Transient central cholinergic activation enhances sympathetic nervous system activity but does not improve hemorrhage-induced hypotension in alcohol-intoxicated rodents

Assessment of Cardiovascular Function and Microvascular Permeability in a Conscious Rat Model of Alcohol Intoxication Combined with Hemorrhagic Shock and Resuscitation

Hypotension, cardiac depression, and elevated microvascular permeability are known problems that complicate resuscitation of patients following traumatic injury, particularly those who are also intoxicated from alcohol consumption. A conscious rat model of combined alcohol intoxication and hemorrhagic shock has been used to study the hemodynamic mechanisms involved. Here, we describe using this model to study microvascular leakage and cardiac electrical activity.

Sphingosine-1-Phosphate Treatment Can Ameliorate Microvascular Leakage Caused by Combined Alcohol Intoxication and Hemorrhagic Shock

Fluid resuscitation following hemorrhagic shock is often problematic, with development of prolonged hypotension and edema. In addition, many trauma patients are also intoxicated, which generally worsens outcomes. We directly investigated how alcohol intoxication impacts hemorrhagic shock and resuscitation-induced microvascular leakage using a rat model with intravital microscopic imaging. We also tested the hypothesis that an endothelial barrier-protective bioactive lipid, sphingosine-1-phosphate (S1P), could ameliorate the microvascular leakage following alcohol intoxication plus hemorrhagic shock and resuscitation. Our results show that alcohol intoxication exacerbated hemorrhagic shock and resuscitation-induced hypotension and microvascular leakage. We next found that S1P effectively could reverse alcohol-induced endothelial barrier dysfunction using both cultured endothelial cell monolayer and in vivo models. Lastly, we observed that S1P administration ameliorated hypotension and microvascular leakage following combined alcohol intoxication and hemorrhagic shock, in a dose-related manner. These findings suggest the viability of using agonists that can improve microvascular barrier function to ameliorate trauma-induced hypotension, offering a novel therapeutic opportunity for potentially improving clinical outcomes in patients with multi-hit injuries.

Acute alcohol intoxication-induced microvascular leakage

BACKGROUND

Alcohol intoxication can increase inflammation and worsen injury, yet the mechanisms involved are not clear. We investigated whether acute alcohol intoxication increases microvascular permeability and investigated potential signaling mechanisms in endothelial cells that may be involved.

METHODS

Conscious rats received a 2.5 g/kg alcohol bolus via gastric catheters to produce acute intoxication. Microvascular leakage of intravenously administered fluorescein isothiocyanate (FITC)-conjugated albumin (FITC-albumin) from the mesenteric microcirculation was assessed by intravital microscopy. Endothelial-specific mechanisms were studied using cultured endothelial cell monolayers. Transendothelial electrical resistance (TER) served as an index of barrier function, before and after treatment with alcohol or its metabolite acetaldehyde. Pharmacologic agents were used to test the roles of alcohol metabolism, oxidative stress, p38 mitogen-activated protein kinase (MAPK), myosin light-chain kinase (MLCK), rho kinase (ROCK), and exchange protein activated by cAMP (Epac). VE-cadherin localization was investigated to assess junctional integrity. Rac1 and RhoA activation was assessed by ELISA assays.

RESULTS

Alcohol significantly increased FITC-albumin extravasation from the mesenteric microcirculation. Alcohol also significantly decreased TER and disrupted VE-cadherin organization at junctions. Acetaldehyde significantly decreased TER, but inhibition of alcohol dehydrogenase or application of a superoxide dismutase mimetic failed to prevent alcohol-induced decreases in TER. Inhibition of p38 MAPK, but not MLCK or ROCK, significantly attenuated the alcohol-induced barrier dysfunction. Alcohol rapidly decreased GTP-bound Rac1 but not RhoA during the drop in TER. Activation of Epac increased TER, but did not prevent alcohol from decreasing TER. However, activation of Epac after initiation of alcohol-induced barrier dysfunction quickly resolved TER to baseline levels.

