Clinical and Experimental Pharmacology and Physiology (1992) 19, 599-602
SHORT COMMUNICATION
XANTHINES INHIBIT HUMAN PLATELET AGGREGATION INDUCED BY PLATELET-ACTIVATING FACTOR N. L. A. Misso and P. J. Thompson
Department of Medicine, University of Western Australia, Queen Elizabeth I1 Medical Centre, Nedlands, Western Australia (Received 23 January 1992; revision received 16 April 1992)
SUMMARY 1 . Platelet-activating factor (PAF) may be involved in the pathogenesis of asthma, and therefore the effects of the anti-asthma drugs theophylline and enprofylline on human platelet aggregation and adenosine triphosphate (ATP) release induced by PAF and adenosine diphosphate (ADP) were studied. 2. Enprofylline (50% inhibitory concentration [IC~O] = 94.8 zk 13.2 pmol/ L) was more potent than theophylline ( I G o = 934.1 & 40.1 pmol/ L) as an inhibitor of PAF-induced aggregation, and the xanthines were twice as potent as inhibitors of PAF-induced aggregation when compared with ADP-induced aggregation. ATP release was 1.4 times more sensitive to inhibition by the xanthines than aggregation. 3. Although high concentrations of xanthines inhibited platelet aggregation and ATP release induced by PAF, therapeutic concentrations are unlikely to inhibit PAF-induced effects.
Key words: platelet-activating factor, platelet aggregation inhibitors, xanthines.
INTRODUCTION Platelet-activating factor (PAF; 1-0-alkyl-2-acetyl-mglycero-3-phosphocholine) is a mediator of inflammation (Page et al. 1984) and a potent stimulus for bronchoconstriction (Cuss et al. 1986) and platelet aggregation and release (McManus et al. 1981). The release of PAF from inflammatory cells may be important in the pathogenesis of asthma (Braquet et al. 1987; Smith 1991), and this has stimulated research into the effects of anti-asthma drugs on PAF-induced responses (Pretolani et al. 1989).
Theophylline is routinely used in the clinical management of asthma, and has been reported to inhibit PAF-induced bronchoconstnction and bronchial hyperreactivity in the guinea-pig (Morley et al. 1988; Pretolani et al. 1989). Enprofylline (3-propylxanthine) is 3-4 times more potent than theophylline as a bronchodilator (Lunell et al. 1984), and unlike theophylline is not an adenosine receptor antagonist (Vinge et al. 1984). Xanthines are known to inhibit ADP-induced platelet aggregation (Ardlie et al. 1967; Vinge et al.
Correspondence:Associate Professor P. J. Thompson, University Department of Medicine, Queen Elizabeth I1 Medical Centre, Nedlands, WA 6009, Australia.
N . L. A. Miss0 and P. J. Thompson
600
1984), but their effects on PAF-induced platelet aggregation have not previously been reported. In the present study the effects of theophylline and enprofylline on PAF-induced human platelet aggregation and ATP release were compared with their effects on platelet responses to ADP.
mined by interpolation on the linear portion of the sigmoidal curves of per cent inhibition versus xanthine concentration for each subject. The mean ( *s.e.m.) ICSO values for platelet preparations from eight subjects were then calculated.
