Inhibition of human platelet function by verapamil

Platelet activation in essential hypertension: implications for antiplatelet treatment

Essential hypertension is associated with increased risk of arterial thrombotic disease. Among other factors, enhanced platelet activity contributes significantly to this phenomenon. An increased level of circulating monocyte-platelet aggregates (MPAs) represents one of the most robust markers of platelet activation; furthermore, these aggregates are also believed to contribute to the pathophysiology of atherothrombotic disease. Putative mechanisms that contribute to platelet activation in essential hypertension include endothelial dysfunction, neurohumoral (sympathetic and renin-angiotensin systems) overactivity, decreased platelet nitric oxide (NO) biosynthesis, and platelet degranulation secondary to increased shear. Current recommendations are that hypertensive patients receive aspirin therapy only if their calculated cardiovascular risk is high and their blood pressure (BP) is adequately controlled. By contrast, the use of antiplatelet treatment in low-risk hypertensive patients is not established and merits further investigation. Moreover, the place of alternative antiplatelet agents other than aspirin, such as clopidogrel, is unclear at present. Some experimental evidence suggests that clopidogrel may confer an additive protective effect over and above aspirin in hypertensive patients, by virtue of effects on the evolution of the atherosclerotic process. This now needs to be investigated in long-term clinical outcome studies.

Is the effect of antihypertensive drugs on platelet aggregability and fibrinolysis clinically relevant?

Hypertension is associated with decreased fibrinolytic potential, mainly expressed as elevated plasma plasminogen activator inhibitor type 1 (PAI-1) levels, and increased platelet aggregability, which may account in part for the increased risk of atherosclerosis and its clinical complications in hypertensive patients. The effects of antihypertensive drugs on this prothrombotic state have been investigated and controversial findings have been reported, possibly because of differences in study designs, patients selected, and methodology used. Scarce and conflicting data exist about the effects of diuretics and beta-adrenoceptor antagonists on the fibrinolytic system, whereas ACE inhibitors have generally been reported to improve the fibrinolytic balance by decreasing plasma PAI-1 levels, calcium channel antagonists have been shown to increase tissue plasminogen activator (tPA) activity, and angiotensin II type 1 (AT(1)) receptor antagonists seem to exert neutral effects. beta-Adrenoceptor antagonists, calcium channel antagonists, and AT(1)-receptor antagonists have been reported to exert anti-aggregatory effects on platelets, while contrasting data exist about the influence of ACE inhibitors. Clinical implications of the changes induced by antihypertensive drugs on the fibrinolytic balance and platelet function are still debated. In particular, the question of whether these changes may translate into different degrees of cardiovascular protection in hypertensive patients remains unanswered. While awaiting more information from clinical trials, the choice of antihypertensive drugs, particularly in high-risk patients, should take into account effects beyond their BP-lowering efficacy. Selected agents should have a favorable, or at least neutral, impact on fibrinolytic function and platelet activity.

Pharmacological modulation of platelet function in hypertension

Platelets exert a considerable influence on human morbidity and mortality. The rationale for their study in hypertension follows the observation that the major consequences of hypertension are stroke and myocardial infarction. However, the etiology of these consequences in hypertension is, paradoxically, not hemorrhagic (as might be expected from the effects of high blood pressure), but occlusive, with thrombus being the culprit lesion. Mechanisms of platelet activation include high shear force, activation of the renin-angiotensin system, endothelial changes, and the presence of comorbidity, such as atrial fibrillation. The treatment of high blood pressure brings about a reversal of the changes seen in the cell. This could be in part due to the direct effect of the drug on the megakaryocyte and/or the platelets themselves, or it might simply be due to the reduction in blood pressure. Some drugs, such as calcium channel antagonists and angiotensin II receptor blockers, however, might have direct effects on platelet biochemistry other than reducing blood pressure. Finally, antiplatelet drugs are becoming an important part of the management of high-risk hypertensives, which aim to minimize vascular complications.

Effects of diltiazem and verapamil on ADP-induced rabbit platelet shape change and aggregation

1. The effects of diltiazem and verapamil (two structurally different calcium channel blockers) were examined on the rabbit platelets shape change and aggregation induced by adenosine-5'-diphosphate (ADP). 2. ADP was a much more potent stimulator on inducing platelet shape change (ED50 = 1 x 10(-7)) than platelet aggregation (ED50 = 1.78 x 10(-6)). 3. Both drugs similarly inhibited ADP-induced platelet shape change and aggregation at concentrations more than 300 microM. 4. There were no significant differences in inhibitory effects of either diltiazem or verapamil on ADP-induced platelet shape change and aggregation. 5. The inhibitory effects of diltiazem and verapamil on ADP-induced platelet shape change and aggregation at high concentrations may be due to their non specific properties.

