The efflux rate of norepinephrine from platelets and its relation to blood pressure

The role of platelets in essential hypertension

Recent research is helping us understand the complex interactions that occur between platelets and their environment. The several intracellular events that occur during platelet activation are being identified as ar their effects on other platelets, the endothelium and coagulation factors. Heightened platelet activation is seen early in essential hypertension and probably plays an important role in the initiation and progression of atherosclerosis and the disorders associated with it. This review identifies some of the changes in platelet structure and function in essential hypertension and their role in the pathogenesis of hypertensive vascular disease.

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.

Platelet sodium and potassium ATPase [corrected] activity and noradrenaline efflux rate in relation to autonomic and peripheral nerve function in insulin-dependent diabetic patients

Low sodium and potassium adenosine triphosphatase (ATPase) activity has been proposed as a mechanism behind diabetic neuropathy. In this study the platelet ATPase activity and platelet noradrenaline efflux rate were determined in 47 insulin-dependent diabetes mellitus (IDDM) patients and 20 controls. Ulnar motor conduction velocities, tested in a subgroup, were lower in patients than in controls (52.7 +/- 1.3 m s-1 vs. 61.3 +/- 1.4 m s-1; P less than 0.001). Platelet ATPase activity tended to be increased in the patients compared with the controls (29.9 +/- 1.0 x 10(-3) min-1 vs. 26.9 +/- 1.1 x 10(-3) min-1; NS). In ulnar nerve function tested subjects, ATPase activity was higher in patients than in controls (31.2 +/- 1.7 x 10(-3) min-3 vs. 25.9 +/- 1.3 x 10(-3) min-1; P less than 0.01). The platelet noradrenaline efflux rate tended to be higher in patients with lower brake indices, a sign of autonomic neuropathy, than in controls (29.0 +/- 3.0 x 10(-3) min-1 vs. 21.2 +/- 0.9 x 10(-3) min-1; P less than 0.05). The platelet ATPase activity was not decreased in IDDM patients, however, a connection between diabetic autonomic neuropathy and platelet transmittor leakage was indicated.

Effects of ouabain on 86Rb-uptake, 3H-5-HT-uptake and aggregation by 5-HT and ADP in human platelets

In the search of sensitive models for actions of digitalis-like substances on intact cells or tissues, the effects of ouabain on human platelets were investigated. In a concentration-dependent manner ouabain 10(-8)-10(-4) M inhibited Na+-K+-ATPase activity measured as uptake of 86Rubidium (86Rb), with about 90% inhibition of the total uptake at ouabain greater than or equal to 10(-6) M. An almost identical concentration-effect curve was found for platelet uptake of 3H-serotonin (3H-5-HT). The platelet shape change reaction to exogenous 5-HT (1 X 10(-6) M) was suppressed by ouabain (10(-8)-10(-4) M) in a concentration-dependent manner, but with no clear maximum effect within the range tested. Aggregation induced by adenosine-di-phosphate (ADP 2 X 10(-6) M) was enhanced by ouabain 10(-8)-10(-6) M. At the highest concentration tested the rate of aggregation was increased by 31% and the change in light transmission by 54%. At low concentrations (less than 10(-9) M) of ouabain, there was a tendency towards increased aggregation as well as increased uptake of 86Rb, which may be a parallel to observations of positive inotropic effects of low concentration of glycosides, which do not inhibit Na+-K+-ATPase. The results show that human platelets can be used as a model tissue for studying effects of cardiac glycosides. This suggests that it may be useful for further investigations of the biological effects of agents with a similar effect profile, e.g. endogenous digitalis-like substances.

[3H]noradrenaline efflux from platelets and synaptosomes of ethanol-treated rats

Platelets have the ability to take up, store and release biogenic amines and have therefore been used as models for neurons in studies of neuropsychiatric disorders and hypertension. We have studied the spontaneous efflux of [3H]noradrenaline from platelets and synaptosomes of rats chronically treated with ethanol. Male control rats had a more rapid [3H]noradrenaline efflux both from synaptosomes and platelets than female control rats. Ethanol treatment increased efflux in female rats, again both in platelets and synaptosomes. These results suggest that a parallelism exists in noradrenaline release between synaptosomes and platelets, both basal and in situations which stimulate the release.