CONCLUSIONS

Our results suggest that alcohol intoxication increases microvascular permeability to plasma proteins. The data also suggest the endothelial-specific mechanism involves the p38 MAPK, Rac1, and reorganization of VE-cadherin at junctions. Last, activation of Epac can quickly resolve alcohol-induced endothelial barrier dysfunction.

Alcohol abuse and the injured host: dysregulation of counterregulatory mechanisms review

Traumatic injury ranks as the number one cause of death for the younger than 44 years age group and fifth leading cause of death overall (www.nationaltraumainstitute.org/home/trauma_statistics.html). Although improved resuscitation of trauma patients has dramatically reduced immediate mortality from hemorrhagic shock, long-term morbidity and mortality continue to be unacceptably high during the postresuscitation period particularly as a result of impaired host immune responses to subsequent challenges such as surgery or infection. Acute alcohol intoxication (AAI) is a significant risk factor for traumatic injury, with intoxicating blood alcohol levels present in more than 40% of injured patients. Severity of trauma, hemorrhagic shock, and injury is higher in intoxicated individuals than that of sober victims, resulting in higher mortality rates in this patient population. Necessary invasive procedures (surgery, anesthesia) and subsequent challenges (infection) that intoxicated trauma victims are frequently subjected to are additional stresses to an already compromised inflammatory and neuroendocrine milieu and further contribute to their morbidity and mortality. Thus, dissecting the dynamic imbalance produced by AAI during trauma is of critical relevance for a significant proportion of injured victims. This review outlines how AAI at the time of hemorrhagic shock not only prevents adequate responses to fluid resuscitation but also impairs the ability of the host to overcome a secondary infection. Moreover, it discusses the neuroendocrine mechanisms underlying alcohol-induced hemodynamic dysregulation and its relevance to host defense restoration of homeostasis after injury.

Delayed resuscitation with physostigmine increases end organ damage in alcohol intoxicated rats

Previous studies from our laboratory have identified a role for blunted central sympathetic activation in the acute alcohol intoxication (AAI)-induced impairment of the counterregulatory response to hemorrhagic shock (HS). Immediate fluid resuscitation (FR) with acetylcholinesterase inhibitors restores the neuroendocrine and pressor responses to FR in AAI + HS. We hypothesized this intervention would remain beneficial after delay and that restoration of mean arterial blood pressure (MABP) during FR would attenuate organ damage. Male Sprague-Dawley rats received a primed constant alcohol infusion (2.5 g · kg + 0.3 g · kg · h for 15 h) or isocaloric dextrose (DEX) before HS (40 mmHg for 60 min) and FR with lactated Ringer's (LR) solution ± physostigmine (PHYS; 100 µg · kg) immediately or after a 60-min delay after HS. Immediate LR solution elevated MABP in DEX + HS. Acute alcohol intoxication delayed the initial MABP recovery. Delayed LR solution did not further increase MABP in DEX- or AAI + HS. LR solution + PHYS increased MABP in DEX- and AAI + HS after immediate and delayed FR. No differences were noted in markers of organ dysfunction (alanine aminotransferase [ALT], aspartate aminotransferase, blood urea nitrogen, creatinine) after DEX + HS, and this was unaltered by immediate or delayed LR solution + PHYS. Acute alcohol intoxication + HS increased ALT, which was attenuated by immediate LR solution + PHYS. In contrast, delayed LR solution + PHYS exacerbated tissue injury in AAI + HS, as reflected by increased ALT, aspartate aminotransferase, blood urea nitrogen, creatinine, and liver protein carbonylation over time-matched LR solution. In conclusion, PHYS enhanced blood pressure recovery independent of time of FR and presence of AAI. However, in AAI + HS, delayed LR solution + PHYS accentuated organ damage and dysfunction. These findings suggest that although enhancing the sympathetic response can improve hemodynamic recovery during AAI, it may compromise tissue perfusion and enhance tissue injury.

