Effects of dipyridamole and adenosine on vasoactive peptides calcitonin gene-related peptide and atrial natriuretic peptide in humans: role of sympathetic activation

1. It has been observed that dipyridamole (DIP) administration produces equivalent cardiovascular effects at lower systemic adenosine (ADO) plasma concentrations than those obtained with exogenous ADO infusion. This observation led to the identification of DIP for additional 'ischaemia-inducing' mechanisms, possibly based on sympathetic activation. 2. In turn, exogenous ADO administration has proven to elicit a complex neurohumoral response, including an increase in the plasma concentration of catecholamines, associated with augmented levels of the vasoactive peptides calcitonin gene-related peptide (CGRP) and atrial natriuretic peptide (ANP). More particularly, increases in CGRP seem to be dependent on sympathetic activation, while changes in ANP do not. 3. In order to clarify some aspects of the activity of DIP on neurohumoral systems, the effects of administration of DIP and ADO on plasma levels of noradrenaline (NA), CGRP and ANP were studied in healthy volunteers. Haemodynamic parameters were also monitored. 4. Infusion of exogenous ADO produced plasma levels of ADO as high as 1893+/-386 nmol/L, together with a significant increase in plasma levels of CGRP, ANP and NA. Similarly, the infusion of DIP produced augmented plasma concentrations of the examined parameters, with a peak plasma ADO concentration of 470+/-49 nmol/L. 5. At a given ADO plasma concentration of 450+/-10 nmol/L, the increase in CGRP and NA levels with DIP infusion was significantly higher than that observed following the infusion of ADO, whereas the increase in the plasma concentration of ANP following DIP infusion was very similar to that seen following ADO infusion. 6. The physiological background of these findings is based on evidence that DIP displays a greater sympathoexcitatory activity than does exogenous ADO and only the increase in plasma CGRP seems to be mediated, although indirectly, by beta-adrenoceptor stimulation. The exact mechanism of DIP-dependent sympathetic activation remains to be elucidated.

Systemic adenosine increase during cold pressor test is dependent on sympathetic activation

1. Following local vasoconstriction-inducing stimuli, such as the cold pressor test (CPT), significant changes occur in haemodynamics, with a rise in arterial blood pressure and heart rate (HR) due to the activation of the sympathetic nervous system. Among the compensatory mechanisms to local ischaemia, the endogenous nucleoside adenosine (ADO) has been suggested to play a relevant role by contributing to sympathetic stimulation. The possibility was investigated that CPT-induced increases in plasma ADO levels were not only an expression of the increased production of ADO in the ischaemic area, but also a consequence of systemic sympathoexcitatory mechanisms, thus showing a bidirectional involvement of the mechanisms of ADO formation. 2. The CPT was performed in 15 volunteers and mean arterial blood pressure (MABP) and HR were evaluated, together with plasma levels of noradrenaline (NA) and ADO in the tested and contralateral arm. The 15 subjects were then divided into three groups of five that were treated with either 5 mg transdermal clonidine weekly, 100 mg atenolol daily or 600 mg aminophylline twice daily. After 1 week treatment, the same test was repeated in the respective groups. 3. The CPT induced a rise in MABP and HR and an increase in plasma levels of NA and ADO. Increases in ADO were more pronounced in the tested arm. Clonidine blunted the haemodynamic response and NA release, while increases in ADO increase were reduced to a greater extent in the contralateral arm rather than the tested arm. Atenolol only affected MABP and HR without any effect on NA and ADO levels. Theophylline did not show any effect on CPT-induced changes. 4. In conclusion, local vasoconstriction and ischaemia induced in one arm following CPT are associated with haemodynamic changes dependent on the activation of the sympathetic system. The observed increase in plasma levels of ADO seems to be, in part, a direct expression of local responses to ischaemia (pre-dominant in the tested arm), but also appears as the consequence of systemic sympathoexcitatory mechanisms. Such increases in ADO are not dependent on a beta 1-adrenoceptor-mediated mechanism. Finally, theophylline, at a therapeutic dose, has no effect on the response to CPT.

