CHAPTER Introduction 1.1 Definition of hemorrhagic shock A variety of definitions of hemorrhagic shock have arisen as more understanding of the mechanisms involved has been developed. Shock is “a momentary pause in the act of death” (John Warren 1); “Shock is the manifestation of the rude unhinging of the machinery of life” (Samuel V. Gross, 1872). A modern definition of shock would acknowledge that, firstly; shock is inadequate tissue perfusion and inadequate removal of cellular waste products and secondly, that shock is a failure of oxidative metabolism that can involve defects of oxygen (1) delivery, (2) transport or (3) utilization or combinations of all three. The diagnoses of clinical signs of shock are primarily related to organ failure but organ failure is secondary to failure of the cells (Pope et al., 1999). Shock as described by many authors is a “vicious cycle”. They may cascade in a variety of ways such as the decreased in cardiac output, which leads to a decreasing blood pressure, which on turn leads to decreasing tissue perfusion (Pope et al., 1999). 1.2 Organs involvement in prolonged hemorrhagic shock The organ sequentially affected in the organ failure induced by shock is the kidney. Renal failure may ensue as a consequence of shock and depending on the state of volume resuscitation and other factors may have the following characteristic: Initial high level of urine output Low pressure in the renal tubules producing sodium retention Renal dysfunction and failure. Renal failure is a complication of severe shock and is associated with a mortality rate of more than 50 percent. Vigorous fluid resuscitation has improved the situation by reducing the incidence of renal failure; early and adequate resuscitation can avoid this dreaded consequence of shock (Pope et al., 1999). The gastrointestinal consequences of shock include increased acid production and increased permeability of the gastric mucosa. The increased permeability allows tissue penetration by acids, bacteria and endotoxins. In the past, these complications resulted in the late morbidity from hemorrhagic gastritis, which has a high mortality rate (Pope et al., 1999). The liver, like all other organs, responds to shock. The effect on the liver is not well delineated but does result in major changes in bilirubin, isoenzymes, protein synthesis and perhaps most importantly the reticuloendothelial system. Decreased consciousness and changes in neural control mechanisms are the responses of the central nervous system to shock (Pope et al., 1999). 1.3 Physiologic responses to prolonged hemorrhagic shock Acute hemorrhage produces a decrease in arterial systolic, diastolic and pulse pressures along with an increase in the pulse rate and a decrease in the cardiac stroke volume. The cutaneous veins are generally collapsed and fill slowly when compressed centrally (Berne, 1983). The early stages of hemorrhage result in the initiation of a number of feedback mechanisms tend to maintain arterial blood pressure in the presence of a decrease in circulating blood volume and a modest decrease in cardiac output. Some of the regulatory mechanisms include cerebral ischemic responses, reabsorption of tissue fluids at the level of capillaries, release of endogenous vasoconstrictor substances such as vasopressin and renal conservation of salt and water (Chien, 1967). In the early stages of moderate hemorrhage, the changes in total renal vascular resistance are slight because intrinsic autoregulatory mechanisms within the kidney tend to maintain renal blood flow. The intense splanchnic and renal vasoconstriction may protect the heart and brain but can eventually lead to ischemic injury of the kidney and bowel resulting in kidney failure and further vascular injury and loss of fluids from the vascular compartment into the interstitial space (Pope et al., 1999). When the arterial pressure falls below 60 mmHg as during serve hemorrhage, hypoxia of the peripheral chemoreceptors in the carotid body results in activation of chemoreceptor reflexes. This results in increased of breathing frequency. At very low levels of arterial pressure at below 40 mmHg, inadequate cerebral blood flow produces an extremely strong activation of the sympathetic nervous system and intense vasoconstriction in response to cerebral ischemia (Pope et al., 1999). A number of endogenous vasoconstrictors are released during hemorrhage. As a direct response to sympathetic nervous system activation, the release of epinephrine and norepinephrine from the adrenal medulla reinforces the actions of direct sympathetic nervous system innervations of the heart and peripheral circulation. Vasopressin, which is a potent vasoconstrictor, is actively secreted by the posterior pituitary gland in response to hemorrhage. Diminished renal perfusion results in the secretion of rennin from the juxtaglomerular apparatus and the subsequent conversion of angiotensinogen to angiotensin, which is also a powerful vasoconstrictor (Pope et al., 1999). 