Effects of gallopamil on epinephrine and norepinephrine plasmatic levels and on TxB2 and beta-tg release in patients with coronary artery disease during adrenergic stimulus with cold pressor test

We investigated the effect of gallopamil administration during a cold pressor test (CPT) in 18 patients suffering from chronic angina (CA) and in 21 healthy subjects. CA patients showed increased basal levels of beta-thromboglobulin and thromboxane B2 compared to control patients and normal plasma levels of catecholamines. CPT caused plasma catecholamines, beta-thromboglobulin and TxB2 levels to rise. This rise was greater in CA patients than in control patients. Administration of gallopamil (50 mg kg-1 three times a day for 30 days) reduced plasma levels of catecholamines, beta-thromboglobulin and TxB2 blood concentrations either under basal conditions or after CPT. Our data suggest that gallopamil is able to modulate the response induced by adrenergic stress.

Inhibitory effect of clentiazem (TA-3090) on platelet aggregation--alone and in combination with aspirin or ticlopidine

Clentiazem (a novel calcium antagonist) and its basic metabolites (MB1-MB7) showed inhibitory effects on collagen-induced platelet aggregation in human platelets. All the basic metabolites (IC50:8-22 micrograms/ml) had much stronger inhibitory effects than clentiazem itself (IC50:53 micrograms/ml), but the acidic metabolites (MA1-MA4) had no inhibitory effects even at 300 micrograms/ml. Other calcium antagonists (diltiazem, verapamil, nicardipine and nimodipine) also showed similar inhibitory effects although nicardipine and nimodipine were less active than the other drugs. The inhibitory effect of clentiazem was enhanced in the presence of aspirin or ticlopidine. Diltiazem and nicardipine also exhibited a similar potentiation of the anti-platelet effect in combination with aspirin or ticlopidine. Clentiazem also inhibited collagen-induced thromboxane B2 production by the platelets, and this inhibition by clentiazem was additively enhanced by the presence of aspirin. When both clentiazem and aspirin were orally administered to rats, platelet aggregation was additively inhibited. These results indicate that a combination therapy with clentiazem plus aspirin or clentiazem plus ticlopidine may be useful for the prevention and/or treatment of thrombotic disorders.

Platelet function defects associated with hemorrhage or thrombosis

Platelet dysfunction, especially acquired forms, is a common cause of hemorrhage, especially when associated with trauma or surgery. Although the hereditary platelet function defects are generally quite rare, hereditary storage pool disease is common enough to be suspected in an individual, usually a child, with characteristic historical and clinical findings. The acquired platelet function defects, especially those resulting from drugs, are common and should promptly be suspected in patients developing easy and spontaneous bruising, mild-to-moderate mucosal membrane hemorrhage, or unexplained bleeding associated with trauma or surgery. The template bleeding time is generally useful as a screening test of platelet function, but a normal template bleeding time, in the face of a suggestive history, suggestive clinical findings, or in a patient frankly bleeding, is not reliable, and platelet aggregation or lumiaggregation should be done in appropriate clinical situations. Also, prolongation of the template bleeding time is an unreliable predictor of clinical bleeding propensity. The mainstay of therapy for almost all these defects, if bleeding is significant, is the liberal infusion of appropriate numbers of platelet concentrates. The acquired platelet function defects should also be managed by attempts to treat or control the underlying disease, if possible, and offending drugs or potentially offending drugs should immediately be stopped.

Comparative effects of the dihydropyridine-type calcium-agonists (-)-S-Bay K 8644, (+/-)-Bay-W 5035 and (+/-)-Bay-T 5006 on human platelet aggregability

1. Human platelet aggregation induced by collagen is concentration-dependently inhibited by dihydropyridine (DHP)-type calcium(Ca)-agonists. 2. There was no significant difference between the maximal anti-aggregatory effects or the anti-aggregatory potencies of (-)-S-Bay-K 8644 (EC50: 5.3 +/- 1.5 x 10(-5) M), (+/-)-Bay-W 5035 (EC50: 14.9 +/- 8.8 x 10(-5) M) or (+/-)-Bay-T 5006 (EC50: 2.7 +/- 1.9 x 10(-5) M) (P > 0.05). 3. Antiaggregatory effects of DHP-type Ca-agonists seem to be independent of Ca-channel activation.

The effect on platelet behaviour of treatment with atenolol and the combination of atenolol and nifedipine in healthy volunteers

Two double blind crossover studies have been carried out in healthy volunteers to determine the effect on platelet behaviour and on blood coagulation of treatment with atenolol and with a fixed combination of atenolol and nifedipine. The first study compared atenolol with placebo, whilst the second compared atenolol alone with the combination of atenolol and nifedipine. Treatment with atenolol alone had no effect on platelet behaviour. In contrast, treatment with the combination of the beta blocker and the calcium antagonist gave rise to a small but significant increase in platelet dense granule release ex vivo in response to a wide range of agonists. This was not associated with any change in serum levels of thromboxane B2 nor with any alteration in plasma concentrations of platelet factor 4 and beta-thromboglobulin. Neither treatment had any effects on coagulation parameters. The mechanism and clinical significance of the observed increase in the extent of the platelet release reaction during combined atenolol/nifedipine treatment remain speculative.

