Jamie Y. Jeremy

Effect of ethanol on vascular prostacyclin (prostaglandin I2) synthesis, platelet aggregation, and platelet thromboxane release

A series of experiments with platelets from healthy volunteers showed a concentration related inhibitory effect of ethanol on platelet aggregation and release of thromboxane A2. This effect was observed at blood alcohol concentrations ranging between 66 and 132 mg/dl (14.3 and 28.6 mmol/l), which are commonly found in alcoholics. Investigations carried out by incubating ethanol with platelet rich plasma in vitro also showed an inverse linear correlation between ethanol concentration and platelet thromboxane synthesis. In contrast, the incubation of a wide range of concentrations of ethanol with human endothelial cells and rat aortic rings did not alter the ability of these systems to synthesise prostacyclin (prostaglandin I2). This finding of a selective inhibition of thromboxane A2 synthesis and platelet aggregation without an alteration of prostaglandin I2 synthesis may provide an explanation for the reported ethanol mediated protection against vascular disease. This effect of ethanol may also be relevant to the induction of acute gastrointestinal haemorrhage that occurs after bouts of excessive alcohol consumption.

Simulating the Diabetic Environment Modifies In Vitro Prostacyclin Synthesis

To explain the contradictory data on the secretion of prostacyclin (PGI2) in clinical and experimental diabetes, we have investigated the effect of each of the major metabolic abnormalities in uncontrolled diabetes on vascular PGI2 synthesis. An increase in fatty acid concentrations caused a dose-dependent inhibition of PGI2 synthesis by rat aortic rings in vitro; linoleic and linolenic acids were consistently the most and palmitic the least inhibitory. Glucose had a stimulatory effect between 10 and 30 mmol/L, but a progressive fall in stimulation occurred at higher concentrations. Insulin was inhibitory at 10 and 50 mU/L; however, in combination with glucose (10 mmol/L) it was significantly stimulatory (at 10 mU/L) when the aortic rings were preincubated for 2 and 4 h. A pH of 7.0 or less was significantly inhibitory, whereas ketone bodies had no significant effect on PGI2 synthesis. These data show for the first time that altered metabolic factors in uncontrolled diabetes may have different effects on vascular PGI2 synthesis. These data help explain the variability of observations related to PGI2 synthesis and secretion in diabetes, and advocate a more detailed definition of the metabolic status of patients/animals used in such experiments.

Eicosanoid synthesis and release from primary cultures of rat central nervous system astrocytes and meningeal cells

Primary cultures of astrocytes and meningeal cells derived from neonatal rat brain synthesize and release thromboxane A2 and prostacyclin, respectively. Exogenously supplied arachidonic acid and the calcium ionophore, A23187, promote the release of eicosanoids; these effects are blocked by indomethacin and the calcium chelator, ethyleneglycoltetraacetic acid. The finding that astrocytes synthesize and release thromboxane A2 is discussed in the light of our recent findings of receptor-linked membrane phospholipid turnover in these cells.

Heparin-induced platelet aggregation in anorexia nervosa and in severe peripheral vascular disease

Abstract. We have previously demonstrated that platelets obtained from patients with anorexia nervosa or severe peripheral vascular disease are hyperaggregable. Since conventional heparins are known to activate platelets in vitro and occasionally induce thrombosis and consumptive thrombocytopenia in vivo, we have investigated the direct effect of a conventional heparin on platelets obtained from patients with anorexia nervosa or severe peripheral vascular disease. Heparin at therapeutic concentrations was found to induce platelet aggregation of such platelets in vitro. In contrast, a recently developed low molecular weight heparinoid (Org 10172), at therapeutic concentrations, had no effect on these hyperaggregable platelets. We conclude that: (i) heparin may be potentially harmful to patients with hyperaggregable platelets; (ii) thrombocytopenia and thrombosis associated with heparin therapy may be mediated through a direct effect of heparin on platelets; (iii) it is unlikely that heparin induced thrombocytopenia is always mediated by classical immunological mechanisms, especially in patients with hyperaggregable platelets; and (iv) low molecular weight heparinoids may be safer anticoagulants in patients with platelet hyperaggregability.

