Andre Esanu

Platelet Activating Factor (PAF)

SummaryThis review is an attempt to summarise recent data on platelet activating factor (PAF) and PAF antagonists from 1988 to the present. This period saw a burst in research activity focused predominantly on the effect of PAF in various organs. The effect of PAF and its antagonists was further intensively studied in vitro on isolated platelets, leucocytes, macrophages and endothelial cells. From these and earlier data, based on the catastrophe theory of Thorn and Zeeman, a new concept on the interaction between PAF and various cytokines could be recognised as an important mechanism of action of the phospholipid mediator, suggesting the existence of an autocatalytic feedback network through which PAF can influence cellular function under certain pathophysiological conditions. This mechanism can be regarded as the culmination of our recent knowledge on the role of PAF, and may influence the possible clinical implications of PAF antagonists in the near future.It is recognised that PAF is released in shock and ischaemic states, and that PAF antagonists can protect the heart and brain against ischaemic injury. Therefore, in contrast to the previous period, which was predominantly devoted to the elucidation of the role of PAF in immediate hypersensitivity reactions, studies performed on cerebral, myocardial and intestinal ischaemia as well as in various shock conditions have concentrated on entirely new aspects of the effect of PAF antagonists, emphasising the significance of the inflammatory process and cell-to-cell interactions in these pathophysiological states. This has led to a re-evaluation of the experimental data previously accumulated.At the same time, these new trends in PAF and PAF antagonist research have explored further possibilities for the application of PAF antagonists in clinical practice. Attention has been focused on the physiological role of PAF as a signal molecule, especially between the neuroendocrine system and related sensory organs. The recognition of the significance of PAF in mammalian reproduction is fascinating and may lead to new clinical applications of PAF antagonists.It appears probable that, like eicosanoids, PAF is involved in a great variety of membrane-dependent processes that play a fundamental role in the maintenance of homeostasis. PAF research has provided several potent natural and synthetic antagonists which may facilitate the clinical application of these drugs in the near future.

Evidence that mammalian lignans show endogenous digitalis-like activities.

Enterolactone, a lignan that has been identified in biological samples from man and several mammals, shares with ascorbic acid and cardiac glycosides a gamma-butyrolactone. It displaces 3H-ouabain from its binding sites on cardiac digitalis receptor and inhibits, dose dependently, the Na+, K+-ATPase activity of human and guinea-pig heart. The time dependence of this inhibition resembles that of dihydroouabain, a cardiac glycoside in which the lactone ring does not contain conjugated double bonds. The active concentrations of enterolactone as inhibitor of Na+,K+-ATPase are in the 10(-4) M range and, at those concentrations, the cross-reactivity with antidigoxin antibodies is low. Lignans may contribute to the putative digitalis-like activity found in tissues, blood and urine of several mammals including man.

Inhibition of the erythrocyte Na+, K+-pump by mammalian lignans.

Several mammalian lignans, particularly enterolactone, 3-oxy-methyl enterolactone and prestegane B are able to inhibit Na+, K+-pump activity in human red cells with IC50 of about 1 mM. The inhibition of Na+, K+-pump activity by mammalian lignans have the following properties: the IC50 for ouabain remains unchanged suggesting a noncompetitive inhibition. The apparent affinity for internal Na+ and maximal rate of cation translocation are both diminished. The above inhibition of the Na+, K+-pump was obtained at doses 2-3 orders of magnitude higher than those required for ouabain. However we cannot exclude that a glycosyl- (and/or butenolide)-derivative of enterolactone could be one endogenous ouabain-like factor.

Cicletanine attenuates overdrive pacing-induced global myocardial ischemia in rabbits: possible role of cardiac cyclic nucleotides.

BackgroundThis study examined whether cicletanine, an antihypertensive drug with cGMP phosphodiesterase inhibitory effect, could alleviate ventricular overdrive pacing-induced myocardial ischemia in chronically instrumented rabbits. MethodsAn electrode-catheter implanted into the right ventricle was used for pacing (500 bpm over 5 min) and for measuring intracavital ST-segment elevation and ventricular effective refractory period (VERP). PQ and QT intervals were measured in the chest-lead ECG, and dP/dtmax as well as left ventricular end-diastolic pressure (LVEDP) were recorded through a left intraventricular catheter. In separate groups, mean arterial blood pressure (MABP) was monitored from the right carotid artery. Experiments were performed on conscious rabbits after a week of convalescence. In anesthetized, open-chest rabbits, samples were taken from the left ventricle before and after drug treatment and/or overdrive pacing for determination of cGMP and cAMP contents by radioimmunoassay. ResultsIntravenous cicletanine, 30 mg/kg body weight, did not change resting MABP, dP/dtmax, and LVEDP, but it did reduce heart rate and prolonged PQ and QT intervals and VERP. Overdrive pacing produced intracavital ST-segment elevation, increased LVEDP, and decreased dP/dtmax and MABP. Cicletanine administered 15 minutes before pacing significantly attenuated ST-segment elevation, increased LVEDP, and decreased dP/dtmax and MABP. In anesthetized animals, cicletanine itself slightly increased cardiac cGMP and cAMP contents. Overdrive pacing moderately increased cGMP and profoundly elevated cAMP, and in overpaced rabbits, cicletanine further increased cGMO and markedly attenuated cAMP content increased by overdrive pacing. ConclusionsThese results suggest that in correlation with alterations of cardiac cycle nudeotide contents, cicletanine protects the heart against pacing-induced myocardial ischemia.

