Leukotriene D4-induced hypotension is reversed by thyrotropin-releasing hormone

Novel neuroprotective tripeptides and dipeptides

It has long been recognized that thyrotropin-releasing hormone (TRH) and certain TRH analogues are neuroprotective in a variety of animal models of CNS trauma. In addition to these neuroprotective actions, TRH and most TRH analogues have other physiological actions that may not be desirable for treatment of acute injury, such as analeptic, autonomic, and endocrine effects. We have developed a series of dual-substituted TRH analogues that have strong neuroprotective actions, but are largely devoid of these other physiological actions. In addition, we have developed a family of cyclized dipeptides (diketopiperazines), structurally somewhat related to a metabolic product of TRH, that appear even more effective as neuroprotective agents in vitro and in vivo, and may have nootropic properties. Here, we review these novel tripeptide and dipeptide compounds.

Aminopeptidases in visceral organs during alterations in body fluid volume and osmolality

Enzymatic cleavage of some peptides in the local environment could be included among the mechanisms related to the regulation of hydrosaline balance. In order to examine this hypothesis, we measured representative aminopeptidase activities in visceral organs of rats after applying certain hydrosaline challenges. Decreased levels (about 30%) of particulate puromycin-insensitive-neutral aminopeptidase in the renal medulla and of soluble acid aminopeptidase in the lung were observed under hyperosmolality and hypovolemia. Decreased levels (more than 45%) of particulate type-I-pyroglutamyl aminopeptidase in the heart were observed under altered volemia. These results indicate that aminopeptidases at these anatomical locations might be involved in the regulation of body fluid volume and osmolality.

The opioid/anti-opioid balance in shock: a new target for therapy in resuscitation

Teleologically, pain is of paramount importance for survival and induces the organism to cope in an active way with aggressions from a basically hostile environment. While the activation of endogenous analgesic (opioid) systems typically occurs in conditions of surrender (pre-terminal conditions, sustained tortures, etc.), the activation of endogenous anti-analgesic systems triggers mechanisms of active or passive defence (such as camouflage) aimed at survival. The distinctive features of the main anti-analgesic systems (melanocortinergic, cholecystokininergic, thyroliberinergic) and the dramatic results obtained in experimental pre-terminal conditions (hemorrhagic shock, prolonged respiratory arrest) with the administration of their neuropeptide transmitters (ACTH and several ACTH-fragments, including alpha-MSH, CCK peptides and thyrotropin-releasing hormone) are here reviewed. The study of the mechanisms underlying the resuscitating effects of these neuropeptides has led to the discovery of the (often extremely potent) resuscitating effect of other drugs (protoveratrines, nicotine, centrally-acting cholinergic agents, ganglion-stimulating drugs). It is particularly remarkable that in pre-terminal conditions these neuropeptides and drugs have highly impressive effects on cardiocirculatory parameters at doses that are almost or actually inactive under normal conditions, and that their resuscitating effect is obtained without the need for any other supportive treatment and at dose-levels well below toxic ranges. Finally, in hemorrhage-shocked animals, the treatment with anti-analgesic neuropeptides shortly after bleeding considerably extends the time-limit for an effective and definitively curing blood reinfusion. This would be of self-evident importance in clinical practice, because an extremely simple, non-toxic first-aid treatment in the field, shortly after a massive hemorrhage, could resuscitate the patient for a period sufficient to effectively set up the most appropriate in-hospital treatment.

Hemodynamic and neural mechanisms of action of thyrotropin-releasing hormone in the rat

The mechanisms mediating the effects of thyrotropin-releasing hormone (TRH) on the cardiovascular system were studied in the conscious rat. Intracerebroventricular (i.c.v.) injection of TRH (8 pmol-80 nmol/kg) induced dose-dependent increases in mean arterial pressure, heart rate, and cardiac index. Hindquarter blood flow increased due to vasodilation, while an increase in renal and mesenteric vascular resistance caused a decrease in blood flow in the respective organs. The plasma levels of norepinephrine and epinephrine were increased by TRH, while there was no change in plasma renin activity or vasopressin. The cardiovascular actions of i.c.v. TRH were not influenced by blockade of the renin-angiotensin system or vasopressin receptors. The ganglion blocker chlorisondamine and the alpha 1- and alpha 2-adrenoreceptor antagonist phentolamine (2 mg/kg i.v.) abolished the increase in blood pressure and mesenteric vasoconstriction after i.c.v. TRH. Propranolol (2 mg/kg i.v.) blocked the TRH-induced increase in cardiac index, heart rate, and hindquarter blood flow. The hindquarter vasodilation induced by TRH was also blocked by the selective beta 2-adrenoceptor antagonist ICI 188,551 (1 or 2 mg/kg i.v.), while the beta 1-adrenoceptor blocker practolol (10 mg/kg i.v.) had no effect on the hindquarter vasodilation produced by TRH but totally blocked the increase in cardiac index. In adrenal demedullated rats, the systemic hemodynamic effects of i.c.v. TRH were diminished along with the decrease in renal blood flow and increase in renal vascular resistance; however, the increase in hindquarter blood flow was attenuated only in adrenal demedullated rats pretreated with the sympathetic blocker bretylium. The renal vasoconstriction induced by i.c.v. TRH was not abolished by renal denervation. In sinoaortic debuffered rats, the pressor, tachycardic, and mesenteric vasoconstrictor responses to centrally administered TRH were significantly potentiated. Taken together, these data suggest that the putative neurotransmitter TRH may play a role in central regulation of cardiac functions and organ blood flow distribution through both the sympathetic nerves and the adrenal medulla. A pivotal role for beta 2-adrenoceptors in mediation of hindquarter vasodilation is also demonstrated.