Discontinuous alterations of platelet structure and function by bound, ionizable verapamil

We have investigated several effects of verapamil (5-[(3,4-dimethoxyphenethyl)methylamino]-2-(3,4-dimethoxyphenyl)-2- isopropylvaleronitrile) on platelet structure and function. At concentrations below a threshold of approximately 4.2 x 10(7) molecules.cell-1, verapamil binds to platelets according to a typical Langmuir adsorption isotherm (i.e., binding is saturable and noncooperative). By extrapolation, we calculate that saturation would occur at 6.8 x 10(7) +/- 1.9 x 10(7) molecules.platelet-1, with one bound verapamil molecule per two membrane phospholipids. Saturation is never achieved, however, because past the threshold surface concentration, the adsorption isotherm becomes discontinuous and further adsorption becomes a linear function of the concentration of drug in solution. We attribute this discontinuity to disorganization of the membrane bilayer which is stretched beyond cohesion by insertion of too many amphiphilic molecules of a length shorter than that of the phospholipid. The partitioning of verapamil between the bulk aqueous phase and the newly created lipid phase would then account for the linear portion of the adsorption isotherm. The discontinuity of binding is accompanied by discontinuities in the verapamil-dependent swelling of platelets and the verapamil-dependent inhibition of both ADP-inducible binding of fibrinogen to, and aggregation of, platelets. In contrast, N-methylverapamil (5-[(3,4-dimethoxyphenyl)- methylamino]-2-(3,4-dimethoxyphenyl)-2-isopropylvaleronitrile), a hydrophilic quaternary amine derivative of verapamil, neither swells platelets nor inhibits the ADP-dependent processes that we investigated. From this we conclude that deprotonated verapamil is the operative species of the drug. Collectively, these data suggest that verapamil alters platelet structure and function by mechanisms involving disorganization of the platelet plasma membrane.

Calcium-channel blocking agents verapamil and diltiazem are inhibitors of vasopressin-induced human platelet activation

1. This study investigated the influences of calcium-channel blocking agents verapamil and diltiazem on platelet responses induced by arginine vasopressin (AVP) and lysine vasopressin (LVP). 2. The substances inhibited platelet aggregation induced by both low and high AVP concentrations, LVP and adrenaline plus AVP. IC50 values of each drug are lower than those determined for ADP- and collagen-elicited aggregation. Verapamil and diltiazem also decreased AVP-induced thromboxane B2 synthesis. 3. Other series of experiments showed that the addition of ethyleneglycol-bis-(beta-amino-ethyl ether) N, N'-tetra-acetic acid to platelet-rich plasma samples also prevented the platelet response to vasopressin polypeptides. 4. Our data provide evidence that the effects of verapamil and diltiazem on vasopressin-induced platelet responses may be directly related to inhibition of extracellular calcium entry.

Studies on inhibition of human platelet response by diltiazem

1. Aim of the present investigation was to investigate the effects of calcium blocking agent diltiazem on human platelet response to aggregating agents. 2. Results showed that diltiazem inhibits platelet aggregation induced by ADP, arginine vasopressin, adrenaline, collagen, Na arachidonate, thrombin and phorbol ester PMA in a dose-dependent way. 3. Diltiazem decreased also beta-thromboglobulin release and Thromboxane B2 production from stimulated platelets. 4. Intraplatelet cyclic AMP levels were not modified by the substance. 5. Data provide evidence that the modulation of human platelet function by diltiazem could be also related to inhibition of protein kinase C.

Verapamil is a potent inhibitor of 5-HT-induced platelet aggregation

We have studied the effects of verapamil, diltiazem and amlodipine on 5-HT-induced platelet aggregation and compared the results with those obtained for other platelet aggregating agents. Experiments were carried out using both human whole blood and platelet-rich plasma (PRP). Verapamil (but not diltiazem or amlodipine) inhibited 5-HT-induced platelet aggregation at much lower concentrations (IC50 = about 1 microM) than were required for inhibition of aggregation induced by other aggregating agents. Like some other selective inhibitors of 5-HT-induced platelet aggregation, it was not possible to completely overcome the inhibition by increasing the concentration of 5-HT. The antiaggregatory effects of verapamil were similar, but not identical, in whole blood and PRP. These results show that the Ca2+ channel blocker verapamil has some selectivity as an inhibitor of 5-HT-induced platelet aggregation and that this behaviour as a 5-HT antagonist should be taken into account when interpreting any therapeutic benefit ascribed to this drug.