Cicletanine improves myocardial function deteriorated by ischemia/reperfusion in isolated working rat hearts

Summary The effect of cicletanine, a novel furopyridine antihypertensive drug was compared with that of nitren-dipine, a dihydropyridine slow calcium channel blocker, on cardiac function and reperfusion-induced ventricular arrhythmias in isolated working rat hearts subjected to 10-min ischemia induced by ligation of the left main coronary artery followed by 10-min reperfusion. Before ischemia, cicletanine and nitrendipine, perfused at concentrations of 3 ± 10-5, 6 ± 10-5, 10-4, and 2 ± 10-4or 10-8 M, respectively, did not influence heart rate (HR), LV developed pressure (LVDP), its first derivative (LVdP/dtmax) and LV end-diastolic pressure (LVEDP), whereas aortic flow (AF) was decreased by 2 ± 10 -4 M cicletanine only. Coronary flow (CF) remained unchanged by various cicletanine concentrations but was slightly increased by nitrendipine. In the concentration range of 3 ± 10-5–10-4 M, cicletanine improved AF either in ischemia or during reperfusion, whereas 2 ± 10 -4 M had no such effect. Nitrendipine slightly attenuated ischemia/reperfusion-induced decrease in AF. Cicletanine and nitrendipine enhanced LVDP during ischemia. Ischemia-induced deterioration of LVdP/dtmax was reduced by cicletanine, during reperfusion, but this parameter was reduced by nitrendipine and the highest cicletanine concentration. Cicletanine decreased LVEDP significantly during ischemia and reperfusion, but nitrendipine had no such effect. All cicletanine concentrations reduced the incidence of irreversible ventricular fibrillation (VF) during reperfusion, an effect roughly concentration dependent in the range of 3 ± 10-5–10-4 M, whereas nitrendipine had no influence on arrhythmias.

Isaxonine base is a strong perturber of phospholipid bilayer order and fluidity—A differential scanning calorimetry and spin labeling study

The effects of the neurotropic drug isaxonine on fully hydrated dipalmitoyl-phosphatidyl-choline (DPPC) bilayers has been studied in the temperature range 0 degree-60 degrees, using differential scanning calorimetry and electron spin resonance spectroscopy, with two stearic acid spin labels. At low concentration (1% mol/mol), isaxonine is trapped in the polar interface and enhances the phospholipid multibilayers organization in the gel state. In contrast, at high concentration (30% mol/mol), the drug disorganizes the phospholipidic structures and may induce domain formation by phase separation. The strong interactions of isaxonine at the lipid-water interface change the ionization state of the stearic acid spin labels which become totally ionized. Then isaxonine acts as a modifier of the surface pH of the bilayer. The strong membrane effects of isaxonine may explain in part its pharmacological properties in vivo.

Vascular remodeling and antihypertensive therapy: the example of cicletanine.

&NA; Hypertension causes major structural vascular modifications including increase of wall:lumen ratio, vessel wall hypertrophy, and rarefaction of arterioles and small arteries. These structural alterations are accompanied with vascular dysfunctions. The stimuli responsible for this vascular remodeling are numerous and comprise physical, humoral, and locally synthetized factors. Among them, vasoactive substances such as vasoconstrictors (e.g., angiotensin II and endothelin) or vasodilating substances (e.g., prostacyclin and nitric oxide) may play a role as important as those of growth factors. The vascular protection afforded by antihypertensive drugs depends on their mode and their site of action and probably on their direct interference with the local hemodynamic and growth‐affecting factors. This is well illustrated by cicletanine, a new antihypertensive agent that provides a good example of protection of the vasculature. This also emphasizes that vascular remodeling is important in hypertension and must be considered as an essential property of any new antihypertensive drug.

PAF-Acether Induced Arterial Thrombosis and the Effect of Specific Antagonists

Platelet-vessel wall interactions and local thrombosis are investigated in vivo in a branch of the mesenteric artery of the guinea pig, using optoelectronic registration and ultrastructural control. Following an electrical challenge resulting in changes of cell membrane polarization, subsequent superfusion by PAF-acether or a stable analogue, (1-O-alkyl-2-N-methylcarbamyl-sn-glycero-3-phosphocholine, 10(-8) M focal concentration (f.c.)) for a restricted period results in endothelial cell retraction and bleb formation followed by platelet adhesion and the development of a thrombus which over time becomes invaded by leukocytes and eventually occludes the vascular lumen. It was demonstrated in a previous investigation that these phenomena are triggered by the generation of endogenous PAF-acether by the endothelial cells. Specific PAF-acether-antagonists, such as BN 52021 a ginkgolide, but also synthetic molecules, derivatives of the triazolo-pyridino-diazepine group (BN 50727, BN 50755 and BN 50789), significantly inhibit platelet-vessel wall interactions and thrombosis, but not the formation of blebs in the endothelial cells. Hydrogen peroxide (10(-5)M f.c.) not only primes the effect of PAF-acether, but is by itself capable of inducing thrombosis through a PAF-acether-mediated mechanism. Inhibition of acetyl hydrolase by PMSF (phenyl-methyl-sulfonyl-fluoride, 10(-5)M f.c.) invariably results in a significant enhancement of thrombosis, while conversely, inhibition of acetyl transferase by 27584 RP (4-(naphtylvinyl)pyridine hydrochloride, 10(-6)M f.c.) inhibits thromboformation indicating that the remodeling pathway is involved.