Increased platelet adhesiveness in hypertensive cardiovascular disease

Aberrant adenylyl cyclase/cAMP signal transduction and G protein levels in platelets from hypertensive patients improve with antihypertensive drug therapy

We have previously demonstrated a decreased expression of Gi alpha 2 protein in platelets from spontaneously hypertensive rats that was associated with an altered responsiveness of adenylyl cyclase to hormone stimulation and inhibition. In the present studies, we have used platelets from hypertensive patients and examined the hormonal regulation of adenylyl cyclase as well as the levels of G proteins and their modulation by antihypertensive drug therapy. We performed these studies in platelets from four groups of subjects: normotensive subjects (group 1), untreated mildly essential hypertensive patients (group 2), and treated moderately to severely hypertensive patients whose blood pressure was uncontrolled (group 3) or controlled with drug treatment (group 4). GTP gamma S, 5'-(N-ethylcarboxamido)adenosine (NECA), and prostaglandin E1 stimulated adenylyl cyclase activity to a greater extent in hypertensive patients (group 2). This effect was partially corrected (by approximately 50% to 80%) in the patients under antihypertensive drug therapy (groups 3 and 4). In addition, inhibition of adenylyl cyclase mediated by a ring-deleted analogue of atrial natriuretic factor (C-ANF4.23) observed in control normotensive subjects was blunted in hypertensive patients (group 2) and was not corrected in treated patients. Gi alpha levels determined by immunoblotting were in the same range for the four groups, whereas Gi alpha 2 and Gi alpha 3 levels were decreased by 70% and 60%, respectively, in hypertensive patients (group 2) compared with normotensive subjects. Antihypertensive drug therapy (groups 3 and 4) partially restored Gi alpha 2 levels toward normal (group 1) by about 60% and 70%, respectively; however, the reduced Gi alpha 3 levels in group 2 hypertensive patients were not improved in group 3 but were raised toward normal levels in group 4 by about 55%. These results suggest that the altered responsiveness of platelet adenylyl cyclase to hormones in hypertension and the normalization of the response with antihypertensive drug therapy could partly be due to the ability of the latter to modulate Gi alpha protein expression. These effects on platelet function may underlie the beneficial effects of antihypertensive agents on some of the complications of hypertension.

Platelets from spontaneously hypertensive rats exhibit decreased expression of inhibitory guanine nucleotide regulatory protein. Relation with adenylyl cyclase activity

We have recently demonstrated an enhanced expression of inhibitory guanine nucleotide regulatory protein (Gi) in the heart and aorta from spontaneously hypertensive rats (SHR) as compared with control Wistar-Kyoto (WKY) rats; this enhanced Gi expression was associated with an increased inhibition of adenylyl cyclase by inhibitory hormones and decreased stimulation of adenylyl cyclase by stimulatory hormones. In the present studies, we have determined the levels of stimulatory and inhibitory guanine nucleotide regulatory proteins (Gs and Gi, respectively) in platelets from SHR by cholera toxin- and pertussis toxin-catalyzed ADP-ribosylations, respectively, as well as by immunoblotting techniques using specific antibodies for Gs and Gi. Cholera toxin catalyzed the ADP-ribosylation of a single protein of M(r) 45,000 in rat platelets from SHR and WKY rats, and the labeling of this band was not altered in SHR as compared with WKY rats. Pertussis toxin, on the other hand, catalyzed the ADP-ribosylation of a single protein band of M(r) 41,000 in platelets from SHR and WKY rats, and unlike the response in heart and aorta, the labeling of this band was significantly decreased in SHR as compared with WKY rats. Furthermore, immunoblotting experiments using AS/7 antibody, which is specific for Gi alpha-1 and Gi alpha-2, showed a decrease in Gi alpha-2 in platelets from SHR as compared with WKY rats. In addition, the inhibitory effects of angiotensin II and atrial natriuretic factor on adenylyl cyclase and cAMP levels were completely abolished in SHR platelets, whereas the stimulatory effects of GTP, N-ethylcarboxamide adenosine, prostaglandin E1, and forskolin on adenylyl cyclase and cAMP levels were enhanced.(ABSTRACT TRUNCATED AT 250 WORDS)