Verapamil. An updated review of its pharmacodynamic and pharmacokinetic properties, and therapeutic use in hypertension

Although verapamil is a well-established treatment for angina, cardiac arrhythmias and cardiomyopathies, this review reflects current interest in calcium antagonists as anti-hypertensive agents by focusing on the role of verapamil in hypertension. Verapamil is a phenylalkylamine derivative which antagonises calcium influx through the slow channels of vascular smooth muscle and cardiac cell membranes. By reducing intracellular free calcium concentrations, verapamil causes coronary and peripheral vasodilation and depresses myocardial contractility and electrical activity in the atrioventricular and sinoatrial nodes. Verapamil is well suited for the management of essential hypertension since it produces generalised systemic vasodilation resulting in a marked reduction in systemic vascular resistance and, consequently, blood pressure. Evidence from clinical studies supports the role of oral verapamil as an effective and well-tolerated first-line treatment for the management of patients with mild to moderate essential hypertension. Clinical studies have shown that verapamil is more effective the higher the pretreatment blood pressure and some authors have found a more pronounced antihypertensive effect in older patients or in patients with low plasma renin activity. Sustained release verapamil formulations are available for oral administration which, as a single daily dose, are as effective in lowering blood pressure over 24 hours as equivalent doses of conventional verapamil formulations given 3 times daily. As a first-line antihypertensive agent, oral verapamil is equivalent to several other calcium antagonists, beta-blockers, diuretics, angiotensin-converting enzyme (ACE) inhibitors and other vasodilators, and is not associated with many of the common adverse effects of these treatments. Verapamil may be preferred as an alternative first-line antihypertensive treatment to diuretics in elderly patients because it has similar efficacy in these patients without causing the adverse effects commonly linked with diuretic treatment. Furthermore, because verapamil does not cause bronchoconstriction, it may be used in preference to beta-blockers in patients with asthma or chronic obstructive airway disease. Reflex tachycardia, orthostatic hypotension or development of tolerance is not evident following verapamil administration. As a second- or third-line treatment for patients refractory to established antihypertensive regimens, verapamil produces marked blood pressure reductions when combined with diuretics and/or ACE inhibitors, beta-blockers and vasodilators such as prazosin.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of some calcium antagonists on aggregation by adrenalin and serotonin and on alpha-adrenoceptor radioligand binding in human platelets

The inhibitory effects of verapamil, nifedipine and diltiazem, representatives of different classes of calcium antagonists, were studied on aggregation of human platelets induced by adrenalin and serotonin (5-HT). For references, the alpha-adrenoceptor-antagonists phentolamine (alpha 1 and alpha 2) and rauwolscine (alpha 2), and the 5-HT 2-receptor-antagonist ketanserin were included. Verapamil in the concentration range 10(-6) 10(-4) M inhibited both adrenalin- and serotonin-induced aggregation in a concentration-dependent manner, whereas nifedipine and diltiazem had little or no effect. Phentolamine and rauwolscine were clearly weaker than verapamil as antagonists of serotonin, and ketanserin lacked effect on adrenalin-induced aggregation. Binding studies with [3H]dihydro-alpha-ergocryptine and [3H]rauwolscine on human platelet membranes showed equal numbers of binding sites, suggesting that only alpha 2-adrenoceptors were present. In the same concentration range as inhibition of aggregation was obtained, verapamil inhibited binding of either radioligand. Nifedipine, diltiazem and 5-HT were all poor inhibitors of radioligand binding. The results suggest that verapamil at high concentrations not only has alpha-adrenoceptor antagonistic properties but also exerts 5-HT-receptor blocking effects. This was not found with the other calcium channel blockers examined (nifedipine, diltiazem).

Increased platelet aggregation and release reaction in myotonic dystrophy

Platelet aggregation (PA) induced by (-)-epinephrine and adenosine diphosphate (ADP) was studied in 16 patients with myotonic dystrophy (MyD) and 14 healthy subjects. Plasma beta-thromboglobulin level (beta-TG), a useful marker of in vivo platelet release reaction, as well as in vitro 5-[14C]hydroxytryptamine (5-HT) release, were also studied. The extent of PA induced by (-)-epinephrine at 1 and 3 min and by ADP at 3 min was significantly higher in the patients than in controls. Plasma beta-TG and ADP- or epinephrine-induced platelet 5-HT release were also increased in MyD patients. These results suggest that an abnormality in release as well as in alpha 2-receptor functioning occurs in the platelets of MyD patients. The relation of this abnormality to changes in Ca2+ fluxes through the platelet membrane is discussed.

