic reticulum (SR) preparation and measurement of SR Ca2+-ATPase activity9. Cardiac mitochondria preparation and measurement of mitochondria! malondialdehyde (MDA) contentResultsPart 1. Mechanism of LPS-induced alterations of vascular response1. LPS treatment increased about 20-fold serum NO2/NO3- level.2. The plasma ET-1 levels were markedly increase by LPS treatment.3. The concentration-contraction curves evoked by phenylephrine (PE) in aortic rings from LPS-treated rats was significantly decreased compared to the rings from control rats. Aminoguanidine (AMG), a specific inducible NO synthase inhibitor (100 M, 1 mM), inhibited the effect of LPS, while 10M AMG did not inhibit the effect of LPS.4. In the hearts isolated from LPS-treated rats, there was a marked increase in coronary perfusion pressure (CPP). Pretreatment with AMG (100 mg/kg, i.p.) did not reverse the increase of CPP induced by LPS. Pretreatment with BQ-123 (1 mg/kg, i.p.), a selective ETA receptor antagonist abolished LPS-induced increase in CPP. Combination pretreatment with BQ-123 and AMG exhibited the same effect of single pretreatmentwith BQ-123 on CPP.Part 2. Mechanism of LPS-induced cardiac depression1. Contributions of NO and ET-1 in LPS-induced cardiac contractile disturbanceLPS induced decreases in left ventricular developed pressure (LVDP), the product of LVDP and heart rate (LVDPxHR), and maximal rate of ris...