Reactive oxygen species, vascular disease and cardiovascular surgery.

Oxidant stress [ OS ] is a condition in which cells are exposed to excessive levels of either molecular oxygen or chemical derivatives of oxygen called reactive oxygen species [ROS], principal amongst which is superoxide [O2-]. It is becoming increasingly apparent that O2- is a key risk factor for cardiovascular disease [CVD], including atherogenesis, reperfusion injury, angina, restenosis following balloon angioplasty and vein graft failure. When one considers the multiplicity of effects of O2-, this is perhaps not surprising, as it promotes vascular smooth muscle cell proliferation, damages the endothelium, promotes lipid oxidation and activates blood cells. However, perhaps the key reaction of O2- is that with nitric oxide [NO] to form peroxynitrite [ONOO] resulting in a depletion of endogenous vascular NO. Reduced NO formation is also now firmly associated with the aetiology of CVD and as such NO donors may become a major class of drugs. Furthermore, risk factors for CVD, in particular diabetes mellitus [DM], dyslipidaemia, and hyperhomocysteinaemia are all associated with OS. As such, it is becoming increasingly apparent that novel antioxidant therapies, including the gene transfer of antioxidant enzymes, are potentially valuable in the treatment of CVD. In this review, the aetiology of OS and CVD is discussed with particular emphasis on NO. The interactions of risk factors and how this pathophysiology relates to the design of effective novel strategies to treat CVD is also considered. Particular emphasis is also placed on OS and cardiovascular surgery.

Adrenergic modulation of vascular prostacyclin (PGI2) secretion

An in vitro model for the study of adrenoreceptor-prostacyclin (PGI2) relationships in the rat aorta is described. PGI2 synthesis was stimulated by adrenergic agonists (rank order of potency: epinephrine greater than norepinephrine greater than phenylephrine greater than methoxamine). Isoproterenol, UK 14304, clonidine and salbutamol were without effect. Epinephrine (3 X 10(-7) M)-stimulated PGI2 synthesis was inhibited by adrenoreceptor antagonists (rank order of potency: yohimbine greater than prazosin greater than phentolamine greater than corynanthine much greater than propranolol). The absence of calcium in incubation media abolished epinephrine-stimulated PGI2 synthesis as did the calcium channel blocker, verapamil, in a dose-dependent manner. Calcium ionophore A23187 (10(-5) M)-stimulated PGI2 synthesis was inhibited by verapamil (in a dose-dependent manner), but not by prazosin, phentolamine or yohimbine. It is concluded that epinephrine-mediated rat aortic PGI2 synthesis is alpha-adrenoceptor- and not beta-adrenoceptor-mediated, calcium-dependent, and that the alpha-adrenoceptor antagonists evaluated do not have verapamil-like calcium channel blocking activities. These findings may be relevant to contraction-relaxation cycles of vascular tissue.

The rat urinaryprostacyclin as well as other

It has hitherto been assumed that urinary prostanoid excretion reflects renal and/or systemic prostanoid synthesis. Since the bladder forms an integral part of the urinary tract, we investigated whether this organ was capable of synthesising prostanoids. The rat urinary bladder was found to generate large amounts of 6-oxo-prostaglandin FjCC (the stable, spontaneous metabolite of prostacyclin) in vitro; it also produced smaller amounts of prostaglandin E2 and thromboxane B2 (the stable, spontaneous metabolite of thromboxane A2). Distension of the bladder and changes in pH and osmolarity of the incubate were found to markedly alter the production of these prostanoids. Urinary prostanoids may, therefore, reflect not merely renal and/or systemic prostanoid synthesis but also local synthesis and release by the bladder. The presence of these prostanoids in the bladder suggests that they may play a local role in cytoprotection and the regulation of bladder tone.