Platelet ultrastructural morphology and its relevance in essential hypertension

A preliminary study of platelet ultrastructural morphology by transmission electron microscopy in patients with established essential hypertension was carried out. Platelets of hypertensive patients were found to be in various stages of 'platelet activation'. According to the degree of transformation observed, platelets were classified into six forms: discoid, pseudotubular, membranous, saccular, pseudopodical and hyaline and correlated to the severity of hypertension. The pseudopodical and hyaline types platelets were particularly observed in severely hypertensive patients, and/or those with clinical evidence of target organ damage. Platelet ultrastructural changes may form the basis of a better understanding of the aetiology and pathogenesis of essential hypertension, and may serve as a marker in assessing the extent of underlying in vivo endothelial injury and tendency to thrombotic complications.

Serotonin-induced platelet aggregation predicts the antihypertensive response to serotonin receptor antagonists

The 5-HT2-receptor antagonist ketanserin (20-40 mg b.i.d.) was administered to 62 patients of both sexes with uncomplicated primary hypertension. After 4 weeks of treatment about 50% of the patients had reached the target diastolic blood pressure of 90 mm Hg or below. Interindividual variability was large. In a retrospective analysis the variability could not be explained by sex or the dose of ketanserin. There was a weak association between age and systolic blood pressure response (r = 0.24; P = 0.06), which could be entirely accounted for by the higher base line blood pressure in the elderly patients. In one group of patients (n = 12), the ex vivo aggregation to serotonin (10(-6) M) was studied during treatment with placebo and ketanserin. Ketanserin completely inhibited 5-HT-induced aggregation in all patients. There was a close correlation between the area under the 5-HT-induced platelet aggregation curve during placebo and the subsequent reduction in diastolic blood pressure after 4 weeks of treatment with ketanserin. The present data suggest that the blood pressure response to ketanserin can be predicted from the ex vivo sensitivity of platelets to serotonin. By implication, they also support a role for serotonergic mechanisms in hypertension.

Enhanced thromboxane formation by blood but not whole platelets from spontaneously hypertensive rats

Increased serum levels of immunoreactive thromboxane B2 (iTXB2) were observed in spontaneously hypertensive rats of the Okamoto-Aoki strain (SHR) compared with normotensive Wistar-Kyoto rats (WKY). Serum iTXB2 levels in whole blood allowed to clot at 37 degrees C for 1 hour were significantly greater in SHR than WKY at 8, 16-20, and 38 weeks of age, whereas formation of iTXB2 by thrombin-stimulated whole platelets from 6 16-week-old SHR and 6 age-matched WKY was 399 +/- 44 and 377 +/- 38 ng/10(9) platelets/30 min, respectively. No significant difference in radioconversion of exogenous arachidonic acid to TXB2 was observed in whole platelets from SHR (18.2 +/- 2.5%, n = 4) and WKY (20.1 +/- 3.0%, n = 4) at 16 weeks of age. These results support the proposal that enhanced ability of blood from SHR to generate iTXB2 is independent of the stage of hypertension development. This enhancement probably depended on factors or blood elements other than platelets since no difference in formation was observed on stimulation of whole platelets.

Relationship of platelet specific proteins and other factors to atherosclerosis in various stages of hypertension

There is considerable evidence from previous studies that platelets play an important role in the development and progression of atherosclerosis in hypertension, more so in relation to the stage of hypertension. Seventy one hypertensive patients (WHO stage I: 39, stage II: 23, stage III: 9) aged 19-84 (mean age: 56, 59 and 62 respectively for each stage) and 37 normal controls (aged 22-72 with a mean age of 52) were involved in this study. Hematocrit, beta-thromboglobulin (beta-TG), platelet factor 4 (PF4), beta-TG/PF4 ratio, total cholesterol (TC), low density lipoprotein-C, and triglycerides were higher in the hypertensive group while platelet count, circulating platelet aggregates, and high density lipoprotein-C were higher in the normotensive group. Among the hypertensives, stage III patients showed the highest beta-TG, PF4, beta-TG/PF4 ratio, triglycerides, and stage I with the least elevation. There were no significant differences noted in the ADP or epinephrine-induced platelet aggregation in both the normal and hypertensive patients. Other parameters such as heart rate, serum sodium, potassium, renal and liver function tests, plasma renin activity, aldosterone, fibrinogen thromboxane B2 and 6-Keto-PGF1 alpha, showed no significant differences in both groups. This study clearly showed that beta-TG/PF4 ratio and triglycerides are closely related to the stage of hypertension and are good indicators of in vivo platelet activation in hypertensives which may account for the acceleration of hypertensive vascular complications secondary to atherogenesis.