Carnitines increase plasma levels of adenosine and ATP in humans

In order to help to clarify the mode of action of carnitine derivatives, plasma levels of adenosine, ATP and inosine were evaluated following the infusion of 0.75, 0.50 and 0.25 mg/kg/min propionyl-L-carnitine (PLC) for 30 min in patients affected with peripheral arterial disease. Moreover, the effects of 0.75 mg/kg/min acetyl-L-carnitine (ALC) and L-carnitine (LC) were studied in the same conditions. Finally, the activity of 7.5 mg/kg/min PLC administered for 3 min was also evaluated. PLC and ALC produced a significant increase in plasma levels of adenosine and ATP, whereas LC induced less relevant changes. The administration of the compounds did not affect the adenosine/inosine ratio. Peak plasma levels of adenosine preceded in any case those of ATP. The possibility can be suggested that the pharmacological activity of PLC, ALC, and LC may be mediated, at least in part, by an interference with the endogenous purine system. Since these effects may be related to physiological mechanisms of tissue protection, new pharmacological perspectives for the compounds may arise.

[Usefulness of blood and plasma viscosity measurements in the evaluation of arteriopathic patients]

Blood viscosity represents the internal friction of the blood, in other terms, its resistance to be deformed and to flow under a tangential force. This function is conditioned by the intrinsic characteristics of blood and the geometry of the system in which flow occurs. Thus, any in vitro (or ex vivo) system of measurement of blood viscosity represents a relatively approximate evaluation of the ratio between intertial forces and viscous resistance. However, the possibility of measuring blood viscosity in a reproducible way provided the opportunity of identifying some clinical conditions associated with a worsening in rheologic parameters. Thus, "primary" and "secondary" hyperviscosity syndromes have been defined, on the basis of the pathogenetic responsibility of rheologic or hemodynamic factors, respectively. The possibility of performing in vitro measurement of blood viscosity provided the opportunity of planning several clinical pharmacology studies with theoretically active drugs. On such a basis, the rationale has arisen for a "rheologic therapy" of circulatory disturbances. Moreover, the evaluation of rheologic parameters is extremely useful since epidemiological studies have shown that viscosity may represent a marker and a prognostic factor for the progression of ischemic diseases as well as an independent risk factor, thus suggesting the opportunity of performing controlled clinical studies to evaluate the impact of hemorheological changes in the prevention of cardiovascular events.

Experimental model of short-time exercise-induced preconditioning in POAD patients

Regular physical exercise improves walking performance in patients affected with peripheral obliterative arterial disease (POAD). The mechanisms underlying the phenomenon are still controversial. In order to verify the hypothesis that physical conditioning of lower limbs on a treadmill and ischemic preconditioning of the heart could share some biological aspects, 14 POAD subjects underwent a training program on the treadmill consisting of five repeated submaximal exercises at five-minute and two-hour intervals preceding the maximal tolerance test. Moreover, a protocol with two daily submaximal walking exercises over one week was also performed. Pain-free and total walking distance were measured before and after they performed the program. Moreover, plasma levels of adenosine and adenosine triphosphate (ATP) were measured and polymorphonuclear (PMN) leukocyte activity was studied together with rheologic parameters. Pain-free distance was prolonged by 15.4% and 14.3%, and total distance was prolonged by 23.1% and 26.9%, in the exercises with five-minute and two-hour intervals, respectively. After one week of daily exercises, the onset of pain and the end of the test were delayed by 24% and 43.7%, respectively. An improvement in blood rheology and a reduced PMN reactivity were also observed with the three protocols, associated with an increase in plasma levels of adenosine and ATP. Similarly to ischemic preconditioning in the heart, the possibility is suggested that an adenosine-mediated mechanism may contribute to the development of physical conditioning in treadmill-trained POAD patients.

Pharmacological preconditioning of ischaemia

Five repeated submaximal treadmill exercises at 2 h intervals following a maximal test prolong walking distance and reduce haemorheological derangement in a second maximal test in patients affected with peripheral obliterative arterial disease (POAD). An increase in adenosine plasma levels is observed during maximal tests, thus suggesting the possibility of an ischaemic preconditioning of lower limbs. The intravenous infusion of 50-100-200 mg buflomedil, and the oral administration of 300-600-900 mg of the drug in POAD patients, also produce an increase in plasma levels of adenosine. Finally, 600 mg buflomedil orally at 12 h intervals produced pulse increase in adenosine plasma levels without any accumulation of the drug or adenosine for at least one week. The possibility of a pharmacological preconditioning of ischaemia is suggested.

Pharmacological preconditioning of ischemic heart disease by low-dose dipyridamole

Fourteen patients affected with coronary artery disease underwent two consecutive dipyridamole echocardiographic stress tests, in basal conditions and after repeated low doses of intravenous dipyridamole, following the observation that pulse increases in adenosine plasma levels due to repeated intravenous administration of dipyridamole mimic the mechanism of ischemic preconditioning. Echocardiographic, electrocardiographic, haemodynamic parameters, and adenosine plasma levels were measured. After the second test, six patients were completely negative, and in those eight still positive the onset of dyssynergy was delayed.