RESULTS METHODS
The maximum aggregation response to PAF was PAF (1-O-hexadecyl-2-acetyl-sn-glycero-3-phosphocho- obtained at a mean (*s.e.m.) minimum PAF concenline; Sigma Chemical Co, St Louis, MO, USA) was (a) dissolved in phosphate-buffered saline (PBS); pH 7.4 100 containing 0.25% human serum albumin. Luciferase-luciferin (Sigma) was dissolved in sterile water 80 and theophylline (Sigma) and enprofylline (Astra Pharmaceuticals, Sydney, Australia) were dissolved 60 in PBS. Blood was taken from eight fasting, healthy volunteers who gave informed consent and had received no 40 medication in the previous 10 days. The blood was anticoagulated with a 1 O : l volume of 3.8% sodium 20 citrate, and was centrifuged at 150 g for 10 rnin to generate platelet-rich plasma (PRP). Platelet-poor 0 plasma (PPP) was obtained by centrifuging the remaining blood at 1200 g for 10 min. Platelet aggregation ;-20 and ATP release were measured simultaneously in a I I el c -5 -4 -3 -2 Chrono-log lumiaggregometer (Model 550) with a a m dual pen recorder. For each assessment of platelet aggregation and ATP release, P R P (450 pL), luck ferase-luciferin (25 pL) and PBS (control) or xanthine (50 pL) were incubated at 37OC for 2 min. PAF or ADP was then added to give the required final concentration. The pre-incubation time of 2 min was chosen because tests in our laboratory have demonstrated no substantial increase in the degree of inhibition with incubation times of up to 10 min and because other studies (Ardlie et al. 1967; Vinge et al. 1984) have used 2-3 min pre-incubation times. The extent of platelet aggregation was measured by calculating the maximum light transmission achieved as a percentage of the light transmission through autologous PPP. ATP release was calibrated by adding a known amount of ATP to a sample of PRP containing luciferase-luciferh. Each xanthine was assessed on separate occasions -20 L , 1 I I using P RP preparations from the same eight subjects. -5 -4 -3 -2 The minimum concentrations of PAF and ADP that log [XanIhine concentration] (rnoltL) produced a maximum aggregation response were first determined for each individual. This predetermined Fig. 1. Inhibition of (a) PAF-induced platelet aggregation PAF or ADP concentration was subsequently used to and (b) ATP release by theophylline ( a and) enprofylline evaluate the inhibition of aggregation and ATP release (-+--). Inhibition of responses by xanthines were calculated by each xanthine as a percentage of the response as percentages of the responses obtained in control tests obtained in control tests with PBS. The concentration with PBS. Values are mean ( fs.e.rn.) results of experiments was deterwith PRP preparations from eight subjects. producing a 50% inhibition of response (IC~O) I
0
Xanthines and PA F-induced platelet activation
tration of 56.5f4.9 nmol/L (range 35.3-106). ATP release reached a mean maximum of 4.6 k 0.6 pmol/ L at a mean PAF concentration of 60.1 f9.3 nmol/ L. A secondary wave of aggregation was observed for all control aggregations and also with the lower concentrations of xanthines. At higher concentrations only a reversible primary wave of aggregation was observed. Enprofylline concentrations of 26 to 515 pmol/L inhibited PAF-induced platelet aggregation and ATP release, while theophylline was effective at concentrations ranging from 500 to 2770 pmol/L (Fig. 1). As an inhibitor of aggregation, enprofylline (1C50 = 94.8 f 13.2 pmol/ L) was 10 times more potent than theophylline (Ic50 = 934.1 f40.1 pmol/ L). ATP release was 1.4 times more sensitive to inhibition by the xanthines than platelet aggregation, and enprofylline (I& = 68.4+4.7 pmol/L) was again 10 times more potent than theophylline (Ic50 = 671.5 k 6 7 . 1 pmol/ L). The minimum ADP concentrations (7.5-15 pmol/ L) producing maximum responses were used to stimulate platelets in the presence of three concentrations of xanthines. ADP-induced responses were less sensitive to inhibition by the xanthines than PAF-induced responses. Enprofylline (IGo = 218 42 pmol/ L) was nine times more potent than theophylline ( I G o = 199931 172 pmol/L) as an inhibitor of ADP-induced platelet aggregation. ATP release stimulated by ADP was 2.1 times more sensitive to inhibition by the xanthines than platelet aggregation, and enprofylline (I& = 102 f21 pmol/ L) was again nine times more potent than theophylline ( I G o = 940 245 pmol/ L).