Receptor-dependent and -independent protein phosphorylation in platelets of spontaneously hypertensive rats

This study investigates the role of protein kinase C and of myosin light chain kinase in mediating platelet hyperresponsiveness in spontaneously hypertensive rats (SHR). For this purpose, 32P-labeled washed platelets of both SHR and normotensive controls Wistar-Kyoto (WKY) were challenged either with a receptor-mediated agonist (thrombin) or with direct activators of myosin light chain kinase and protein kinase C. Such enzymatic activities were assessed by measuring changes in 32P-labeling of their respective target proteins, namely myosin light chain (20 KDa) and the 47 KDa protein. In resting platelets, the patterns of protein phosphorylation were similar between SHR and WKY, suggesting that the two cell types were in a comparable quiescent status. By contrast, in both dose-response and time-course studies, thrombin promoted a significantly greater phosphorylation of the 20- and 47 KDa proteins in platelets of SHR compared with that for WKY. Sensitivity of myosin light chain kinase to the calcium ionophore A23187 and of protein kinase C to both phorbol ester and dioctanoylglycerol was apparently not different between the two cell types. The data indicate that the exaggerated thrombin-induced protein phosphorylation observed for platelets of SHR is not linked to alterations in protein kinase C and/or myosin light chain kinase per se. These results therefore suggest that platelet hyperresponsiveness in SHR is likely to be related, at least in part, to abnormalities in receptor-mediated transmembrane signalling.

Platelet calcium and quenched-flow aggregation kinetics in essential hypertension

Abnormal platelet function may contribute to the complications of essential hypertension. We have studied the kinetics of platelet aggregation induced by adenosine diphosphate (ADP) or epinephrine, plasma beta-thromboglobulin, and basal, cytosolic, and free calcium, as correlates of platelet function. Fifteen untreated patients with essential hypertension and without detectable atherosclerosis, 18-40 years old, were compared with 30 matched normotensive control subjects. Maximal rates of platelet aggregation (Vmax) with ADP and epinephrine were significantly higher in patients than in control subjects (p less than 0.03), as assessed by quenched-flow aggregometry. However, significance was lost when Vmax was corrected for the platelet count. Paradoxically, the activation constants (Ka) for ADP were higher in patients than in control subjects (p less than 0.03). With ADP as the inducing agent, onset time (t) or lag period before aggregation begins was longer in patients than in control subjects (p less than 0.02). beta-thromboglobulin levels, an index of in vivo platelet activation, were not significantly different between the two groups (p = 0.13). The mean platelet cytosolic free calcium concentration was higher in patients (213 +/- 19 nM) than in control subjects (172 +/- 14 nM), but this difference was not statistically significant (p = 0.07). However, there was a close correlation between the free calcium level and systolic, diastolic, and mean blood pressure (p less than 0.003, p less than 0.04, p less than 0.004, respectively). No difference in platelet volume between the two groups was found. Our data suggest that platelets in the early stages of essential hypertension display an overall increased aggregation potential but a diminished sensitivity to ADP.(ABSTRACT TRUNCATED AT 250 WORDS)

Megakaryocytopoiesis in spontaneously hypertensive rats (SHR)

Bone marrow megakaryocytes and their progenitors were studied in SHR in order to obtain more information about megakaryocytopoiesis in hypertension since it is known that various anomalies of platelet function occur in hypertension. Megakaryocytopoiesis under steady state conditions and following stimulated erythropoiesis and thrombocytopenia was not found to be significantly different in SHR from that in normotensive Wistar controls.