Pharmacokinetics and pharmacodynamics of neutrophil-associated ciprofloxacin in humans

OBJECTIVE

To study the possibility that the penetration of the antibiotic ciprofloxacin into polymorphonuclear leukocytes (PMN) may be associated with some changes in cell reactivity.

DESIGN

Superoxide anion and chemiluminescence generation induced by formyl-methionyl-leucyl-phenylalanine (fMLP) and platelet-activating factor (PAF) were studied ex vivo in 12 healthy volunteers (mean age, 53.15 +/- 16.3 years; mean body weight, 71.23 +/- 6.9 kg) at fixed intervals up to 72 hours from the administration of a single oral dose of 250 mg ciprofloxacin. Cytosolic free calcium levels ([Ca2+]i) in resting and stimulated cells were also evaluated. The dynamic parameters of the effects on PMNs were compared with the kinetic profile of the drug in plasma and in PMNs.

RESULTS

Superoxide generation induced by the stimulating agents increased significantly, reaching a peak after 12 hours (+116% [p < 0.001] for fMLP and +66% [p < 0.05] for PAF). Similarly, chemiluminescence production showed a threefold increase in the response to the stimulating agents 12 hours after drug administration (p < 0.001). The increase in [Ca2+]i in stimulated PMNs was significantly potentiated (p < 0.001). The mathematic analysis of the effects of ciprofloxacin showed that time to maximal activity was between 10.4 hours (PAF-dependent [Ca2+]i increase), and 15 hours (fMLP-induced superoxide anion and chemiluminescence production). The ratio of PMNs to plasma ciprofloxacin concentration increased progressively, from 0.5 at 30 minutes to 10.4 after 24 hours. In addition, time to maximal activity and half-life differed in PMNs and in plasma (4.66 versus 1.90 hours and 13.03 versus 7.28 hours, respectively).

CONCLUSIONS

Ciprofloxacin administration induced a long-lasting enhancement of PMN reactivity to fMLP and PAF. The levels of the drug in the cells were greater and more sustained in the time than those in plasma.

Increase in plasma levels of adenosine and adenine nucleotides after intravenous infusion of buflomedil in humans

To clarify the mode of action of the vasoactive agent buflomedil, we evaluated plasma levels of adenosine and adenine nucleotides after intravenous (i.v.) infusion in humans of 50, 100, and 200 mg of the drug in 20 min. Buflomedil induced an increase of the same order of magnitude in plasma levels of adenosine and adenine nucleotides. Maximal adenosine increase (84%) was observed at the end of the infusion period, whereas peak plasma levels of ATP and ADP (69 and 55%, respectively) and of AMP (61%) were detected 10 and 5 min after discontinuation of infusion, respectively. Although the exact mode of action of buflomedil at the molecular level remains unclear, some indirect findings suggest that the increase in adenosine may be due to enhanced release rather than to inhibition of cell uptake. Because such activity of buflomedil consists of enhancement of physiologic mechanisms of vasodilation and tissue protection occurring in the course of ischemic events, new pharmacologic perspectives for the drug may arise.

Inhibition of neutrophil function in vitro by nimesulide. Preliminary evidence of an adenosine-mediated mechanism

Nimesulide (CAS 51803-78-2) is a methane sulphoanilide derivative provided with specific anti-inflammatory activity. In human polymorphonuclear leucocytes (PMNs), the activity of nimesulide has been suggested to be based on the inhibition of the oxidative burst. However, the effect of the compound on PMNs function seems to be very complex. In order to give a major insight into the mechanism of action of nimesulide, the effect of the drug was studied in vitro on human PMNs functions, such as free radical generation and enzyme release, and on cytosolic free calcium levels, following the activation with specific stimuli. Moreover, the hypothesis that nimesulide could act by interfering with the adenosine cell receptor system was also evaluated. Nimesulide (1-50 mumol/l showed a dose-dependent inhibitory activity on superoxide anion and chemiluminescence production from PMNs stimulated with the oligopeptide fMLP, the ionophore A23187, and the phorbol ester PMA. Enzyme release was significantly reduced, when fMLP and A23187 represented the stimulating agents, while no effect at all was observed with PMA. Studies with the fluorescent calcium chelating dye FURA 2/AM showed that nimesulide was able to reduce free cytosolic calcium increase produced by fMLP and the ionophore ionomycin. The preincubation of cells with the specific adenosine receptor antagonist theophylline was able to significantly reverse the inhibitory activity of nimesulide, either on free radical production and enzyme release, and on free cytosolic calcium increase sustained by fMLP and the ionophores.(ABSTRACT TRUNCATED AT 250 WORDS)