*
*
DISCUSSION This study has investigated the previously unknown effect of xanthines on PAF-induced responses in human platelets, and has shown that platelet aggregation and ATP release induced by PAF were more potently inhibited by enprofylline and theophylline than the corresponding responses induced by ADP. The inhibitory effects on ADP-induced platelet responses were comparable to those reported by Vinge et al. (1984). ADP, thrombin and collagen activate platelets by mobilizing intracellular calcium (Ardlie 1982), and PAF also induces an influx of calcium into platelets (Valone & Johnson 1985; Selak & Smith 1989). Xanthines and other phosphodiesterase inhibitors probably inhibit platelet activation by elevating intracellular cyclic A M P concentrations and reducing cytoplasmic free calcium mobilization (Ardlie et al. 1967; Lanza el al. 1987). The concentrations of theophylline that
60 1
inhibited aggregation in the present study have previously been shown to inhibit platelet phosphodiesterase (Mills & Smith 1971), although an increase in platelet cyclic AMP concentrations has not always been demonstrated (Burns & Dodge 1984). PAFinduced aggregation may require a greater increase in intracellular calcium than is required for ADP-induced aggregation (Ardlie 1982), which possibly explains the greater potency of the xanthines as inhibitors of the response to PAF. Enprofylline is 5-6 times more potent than theophylline as an inhibitor of human platelet cyclic AMP phosphodiesterase (Kilian et ~ l1985), . which possibly explains the greater potency of enprofylline compared with theophylline as inhibitors of PAF- and ADPinduced responses in the present study and ADPinduced aggregation in the study by Vinge et al. (1984). A greater mobilization of intracellular calcium is required for secondary aggregation and release to occur (Detwiler et al. 1978), and since xanthines reduce intracellular calcium, this may explain the observation that platelet ATP release was more potently inhibited than aggregation for both PAF and ADP. Inhibition of PAF-induced platelet aggregation and ATP release by xanthines is not mediated by specific antagonism of the PAF receptor, and is only apparent at concentrations higher than those used clinically. Consistent with this, we have previously shown that ex vivo platelet responses to PAF are not inhibited 2.5 h after the administration of an oral therapeutic dose of theophylline (Thompson et al. 1987). PAF-induced effects in asthma are therefore unlikely to be inhibited at therapeutic plasma concentrations of theophylline (55-110 pmol/L) or enprofylline (5-25 pmol/L), a suggestion further supported by the observation that therapeutic concentrations of theophylline had no effect on bronchoconstriction or airway hyperresponsiveness induced by inhaled PAF (Chung et al. 1989).
REFERENCES Ardlie, N. G., Clew, G., Schulz, B. G . & Schwartz, C. J.
(1967) Inhibition and reversal of platelet aggregation by methylxanthines. Thrombosis et Diathesis Haemorrhagica, 18, 670-673. Ardlie, N. G. (1982) Calcium ions, drug action and platelet function. Pharmacology and Therapeutics, 18,249-270. Braquet, P., Touqui, L., Shen, T. Y. & Vargaftig, B. B. (1 987) Perspectives in platelet-activating factor research. Pharmacological Reviews, 39, 97- 145. Burns, G. B. & Dodge, J. A. (1984) Theophylline inhibits
602
N. L. A . Miss0 and P. J. Thompson
platelet aggregation, prostaglandin and thromboxane production by a mechanism which is independent of cyclic AMP. Agents and Actions, 14, 102-108. Chung, K. F., Lammers, J. W., McCusker, M., Roberts, N. M., Nichol, G . M. & Barnes, P. J. (1989) Effect of theophylline on airway responses to inhaled plateletactivating factor in man. European Respiratory Journal, 2,763-167. Cuss, F. M., Dixon, C. M. S. & Barnes, P. J. (1986) Effects of inhaled platelet activating factor on pulmonary function and bronchial responsiveness in man. Lancet, 2, 189- 192. Detwiler, T. C., Charo, I. F. & Feinman, R. D . (1978) Evidence that calcium regulates platelet function. Thrombosis and Haemostasis, 40, 207-21 1, Kilian, U., Amschler, H., Beume, R., Eltze, M., Kolassa, N. & Schudt, C. (1985) Bronchospasmolytic effects of substituted 6-phenyl-3-(2H)-pyridazinones in comparison to theophylline and enprofylline. In: Anti-asthma Xanthines and Adenosine, (Ed. K . E. Andersson & C. G. A. Persson), pp. 209-213. Excerpta Medica, Amsterdam. Lanza, F., Beretz, A., Stierle, A., Corre, G . & Cazenave, J. P. (1987) Cyclic nucleotide phosphodiesterase inhibitors prevent aggregation of human platelets by raising cyclic A M P and reducing cytoplasmic free calcium mobilization. llrombosis Research, 45,477-484. Lunell, E., Andersson, K. E., Persson, C. G. A. & Svedmyr, N. (1984) Intravenous enprofylline in asthma patients. European Journal of Respiratory Diseases, 65, 28-34. McManus, L. M., Hanahan, D. J. & Pinckard, R. N. (198 1) Human platelet stimulation by acetyl glyceryl ether phosphorycholine. Journal of Clinical Investigation, 67,903906. Mills, D. C. B. & Smith, J. B. (1971) The influence on
platelet aggregation of drugs that affect the accumulation of adenosine 3':S-cyclic monophosphate in platelets. Biochemical JournaI, 121, 185-196. Morley, J., Sanjar, S., Boubekeur, K., Aoki, S. & Kristersson, A. (1988) Pharmacological evaluation of prophylactic anti-asthma drugs by reference to the pathological sequelae of exposure to allergen or platelet activating factor. Agents and Actions Supplement, 23, 187-194. Page, C. P., Archer, C. B., Paul, W. & Morley, J. (1984) PAF-acether: A mediator of inflammation in asthma. Zkends in Pharmacological Sciences, 5, 239-241. Pretolani, M., Lopez-Ferrer, P. & Vargaftig, B. B. (1989) From anti-asthma drugs to PAF-acether antagonism and back - Present status. Biochemical Pharmacology, 38, 1373-1384. Selak, M. A. & Smith, J. B. (1989) Platelet-activating factor-induced calcium mobilization in human platelets and neutrophils: Effects of PAF-acether antagonists. Journal of Lipid Mediators, 1, 125-137. Smith, L. J. (1991) The role of platelet-activating factor in asthma. American Review of Respiratory Disease, 143, SIOO-102. Thompson, P. J., Misso, N., Heitz, M., Gillon, R. L. & Phillips, M. J. (1987) The effect of xanthines on platelet responses to platelet activating factor (PAF). American Review of Respiratory Disease, 135, (Suppl.), A395 Valone, F. H. & Johnson, B. J. (1985) Modulation of cytoplasmic calcium in human platelets by the phospholipid platelet-activating factor 1-0-alkyl-sn-glycero-3-phosphocholine. Journal of Immunology, 134, 1120-1 124. Vinge, E., Andersson, K. E. & Persson, C. G. A. (1984) Effects on aggregation of human platelets of two xanthines and their interactions with adenosine. Acta PhysioIogica Scandinavica, 120, 117-121.
SHORT COMMUNICATION
XANTHINES INHIBIT HUMAN PLATELET AGGREGATION INDUCED BY PLATELET-ACTIVATING FACTOR N. L. A. Misso and P. J. Thompson
Department of Medicine, University of Western Australia, Queen Elizabeth I1 Medical Centre, Nedlands, Western Australia (Received 23 January 1992; revision received 16 April 1992)
SUMMARY 1 . Platelet-activating factor (PAF) may be involved in the pathogenesis of asthma, and therefore the effects of the anti-asthma drugs theophylline and enprofylline on human platelet aggregation and adenosine triphosphate (ATP) release induced by PAF and adenosine diphosphate (ADP) were studied. 2. Enprofylline (50% inhibitory concentration [IC~O] = 94.8 zk 13.2 pmol/ L) was more potent than theophylline ( I G o = 934.1 & 40.1 pmol/ L) as an inhibitor of PAF-induced aggregation, and the xanthines were twice as potent as inhibitors of PAF-induced aggregation when compared with ADP-induced aggregation. ATP release was 1.4 times more sensitive to inhibition by the xanthines than aggregation. 3. Although high concentrations of xanthines inhibited platelet aggregation and ATP release induced by PAF, therapeutic concentrations are unlikely to inhibit PAF-induced effects.
Key words: platelet-activating factor, platelet aggregation inhibitors, xanthines.
INTRODUCTION Platelet-activating factor (PAF; 1-0-alkyl-2-acetyl-mglycero-3-phosphocholine) is a mediator of inflammation (Page et al. 1984) and a potent stimulus for bronchoconstriction (Cuss et al. 1986) and platelet aggregation and release (McManus et al. 1981). The release of PAF from inflammatory cells may be important in the pathogenesis of asthma (Braquet et al. 1987; Smith 1991), and this has stimulated research into the effects of anti-asthma drugs on PAF-induced responses (Pretolani et al. 1989).