Profile of an ideal antianginal agent

Many antianginal agents are available for the treatment of coronary artery disease. These agents act by influencing the determinants of myocardial oxygen supply and demand. The 3 main classes of agents are the nitrates, beta-adrenergic blocking agents and the calcium entry blockers. Agents from all 3 classes have shown efficacy in treating both symptomatic and asymptomatic myocardial ischaemia. However, some patients cannot be treated with these agents because of side effects or contraindications. An ideal antianginal drug should effectively treat both angina and silent ischaemia. Additionally, it should be free of side effects, allow for maintenance of physical performance and be metabolically neutral. New agents are being developed which strive for this goal.

Study of in vivo platelet activation in uncomplicated essential hypertension

Beta-thromboglobulin (BTG) and platelet factor 4 (PF4), platelet alpha-granule specific proteins, and serotonin (5-HT) which is stored in dense granules are released when platelets are activated. To investigate in vivo platelet activation in uncomplicated essential hypertension, platelet 5-HT and PF4 contents, plasma BTG and PF4 concentrations, as well as urinary BTG levels were assessed in normotensive and hypertensive subjects. Plasma BTG and PF4 concentrations and urinary BTG levels were comparable in both groups. Mean platelet 5-HT content was significantly decreased in hypertensive subjects without modification of the intraplatelet PF4 content. These data suggest first of all that the decrease in platelet 5-HT content is due mainly to the inhibition of platelet 5-HT uptake previously described, and second of all that no significant in vivo platelet activation occurs in essential hypertensive subjects devoid of cardiovascular complications.

Sodium and noradrenaline effluxes from platelets in male relatives to hypertensive individuals

The efflux rate constants for noradrenaline efflux and 22Na-efflux from platelets, the number of ouabain binding sites on platelets and the influence of plasma on ouabain binding to platelets were determined in 35 normotensive men belonging to families with a high incidence of essential hypertension and 31 men from families, where no hypertension was registered in the close relatives. The earlier finding of a higher efflux rate constant for noradrenaline in relatives was confirmed. There was a significant negative correlation between the total 22Na-efflux rate constant and the noradrenaline rate constant in relatives as well as between the number of ouabain binding sites and the noradrenaline rate constant. No such correlations were registered in the controls. The ouabain resistant 22Na-efflux rate constant was lower in relatives, but the ouabain sensitive 22Na-efflux rate constant did not differ between the groups, nor did the total number of ouabain binding sites on platelets. Number of ouabain-binding sites measured at a low concentration of (3H)-ouabain in the presence of deproteinized plasma samples was the same in both groups, contradicting the presence of an endogenous ouabain-like plasma factor in the relatives.

Blood platelets in human essential hypertension

Blood platelets of patients with essential hypertension display signs of both increased sensitivity in vitro to aggregating stimuli believed to contribute to thrombosis and of activation in vivo possibly expressing the release of vasoactive products. The mean features of the modified platelet profile in hypertension include an increased alpha 2-adrenergic receptor density, an enhanced rate of adhesion/aggregation in particular in response to ADP and arachidonic acid, a greater sensitivity for thrombin and adrenaline to stimulate increases in cytoplasmic-free Ca2+, increased resting levels of cytoplasmatic-free Ca2+, a reduced content of serotonin often combined with a defective uptake mechanism, a facilitated efflux rate of noradrenaline, an exaggerated release reaction in vivo as indicated by the increased plasma levels of Beta-thromboglobulin and a shortened platelet life span. These changes occur to various extents in some, but not all, hypertensive patients and are not always strictly related to the degree of blood pressure increase. On the contrary, platelet cyclooxygenase and thromboxane synthetase activity are in the normal range.