Systemic administration of a centrally acting acetylcholinesterase inhibitor improves outcome from hemorrhagic shock during acute alcohol intoxication

Previously, we have demonstrated that acute alcohol intoxication impairs hemodynamic counter-regulation to hemorrhage in unanesthetized rats, and that this phenomenon is associated with an impaired neuroendocrine response to blood loss. Moreover, we demonstrated that central acetylcholinesterase inhibition restores the hemodynamic and neuroendocrine responses to hemorrhage in alcohol-intoxicated rats. We hypothesized that similar responses could be elicited by systemic administration of physostigmine, an acetylcholinesterase inhibitor that penetrates the blood brain barrier. The relevance of this approach was to establish effectiveness of a more clinically applicable route of drug administration than that used previously. Chronically catheterized adult male Sprague-Dawley rats (250-275 g) were administered a bolus of physostigmine (i.v., 100 microg/kg) at rest, and in a separate study, simultaneously with Ringer's lactate solution after an overnight intragastric infusion of 30% alcohol (approximately 7 g/kg for 15 h) or 52% isocaloric dextrose and fixed-pressure hemorrhage. I.v. physostigmine administration immediately increased sympathetic outflow via activation of central nicotinic receptors and improved the pressor response to fluid resuscitation in both dextrose controls and alcohol-intoxicated animals. The improved hemodynamic recovery achieved with physostigmine was also associated with attenuation of the rises in the markers of liver and renal damage alanine aminotransferase and blood urea nitrogen in alcohol-intoxicated animals. Additional studies are warranted to determine the effect of central acetylcholinesterase inhibition on tissue injury and survival after severe blood loss, as well as its effects on long-term metabolic and inflammatory responses.

Central acetylcholinesterase inhibition improves hemodynamic counterregulation to severe blood loss in alcohol-intoxicated rats

Acute alcohol intoxication results in impaired hemodynamic counterregulation to blood loss and is associated with an attenuated hemorrhage-induced release of catecholamines and AVP. We speculated that restoration of the neuroendocrine response to hemorrhage would improve mean arterial blood pressure (MABP) recovery during acute alcohol intoxication. Previously, we demonstrated that intracerebroventricular (i.c.v.) choline, a precursor of acetylcholine, transiently increases sympathetic nervous system (SNS) outflow but is not capable of improving neuroendocrine and hemodynamic compensation to hemorrhage in alcohol-treated rats. We hypothesized that prolongation of the observed effect via i.c.v. neostigmine, an acetylcholinesterase inhibitor, would enhance SNS outflow, restore the neuroendocrine response, and in turn improve hemodynamic responses to hemorrhage during acute alcohol intoxication. I.c.v. neostigmine (1 microg) increased MABP, catecholamines, and AVP within 5 min and reversed hypotension due to 40% hemorrhage and intragastric alcohol (30% wt/vol, 2.5 g/kg) administration in chronically catheterized male Sprague-Dawley rats (225-250 g body wt). Acute alcohol intoxication before 50% hemorrhage decreased basal MABP, accentuated hypotension midhemorrhage, suppressed the hemorrhage-induced release of norepinephrine and AVP, and prevented restoration of MABP to basal levels after fluid resuscitation with lactated Ringer solution. I.c.v. neostigmine (0.5 microg) produced a sustained increase in MABP beginning at 30 min of hemorrhage that persisted throughout fluid resuscitation in control and alcohol-treated animals. I.c.v. neostigmine enhanced epinephrine responses and restored the hemorrhage-induced release of norepinephrine and AVP in alcohol-treated rats. These results demonstrate that inhibition of acetylcholinesterase in the central nervous system enhances SNS outflow, restores the neuroendocrine response to severe blood loss, and thereby improves hemodynamic counterregulation during acute alcohol intoxication. This study provides evidence for a central (and not peripheral) role of alcohol in impairing hemodynamic stability during hemorrhagic shock.