The 5-hydroxytryptamine system of blood platelets: physiology and pathophysiology

The 5-hydroxytryptamine (5HT)-system of human blood platelets consists of a relatively specific uptake mechanism for 5HT at the plasma membrane, intracellular storage organelles (dense bodies), a metabolizing enzyme (monoaminoxidase B) and a 5HT2-receptor whose stimulation leads to activation of the phosphatidylinositide turnover, a rise in free cytoplasmic Ca2+, phosphorylation of proteins and a shape change reaction. There is neither a relevant 5HT-biosynthesis nor a marked physiological 5HT-turnover in platelets. Under physiological conditions the platelet 5HT-system may have a role as a scavenger for free extracellular 5HT and in hemostasis. Disturbances which have been described in pathophysiological states include impairment of 5HT-uptake (hypertension, migraine), impairment of 5HT-storage (storage pool deficiencies, thromboembolic disorders, hypertension) and increased sensitivity to activating agents like 5HT (cardiovascular disorders, diabetes). Besides their role in physiology and pathophysiology platelets may be useful partial models for vascular smooth muscle cells.

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.

The relation between protein carboxyl methylating and demethylating enzyme activities and noradrenaline efflux in platelets

To test the hypothesis that the raised noradrenaline efflux rate constant in platelets observed in hypertensive individuals and some normotensive relatives to hypertensives could be ascribed to disturbances in the protein carboxyl methylation, protein carboxyl methylase activity and noradrenaline efflux rate constant were measured in platelets from 33 young healthy males. Sixteen had inheritance of hypertension. Protein methyl esterase activity and the noradrenaline efflux rate constant were measured in 43 men, 22 of whom had inheritance of hypertension. No correlation was found between unstimulated noradrenaline efflux from platelets and enzyme activity.

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.

Leukocyte ionized calcium and sodium content and blood pressure in humans

The relationship between leukocyte ionized calcium concentration, sodium content, and blood pressure was studied in normotensive subjects with (n = 17) and without (n = 21) a family history of hypertension and in untreated patients with essential hypertension (n = 22). There was a highly significant correlation between intracellular ionized calcium and mean supine blood pressure (measured on the same occasion) in normal subjects with no family history of hypertension (r = +0.73, p less than 0.01). This relationship was lost in normal subjects with a family history of hypertension, and in hypertensive patients there was a nonsignificant negative correlation between intracellular ionized calcium and blood pressure (r = +0.08 and -0.31, respectively). Intracellular ionized calcium was similar in the normotensive groups (both, 126 +/- 7 nmol/L) and slightly but nonsignificantly elevated in hypertensive patients (143 +/- 10 nmol/L; p = 0.09). There was no correlation between intracellular ionized calcium and sodium content in any group (r less than 0.1). These results indicate that while leukocyte ionized calcium in normotensive subjects with no family history of hypertension may reflect smooth muscle contractility resulting in the positive correlation between leukocyte ionized calcium and blood pressure, this relationship is lost in hypertensive patients and subjects predisposed to hypertension. This may be due to an altered relationship between leukocyte and smooth muscle calcium handling in these subjects or to non-calcium-mediated influences on blood pressure.

Distribution of the methyl acceptor proteins within platelet granules

Fractionation of platelet alpha-granules and dense bodies demonstrated that methyl acceptor proteins (MAP) were present in the alpha-granule fractions, but seemed to be low or absent in the dense body fraction. MAP activity in four intragranular proteins (albumin, fibrinogen, beta-thromboglobulin and platelet derived growth factor) was low or absent and could not explain the difference in methyl acceptor proteins distribution between alpha-granules and dense bodies.

The relation between in vitro efflux of noradrenaline from platelets and vas deferens in man

The efflux of isotope-labelled noradrenaline from platelets and vas deferens was compared in 29 healthy males. Platelets and a preparation of tissue from vas deferens were incubated with isotope-labelled noradrenaline until equilibrium in the uptake was obtained. The spontaneous efflux of noradrenaline in buffer was measured for 20 min. There was a significant positive correlation between the efflux of noradrenaline from platelets and vas deferens (r = 0.56, P less than 0.001).

Role of alpha adrenoceptors in hypertension and in antihypertensive drug treatment

Recent plethysmographic experiments suggest that both postsynaptic alpha 1 and alpha 2 adrenoceptors in human resistance vessels play an important role in the maintenance and regulation of vascular tone. Central alpha 2 adrenoceptors are assumed to be involved in the central regulation of blood pressure. Radioligand binding studies on the density and characteristics of alpha adrenoceptors have not revealed consistent differences between normotensive and hypertensive subjects, with the exception of pheochromocytoma, in which a consistent down regulation of alpha 2 adrenoceptors in thrombocytes has been demonstrated. The radioligand binding studies are limited, since they cannot be performed on vascular tissues. Alpha adrenoceptors are vitally important as targets of several antihypertensive drugs; the activities of these agents and the principles and clinical relevance of mechanisms involving alpha adrenoceptors are reviewed.