Dihydropyridine agonist Bay K 8644 inhibits platelet activation by competitive antagonism of thromboxane A2-prostaglandin H2 receptor

The dihydropyridine calcium channel antagonists, such as nifedipine, inhibit platelet aggregation in vitro and ex vivo, but the mechanism by which this occurs is uncertain. Bay K 8644 (BAY) is a substituted dihydropyridine that has effects on voltage-dependent calcium channels in cardiac and smooth muscle that are opposite the effects of nifedipine. To evaluate the mechanism responsible for dihydropyridine-induced inhibition of platelet function, we studied the in vitro effects of BAY on human platelet aggregation and secretion plus several related biochemical parameters, including cytoplasmic ionized calcium ([Ca2+]i). BAY exerted concentration-dependent effects on platelet aggregation and secretion of [14C]serotonin. BAY (1-10 microns) inhibited the second wave of platelet aggregation and secretion stimulated by adenosine diphosphate or epinephrine and blocked shape change, aggregation, and secretion induced by the thromboxane A2 (TXA2) mimic, U46619. BAY also inhibited U46619-induced phosphorylation of the approximately 40,000-dalton cytoplasmic protein substrate of protein kinase C (40K protein), formation of TXA2, and rise in [Ca2+]i, all biochemical consequences of platelet activation. The (+)-(R) enantiomer of BAY [BAY(+)] was predominantly responsible for the inhibitory effects of racemic BAY. Nifedipine had the same inhibitory effects on platelet function and biochemistry, except it was approximately 10 times less potent than BAY. Since these results suggested inhibition of the TXA2-prostaglandin H2 (PGH2) receptor, we measured binding of [3H]U46619 to intact platelets. BAY, BAY(+), and nifedipine all functioned as competitive antagonists of [3H]U46619 binding (BAY Ki = 1.47 microM). They did not inhibit binding of [3H]yohimbine to platelet alpha 2-adrenergic receptors. At 1-10 nM BAY, BAY(+) and the (-)-(S) enantiomer of BAY [BAY(-)] all resulted in slight stimulation of platelet function and biochemical events. No significant increase in [3H]U46619 binding was demonstrable, however. Therefore, dihydropyridines that function as either calcium channel agonists or antagonists in cardiac or smooth muscle exert concentration-dependent effects on platelet function. In nanomolar concentrations, they augment, and in micromolar concentrations, they inhibit platelet activation induced by TXA2 or U46619. These data indicate that dihydropyridines do not inhibit TXA2-induced platelet activation by an effect on voltage-dependent calcium channels; they define the mechanism of inhibition as competitive antagonism of the TXA2-PGH2 receptor. The mechanism responsible for augmentation of platelet activation is uncertain.(ABSTRACT TRUNCATED AT 400 WORDS)

Antiplatelet effects of oral diltiazem, propranolol, and their combination

1. The antiplatelet effects of a single oral dose of the calcium entry blocker diltiazem (60 mg), the beta-adrenoceptor blocker propranolol (40 mg), and their combination were studied in five healthy subjects. 2. Platelet aggregation and ATP release induced by adrenaline and ADP and ADP induced platelet thromboxane A2 generation were significantly inhibited (P less than 0.05) by either diltiazem or propranolol, although propranolol tended to have greater inhibitory effects on platelet function than diltiazem that did not reach statistical significance. 3. Combination therapy resulted in additive antiplatelet effects that were significantly (P less than 0.05) greater than either drug alone. 4. These data indicate that combined administration of a calcium entry blocker and a beta-adrenoceptor blocker results in additive inhibitory effects on platelet function. These effects may mediate part of the therapeutic efficacy of combination therapy in patients with coronary artery disease.

The effect of calcium channel blockers on blood platelet function, especially calcium uptake

The effects of organic and inorganic calcium antagonists on washed platelets from rat and human have been studied. Platelet aggregation was assessed by turbidimetry. Endogenous serotonin release was measured on the same sample by means of electrochemically treated carbon fiber electrodes. The organic calcium antagonist, nitrendipine, and the inorganic calcium channel blockers (Co2+, Mn2+, Cd2+, La3+) drastically inhibited rat and human platelet aggregation induced by thrombin, ADP or adrenaline in the presence of 0.32 mM Ca2+. In our conditions, the thrombin-induced release of endogenous serotonin was found to be external Ca2+-dependent and completely inhibited by 20 microM nitrendipine or 1 mM Cd2+. In addition, Ba2+ or Sr2+ ions can be substituted for Ca2+ to bring about platelet aggregation as well as endogenous serotonin secretion. In Ba2+ or Sr2+-containing media, rat platelet aggregation and/or serotonin secretion can be inhibited by either nitrendipine or Cd2+. Finally, we have also studied the thrombin- and external Ca2+-dependence of radiolabeled calcium uptake by rat platelets. We found that the thrombin-induced 45Ca uptake was inhibited by either 18 microM nitrendipine or 1 mM Cd2+. These results provide strong evidence for the existence of an influx of divalent cations (Ca2+, Sr2+, Ba2+) triggering platelet function. They also suggest, although they do not prove, that the translocation of these cations occurs through an agonist-operated channel as proposed by Hallam and Rink (FEBS Lett. 186 (1986) 175-179).

