Hans Strobach

Direct inhibition of platelet function by organic nitrates via nitric oxide formation

This study investigates the mechanisms of platelet inhibition by the nitrate esters isosorbide dinitrate, isoidide dinitrate, isomannide dinitrate, isosorbide 2-mononitrate and isosorbide 5-mononitrate as compared to the spontaneous nitric oxide (NO)-donor linsidomine, the active metabolite of molsidomine. Nitrates and linsidomine dose-dependently inhibited aggregation, ATP secretion and thromboxane formation of washed human platelets at a rank order of potency, identical with that for stimulation of cyclic GMP in cultured rat lung fibroblasts. While linsidomine (0.1 mM) caused a 3-fold platelet cGMP elevation, there was a weak (< or = 30%) but significant cGMP stimulation by organic nitroesters, which was tightly correlated with inhibition of platelet aggregation (r = 0.926, P = 0.008). Zaprinast (2 microM) potentiated, while methylene blue (1 microM) and oxyhemoglobin (10 microM) reversed the antiaggregatory effects. Linsidomine (0.5 microM-0.1 mM) dose-dependently released NO in a cell-free system. No spontaneous NO release was detected with organic nitroesters (0.1 mM). These data suggest that, to some extent, bioactivation of organic nitroesters occurs in platelets, resulting in platelet inhibition via the NO/cGMP system.

Nitric oxide but not prostacyclin is an autocrine endothelial mediator

Using porcine aortic endothelial cells, the present study investigates whether stimulation of prostacyclin (PGI2) and nitric oxide also causes elevation of the respective second messengers cAMP and cGMP in the endothelial generator cells. The calcium ionophore A23187 at 0.3-3 microM increased endothelial cGMP levels up to 27-fold in an L-arginine-dependent manner as assessed through complete inhibition by NG-monomethyl-L-arginine (100 microM). The 36-fold PGI2 stimulation by 3 microM A23187 was not accompanied by an intracellular increase in cAMP or an enhanced cAMP efflux. Correspondingly, the PGI2 mimetic iloprost (10 pM-100 microM) did not change endothelial cAMP levels. However, forskolin (1-100 microM) and prostaglandin E2 (PGE2) (0.1-10 microM) produced concentration-dependent increases in cAMP with a 9-fold and 8-fold stimulation at 100 microM forskolin and 10 microM PGE2, respectively. These results demonstrate that in contrast to NO, PGI2 acts as a strictly paracrine hormone without affecting the respective second messenger cAMP in the endothelial generator cells.

Prostacyclin rather than nitric oxide lowers human umbilical artery tone in vitro

This study was designed to determine vasodilator activities of two endothelium-derived relaxing factors: prostacyclin (PGI2) and nitric oxide (NO) in human umbilical arteries. Isolated vessel segments were contracted by submaximal concentrations of serotonin and bradykinin. These contractions were enhanced after inhibition of prostaglandin formation by the cyclooxygenase inhibitor indomethacin and after removal of the endothelium, both resulting in a pronounced decrease in PGI2 formation. Contractions remained unchanged after treatment of the vessels with nitro-L-arginine, a selective inhibitor of endogenous NO biosynthesis. The efficacy of inhibition of NO biosynthesis was established by a more than 60% reduction in cyclic GMP accumulation. Even inhibition of stimulated NO formation by histamine did not change vascular tone. These data suggest that PGI2 rather than NO is an endothelium-derived relaxing factor in human umbilical arteries.

Incomplete Inhibition of Platelet Secretion by Low-dose Aspirin.

This study determines the effects of low-dose oral acetylsalicylic acid (Aspirin) on platelet aggregation and dense granule secretion in relation to inhibition of thromboxane formation. In addition, possible modifications of inhibition of platelet function by iloprost and linsidomine (SIN-1) were also studied. Aspirin (40 mg/day) was administered to 15 healthy male non-smoking volunteers for 8 consecutive days. Platelet function was measured ex vivo before and 12 h after the last drug intake. At the same times urinary excretion of thromboxane B(2) (TXB(2)) and 6-oxo-PGF(1α) were determined. Asirin treatment resulted in a significant (P < 0.01), by 65%, reduction of urinary TX B(2) excretion, whereas urinary excretion of 6-oxo-PGF(1α) remained unchanged, demonstrating the expected selectivity of low-dose aspirin for the platelet cyclooxygenase. There was also a nearly complete (93-95%) inhibition of collagen (0.3-5 μg/ml)-induced thromboxane formation in platelets. However, collagen-induced ATP-and 5-HT secretion was much less inhibited by aspirin and was reduced by only 20% at 5 μg/ml collagen. Similar results were obtained with ADP while thrombin (> 1.2 IU/ml)-induced 5-HT secretion was not antagonized at all. Also aspirin did not affect the inhibition of platelet function by iloprost and linsidomine (SIN-1). It is concluded that stimulation of platelet secretion by stronger stimuli, such as collagen or thrombin, is largely cyclooxygenase-independent and may not be detected by measuring serum thromboxane levels. This may be relevant for clinical situations with shear-stress-induced platelet activation, for example at carotis stenoses or other types of atherosclerotic plaques.