Assessment of platelet function in patients with Raynaud's syndrome.

Platelet function was studied in 11 patients with Raynauds syndrome and 11 healthy controls. Platelets obtained from patients with Raynauds syndrome were significantly more responsive to adrenaline, produced more thromboxane A2, and were resistant to prostaglandin inhibitors (prostacyclin and prostaglandin E1) of platelet aggregation. Platelets from control subjects and patients with Raynauds syndrome were more resistant to prostaglandin inhibitors when reactions were carried out at 27 degrees C rather than at 37 degrees C. Patients with Raynauds syndrome also had significantly increased plasma concentrations of beta-thromboglobulin, fibrinogen, and circulating platelet aggregates. In an attempt to elicit local platelet responses, the forearms of control subjects and patients with Raynauds syndrome were cooled in water tanks and platelet function tests performed before and after cooling. No significant difference in the results was observed. The potential role of platelets in the pathogenesis of Raynauds syndrome is discussed.

The effect of sildenafil on corpus cavernosal smooth muscle relaxation and cyclic GMP formation in the diabetic rabbit

Sildenafil, a type V phosphodiesterase inhibitor, enhances smooth muscle relaxation in normal human and rabbit corpus cavernosum. We investigated the in vitro effects of sildenafil on non-adrenergic, non-cholinergic and nitric oxide (NO)-mediated cavernosal smooth muscle relaxation in diabetic rabbits, since alterations in this pathway are recognised in diabetic erectile dysfunction. Diabetes mellitus was induced in male New Zealand White rabbits with alloxan. Cavernosal strips from age-matched control, 3- and 6-month diabetic animals were mounted in organ baths. Relaxation responses to electrical field stimulation (1-20 Hz) or sodium nitroprusside (10(-8)-10(-4) M) were assessed in the absence and presence of sildenafil (10(-8) and 10(-7) M). The effect of sildenafil on cGMP formation by the corpus cavernosum was also assessed following stimulation with sodium nitroprusside, A23187 and acetylcholine. Sodium nitroprusside-stimulated relaxations were significantly (P<0.03) impaired in the corpus cavernosum from both diabetic groups, (IC(50)=4.6 x 10(-6) M following 3 months of diabetes mellitus and 4.0 x 10(-6) M following 6 months of diabetes mellitus; compared to 7.5 x 10(-7) M for pooled age-matched controls). Sildenafil (10(-7) M) significantly enhanced sodium nitroprusside-stimulated relaxation in control (P<0.05) and diabetic groups (P<0.03). Electrical field stimulation-mediated relaxations of the corpus cavernosum were significantly impaired after 6-month diabetes mellitus and enhanced by sildenafil (10(-8) M). cGMP formation by the diabetic corpus cavernosum was impaired significantly, but restored towards normal by sildenafil. We suggest that the impairment of NO-mediated relaxation of the corpus cavernosum reflect, at least in part, a defect in guanylyl cyclase activity. These findings support the use of sildenafil as an effective, orally administered, treatment for diabetic erectile dysfunction.

Fibrinogen mediated activation of platelet aggregation and thromboxane A2 release: pathological implications in vascular disease.

The effect of a human fibrinogen preparation on in vitro platelet aggregation was assessed. Platelets were obtained from healthy volunteers. Human fibrinogen induced platelet aggregation in 65% of platelet rich plasma samples and enhanced submaximal platelet aggregation induced by heparin or by several conventional agonists in all samples. Aggregation induced by fibrinogen alone was reversed by the in vitro addition of human albumin. Fibrinogen induced aggregation was associated with the release of the vasoconstrictor, thromboxane A2. Preincubation with indomethacin inhibited both the aggregation and the release of thromboxane A2. Fibrinogen had no effect on in vitro vascular prostaglandin I2 synthesis (rat aortic rings) during a 60 minute incubation. The observed effects of fibrinogen on platelet function may be relevant to clinical conditions in which hyperaggregability of platelets is associated with hyperfibrinogenemia and thrombosis.