Effects of ethanol and acetaldehyde on cultured rabbit aortic myocytes and human platelets in vitro

The recently reported finding that a moderate alcohol consumption may lower the risk of cardiovascular disease prompted a study of the effects of ethanol and acetaldehyde on proliferation, viability and collagen secretion of rabbit aortic myocytes in culture and on the spontaneous efflux reaction of human platelets in vitro. Ethanol had no effects on any of the systems and acetaldehyde did not influence platelets significantly. Fifty mumol 1(-1) acetaldehyde diminished the proliferation and collagen secretion of arterial myocytes by 20% (P less than 0.01) and 100 mumol 1(-1) acetaldehyde by 39% (P less than 0.001) without affecting cell mass or cell death. A metabolic degradation, and some evaporation, of acetaldehyde was taking place and 50 mumol 1(-1) acetaldehyde was halved after approximately 2 h. The more 'physiological' concentration of acetaldehyde (5 mumol 1(-1] influenced cell proliferation significantly (P less than 0.001) if the concentration was restored by 6-h intervals and the incubation time increased from 24 to 48 h. The weak aldehydedehydrogenase inhibitor chlorpropamide did not accentuate the effects of acetaldehyde.

Decreased uptake of 3H-serotonin and endogenous content of serotonin in blood platelets in hypertensive patients

The uptake and content of serotonin in blood platelets were studied in patients with essential hypertension and in five families in which at least one member was hypertensive. Blood was obtained from male and female normotensive volunteers and hypertensive patients who were free of medication. Lineweaver-Burk plots of 3H-serotonin uptake from both control subjects and hypertensive patients were linear, which suggested simple Michaelis-Menten uptake kinetics. The maximal uptake velocity (Vmax) in hypertensive patients was significantly lower than in control subjects (control = 41.7 +/- 3.3 pmol/min/10(8) platelets, n = 17; hypertensive = 26.6 +/- 3.0 pmol/min/10(8) platelets, n = 16; p less than 0.005). The affinity constant (Km) was slightly but significantly lower in hypertensive patients (control = 0.70 +/- 0.08 microM; hypertensive = 0.46 +/- 0.08 microM; p less than 0.05). The serotonin content in blood platelets determined by high pressure liquid chromatography with electrochemical detection was significantly lower in hypertensive patients (control = 165.0 +/- 12.9 nmol/10(11) platelets, n = 29; hypertensive = 105.9 +/- 10.4 nmol/10(11) platelets, n = 27; p less than 0.001). In the five families investigated, the lowered serotonin content was observed in some normotensive members. The reduced number of carriers of serotonin uptake and the slight decrease in the affinity constant observed in platelets of patients with essential hypertension suggest that serotonin metabolism is altered in essential hypertension and that blood platelets may be a useful model in studying the serotonergic modifications at the molecular level.

Efflux of granule constituents from platelets exposed to different mechanical forces

The efflux of 14C-norepinephrine, 3H-serotonin and beta-thromboglobulin from platelets was compared with the extent of lysis in two different systems of incubation, in which Ca2+ and conventional platelet activating agents were omitted. In the first system platelets were incubated in buffer at 37 degrees C and gently shaken. Cell lysis was low and efflux of 14C-norepinephrine was 71-97% of total initial uptake after 24 hours. In the other system rotating plastic tubes at 20 degrees C were used which exposed the platelets to higher mechanical stress. Cell lysis and aggregation was already high after 10-30 minutes, but the efflux of 14C-norepinephrine and 3H-serotonin was lower and not parallel to cell lysis. The release of 14C-norepinephrine was higher than that of 3H-serotonin in both systems. There was a considerable release of beta-thromboglobulin from platelets remaining in plasma despite of a constant pH. These findings indicate that platelet lysis is independent of the release of platelet granule constituents.

