Endothelial dysfunction in sepsis

The endothelium takes part in the regulation of numerous physiological functions and lies at the interface of circulating blood and the vessel wall. Under physiological conditions, it is responsible for anticoagulant and anti-adhesive properties, and it regulates vasomotor tone and vascular homeostasis. Endothelial dysfunction has been associated with many pathophysiological processes, such as inflammation and oxidative and nitrosative stresses. Endothelial cells are precociously exposed to circulating signaling molecules and physical stresses, like in sepsis and septic shock. Septic shock is associated with hypotension and frequently with disseminated intravascular coagulation contributing to multiple organ failure and a high mortality rate. Impairment of endothelial function leads to phenotypic and physical changes of the endothelium, with deregulated release of potent vasodilators nitric oxide and prostacyclin, reduction of vascular reactivity to vasoconstrictors, associated with leukocytes' and platelets' aggregation, and increase in inducible nitric oxide synthase expression that can exert a negative feedback on endothelial nitric oxide synthase expression, with subsequent deregulation of nitric oxide signaling. Endothelial dysfunction therefore plays a major role in the pathophysiology of septic shock and organ dysfunction, and has been suggested to be a predictor of mortality in sepsis. Thus, early detection of endothelial dysfunction could be of great interest to adapt treatment in initial stage of sepsis. Current therapeutics used in sepsis mostly aim at controlling inflammation, vascular function and coagulation. Fluid administration, vasopressors, vasodilators and recombinant human activated protein C are also part of the treatments with the ultimate goal to exert beneficial effects on organ function and survival.

Mechanisms of vascular hyporesponsiveness in septic shock

PURPOSE

To define some of the most common characteristics of vascular hyporesponsiveness to catecholamines during septic shock and outline current therapeutic approaches and future perspectives.

METHODS

Source data were obtained from a PubMed search of the medical literature with the following MeSH terms: Muscle, smooth, vascular/physiopathology; hypotension/etiology; shock/physiopathology; vasodilation/physiology; shock/therapy; vasoconstrictor agents.

RESULTS

NO and peroxynitrite are mainly responsible for vasoplegia and vascular hyporeactivity while COX 2 enzyme is responsible for the increase in PGI2, which also contributes to hyporeactivity. Moreover, K+ATP and BKCa channels are over-activated during septic shock and participate in hypotension. Finally, other mechanisms are involved in vascular hyporesponsiveness such as critical illness-related corticosteroid insufficiency, vasopressin depletion, dysfunction and desensitization of adrenoreceptors as well as inactivation of catecholamines by oxidation.

CONCLUSION

In animal models, several therapeutic approaches, targeted on one particular compound have proven their efficacy in preventing or reversing vascular hyporesponsiveness to catecholamines. Unfortunately, none have been successfully tested in clinical trials. Nevertheless, very high doses of catecholamines ( > 5 μg/kg/min), hydrocortisone, terlipressin or vasopressin could represent an alternative for the treatment of refractory septic shock.

Ergotamine and dihydroergotamine: history, pharmacology, and efficacy

Ergotamine and dihydroergotamine share structural similarities with the adrenergic, dopaminergic, and serotonergic neurotransmitters. As a result, they have wide-ranging effects on the physiologic processes that they mediate. Ergotamine and dihydroergotamine are highly potent at the 5-HT1B and 5-HT1D antimigraine receptors and, as a consequence, the plasma concentrations that are necessary to produce the appropriate therapeutic and physiologic effects are very low. The broad spectrum of activity at other monoamine receptors is responsible for their side effect profile (dysphoria, nausea, emesis, unnecessary vascular effects). Both ergotamine and dihydroergotamine have sustained vasoconstrictor actions. In acute migraine treatment, their mechanisms of action involve constricting the pain-producing intracranial extracerebral blood vessels at the 5-HT1B receptors and inhibiting the trigeminal neurotransmission at the peripheral and central 5-HT1D receptors. The scientific evidence for efficacy is stronger for dihydroergotamine than for ergotamine. Their wide use is based on long-term experience.

The early use of ergotamine in migraine. Edward Woakes' report of 1868, its theoretical and practical background and its international reception

Although ergot had been used in obstetrics for several centuries, it was proposed for the treatment of migraine only in the 19th century. The British ENT-surgeon Edward Woakes (1837-1912) recommended ergot as a vasoconstricting agent for migraine and other neurogenic conditions associated with vasodilatation in 1868. He subscribed to the theory of vasodilatation by sympathetic deficit, presented in the early 1850s by Brown-Séquard and Claude Bernard. Du Bois-Reymond proposed vasoconstriction by sympathetic overactivity as the cause of migraine in 1860; Brown-Séquard opposed this in favour of vasodilatation. Vasodilatation due to sympathetic deficit in migraine was again supported by Möllendorf, with clinical evidence, in 1867. Woakes' paper of 1868 introduced ergot as a vasoconstrictor for the same condition. Reception abroad was prompt. A German version appeared in 1869, and Eulenburg cited Woakes in his textbook of 1871. Eulenburg presented the use of ergot for migraine as a routine measure in the second edition of his textbook in 1878, and in a paper published in 1883. The method was internationally accepted, but it became really popular only after the isolation of pure ergotamine in 1918, resulting in the first reliable compounds with stable properties and predictable effects. Contrary to Woakes' theory, in the early 20th century ergot was used for migraine because of its well-documented adrenolytic properties, as migraine was by then again believed to be a sympathotonic and vasospastic condition.

From serotonin receptor classification to the antimigraine drug sumatriptan

After the synthetic serotonin 5-hydroxytryptamine (5-HT) became available in the early 1950s, attempts were soon under way to study the nature of 5-HT receptors. Using the guinea-pig isolated ileum, Gaddum and Picarelli (1957) suggested that 5-HT-induced contractions were mediated by a morphine-sensitive "M" receptor located on the parasympathetic ganglion and a dibenzyline-sensitive "D" receptor located on the smooth muscle. Though this classification ws used during the next three decades, it was realized that some effects of serotonin, for example vasoconstriction within the carotid vascular bed, were not mediated by either "M" or "D" receptors. When radioligand binding studies led to the identification of 5-HT1 and 5-HT2 "receptors" in the rat brain membranes, it became increasingly apparent that the two receptor classifications were not identical. Thus, a new framework for serotonin receptor nomenclature and classification was proposed: 5-HT1-like (5-HT1), 5-HT2 (formerly "D") and 5-HT3 (formerly "M") receptors. At the present time, several subtypes of 5-HT1 receptors as well as a 5-HT4 receptor are also recognized. As the serotonin receptor classification was emerging to indicate that carotid vasoconstriction by serotonin is mediated by a subtype of 5-HT1 receptors, on the migraine front it was being suggested that the disease is associated with vasodilation within the cranial extracerebral circulation and deranged serotonin metabolism and that certain antimigraine drugs caused a selective carotid vasoconstriction, probably via serotonin receptors. Therefore, Humphrey and colleagues conceived that synthesis of serotonin derivatives may lead to a compound that would elicit highly selective carotid vasoconstriction and abort migraine attacks. Indeed, via the synthesis of 5-carboxamidotryptamine and AH25086, sumatriptan was designed. The drug acts as an agonist at the vasoconstrictor 5-HT1 receptor subtype and has proved highly effective in the therapy of migraine attacks.