Effects of novel aggregation inhibitors on serotonin uptake by human blood platelets

Novel aggregation inhibitors blocked serotonin uptake by human blood platelets in concentrations ranging from 0.7 +/- 0.1 microM to 237.5 +/- 35.7 microM; a modified procedure, validated by kinetic analysis, was employed in which pH drift was minimized to 0.03 during the active assay period. Structural features in carbamoylpiperidine and nipecotoylpiperazine derivatives which actually constitute molecular probes, and show remarkable specificity for aggregation-inhibitory target sites, disclosed striking differences between the latter and serotonin receptors or other loci affecting serotonin uptake.

Nicardipine in the treatment of Raynaud's phenomenon. Dissociation of platelet activation from vasospasm

A new calcium channel blocker, nicardipine, was studied for treatment of Raynaud's phenomenon in a double-blind, placebo-controlled, crossover trial during the winter months. Clinical response was assessed by a patient-kept diary of symptoms and finger systolic pressure that was measured at room temperature and during cold challenge. In vivo platelet activation was determined by measuring plasma levels of the platelet-specific proteins, beta-thromboglobulin and platelet factor 4. When treatment with placebo was compared with treatment with nicardipine, no significant differences were found in the number of Raynaud's attacks per day, the severity of attacks, change in character in Raynaud's phenomenon, use of hands in winter months, patient assessment of medication or objective measurements of finger systolic pressure, and critical closing temperature. There was a reduction of plasma levels of beta-thromboglobulin and platelet factor 4 in the overall study group while taking nicardipine compared with that during the placebo period (mean change 5.0 +/- 2.4 ng/ml, P = 0.054, and 1.4 +/- 0.6 ng/ml, P less than 0.01, respectively). These results demonstrate that while nicardipine was not effective in reducing the episodes of Raynaud's phenomenon, it did inhibit in vivo platelet activation. These findings suggest that platelet activation is not the primary event in the pathogenesis of acute vasospasm in Raynaud's phenomenon, since reduction of platelet activation by the drug did not change the severity of vasospasm.

Effects of oral diltiazem on platelet function: alone and in combination with "low dose" aspirin

The effects of 3 days of oral diltiazem, "low dose" aspirin (40 mg/day), and their combination on platelet function was studied in 5 normal subjects. Both drugs inhibited platelet aggregation and ATP release induced by collagen, epinephrine and threshold concentrations of ADP. Aspirin and diltiazem decreased thromboxane A2 generation during ADP induced aggregation by 94 percent and 53 percent respectively, however both agents inhibited aggregation similarly, which suggests that diltiazem's anti-platelet effect was due to mechanisms other than inhibition of thromboxane metabolism alone. Combination therapy resulted in a partially additive inhibitory effect on ADP induced aggregation and thromboxane A2 generation. Two subjects had bleeding times over 15 minutes after receiving combination therapy.

ATP-secretion occurs as an initial response in collagen induced platelet activation

The time course and degree of collagen induced platelet shape change, aggregation and ATP-secretion in citrated human platelet rich plasma was monitored with a lumi-aggregometer. Onset of ATP-secretion was measurable within seconds after collagen challenge and always distinctly before onset of shape change. In absence of platelet aggregation, the degree of collagen stimulation was correlated to onset of secretion, the secretion rate and onset of subsequent shape change. It is concluded that the secretion originated from the collagen-adherent platelets. Oral aspirin [1000 mg] did neither delay onset of secretion nor reduce the final amounts of ATP indicating that formation of TxA2 is not a necessary prerequisite for dense granule secretion during the primary activation process. EDTA and EGTA decreased the initial secretion rate, but may have interfered with the adhesion process. Micromolar concentrations of verapamil and yohimbine also reduced secretion, while the effect of theophylline was 100 times weaker. The results suggest that the primary activation process is vulnerable at the alpha-receptor-level and that the initial ATP-secretion requires Ca2+-entry rather than intra-platelet Ca2+-mobilization.

Disparate effects of the calcium-channel blockers, nifedipine and verapamil, on alpha 2-adrenergic receptors and thromboxane A2-induced aggregation of human platelets