Plasma protein changes in primary hypertension in humans and rats

To determine whether plasma protein changes may be associated with primary hypertension, we analyzed plasma proteins from essential hypertensive (EHT) patients and genetically hypertensive rats using two-dimensional electrophoresis. An additional plasma protein, having a molecular weight of 13,000 daltons and an isoelectric point of 4.5, was found in 82% of the patients with borderline or moderate hypertension (n = 29) and in all permanently hypertensive patients (n = 12). This protein was detected in 36% of normotensive (NT) subjects (n = 50). In the latter, the influence of family history, sex, and secondary hypertension were studied. Plasma proteins were also studied in spontaneously hypertensive rats (SHR). In all plasma from young male (n = 10) and female (n = 6) SHR, two additional proteins (molecular weight = 16,000 daltons, pHi = 4.7 and 5.1) were detectable. These plasma proteins were not detectable in male Wistar Kyoto rats (WKY) and in 50% of female WKY, and their frequency was 10% (n = 10) and 0% (n = 3) in normal male WKY and in male WKY rendered hypertensive by methylprednisolone, respectively. We conclude that these alterations of plasma proteins may be considered a biochemical feature of primary hypertension.

Effects of thrombin, acetyl salicylic acid and prostaglandins on spontaneous efflux of norepinephrine and beta-thromboglobulin from platelets in calcium/magnesium deficient environment

The effect of different substances on the spontaneous efflux of 14C-norepinephrine (14C-NE) and beta-thromboglobulin (beta TG) from platelets in Ca2+ - and Mg2+ - free buffer was tested. Addition of increasing concentrations of thrombin (0.015-0.625 U/ml) resulted in an immediate increase of 14C-NE detectable in the supernatants, but the slopes of the efflux curves from 10 min after thrombin addition were almost identical and parallel with the slope before thrombin addition. Also beta-thromboglobulin was steadily released after thrombin addition. Prostaglandin E1 and prostacyclin in concentrations that gave a 30-fold increase in cAMP accumulation resulted in a moderate decrease in efflux of 14C-NE from 10 to 30 min but did not influence the efflux thereafter up to 80 min. Acetyl salicylic acid in vivo and in vitro (200-500 mumol/l) did not influence the efflux of 14C-NE or beta TG. The conclusions are that factors known to exert considerable effects on aggregation, desaggregation and acute release produce only transitory effects in a Ca2+ - and Mg2+ - free environment and that the basal efflux of alpha-granule components seems to be a stable process.

Spontaneous efflux of platelet alpha-granule components in relation to 14C-norepinephrine in buffers

In an investigation of the spontaneous efflux of beta-thromboglobulin (beta-TG), 14C-norepinephrine (14C-NE) and growth stimulating activity (GSA), measured as incorporation of 3H-thymidine in rabbit aortic smooth cells, from platelets resuspended in Ca++- and Mg++-free buffer a continuous increase in 3H-thymidine incorporation was seen during 120 min. There was good stoichiometric parallelism between beta-TG and 14C-NE during the 80 minutes the experiment lasted, indicating that norepinephrine is incorporated in alpha-granules and release by exocytosis.

Liberation of catecholamines and 5-hydroxytryptamine from human blood-platelets

Spontaneous and drug-induced liberation of 14C-5-hydroxytryptamine (14C-5HT), 3H-dopamine (3H-DA) and 3H-noradrenaline (3H-NA) from normal and reserpinized human blood-platelets has been determined from measurements of the amine contents before and after incubation in tris-buffer. In normal platelets the spontaneous liberation of 3H-catecholamines was more marked than that of 14C-5HT, but was less in percent for all these labelled amines than in reserpinized platelets. Thrombin lowered amine contents more in normal than in reserpinized platelets. The initial thrombin-induced decrease of 14C-5HT, in contrast to that of 3H-catecholamines, showed a partial recovery after 30 min which was abolished by imipramine. The benzoquinolizine Ro 4-1284 diminished all the amines in normal, but not in reserpinized platelets. In normal platelets tryamine affected 14C-5HT and 3H-DA about equally, whereas 3H-NA much less. Octopamine showed a similar pattern as tryamine, but was less potent. P-chlormethamphetamine (PCMA) and amphetamine decreased 3H-DA less markedly than 14C-5HT and 3H-NA not at all. In reserpinized platelets these arylalkylamines induced a decrease of 14C-5HT but not of 3H-catecholamines. It is concluded that (a) 3H-catecholamines like 14C-5HT are mainly localized in the granular pool of normal human platelets, (b) the pattern of action of a drug on intra- and extragranular amines depends not only on the nature of the drug and the amine to be liberated, but in comparison with previous results also on the species, (c) platelets are not completely satisfactory models for monoaminergic neurons, especially catecholaminergic ones regarding drug-induced amine liberation.