Theophylline is routinely used in the clinical management of asthma, and has been reported to inhibit PAF-induced bronchoconstnction and bronchial hyperreactivity in the guinea-pig (Morley et al. 1988; Pretolani et al. 1989). Enprofylline (3-propylxanthine) is 3-4 times more potent than theophylline as a bronchodilator (Lunell et al. 1984), and unlike theophylline is not an adenosine receptor antagonist (Vinge et al. 1984). Xanthines are known to inhibit ADP-induced platelet aggregation (Ardlie et al. 1967; Vinge et al.
Correspondence:Associate Professor P. J. Thompson, University Department of Medicine, Queen Elizabeth I1 Medical Centre, Nedlands, WA 6009, Australia.
N . L. A. Miss0 and P. J. Thompson
600
1984), but their effects on PAF-induced platelet aggregation have not previously been reported. In the present study the effects of theophylline and enprofylline on PAF-induced human platelet aggregation and ATP release were compared with their effects on platelet responses to ADP.
mined by interpolation on the linear portion of the sigmoidal curves of per cent inhibition versus xanthine concentration for each subject. The mean ( *s.e.m.) ICSO values for platelet preparations from eight subjects were then calculated.
RESULTS METHODS
The maximum aggregation response to PAF was PAF (1-O-hexadecyl-2-acetyl-sn-glycero-3-phosphocho- obtained at a mean (*s.e.m.) minimum PAF concenline; Sigma Chemical Co, St Louis, MO, USA) was (a) dissolved in phosphate-buffered saline (PBS); pH 7.4 100 containing 0.25% human serum albumin. Luciferase-luciferin (Sigma) was dissolved in sterile water 80 and theophylline (Sigma) and enprofylline (Astra Pharmaceuticals, Sydney, Australia) were dissolved 60 in PBS. Blood was taken from eight fasting, healthy volunteers who gave informed consent and had received no 40 medication in the previous 10 days. The blood was anticoagulated with a 1 O : l volume of 3.8% sodium 20 citrate, and was centrifuged at 150 g for 10 rnin to generate platelet-rich plasma (PRP). Platelet-poor 0 plasma (PPP) was obtained by centrifuging the remaining blood at 1200 g for 10 min. Platelet aggregation ;-20 and ATP release were measured simultaneously in a I I el c -5 -4 -3 -2 Chrono-log lumiaggregometer (Model 550) with a a m dual pen recorder. For each assessment of platelet aggregation and ATP release, P R P (450 pL), luck ferase-luciferin (25 pL) and PBS (control) or xanthine (50 pL) were incubated at 37OC for 2 min. PAF or ADP was then added to give the required final concentration. The pre-incubation time of 2 min was chosen because tests in our laboratory have demonstrated no substantial increase in the degree of inhibition with incubation times of up to 10 min and because other studies (Ardlie et al. 1967; Vinge et al. 1984) have used 2-3 min pre-incubation times. The extent of platelet aggregation was measured by calculating the maximum light transmission achieved as a percentage of the light transmission through autologous PPP. ATP release was calibrated by adding a known amount of ATP to a sample of PRP containing luciferase-luciferh. Each xanthine was assessed on separate occasions -20 L , 1 I I using P RP preparations from the same eight subjects. -5 -4 -3 -2 The minimum concentrations of PAF and ADP that log [XanIhine concentration] (rnoltL) produced a maximum aggregation response were first determined for each individual. This predetermined Fig. 1. Inhibition of (a) PAF-induced platelet aggregation PAF or ADP concentration was subsequently used to and (b) ATP release by theophylline ( a and) enprofylline evaluate the inhibition of aggregation and ATP release (-+--). Inhibition of responses by xanthines were calculated by each xanthine as a percentage of the response as percentages of the responses obtained in control tests obtained in control tests with PBS. The concentration with PBS. Values are mean ( fs.e.rn.) results of experiments was deterwith PRP preparations from eight subjects. producing a 50% inhibition of response (IC~O) I
0
Xanthines and PA F-induced platelet activation