Platelet-vessel wall interactions: implication of 5-hydroxytryptamine. A review

The evidence for an impact of platelet-derived 5-hydroxytryptamine (5-HT) on local tissue perfusion is reviewed. By interacting with 5-HT2 serotonergic receptors, 5-HT, directly or through amplification, activates platelets, endothelial and vascular smooth muscle cells producing platelet aggregation, vascular permeability increase and large vessel constriction. Pharmacodissection in experimental animals with selective serotonergic 5-HT2 receptor antagonists, e.g. ketanserin, shows that 5-HT largely contributes to the platelet-mediated increase in vascular permeability, to platelet-vessel wall interaction during hemostasis, to cardiopulmonary dysfunction provoked by thromboembolism and to the platelet-mediated inhibition of peripheral collateral circulation. Clinical results obtained with ketanserin further substantiate an involvement of platelet-derived 5-HT in the pathogenesis of impaired tissue perfusion in some cardiovascular conditions.

Effect of propranolol, practolol and atenolol on human platelet thromboxane formation and plasma levels of prostaglandins 6-keto-F1 alpha and E2

The effects of three different beta-adrenergic blocking drugs, propranolol, practolol and atenolol on platelet thromboxane production and the release of prostacyclin and prostaglandin E2 into the circulation were investigated in healthy volunteers. The beta-adrenergic antagonists were administered intravenously at equipotent doses. The serum TxB2 levels after whole blood clotting and the arterial and venous plasma concentrations of 6-keto-PGF1 alpha and PGE2 were measured before and during a 60 min period after the administration of the drugs, using radioimmunoassay. Practolol and atenolol elicited a significant decrease in platelet thromboxane formation but remained without effect on plasma 6-keto-PGF1 alpha and PGE2 levels. In contrast, propranolol did not influence serum TxB2 concentrations but induced a significant increase in plasma content of 6-keto-PGF1 alpha and PGE2. The results indicate that beta-adrenergic antagonists alter the balance between the proaggregatory, vasoconstricting and antiaggregatory, vasodilating prostanoids in the human cardiovascular system. Although the direction of the action of these drugs seems to differ depending on the selectivity of the beta-adrenoceptor blocking properties the net effect of this action should be beneficial.

Aggregation, membrane potential, and transport in platelets of spontaneously hypertensive rats

Aggregation, plasma membrane potential, and calcium uptake by platelets of spontaneously hypertensive rats (SHR) were studied. The sensitivity of SHR platelets to the action of ADP and collagen was increased. The addition of Ca-ionophore (A-23187) resulted in aggregation of SHR platelets at a Ca2+ concentration in the incubation medium 20%-30% lower than that required for aggregation of platelets from normotensive rats. The plasma membrane of SHR platelets was found to be partially depolarized (by 8-10 mV) as compared to that of normotensive controls. This difference was apparently due to a higher membrane permeability for Na+. It is suggested that the increase in 45Ca uptake by SHR platelets is related to an increased rate of Ca2+ diffusion along a potential-dependent channel, due to partial depolarization of plasma membrane.

Calcium-dependent proteolytic stimulation of adenylate cyclase in platelets from spontaneously hypertensive rats

Abnormalities of platelet aggregation and cyclic nucleotide metabolism are present in hypertension. We observed a greater increase in the level of cyclic adenosine monophosphate (AMP) after prostaglandin E1 (PGE1) stimulation and a lack of decrease of this cyclic nucleotide by epinephrine in platelets from spontaneously hypertensive rats (SHR) as compared to normotensive rats. The difference in cyclic AMP production between SHR and control rats in response to PGE1 is dependent upon platelet exposure to calcium. Since calcium and cyclic AMP are closely related and are both abnormally regulated in hypertension, we have studied the effect of calcium on adenylate cyclase activity. We show here that two forms of endogenous calcium-dependent proteases (membrane-bound and soluble) stimulate the basal activity and the hormonal responsiveness of adenylate cyclase. The sensitivity of calcium-dependent proteolytic control of adenylate cyclase to very-low concentrations of calcium indicates that the regulation may be physiologically important. Furthermore, calcium exerts a greater influence on platelet adenylate cyclase from SHR than on that from normotensive rats. The adenylate cyclase defect seems to be located in the membrane fraction and may, therefore, result from an increase in the activity of the membrane-bound calcium-protease or may be intrinsic to adenylate cyclase itself. The exact site that is sensitive to proteolysis remains to be established.