Calcium-channel blockers inhibit human platelet aggregation in vitro and ex vivo. To further evaluate the mechanism(s) responsible for the inhibition induced by this structurally heterogeneous group of compounds, we studied the effect of nifedipine and verapamil on human platelet aggregation in vitro. Neither 10 microM nifedipine nor 10 microM verapamil consistently inhibited the aggregation response of platelet-rich plasma to threshold concentrations of ADP, sodium arachidonate, epinephrine, or collagen. However, both 10 microM nifedipine and 10 microM verapamil epinephrine-potentiated, thromboxane A2 (TXA2)-induced aggregation of aspirin-incubated, gel-filtered platelets. Aggregation of similarly prepared platelets induced by TXA2 alone was abolished by 10 microM nifedipine but not by 10 microM verapamil. Even 100 microM verapamil gave only partial and inconsistent inhibition of aggregation. Both drugs had essentially the same effects on platelet aggregation induced by the stable endoperoxide and TXA2 mimic, U46619, with or without epinephrine. Neither 10 microM nifedipine nor 10 microM verapamil elevated platelet cyclic AMP. Verapamil (10 microM) inhibited binding of [3H]-yohimbine (an alpha 2-adrenergic receptor antagonist) to intact human platelets (KD 10.5 nM vs 2.4 nM for control platelets) without altering the number of binding sites. In contrast, 10 microM nifedipine had no effect on KD or number of binding sites. These results indicate that nifedipine and verapamil inhibit epinephrine-potentiated, TXA2-induced human platelet aggregation by different mechanisms. Verapamil inhibits the epinephrine contribution to the aggregation response by blocking alpha 2-adrenergic receptor binding. Nifedipine blocks the platelet response to TXA2 without affecting alpha-adrenergic receptor binding. These observations have potential clinical implications with regard to the mechanisms by which calcium-channel blockers inhibit vascular spasm and myocardial ischemia.

Inhibition of whole blood platelet aggregation by nicardipine, and synergism with prostacyclin in-vitro

Platelets are involved in the pathogenesis of vascular disease, and calcium channel blocking agents (CCB) such as nicardipine, are being used in the treatment of such disorders. CCB's are known to have minor anti-platelet actions from studies performed in platelet rich plasma (PRP). Recently it has become possible to study platelet aggregation in whole blood. The effects of nicardipine on whole blood platelet aggregation were studied in-vitro using the Clay-Adams Ultra Flo 100 whole blood platelet counter. Nicardipine inhibited aggregation to 0.5 micrograms/ml collagen, and 0.5 mM arachidonic acid in a dose dependent manner, but had minimal effects on aggregation to 10 microM ADP. Nicardipine also acted synergistically with prostacyclin to inhibit aggregation. The effect of nicardipine on generation of PGI2 and TxA2 from whole blood was studied. Nicardipine did not affect TxA2 production, but significantly increased PGI2 production at high concentration. The effect of nicardipine on vascular PGI2 production was also assessed using umbilical artery rings, but nicardipine had no effect on PGI2 production. This study confirms that CCBs have inhibitory actions on platelet aggregation, and this may be of value in the treatment of vascular disease.

Diltiazem: possible hematologic complications

Calcium blockers are being investigated for a potential role in the prevention and treatment of cerebral vasospasm. They have been found to inhibit platelet function both with in vivo and with in vitro experiments. Clinical evidence of abnormal platelet function is sparse, however. We are presenting three patients who developed increased bleeding times that reverted to normal when the diltiazem was discontinued. This may lead to serious consequences in the presence of an unsecured aneurysm and for patients facing major intracranial surgery.

Effects of atrial natriuretic factor on human platelet function

We examined the hypothesis that atrial natriuretic factor (ANF), a substance with known vasorelaxant activities, shares with other vasodilators the property of inhibiting platelet function. Aggregation of citrated platelet-rich plasma (PRP) from 23 healthy volunteers induced by ADP, adrenaline, arachidonic acid, collagen, gamma-thrombin, the endoperoxide analogue U-44069, serotonin, the calcium ionophore A-23187 or platelet aggregating factor was measured after incubation of PRP with ANF for 3 minutes at concentrations of 4 X 10(-9), 4 X 10(-8) and 4 X 10(-7) M or vehicle as control. ANF decreased ADP-induced aggregation significantly (P less than 0.02), but only at the highest concentration used and to a minor extent (control: 73.6 +/- 11.2%; after ANF 4 X 10(-7) M: 60.0 +/- 17.1%, mean +/- S.D., n = 39) by a selective inhibitory effect on the secondary wave; neither aggregation by all other agents tested nor thromboxane B2 generation induced by ADP and adrenaline was altered by incubation with ANF. Although ANF thus has detectable effects on ADP-induced platelet aggregation in vitro, these data suggest that ANF is unlikely to be a physiologically significant modulator of platelet function.