tration of 56.5f4.9 nmol/L (range 35.3-106). ATP release reached a mean maximum of 4.6 k 0.6 pmol/ L at a mean PAF concentration of 60.1 f9.3 nmol/ L. A secondary wave of aggregation was observed for all control aggregations and also with the lower concentrations of xanthines. At higher concentrations only a reversible primary wave of aggregation was observed. Enprofylline concentrations of 26 to 515 pmol/L inhibited PAF-induced platelet aggregation and ATP release, while theophylline was effective at concentrations ranging from 500 to 2770 pmol/L (Fig. 1). As an inhibitor of aggregation, enprofylline (1C50 = 94.8 f 13.2 pmol/ L) was 10 times more potent than theophylline (Ic50 = 934.1 f40.1 pmol/ L). ATP release was 1.4 times more sensitive to inhibition by the xanthines than platelet aggregation, and enprofylline (I& = 68.4+4.7 pmol/L) was again 10 times more potent than theophylline (Ic50 = 671.5 k 6 7 . 1 pmol/ L). The minimum ADP concentrations (7.5-15 pmol/ L) producing maximum responses were used to stimulate platelets in the presence of three concentrations of xanthines. ADP-induced responses were less sensitive to inhibition by the xanthines than PAF-induced responses. Enprofylline (IGo = 218 42 pmol/ L) was nine times more potent than theophylline ( I G o = 199931 172 pmol/L) as an inhibitor of ADP-induced platelet aggregation. ATP release stimulated by ADP was 2.1 times more sensitive to inhibition by the xanthines than platelet aggregation, and enprofylline (I& = 102 f21 pmol/ L) was again nine times more potent than theophylline ( I G o = 940 245 pmol/ L).
*
*
DISCUSSION This study has investigated the previously unknown effect of xanthines on PAF-induced responses in human platelets, and has shown that platelet aggregation and ATP release induced by PAF were more potently inhibited by enprofylline and theophylline than the corresponding responses induced by ADP. The inhibitory effects on ADP-induced platelet responses were comparable to those reported by Vinge et al. (1984). ADP, thrombin and collagen activate platelets by mobilizing intracellular calcium (Ardlie 1982), and PAF also induces an influx of calcium into platelets (Valone & Johnson 1985; Selak & Smith 1989). Xanthines and other phosphodiesterase inhibitors probably inhibit platelet activation by elevating intracellular cyclic A M P concentrations and reducing cytoplasmic free calcium mobilization (Ardlie et al. 1967; Lanza el al. 1987). The concentrations of theophylline that
60 1
inhibited aggregation in the present study have previously been shown to inhibit platelet phosphodiesterase (Mills & Smith 1971), although an increase in platelet cyclic AMP concentrations has not always been demonstrated (Burns & Dodge 1984). PAFinduced aggregation may require a greater increase in intracellular calcium than is required for ADP-induced aggregation (Ardlie 1982), which possibly explains the greater potency of the xanthines as inhibitors of the response to PAF. Enprofylline is 5-6 times more potent than theophylline as an inhibitor of human platelet cyclic AMP phosphodiesterase (Kilian et ~ l1985), . which possibly explains the greater potency of enprofylline compared with theophylline as inhibitors of PAF- and ADPinduced responses in the present study and ADPinduced aggregation in the study by Vinge et al. (1984). A greater mobilization of intracellular calcium is required for secondary aggregation and release to occur (Detwiler et al. 1978), and since xanthines reduce intracellular calcium, this may explain the observation that platelet ATP release was more potently inhibited than aggregation for both PAF and ADP. Inhibition of PAF-induced platelet aggregation and ATP release by xanthines is not mediated by specific antagonism of the PAF receptor, and is only apparent at concentrations higher than those used clinically. Consistent with this, we have previously shown that ex vivo platelet responses to PAF are not inhibited 2.5 h after the administration of an oral therapeutic dose of theophylline (Thompson et al. 1987). PAF-induced effects in asthma are therefore unlikely to be inhibited at therapeutic plasma concentrations of theophylline (55-110 pmol/L) or enprofylline (5-25 pmol/L), a suggestion further supported by the observation that therapeutic concentrations of theophylline had no effect on bronchoconstriction or airway hyperresponsiveness induced by inhaled PAF (Chung et al. 1989).
REFERENCES Ardlie, N. G., Clew, G., Schulz, B. G . & Schwartz, C. J.