1.4 The role Nitric Oxide play in prolonged hemorrhagic shock Excessive production of nitric oxide (NO) as result of inducible nitric oxide synthase (iNOS) induction has been implicated as the most important factor contributing to the pathophysiology of hemorrhagic shock (Szabo et al., 1994; Moncada et al., 1991; Szabo, 1995). The induction of iNOS in turn metabolizes L-arginine, resulting in excessive formation of NO that may contribute to the vascular impairment and multiple organ damage (Hua et al., 1999). In recent years, numerous efforts and studies have aimed to evaluate the potential of NOS inhibitors in maintaining mean arterial blood pressure (MABP) and increasing the survivability of the shocked animals. NOS inhibitors have been shown to be able to maintain a high MABP, after shock was induced, by antagonizing the vasodilatating effects of NO by inhibiting their release (Szabo et al., 1994; Moncada et al., 1991; Szabo, 1995). A high MABP theoretically would maintain curial organ perfusions and would in turn reduced occurrences of organ ischemia. In a recent study, L-citrulline production in the anteroventral 3rd ventricle (AV3V) region in rats subjected to hemorrhagic shock had significantly increased in control rats. Lcitrulline is an indicator of nitric oxide (NO) synthesis. Thus these findings indicate that NO production in these areas contributes to the hypotension due to hemorrhage (Goren et al., 2001). Maintenance of NO production by endothelia NOS (eNOS) is important in early stages of ischemia and its inhibition could exacerbate organs injury (Guo Weir 1999, Dawson and Dawson 1996). Patients that have survived severe hemorrhagic shock are known to show neurological changes likely due to brain ischemia (Carrillo et al., 1998). Nitric oxide (NO) overproduction by induction of inducible nitric oxide synthase (iNOS) in the brain (Dalkara et al., 1994; Weir et al., 1999; Liaudet et al., 2000; Szabo and Thiemermann, 1994) has been shown to play an important role in secondary neuronal damage (Iadecola 1995). The neuroprotective properties of selective NOS inhibitors arise from their ability to inhibit the mass release of NO after brain injury (Iadecola 1995; Zhang et al., 1996; Cash et al., 2001; Viktorov, 2000; Higuchi et al., 1998). 1.5 The role nitric oxide and prostaglandin E2 in prolonged hemorrhagic shock Hemorrhagic shock produces the bioregulatory molecule nitric oxide (NO) which is generated catalytically by three enzymes (constitutive, neuronal and inducible) collectively termed NO synthase (Teng and Moochhala, 1999). Previous studies have shown that the inflammatory (inducible nitric oxide synthase) iNOS is upregulated in organs such as lungs, livers and kidneys during shock (Thiemermann et al., 1993, AnayaPrado et al., 2003, Mc Donald et al., 2003). The excessive activation of iNOS results in cardiovascular and organ dysfunction in clinical and experimental setting of inflammatory disease of both septic and nonseptic etiology (Ungureanu-Longrois et al., 1995, Harbrecht et al., 1992, Petros et al., 1995, Vallance and Moncada, 1993, Grosjean et al., 1999, Collins et al., 2003, Menezes et al., 2003, Hierholzer et al., 2002, Liu et al., 2002, Cuzzocrea et al., 2002). The inducible NOS are one of the inflammatory mechanisms that contribute to cerebral damage (Pozzilli et al., 1985, Clark et al., 1995, Chen et al., 1992, 1994, Feuerstein et al., 1998, Iadecola, 1997). Some investigators have shown that during hemorrhagic shock, cyclooxgenase-2 (COX2) is up-regulation as a result of an inflammatory response (Tsukada et al., 2000, Knoferl et al., 2001). Two isoforms of COX have been identified namely COX-1 and COX-2. COX-1 is a constitutive isoform that is expressed in most tissues and is responsible for the physiological production of PGs. On the other hand, COX-2 is an inducible isoform that is induced by cytokines, mitogens, and endotoxins in inflammatory cells and is responsible for the elevated production of