Comparative effects of nitroglycerin and nitroprusside on prostacyclin generation in adult human vessel wall

The precise mechanism of vasodilatory actions of nitroso-compounds is not clear. It has been suggested that these drugs might modulate release of the vasodilator, prostacyclin, from cultured endothelial cells and bovine arteries or potentiate actions of prostacyclin. This study was designed to examine the effects of nitroglycerin and nitroprusside on prostacyclin release from adult human vasculature. Saphenous vein ring preparations were incubated with nitroglycerin or nitroprusside and arachidonic acid, the substrate for prostacyclin. Vascular rings incubated with nitroglycerin released significantly more prostacyclin (measured as 6-keto-prostaglandin F1 alpha, a stable hydrolysis product of prostacyclin by radioimmunoassay) compared with the control vascular rings (p less than 0.02). This increase was observed at the therapeutic concentrations of nitroglycerin (5 to 10 ng/ml). However, incubation of saphenous vein rings with nitroprusside in concentrations as high as 1 microgram/ml was not associated with any increase in prostacyclin release. Prior incubation of vascular rings with the cyclooxygenase blocker, indomethacin, inhibited nitroglycerin-induced prostacyclin release. Incubation of vascular rings with the selective thromboxane A2 blocker, OKY 1581, resulted in additional prostacyclin release with nitroglycerin treatment, presumably by inhibiting vessel wall-generated thromboxane A2. Nitroprusside had no significant effect on prostacyclin release from indomethacin-treated or OKY 1581-treated vascular rings. This study suggests significant stimulatory effects of nitroglycerin, but not of nitroprusside, on prostacyclin release from human saphenous vein. Nitroglycerin-induced prostacyclin release may be an important mechanism of its antiischemic actions in human subjects.

Arachidonic acid metabolites, hypertension and arteriosclerosis

The level of arterial blood pressure is set by complete interactions of several mechanisms which influence both blood flow in and resistance of the vascular system. An imbalance favouring elevation of vascular resistance or extracellular volume will result in hypertension. Such alterations may include increased activity of the sympathetic nervous system, of the renin-angiotensin system, or excessive secretion of mineralocorticoids. Of equal importance may be a reduced activity of blood pressure-lowering factors such as prostaglandins and the kallikrein-kinin system. This paper describes the possible significance of prostaglandins in the pathophysiology of hypertension and in degenerative vascular disease, based on their involvement in the control of vascular resistance, renal regulation of extracellular volume and platelet-vessel wall interactions. An abnormality in the biosyn-thesis of certain prostaglandin endoperoxide metabolites may lead to hypertension even without an increase in the activity of the classic blood-pressure-elevating systems. The contribution of prostaglandins for the development of hypertension and degenerative vascular disease may be based on an inherent abnormality of the prostaglandin system, as well as on the effects of major risk factors such as dietary intake of sodium and fat on prostaglandin synthesis. Specific blockade or stimulation of distinct biosynthetic pathways leading to antagonistically acting prostaglandins and nutritional manipulation of precursor fatty acids should lead to a better understanding of the pathomechanisms involved and may offer new strategies for therapy or prevention of these cardiovascular disorders.

Role of blood platelets and prostaglandins in coronary artery disease

In the last decade, several studies evaluating blood platelet function in patients with coronary heart disease have been reported. Although several platelet function abnormalities such as enhanced platelet aggregation, decreased platelet survival and increase in platelet release reaction in the stable condition and during stress in patients with myocardial ischemia have been recognized, the mechanism of these abnormalities is just beginning to be understood. Discovery of certain platelet and endothelium-generated prostaglandins has provided some information as to the possible mechanism of platelet dysfunction. Abnormalities of prostaglandin production and platelet sensitivity to various prostaglandins may have an important bearing on the enhanced platelet aggregation in vivo, genesis of atherosclerosis and probably precipitation of acute ischemic events. Since the discovery of these prostaglandins, the precise mode of action of several commonly used platelet-active drugs has been clarified. Development of new drugs acting at selective steps in the prostaglandin pathways may provide some exciting novel therapeutic procedures in patients with coronary heart disease.