Verapamil attenuates stress-induced gastric ulceration

Since many of the proposed etiologic factors leading to gastric stress ulceration involve stimulation of calcium influx, the effect of verapamil, a potent calcium channel blocker, on the gastric mucosa in cold-restrained inbred rats was assessed. Twenty-nine rats received intraperitoneal normal saline (2 ml) while the experimental group (N = 29) received 1 mg/kg verapamil in an equal volume of normal saline intraperitoneally. All animals were then stressed at 4 degrees C for 4 hr and sacrificed. Gastrin and fatty acid levels were measured and blinded ulcer scoring of the gastric mucosa was carried out. Verapamil-treated animals had decreased frequency and severity of gastric stress ulceration as assessed by ulcer index, ulcer grade, and number of ulcers/animal. In addition, the plasma gastrin levels tended to be lower in the verapamil group. Fatty acid levels were similarly depressed following cold restraint in both groups. Pretreatment with verapamil significantly decreased gastric ulcerative response to cold-restraint stress in the rat. This effect of verapamil pretreatment may be secondary to cytoprotection of the gastric mucosa, preservation of gastric mucosal blood flow, or blockade of calcium-mediated ulcerogenic stimuli.

Beta-adrenoceptor antagonists and human platelets: relationship of effects to lipid solubility

Six beta-adrenoceptor antagonists (propranolol (+ and - isomers); ICI-118,551; oxprenolol; timolol; metoprolol; and practolol (+ and - isomers), chosen to represent a spectrum of physicochemical and pharmacological properties, inhibited the response of human platelets to all aggregating agents tested. For any given aggregating agent the extent of inhibition correlated with the lipid solubility of the beta-adrenoceptor antagonist and showed no relation to other properties of these compounds. Inhibition of aggregation by the beta-adrenoceptor antagonists was manifested as a parallel shift to the right in the dose-response curve. Analysis of these data according to Arunlakshana and Schild (Br. J. Pharmac. 14, 48-58 (1969] showed a dependence of the apparent pA2 on the agonist employed and gave a slope approximating unity when ADP, 9,11-epoxymethanoprostaglandin H2 (U-46619), adrenaline, 1-O-alkyl-2-acetyl-sn-glyceryl-3-phosphorylcholine (PAF) or arachidonate were used as agonists. Slopes significantly greater than unity, and approaching a value of 2, were obtained when this analysis was applied to data obtained using collagen in the presence or absence of aspirin, 12-O-tetradecanoylphorbol-13-acetate (TPA), or a divalent cation ionophore (A-23187) as agonists. Inhibition by (+/-) propranolol of secretion induced by collagen was manifested as a parallel shift to the right in the dose-response curve for collagen. The Schild plot of these data has a slope of unity. (+/-)-Propranolol inhibited thromboxane B2 production induced by collagen but over a similar concentration range had little effect on conversion of arachidonate to thromboxane B2. (+/-)Propranolol had no significant effect on the level of cyclic-3',5'-AMP (cAMP) in unstimulated platelets or on the increase in the level caused by addition of forskolin, but caused partial inhibition of the increase in platelet cAMP induced by prostaglandin E1. It also completely abolished inhibition by ADP of the increase in [cyclic-3',5'-AMP] induced by prostaglandin E1. These data are interpreted on the basis of a model in which interaction of propranolol with phosphatidylserine and phosphatidylinositol causes inhibition of phospholipases C and A2, inhibition of protein kinase C and alteration of membrane receptor properties as a consequence of distortion of their microenvironment.

Antithrombogenic effects of calcium channel blockers: synergism with prostacyclin and thromboxane synthase inhibitors

Four calcium channel blockers (nimodipine, nifedipine, verapamil and diltiazem) of three chemical classes were tested in vitro for inhibition of platelet aggregation using heparinized human platelet rich plasma. Both ADP- and thrombin-induced aggregation were inhibited as was the biosynthesis of thromboxane A2 in response to ADP or thrombin. However, the IC50's for the calcium channel blockers were greater than or equal to 110 microM. Nimodipine was also tested in combination with prostacyclin, the potent platelet antiaggregatory agent, or with a thromboxane synthase inhibitor, U63557A. At concentrations at which neither nimodipine or prostacyclin inhibited platelet aggregation greater than or equal to 10%, the two compounds is combination synergistically inhibited both ADP- and thrombin-induced platelet aggregation. U63557A inhibited biosynthesis of thromboxane A2 by platelets in response to ADP or thrombin, but did not inhibit either ADP- or thrombin-induced platelet aggregation. However, U63557A in combination with a threshold inhibitory concentration of nimodipine resulted in a synergistic inhibition of platelet aggregation induced by ADP or thrombin. These results suggest that calcium channel blockers may be of therapeutic value as a new class of antithrombogenic agents when used in combination with agents that inhibit either platelet aggregation or synthesis of platelet thromboxane A2.

Verapamil does not alter platelet function in patients with recent myocardial infarction

Reports concerning the influence of the calcium antagonist verapamil on platelet function are conflicting. In a randomized double blind trial including 52 patients with acute myocardial infarction the effect of verapamil 120 mg given perorally three times a day for three months was investigated. There were no alterations in cutaneous bleeding time, platelet aggregate ratio or platelet aggregation induced by ADP or collagen. Verapamil administered in therapeutic doses does not seem to affect platelet function in patients with ischaemic heart disease.