(1967) Inhibition and reversal of platelet aggregation by methylxanthines. Thrombosis et Diathesis Haemorrhagica, 18, 670-673. Ardlie, N. G. (1982) Calcium ions, drug action and platelet function. Pharmacology and Therapeutics, 18,249-270. Braquet, P., Touqui, L., Shen, T. Y. & Vargaftig, B. B. (1 987) Perspectives in platelet-activating factor research. Pharmacological Reviews, 39, 97- 145. Burns, G. B. & Dodge, J. A. (1984) Theophylline inhibits
602
N. L. A . Miss0 and P. J. Thompson
platelet aggregation, prostaglandin and thromboxane production by a mechanism which is independent of cyclic AMP. Agents and Actions, 14, 102-108. Chung, K. F., Lammers, J. W., McCusker, M., Roberts, N. M., Nichol, G . M. & Barnes, P. J. (1989) Effect of theophylline on airway responses to inhaled plateletactivating factor in man. European Respiratory Journal, 2,763-167. Cuss, F. M., Dixon, C. M. S. & Barnes, P. J. (1986) Effects of inhaled platelet activating factor on pulmonary function and bronchial responsiveness in man. Lancet, 2, 189- 192. Detwiler, T. C., Charo, I. F. & Feinman, R. D . (1978) Evidence that calcium regulates platelet function. Thrombosis and Haemostasis, 40, 207-21 1, Kilian, U., Amschler, H., Beume, R., Eltze, M., Kolassa, N. & Schudt, C. (1985) Bronchospasmolytic effects of substituted 6-phenyl-3-(2H)-pyridazinones in comparison to theophylline and enprofylline. In: Anti-asthma Xanthines and Adenosine, (Ed. K . E. Andersson & C. G. A. Persson), pp. 209-213. Excerpta Medica, Amsterdam. Lanza, F., Beretz, A., Stierle, A., Corre, G . & Cazenave, J. P. (1987) Cyclic nucleotide phosphodiesterase inhibitors prevent aggregation of human platelets by raising cyclic A M P and reducing cytoplasmic free calcium mobilization. llrombosis Research, 45,477-484. Lunell, E., Andersson, K. E., Persson, C. G. A. & Svedmyr, N. (1984) Intravenous enprofylline in asthma patients. European Journal of Respiratory Diseases, 65, 28-34. McManus, L. M., Hanahan, D. J. & Pinckard, R. N. (198 1) Human platelet stimulation by acetyl glyceryl ether phosphorycholine. Journal of Clinical Investigation, 67,903906. Mills, D. C. B. & Smith, J. B. (1971) The influence on
platelet aggregation of drugs that affect the accumulation of adenosine 3':S-cyclic monophosphate in platelets. Biochemical JournaI, 121, 185-196. Morley, J., Sanjar, S., Boubekeur, K., Aoki, S. & Kristersson, A. (1988) Pharmacological evaluation of prophylactic anti-asthma drugs by reference to the pathological sequelae of exposure to allergen or platelet activating factor. Agents and Actions Supplement, 23, 187-194. Page, C. P., Archer, C. B., Paul, W. & Morley, J. (1984) PAF-acether: A mediator of inflammation in asthma. Zkends in Pharmacological Sciences, 5, 239-241. Pretolani, M., Lopez-Ferrer, P. & Vargaftig, B. B. (1989) From anti-asthma drugs to PAF-acether antagonism and back - Present status. Biochemical Pharmacology, 38, 1373-1384. Selak, M. A. & Smith, J. B. (1989) Platelet-activating factor-induced calcium mobilization in human platelets and neutrophils: Effects of PAF-acether antagonists. Journal of Lipid Mediators, 1, 125-137. Smith, L. J. (1991) The role of platelet-activating factor in asthma. American Review of Respiratory Disease, 143, SIOO-102. Thompson, P. J., Misso, N., Heitz, M., Gillon, R. L. & Phillips, M. J. (1987) The effect of xanthines on platelet responses to platelet activating factor (PAF). American Review of Respiratory Disease, 135, (Suppl.), A395 Valone, F. H. & Johnson, B. J. (1985) Modulation of cytoplasmic calcium in human platelets by the phospholipid platelet-activating factor 1-0-alkyl-sn-glycero-3-phosphocholine. Journal of Immunology, 134, 1120-1 124. Vinge, E., Andersson, K. E. & Persson, C. G. A. (1984) Effects on aggregation of human platelets of two xanthines and their interactions with adenosine. Acta PhysioIogica Scandinavica, 120, 117-121.