PG during inflammation (Dubois et al., 1998, Iadecola, 1997). Prostanoids, including prostaglandins (PGs), prostacyclins, and thromboxanes, are synthesized from these enzymatic pathways (Murakami et al., 1997, Vane et al., 1998, Smith et al., 2000). Cerebral damage also enhances the expression of COX-2 (Nogawa et al., 1997, Miettinen et al., 1997, Planas et al., 1995, Collaco-Moraes et al., 1996, Goodwin et al., 1999). In addition to their role in inflammation, prostanoids have also been shown to modulate vasodilation (Okamoto et al., 1998, Moncada et al., 1993). The inhibition of NO production that could possibly alter the vasodilatory and inflammatory pathway mediated by the COX-2 in prolong rats has not been well documented. Interestingly, studies have shown that pharmacological manipulation of one pathway could result in cross-modulation of the other pathway. However the relevance of these interactions in vivo is controversial. The interaction between NO and COX-2 is likely to play a role in brain diseases associated with inflammation, such as AIDS dementia, multiple sclerosis, brain neoplasm and Alzheimer disease and other pathological conditions such as nephrosis, sepsis or rheumatoid arthritis (Salvemini et al., 1993, Nogawa et al., 1998). AG is well known as an iNOS inhibitor but little is known of its ability to inhibit COX-2 up-regulation via NO inhibition in prolong hemorrhagic shock. Our experiment might shed some light on the interaction between NO and COX-2 in Prolong hemorrhagic shock. 1.6 The role nitric oxide and angiotensin II play in prolonged hemorrhagic shock The renin-angiotensin system is one of the major regulators of arterial blood pressure (Oudat et al., 2003) as it is the most potent pressor substance known (Chesley et al., 1963). In contrast to ANGII function, under physiological conditions, generation of NO from L-arginine by the constitutive NO synthase (NOS) present in vascular endothelial cells keeps the vasculature in a permanent state of active vasodilatation (Rees et al., 1989). Studies have shown that during the event of shock, an inducible isoform of NO synthase (iNOS) is expressed, resulting in excessive formation of nitric oxide (NO) that may contribute to the vascular impairment (Ochoa et al., 1991). Animal studies have also suggested that nitric oxide (NO) overproduction may mediate vascular hyporeactivity and decompensation following hemorrhagic shock (Thiemermann et al., 1993. Zingarelli et al., 1992). We hypothesize that reduced sensitivity to angiotensin II is a result of excessive NO formation. We also hypothesized that treatment of ANGII with NOS inhibitors, would have a beneficial effect on the blood pressure following prolonged hemorrhagic shock. 1.7 The role nitric oxide plays in a combine rat model of prolonged hemorrhagic shock and fluid percussion injury (induction of traumatic brain injury) Nitric oxide (NO) has been implicated as being an important mediator in a variety of pathological conditions, including traumatic brain injury (Wada et al., 1998) and P PPP hemorrhagic shock (Hua and Moochhala, 1999). The role of NO in maintaining homeostasis (Ambrosio et al., 1998) and regulating organ function during traumatic brain injury and hemorrhagic shock is complex. The inducible NO synthase (iNOS) has been hypothesized to play a critical role in the pathophysiologic consequences of secondary brain injury and severe hemorrhage. During traumatic brain injury (Sinz et al., 1999) and hemorrhagic shock (Szabo and Billiar 1999, Shinoda and Whittle, 2001), there is induction of iNOS. The iNOS metabolizes L-arginine resulting in large amounts of nitric oxide (NO) production, which might lead to vascular hypotension and multiple organ damage following hemorrhagic shock (Goren et al., 2001). In the brain, iNOS is produced in large amounts by macrophages and microglia following traumatic brain injury (Shinoda and Whittle, 2001) iNOS-derived NO is acutely detrimental, possibly because of toxic effects of NO metabolites such as peroxynitrite. Excessive production of NO may also be involved in glutamate neurotoxicity and is responsible for neuronal death (Szabo, 1995). 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British Journal of Pharmacology 120, 357-366, 1997b. 138 [...]... evidenced in sham-operated rats (86.95 3 .25 SF) Untreated prolonged hemorrhagic shock rats had a significantly higher GOT level (148. 42 Table 2. 2 2. 