Cardiovascular pharmacology of thyrotropin releasing hormone

Thyrotropin releasing hormone (TRH) and its receptors are present in the cardiovascular nuclei of the brain as well as in the intermediolateral cell column of spinal cord. Anatomical, neurophysiological, functional and pharmacological studies suggest that TRH is a neurotransmitter/neuromodulator in the central nervous system. Administration of TRH to experimental animals or human subjects induces pressor and tachycardic responses and increases plasma levels of catecholamines. These effects are likely to be mediated by a central nervous system activation of the sympathoadrenomedullary system with no involvement of vasopressin or renin-angiotensin system. In the conscious rat, the TRH-induced pressor response is accompanied by an increment in cardiac output and a distinct change in organ blood flow, a hindquarter skeletal muscle vasodilation accompanied by renal and mesenteric vasoconstriction. The role of TRH in hypertension has not been studied. However, the extremely potent pressor and vasoconstrictor properties of TRH makes this tripeptide a candidate for neurotransmitters/modulators involved in the development and/or maintenance of hypertension. The role of TRH in the therapy of shock is at present controversial. Though preliminary experimental work raised hopes and expectations for therapeutic usage of TRH in shock and trauma, the more recent studies have shown no effect or a detrimental effect for TRH in some experimental shock states.

Spontaneously hypertensive rats exhibit supersensitivity to the hypertensive and hyperthermic effects of thyrotropin releasing hormone

The effect of thyrotropin releasing hormone (TRH) on colonic temperature and systolic blood pressure of age-matched spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats was determined. Administration of TRH produced dose-dependent increases in body temperature and systolic blood pressure. TRH-induced changes in both responses were of greater magnitude in SHR rats compared to WKY rats. The results provide the first evidence that SHR rats exhibit supersensitivity to non-neuroendocrinological effects of TRH and that TRH may play a role in the pathophysiology of elevated blood pressure.

Vascular responses to leukotriene B4, C4 and D4 following FPL 55712, indomethacin, saralasin, phentolamine and verapamil in the conscious rat

The pressor and vascular permeability effects of leukotrienes B4 (LTB4), C4 and D4 were investigated in conscious unrestrained rats. Leukotrienes C4 and D4 (3.2-51 nmol kg-1 i.v.) caused an acute dose-dependent elevation of the mean arterial pressure, which was maximal after 2 min and returned to control levels within 14 min. Heart rate was significantly reduced by the higher doses of LTC4 and LTD4. LTB4 (up to a dose of 51 nmol kg-1) was essentially inactive. These effects of LTC4 and LTD4 were abolished by FPL 55712, a putative antagonist of sulphidopeptide leukotrienes and by verapamil, a calcium channel blocker. Indomethacin, phentolamine or saralasin pretreatment failed to modify the pressor response to LTC4 and LTD4. LTC4 and LTD4 furthermore caused an increase in haematocrit values, which was significantly attenuated by FPL 55712, indomethacin and verapamil. The present findings show that the pressor effect of LTC4 and LTD4 is not related to prostanoid release and can be reversed by calcium channel blockade; whereas the effect on vascular permeability seems to require the presence of both cyclo-oxygenase product(s) and calcium.

Evidence for the involvement of sulfidopeptide leukotrienes in the pathogenesis of Pichinde virus infection in strain 13 guinea pigs

Strain 13 guinea pigs inoculated subcutaneously with Pichinde virus developed fever, lost body weight, decreased water and food consumption, and died at 14 +/- 0.6 days. After FPL-55712, a leukotriene D4 antagonist, was administered beginning on PID (post-inoculation day) 3 for 18 days, the magnitude of body weight loss decreased and food intake increased, despite a persistent fever. The treated guinea pigs also survived significantly longer than infected animals receiving placebo injection (21 vs 14 days). Using guinea pig ileum bioassay and radioimmunoassay, we detected significant levels of plasma leukotrienes in Pichinde virus-infected guinea pigs on PID 11 and possibly PID 14. These findings strongly suggest that leukotrienes play a role in the pathogenesis of arenavirus infection and may account in part for the observed cardiac depression, pulmonary edema, and eventual death.