Isradipine inhibits PMN leukocyte function. A possible interference with the adenosine system

The dihydropyridine derivative isradipine is able to inhibit PMN leukocyte function, such as enzyme release and free radical generation, following the activation with specific stimuli. Moreover, the drug prevents calcium influx into the cells as detected by the specific fluorescent dye FURA 2/acetoxymethylester. The specific adenosine receptor antagonist theophylline is able to partially remove the inhibiting activity, thus suggesting a possible interference of isradipine with the adenosine system. Such a cell-protecting activity adds further rationale to the employment of isradipine in those conditions, such as acute and chronic ischaemia and reperfusion damage, in which PMN leukocyte-dependent tissue injury represents a relevant pathogenetic mechanism.

Pharmacodynamics of salmon calcitonin in humans: new markers of pharmacological activity

In order to define the pharmacodynamic profile of salmon calcitonin (sCT) in humans, several markers of the biological activity of the drug have been studied, namely cAMP, adenosine and pO2 in venous blood, and the cytosolic free calcium level in circulating cells. Different dosages and routes of administration (1.5 IU.kg-1 and 0.75 IU kg-1 IM, and 1.5 IU.kg-1 via nasal spray) were compared. sCT caused an increase in cAMP, adenosine and pO2, and a decrease in cytosolic free calcium in neutrophils, lymphocytes and platelets. The peak times of all these parameters ranged between 109 and 182 min, and 101 and 168 min after IM and nasal spray administration respectively. There was greater variability in the values after IM than nasal spray of administration of sCT. It is concluded that adenosine and pO2 in venous blood, and cytosolic free calcium in circulating cells are valuable markers of the activity of sCT and that sCT decreases the cytosolic free calcium level in neutrophils, lymphocytes and platelets. Pharmacodynamic analysis of the biological effects of the drug is highly reliable and valuable in predicting its pharmacological profile. sCT administration via a nasal spray is able to produce significant biological effects, although they are less marked than after IM dosing.

Defibrotide inhibits Ca2+ dependent neutrophil activation: implications for its pharmacological activity in vascular disorders

Defibrotide (DEF) is a polydeoxyribonucleotide agent provided with profibrinolytic and antithrombotic properties. Moreover, an antiischemic, cardioprotective effect of the drug has recently been demonstrated in experimental animals. Increasing evidence exists of the important role played by neutrophils in the development of tissue damage during chronic and acute ischemia and in the early phases of reperfusion. In order to evaluate whether the overall cytoprotective effect of DEF could be based, at least in part, on a neutrophil-involving mechanism, the authors studied the in vitro effects of the drug on human neutrophil activation triggered by several specific stimuli. The drug dose-dependently (10-100 microM) inhibited enzyme release, superoxide anion generation, and chemiluminescence induced in neutrophils by the chemoattractant oligopeptide fMLP and by the divalent cation ionophores A23187 and ionomycin. The increase of extracellular calcium concentration from 0.5 to 2.0 mM antagonized the inhibitory effect of DEF. The use of the fluorescent probe Fura 2/AM led them to show that DEF is able to reduce the cytosolic free calcium increase following specific stimulation by affecting extracellular calcium entrance. Such a behavior resembles that of calcium-antagonistic drugs, thus suggesting that DEF works, at least in part, similarly to calcium entry blockers. Such an activity on cell calcium translocation could represent the underlying molecular mechanism of cytoprotection. Finally, the inhibitory action on neutrophil functions may play a role in tissue protection during ischemic injury.

Differential effect of adenosine on DNA synthesis in lymphoid and myeloid cell lines

In this report, the effects of adenosine on the promyelocytic cell line HL-60 and on T-lymphocytic clones are compared. According to previous reports, adenosine induces a dose-dependent inhibition of DNA synthesis in T-lymphocytes. Conversely, adenosine dose-dependently enhances DNA synthesis in HL-60 cells, as documented with [3H]thymidine uptake studies and flow cytometric cell-cycle analysis. Unlike its effect on lymphocytes, the adenosine effect on HL-60 cells does not seem to be mediated by receptor binding, but it appears to be correlated with an intracellular mechanism following active uptake. Despite the different effects exerted by adenosine on lymphocytes and myeloid cells, a purinergic pathway appears to be more generally involved in the regulation of some phases of cell growth.