Oral Naftidrofuryl Prevents Platelet Hyperreactivity Ex Vivo and Inhibits Functional Desensitization to Prostacyclin in Hypercholesterolemic Rabbits

Among other mediators, platelet-derived serotonin (5-HT) may contribute to thromboembolic complications of atherosclerosis. We determined whether long-term oral treatment with the 5-HT2 antagonist naftidrofuryl (NAF, 50 mg/kg daily for 12 weeks) alters platelet function in cholesterol-fed (1%) rabbits. Hypercholesterolemia resulted in marked platelet hyperreactivity to collagen and ADP. This included increased aggregation, ATP secretion, and thromboxane formation; e.g., collagen-induced (1.2 μg/ml) platelet aggregation was stimulated to 210 ± 10 mm/30 s in cholesterol-fed rabbits as compared with 108 ± 9 mm/30 s in rabbits fed a standard diet (p < 0.05). Inhibition of ADP-stimulated platelet activation by the prostacyclin mimetic iloprost was significantly reduced. NAF did not reduce plasma cholesterol in hypercholesterolemia, but prevented enhanced platelet aggregation, thromboxane formation, and ATP secretion. NAF treatment significantly reduced collagen-induced (1.2 μg/ml) aggregation to 81 ±20 mm/30 s in these animals (p < 0.05). NAF also inhibited functional desensitization of platelets to iloprost, but did not alter the impaired binding of [3H]iloprost to platelet membranes in hypercholesterolemic animals. NAF also did not change any of these parameters in normocholester-olemic rabbits. These data suggest beneficial effects of NAF on platelet hyperreactivity in experimental hypercholesterolemia which may also be relevant for its clinical use.

Excretion of prostacyclin and thromboxane metabolites before, during, and after pregnancy-induced hypertension

The aim of the present study was to assess whether changes in prostacyclin (PGI2) and thromboxane (TXA2) generation precede the manifestation of pregnancy-induced hypertension (PH). The metabolites 6-oxo-PGF1 alpha and TXB2 were measured in the urine of 69 randomly selected pregnant women from 16-20 weeks of gestation (wg) until delivery and more than 6 weeks postpartum. Between 16-20 and 21-24 wg 6-oxo-PGF1 alpha excretion did not change in patients who later developed PIH (n = 6) but increased significantly in the control group (n = 63). In contrast, a marked rise in TXB2 excretion was found in the PIH group but not in controls. Thereafter significant differences between both groups persisted from 25 wg until delivery. The 6-oxo-PGF1 alpha/TXB2 ratio was below the 10th percentile from 21-24 wg until delivery in patients with developing PIH. The excretion of both metabolites was substantially lower in the non-pregnant state without any difference between patient groups. These results show an altered urinary excretion of both 6-oxo-PGF1 alpha and TXB2 preceding the onset of the disease. A pathophysiological role of PGI2 deficiency and increased TXA2 formation in PIH appears substantiated.

Interactions between nitric oxide and prostacyclin in myocardial ischemia and endothelial cell cultures

This study investigates biochemical and functional interactions between NO and PGI2 that generate pathways in two different in vitro assays: porcine aortic endothelial cells (PAEC) and reperfused ischemic Langendorff hearts of rabbits. Using cGMP as an index of NO generation and 6-oxo-PGF1 alpha as an index for PGI2 production in endothelial cells, it is demonstrated that the two metabolic pathways for NO and prostacyclin formation act independent of each other. Moreover, NO appears to have an autocrine function in endothelial cells which does not exist with PGI2, probably because of a lack of PGI2 receptors. Endothelial damage in the course of myocardial ischemia is associated with a marked increase in mediator release whose inhibition has consequences for both myocardial and coronary function: inhibition of NO formation also inhibits PGI2 release and the recovery of coronary vessel tone with only minor if any effect on myocardial contractility. In contrast, inhibition of PGI2-generation results in marked deterioration of myocardial recovery with only minor changes in coronary perfusion. It is concluded from these data that PGI2 in endothelial injury is important for preservation of myocardial function while NO might mainly be involved in control of local vessel tone.

Catecholamine excretion as a function of personality

An experiment was designed to study the relationship between urine catecholamine excretion and the personality traits of ex traversion and neuroticism. Using chance samples of day-urine, we found a significant negative correlation between extroversion and the amount of urine catecholamines. This relationship is modified by smoking habits. The correlation holds only for nonsmokers.

Ist der niedrige Gefäßwandwiderstand im fetoplazentaren Kreislauf von der Prostazyklinbindung abhängig

In vorliegender Studie wurde der Einflus der basalen Prostazyklinbildung auf den Tonus menschlicher Nabelarterien untersucht. Die Hemmung der Prostazyklinbildung fuhrte zu gesteigerter Kontraktilitat der Arteriensegmente. Nach diesen Ergebnissen scheint Prostazyklin ein wirksamer endothelialer Vasodilatator umbilikaler Arterien zu sein.