PAF-acether and leukotriene participation in acute circulatory shock

The fact that PAF-acether elicites acute circulatory collapse in anesthetized dogs supports the hypothesis that its endogenous double is involved in shock state events. The fact that cysteinyl containing leukotrienes has been shown to be released in various shock states, themselves producing noxious effects related to such circulatory disturbances, suggests a possible role of these arachidonic acid metabolites in shock syndrome. The present report summarizes the part played by these mediators in shock developments. More precisely, nanograms PAF-acether IV in anesthetized dogs produced biphasic effects on mesenteric blood flow and inhibited, in dogs and rats, histamine induced gastric acid secretion. These results confirm the distributive component of the PAF-acether circulatory collapse. On the other hand, as leukotrienes, nanograms PAF-acether elicited both plasma extravasation and vasoconstriction in guinea pig skin. Lastly, in conscious mice, lipoxygenase antagonists, but not cyclooxygenase antagonists, inhibited lethal effects of PAF-acether, suggesting a mutual and synergistic action of PAF-acether and leukotrienes in shock state.

Differential hemodynamic effects of leukotriene D4 in anesthetized rats: evaluation by directional pulsed Doppler technique

Leukotriene D4 (LTD4) is the major constituent of slow-reacting substance of anaphylaxis (SRS-A). Cardiovascular depression and hypotensive shock represent the major manifestations that attend systemic anaphylaxis. To further evaluate the hemodynamic effects of LTD4, we measured blood pressure (BP), heart rate (HR) and blood flow (BF) (directional pulsed Doppler flowmeter) to different vascular beds (hindquarter, mesenteric and renal) of the urethane-anesthetized rat. LTD4 (3, 10 and 30 micrograms/kg, i.v.) caused a dose-dependent increase in BP: 15 +/- 3, 20 +/- 4 and 24 +/- 2 mm Hg, respectively, which was maximum after 2 min and returned to control level at 10 min; HR was not significantly altered. BF to different vascular beds was differentially altered: mesenteric (-59%) greater than hindquarter (-38%) greater than renal (-10%). Vascular resistance (VR) increased by 195, 85 and 40% in mesenteric, hindquarter and renal beds, respectively. Thyrotropin-releasing hormone (TRH) (2-5 mg/kg, i.v.) injected after LTD4 increased BP, reversed the decrease in BF and the increase in VR in the mesenteric and hindquarter vascular beds. These data suggest that LTD4 receptors are unevenly distributed in various vascular beds and that the splanchnic area is particularly vulnerable to anaphylaxis-induced ischemia. Furthermore, Thyrotropin Releasing Hormone (TRH) might be useful to antagonize the hemodynamic consequences mediated by SRS-A or leukotriene.

Reversal of leukotriene D4 hypotension by thyrotropin-releasing hormone

Thyrotropin-releasing hormone (TRH) reversed the hypotension produced by leukotriene D4 (LTD4) (5 micrograms/kg, i.v.) in conscious guinea pigs in a dose-dependent fashion at intravenous doses between 0.2 and 2.0 mg/kg. LTD4 hypotension was also reversed by the synthetic TRH analog MK771 (0.2 or 2 mg/kg, i.v.), suggesting a possible receptor-mediated mechanism. Since LTD4 has been implicated as a mediator of anaphylaxis, these results provide a basis for further evaluation of TRH and its analogs in the treatment of anaphylactic shock.

Cardiovascular and sympathetic effects of leukotriene D4 in renovascular and spontaneously hypertensive rats

Leukotriene D4 (LTD4), a constituent of slow-reacting substance of anaphylaxis (SRS-A), elicits a pressor response followed by prolonged hypotension in spontaneously hypertensive (SHR) but not in Wistar-Kyoto (WKY) rats. In order to investigate whether the depressor response to LTD4 in SHR rats is related to hypertension itself, we have studied the cardiovascular and sympathetic effects of LTD4 in 1-kidney, 1-clip hypertensive rats and 1-kidney, normotensive rats. In all groups of conscious rats, intra-arterial administration of LTD4 (0.2-20 micrograms/kg) caused dose-dependent pressor responses of similar degree except in WKY rats, which responded less. Only SHR rats developed a significant and progressive hypotension (-58 +/- 5 mm Hg) following pressor phase. In all but WKY rats, the response phase was attended by an increase in plasma levels of norepinephrine and epinephrine. Only SHR rats showed marked and persistent hemoconcentration following pressor effect. Thus, depressor response of SHR rats to systemic administration of LTD4 does not appear to be merely due to the magnitude of blood pressure elevation and may in part result from microcirculatory changes not present in other hypertensive rats.