36 SF) Creatinine and GOT levels in different groups of rats GOT (SF Units/ml) Creatinine ( mol/L) Normal rats 23 2.49 37.13 85 .23 2. 41 Sham-operated rats 26 1.66 75.14 86.95 3 .25 Untreated rats *816. 82 45.97 *148. 42 2.36 Creatinine and GOT levels in different... creatinine levels in sham-operated animals (26 1.66 75.14 mol/L) when compared with normal rats However, a marked increase in plasma creatinine level was found in salinetreated prolong hemorrhagic shock rats (816. 82 45.97 mol/L) AG-treated rats attenuated the production of creatinine during prolonged hemorrhagic shock Plasma creatinine levels were elevated in prolonged hemorrhagic shock rats receiving... role of NO and the therapeutics effects of conservative fluids and NOS inhibitors 24 3.1 Introduction Hemorrhagic shock is implicated in the induction of inducible nitric oxide synthase that leads to increase production of nitric oxide (NO) We investigated the effects of NO in two rat models of hemorrhagic shock The fixed pressure model in anesthetized rats on survival, mean arterial blood pressure (MABP),... difference in MABP among the rat groups 32 prior to prolonged hemorrhagic shock (data not shown) There was no MABP recording post -prolonged hemorrhagic shock in saline-treated rats as the rats died within this prolonged hemorrhagic shock period The MABP for all pre-treatment rat groups was similar to sham-operated rats In rats treated with L-NAME, MABP was 88 .2 2.5 mmHg, 60 min post -prolonged hemorrhagic shock. .. catheters were inserted into the right carotid artery for arterial blood withdrawal/monitoring and the inferior vena cava via the right femoral vein for administration of fluids and drugs After cannulation at the artery, the distal end of the cannula is tunneled under the skin to exteriorize at the nape of the neck The cannulas were held in place with dental cement and stoppered with a small metal pin Rats... recover before they were subjected to prolonged hemorrhagic shock 3 .2. 3.3 Prolonged hemorrhagic shock in conscious rats (fixed volume) The stabilization period and withdrawal of blood are the same as mentioned above The total amount of blood withdrawn was kept constant (volume of blood = 8ml) Surgical procedure and volume of normal saline received in sham-operated animals and time of 27 infusion of 0.9%sodium... dissolved in 0.9% sodium chloride solution (Sigma) The handling and care of all animals were mentioned in chapter 2 25 3 .2. 1 .2 Animal preparation in anesthetized rats The animals were deprived of food for 24 hours before the experiment but allowed free access to water They were anaesthetized with CRC cocktail (0.3 mL/l00 g body weight) intraperitoneally and were maintained under anaesthesia for the duration... via the right femoral vein for administration of fluids and drugs After cannulation at the artery, the distal end of the cannula is tunneled under the skin to exteriorize at the nape of the 26 neck The cannulas were held in place with dental cement and stoppered with a small metal pin Rats were allowed 48 hours to recover before they were subjected to prolong hemorrhagic shock 3 .2. 3.1 Prolonged hemorrhagic. .. 72 hours) after prolonged hemorrhagic shock, the animals were reanesthetized with CRC and intracardially perfused with a warm (370 C) 2% TTC (2, 3,5,-triphenyltetrazolium chloride) solution Their brains were quickly removed, immersed in the 370 C TTC solution for 15 min to enhance staining and then 29 placed in 10% buffered formaldehyde Six serial coronal sections from each brain were cut at 2 mm intervals... hemorrhagic shock) 2. 3.4 Morphological evaluation The kidneys (Figure 2. 6A), livers (Figure 2. 6C), lungs (Figure 2. 6E) and stomach (Figure 2. 6G) in sham-operated rats appeared structurally normal Severe microscopic injury was encountered in various organs of rats following prolonged hemorrhagic shock in untreated rats There was evidence of leakage of blood and tissue damages in the kidneys, livers, lungs and . between NO and COX -2 in Prolong hemorrhagic shock. 1.6 The role nitr ic oxide and angiotensin II play in prolonged hemorrhagic shock The renin-angiotensin system is one of the major regulators. prolong ed hemorrhagic shock . 1.7 The role nitric oxide plays in a combine rat model of prolonged hemorrhagic shock and fluid percussion injury (induction of traumatic brain injury) Nitric. 1999) and hemorrhagic shock (Szabo and Billiar 1999, Shinoda and Whittle, 20 01), there is induction of iNOS. The iNOS metabolizes L-arginine resulting in large amounts of nitric oxide (NO)