Liberation of catecholamines from blood platelets

1 Platelet-rich plasma (PRP) of guinea-pigs with or without reserpine was preincubated either with [(14)C]-5-hydroxytryptamine ([(14)C]-5-HT) plus [(3)H]-dopamine or with [(14)C]-5-HT plus [(3)H]-noradrenaline ([(3)H]-NA). After isolation on two successive dextran gradients the double-labelled platelets were incubated in Tris-buffer in the presence or absence of various drugs. The decrease in radioactivity in the platelets was measured in order to determine the amount of the amine that had been liberated.2 Spontaneous liberation of the labelled amines was more marked in reserpine-treated platelets than in normal ones and somewhat more pronounced for the (3)H-catecholamines than for [(14)C]-5-HT.3 The reserpine-like benzoquinolizine, Ro 4-1284, caused liberation of all three labelled amines in normal but not in reserpine-treated platelets. More [(3)H]-dopamine was liberated than [(14)C]-5-HT and less [(3)H]-NA.4 The arylalkylamines, tyramine and p-chloromethamphetamine (PCMA), liberated all three labelled amines from normal platelets, and [(14)C]-5-HT and [(3)H]-dopamine, but not [(3)H]-NA from reserpine-treated ones. In normal platelets dopamine was reduced to a greater extent than [(14)C]-5-HT and [(3)H]-NA to a smaller extent, whereas in reserpine-treated platelets [(14)C]-5-HT was more markedly diminished than [(3)H]-dopamine.5 The 5-HT uptake inhibitor, imipramine, had little influence on the spontaneous and drug-induced liberation of [(14)C]-5-HT and [(3)H]-dopamine.6 It is concluded that (3)H-catecholamines like [(14)C]-5-HT are mostly localized in the granular pool of platelets; the three drugs tested liberate [(3)H]-dopamine [(3)H]-NA and [(14)C]-5-HT from the granular pool. Ro 4-1284 does not liberate (3)H-catecholamines and [(14)C]-5-HT from extragranular sites whereas tyramine and PCMA also act on the extragranular pool of [(3)H]-dopamine and [(14)C]-5-HT but not [(3)H]-NA.7 The liberation of catecholamines from platelets differs from that of 5-HT in several respects and platelets are only partly comparable to neurones as far as drug-induced liberation of biogenic amines is concerned.

Genetic markers in spontaneously hypertensive rats

Establishment of various models for hypertensive diseases such as spontaneously hypertensive rats (SHR) and stroke-prone SHR (SHRSP) clarified the importance of genetic factors in the pathogenesis of these diseases and further accelerated studies on their genetic mechanisms. Although various biochemical abnormalities have been detected and can be used as biochemical markers in these models, they have not been always closely related to blood pressure in F2 generation obtained by cross breeding between SHR and normotensive Wistar-Kyoto rats. Recent studies indicate that biomembrane abnormalities detected in erythrocytes and other membranes of SHR and SHRSP may not only be biochemical markers but also related to the pathogenesis of hypertensive diseases.

Net efflux rate of norepinephrine from platelets in normotensive individuals belonging to families with a heavy accumulation of essential hypertension

Platelets have been used as a model of sympathetic neurons to study the storage of norepinephrine in normotensive individuals who all had a first degree relative with essential hypertension and had hypertension in the family for at least two generations. The initial rate of efflux of norepinephrine, k, was determined in 44 young relatives (mean age 29.2 years), in 18 middle-aged relatives (mean age 46.7 years) and in 31 young controls with no known family history of essential hypertension (mean age 29.8 years). From the material of relatives all those with definite hypertension had been a priori excluded. k was significantly higher in the young relatives (22.7 +/- 7.9) than in the middle-aged relatives (17.7 +/- 6.4) (p less than 0.05). 27.3% of the relatives had higher k-values than any of the controls. A significant correlation was found between k-value and diastolic blood pressure in controls but not in young relatives.

A possible role of catecholamines and (Na+ + K+)ATPase in the ethanol withdrawl syndrome

Chronic ethanol intoxication leads to an increase in the intracellular Na+/K+ ratio. It is suggested that this derangement is counteracted by catecholamines via an activation of (Na+ + K+)ATPase. This hypothesis is discussed in relation to the symptomatology of ethanol withdrawal.