Effects of very low versus standard dose acetyl salicylic acid, dipyridamole and sulfinpyrazone on platelet function and thromboxane formation in man

The effects of three conventional antiplatelet regimes, dipyridamole 3 X 75 mg/day (D), sulfinpyrazone 4 X 200 mg/day (S), acetylsalicylic acid 3 X 330 mg/day combined with dipyridamole 3 X 75 mg/day (ASA + D), and very low dose acetylsalicylic acid 100 mg/day (ASA) on platelet function were studied in man following 4 days of treatment. D and S slightly increased mean minimal ADP concentration for irreversible aggregation (n.s.), S reduced aggregation and thromboxane (TXB2) formation on low dose collagen (2P less than or equal to 0.05), ASA and ASA + D increased platelet count (2P less than or equal to 0.05), increased bleeding time (n.s. for ASA, 2P less than or equal to 0.05 for ASA + D), abolished irreversible aggregation on ADP, suppressed TXB2 formation on all (2P less than or equal to 0.001) and aggregation on lower concentrations of collagen (2P less than or equal to 0.01) and abolished aggregation and TXB2 formation on arachidonic acid (2P less than or equal to 0.001). Very low dose ASA suppresses platelet aggregation and TXB2 formation on several stimuli of possible physiologic significance. In the light of a recently proposed critical balance of vascular antiaggregatory prostacyclin and platelet proaggregatory TXA2 very low dose ASA might offer advantages over conventional dosage of ASA and should be evaluated in thromboembolic disorders.

Platelet function in hypertension and effect of therapy

Platelet function was evaluated in 10 hypertensive and 11 normal subjects. In the placebo phase, the plasma beta thromboglobulin level (an index of platelet activation in vivo) was significantly (p less than 0.01) higher in the hypertensive than in the normal subjects; other tests of platelet function gave similar results in the two groups. After control of blood pressure with lofexidine (a centrally acting imidazoline derivative), plasma beta thromboglobulin levels decreased in 9 of the 10 hypertensive patients, but increased in one who showed no change in blood pressure. These studies suggest that enhanced platelet activation in primary hypertension may be associated with increased vascular resistance.

Platelet aggregation measured by the screen filtration pressure method in hypertensive patients

In this study platelet aggregation was measured by the screen filtration pressure method in 2 groups of hypertensives separated according to age. They were perfectly matched for levels of various "risk factors", with normotensive "controls". No significant difference in platelet aggregate filtration pressure (PAFP) was found between hypertensives and normotensives. Contrary to normotensive subjects, there was no significant increase in the PAFP values with age in the hypertensives. This might be due to the significantly higher blood urea values in the hypertensives as compared to the normotensives. An inverse relationship between blood urea concentration and PAFP values was in fact confirmed by in vivo and in vitro studies.

Hypertension and propranolol therapy: effect on blood pressure, plasma catecholamines and platelet aggregation

The effects of propranolol on blood pressure, plasma catecholamine concentration and platelet aggregation were examined in 16 patients with uncomplicated primary hypertension. The patients were studied at rest, during isometric handrip stress and 48 hours after sudden discontinuation of propranolol therapy. Plasma catecholamine concentration and platelet aggregation studies were also carried out in 11 age-matched normotensive and healthy subjects at rest. Plasma catecholamine concentration and platelet aggregation were greater in the hypertensive than in the normotensive subjects, but the difference reached statistical significance for aggregation only. Exercise significantly increased catecholamines and platelet aggregability. The administration of propranolol (240 mg/day) produced a significant decrease in systolic and diastolic blood pressue and in aggregation (the percent of light transmission at 1 microM adenosine diphosphate, at rest) and a significant increase in catecholamine concentration. However, propranolol did not prevent the changes in all these variables with exercise. The abrupt discontinuation of propranolol was not associated with any subjective or objective untoward cardiovascular effect or abnormal changes in plasma catecholamines. However, in some patients the platelet aggregation studies demonstrated a hyperaggregable state, which may be due to a state of supersensitivity of platelets to circulating catecholamines.