Nitroprusside in vitro inhibits platelet aggregation and intracellular calcium translocation. Effect of haemoglobin

The biologically final active compound of nitrovasodilators is now supposed to be nitric oxide (NO), a labile substance identical to EDRF. The effects of nitroprusside on platelet functions were studied in vitro. Platelet aggregation induced by several stimuli (ADP, collagen, arachidonic acid and PAF) was inhibited by increasing concentrations of the drug (1-50 uM); interestingly, the potency of nitroprusside is higher when PAF is employed as stimulating agent in comparison with the other agonists (ED50 = 2 uM for ADP, 2.5 uM for A.A., 4.5 uM for collagen and 0.3 uM for PAF-induced aggregations). The concomitant addition of haemoglobin is able to reverse the inhibitory effect of nitroprusside, according to the view that haemoglobin possesses a high affinity for NO, thus antagonizing the effect of this compound. Nitroprusside was also able to inhibit intracellular calcium translocation, as studied with the Quin 2 technique, induced by PAF and arachidonic acid. Fron these observations the hypothesis may be suggested that nitroprusside inhibits platelet functions by mimicking the endogenous NO, and that the intracellular calcium metabolism is involved in the inhibitory activity of the drug.

Selective 1H-NMR relaxation investigations of membrane-bound drugs in vitro. 3. Calcium-entry blockers and adenosine

Selective proton relaxation rates (SPRR) were measured for selected protons of nimodipine or diltiazem in the presence of neutrophils, allowing detection of binding to the cell membrane. Fast exchange exchange of drug molecules between bound and free environments was shown to be the main factor determining the enhancement of SPRR, whereas viscosity effects could be neglected. The SPRR enhancement was almost completely cancelled out by the presence of adenosine as a cosolute in a dose-dependent fashion, leading to the suggestion that the endogenous mediator 'adenosine' affects binding of calcium-entry blockers to the neutrophil surface.

Adenosine blocks calcium entry in activated neutrophils and binds to flunarizine-sensitive calcium channels

Adenosine is able to prevent Ca+(+) influx into activated neutrophils as detected by the specific fluorescent indicator Quin 2. Such an effect is shown in a similar fashion by the calcium entry blocker flunarizine. The binding interaction between flunarizine and the neutrophil membrane as well as the flunarizine-adenosine antagonism are shown by the 1H-NMR technique, thus supporting evidence of a competition between the agents at the same or a nearby site on the cell membrane.

Adenosine system and cell calcium translocation: interference of calcium channel blockers

Adenosine is able to inhibit in vitro neutrophil functions induced by formyl-methionyl-leucyl-phenylalanine (FMLP) and A23187, but not phorbol 12-myristate 13-acetate (PMA). The inhibiting activity on A23187 is reversed by increasing extracellular Ca2++ concentration. The calcium entry blocker flunarizine shows an activity very similar to that of adenosine. Both adenosine and flunarizine prevent Ca++ influx into activated neutrophils as detected by the fluorescent Ca++ chelator Quin-2. Finally, flunarizine binds to the neutrophil membrane and adenosine competitively inhibits flunarizine binding as assessed by 1H-Nuclear Magnetic Resonance (1H-NMR) technique, thus indicating that the two agents share a common binding site on the cell membrane.

New in vivo model to assess venous endothelial cell functions. Effect of defibrotide

In the past few years there has been increasing interest in the role of the vascular endothelium as an active modulator of biological responses. Endothelial cells exert antithrombotic activity by the release of prostacyclin [23] and adenine nucleotides [16], the availability on the cell surface of heparin-like substances [3], and thrombomodulin-mediated activation of protein C [8]. In addition, endothelium is involved in the regulation of fibrinolysis by releasing soluble factors, such as tissue plasminogen activator (tPA; [10]) and plasminogen activator inhibitor (PAI; [22, 11]), as well as in the control of vascular responsiveness by the production of smooth muscle relaxing and contracting factors. Endothelial cells have also been shown to synthesize and to express procoagulant activities [18]. Many data on endothelial cell functions has been obtained from two experimental models, namely endothelial cell cultures and perfused segments of animal and human vessels. Both are subject to methodological criticism since they only represent in part in vivo conditions, and the necessary experimental manipulations and laboratory procedures greatly modify the naturally occurring cellular functions. In order to overcome such difficulties as far as possible, a new in vivo model has been employed to provide easily assessable and reliable data on the properties of endothelial cells in man. A venous segment was isolated functionally by cannulating a dorsal vein in the hand and a cubital vein in the same arm. Changes observed ex vivo in blood from the cubital vein following infusion into the hand vein of an active drug, can mainly be attributed to its local effect on the venous wall. At the same time, a cubital vein in the other arm was cannulated in order to provide information to distinguish systemic from regional effects.