Platelet function studies in heart disease. VI. Enhanced platelet aggregate formation activity in congestive heart failure: inhibition by sodium nitroprusside

We studied 11 patients with congestive heart failure and 10 normal volunteers for in vivo platelet aggregate formation activity. The patients with heart failure had significantly (p less than 0.01) more circulating platelet aggregates than the normal volunteers. During sodium nitroprusside infusion, the number of circulating platelet aggregates declined to normal levels and in vitro platelet aggregation responses to epinephrine and adenosine diphosphate were also suppressed significantly (p less than 0.01). This was associated with a 30% decline in systemic vascular resistance and a 28% increase in cardiac output. In other in vitro experiments, sodium nitroprusside was found to have direct, dose-related platelet aggregation inhibitory actions. This study suggests that an increase in vascular resistance in certain heart failure patients may in part be related to an increase in circulating platelet aggregates. Direct inhibition of platelet aggregation by sodium nitroprusside may be a mechanism of its beneficial effects in heart failure.

Platelet aggregation in relationship to plasma catecholamines in patients with hypertension

Plasma catecholamine concentration and platelet aggregation were studied in 22 patients with uncomplicated primary hypertension and 13 age-matched normotensive, healthy subjects at rest and in some during isometric handgrip exercise. The effect of norepinephrine (NE) infusion upon platelet aggregation was also examined. Plasma catecholamine concentration was slightly higher in the hypertensive than the normotensive group, but the difference was not significant. However, platelet aggregation to ADP was significantly greater in the hypertensive than the normotensive subjects. Exercise increased significantly both catecholamines and aggregation in both groups. Platelet aggregation was correlated with age (r = 0.62, P less than 0.01) and plasma NE (r = -0.34, P less than 0.05 for the total group of subjects). The infusion of NE increased significantly plasma NE and platelet aggregation and there was an inverse correlation between NE increase and threshold decrease (r = -0.69, P less than 0.05). Thus, plasma catecholamines and important determinants of platelet aggregation. However, in our study, uncomplicated primary hypertension was not associated with abnormal plasma catecholamine concentration. It is likely that the observed abnormal platelet aggregability to ADP represents a secondary phenomenon, possibly related to more advanced atherosclerotic vascular changes in hypertensive than normotensive subjects.

Platelet aggregation in aortic and coronary venous blood in patients with and without coronary disease. 3. Role of tachycardia stress and propranolol

We studied 16 patients with coronary artery disease (CAD) to evaluate platelet aggregation in blood samples withdrawn simultaneously from the aorta and coronary sinus. At rest, mean platelet aggregation in coronary venous blood was significantly lower than that in aortic blood. Platelet counts in coronary venous blood were also lower than in the aortic blood in each of the six CAD patients in whom counts were done. Platelet aggregation was lower in seven patients who were taking propranolol than in the remaining nine who were not taking propranolol. During tachycardia stress, platelet aggregation increased in all patients, but the magnitude of increase was greater in patients not taking propranolol. In four other patients without CAD, platelet aggregation and counts were also studied in the same fashion and were similar in both the aortic and coronary venous blood. These data suggest that in certain CAD patients, platelet consumption or destruction within atherosclerotic vasculature may occur. Propranolol may reduce platelet aggregation at rest and modify excessive aggregation during tachycardia stress in certain CAD patients.

Differences in platelet aggregation in coronary sinus and aortic blood in patients with coronary artery disease: effect of propranolol

Platelet aggregation was studied in aortic and coronary sinus blood samples obtained from 18 patients with coronary artery disease (CAD). Using epinephrine and ADP as aggregating agents, platelet aggregation was lower in coronary venous blood than in aortic blood. In nine patients on long-term propranolol therapy, platelet aggregation was lower in both aortic and coronary venous blood compared to the nine patients not taking propranolol. Four other subjects without angiographic evidence of coronary disease exhibited no difference in platelet aggregation in aortic and coronary sinus blood. These data suggest that platelet aggregation is lower in the coronary venous blood of certain patients with coronary disease and chronic propranolol treatment may reduce aggregation in both aortic and coronary sinus blood.