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In a very interesting recent set of experiments Brunner, Stepanek & Brunner (1980)

have shown that sulphinpyrazone reduces mortality after ligation of the left anterior

descending (LAD) coronary artery in the rat. In this work the LAD is rapidly ligated

between the left auricle and pulmonary cone under light ether anaesthesia. The

animal then recovers, after the thorax has been clo sed , provided that the operation

does not take more than 30 s.

Sulphinpyrazone was administered for three da ys in a dose of 30 mg kg-I S.C. twice

dail y and LAD ligature was performed on the second day oftreatment. There were 91

rats in the drug treated group and 103 in the control group. During the first 30 min

after LAD ligation 14.6% of rats in the control group died and 3.3 % in the sulphin-

EXPERIMENTAL MODELS AND PLATELET REGULATORY DRUGS 217

pyrazone group (P < 0.01). Between 30 min and 21 days 44 .3% died in the control

group and 29 .6% in the sulphinpyrazone group (P < 0.05). The main differences in

mortality recurred during the first 6 days after ligation. Subsequently, the mortality

was low in both groups and there was no statistically significant difference between

them. It is not known to what extent platelets or products of platelet aggregation are

significant in this model or whether direct antiarrythmic effects could be responsible

for the effects seen .

Dipyridamole

Adenosine, when infused into rabbits to give a high blood concentration, inhibits the

rate of formation and embolisation of white bodies after small blood vessels are

injured (Born , Honour & MitchelI, 1964), and since dipyridamole inhibits the uptake

of adenosine into red cells and thus its breakdown (Bunag, Douglas, Imai & Berne,

1964) it was thought that a combination of the two substances could act synergistically in anima I models of white body formation (Emmons, Harrison, Honour &

MitchelI, 1965). However, dipyridamole had activity in its own right and this led to

its first successful trial in the c1inic (Sulli van , Harken & Goslin, 1971).

Inhibition of'platelet thrombiformed as a result of direct injury to the vasculatu re

Dipyridamole has been shown to be active in a number of models involving very

localised injury to vessels in which the thrombi are mainly composed of platelets.

These results are summarised in Table I. Thrombi resulting from chemical and

mechanical (Danese, Voleti & Weiss, 1971) or enzymic endarterectornies (Ma yer &

Hammond, 1973) are also favourably inlluenced by the drug.

Table 1 Positiveeffects of dipyridamole on white body formation in vivo

Species Site ofwhitebody Method of Dose Reference

vesse/ injury

Rabbit Cerebral cortex Mechanical pinching2.7 mg kg-I (i.v.) Emmons, Harrison,

with forceps Honour & MitcheII, 1965

Rabbit Ear arterioles Laserbeam 2.5 mg kg-I (i.v.) Arfors, Hint, Dhall

& Methson, 1968

Rat Mesentericartery Electrical 2.5 mg kg-I (i.v.) Didisheirn, 1968

Rat Mesenteric artery Electrical 1.0 mg kg-I (i.v.) Haarman & Kadatz,

1975

Models involving direct application ofplatelet agon ists in vivo

In some of the earliest studies dipyridamole was shown to be an inhibitor of ADPinduced platelet aggregation in the same models of white bod y formation used to

demonstrate its direct effect on platelet adhesion and aggregation to damaged

endothelium. In these in vivo experiments the lowest concentration of ADP was

determined wh ich , when dripped on to the injured site, caused white bodies to form .

The minimum concentration was raised two -fold 18 min after dipyridamole

(27 mg kg-I i.v.) had been given (Emmons & MitchelI, 1965).

In recent experiments (Holmes, Smith & Freuler, 1977) platelets were continuously

monitored using a Technicon Autocounter. After ADP infusion there was a dose

responsive fall in platelet count wh ich was inhibited by dipyridamole when it was

infu sed ovcr the dose range 0.063-4.0 mg kg-I

. The ID50 calculatcd by regression

analysis was 0.17 mg kg-I min -I

. The drug caused pronounced hypotension but the

218 A. M . WHITE& K. D. BUTLER

platelet effects were shown not to be related to thi s factor. In guinea pigs

the se findings could not be repeated after giving doses of dip yridamole (i.v.) up to

10mg kg-I

. Likew ise dip yridamole at the same do se had no activi ty aga inst the

thrombocytopenias ofthe Arthus and Forssman reaction s.

The intra arterial infusion of collagen lead s to a rapid, partiall y reversible fall in

platelet count in venous blood (Buchanan & Hirsch , 1978). Plat elets pre sumably

adhere to the collagen and ar e permanently removed from the circulation or are

reversibly aggregated due to the release ofmed iators such as TXA2or ADP. The i.v.

administration of dipyridamole (2.5 mg kg") I h before the collagen sti mulus gave a

30% inhibition of a 40% fall in platelet count which was sta tistically significant.

Animal models relating to hum an atherosclerosis

In an interesting and exhaustive study (Harker, Ross, Slichter & Scott, 1976) two

gro ups ofbaboons were infu sed with homocysteine for three months. In the control

group th is led to pronounced endothelial desquamation (approximately 10%)

coupled with reduced platelet survival and marked intimal hyperplasia . Dipyrida mole (30 mg kg-I da ily, p.o.) returned platelet survival and turnover to near

normal values and also significa ntly reduced the intimal hyperplasia .

In the rabbit, results have been at variance with th ese fmding s. Thus Baumgartner

& Studer (1977) could show no effect of dipyridamole over a two week period

(50 mg kg-I twice da ily by stornach tube) on intimal th ickening subsequent to

balloon catheterization ofthe iliac artery.

Exp erim ents relating to myocardial infa rction

Dogs in wh ich represent ati ve platelet population had been prelabelled with 51Cr

were given dip yridamole (5 mg kg") dail y for seven days prior to experiments in

which a 300- 500 micro ampere current was applied to th e intima in the anterior

descend ing or circurnflex corona ry art er y such that the intima was dam aged and

thrombus form at ion took pla ce with in 15-90 min (Moschos, Za hir i, Lyon s, Wei ss,

Oldewurtel, Regan & Newa rk, 1973). In the dipyrid amole-treated group there was a

fall in the ratio of isch aemic to control areas from 3.3 to 2.0 (P < 0.01) and a

significant reduction in the extent of microcirculatory thrombosis without affecting

th e thrombus in the proximal coronar y artery. The decreases in incidence of

a rrhythmias and ventric ular fibrill ation showed very strong trend s but were not

statistica lly significant.

In a related type of experiment (Ha ft, Gershengorm, Kr anz & Oestreicher, 1972)

dogs were pretreated with dipyridamole for four day s (5 mg kg-I) and adrenaline was

infu sed intravenously at a rate of 4 kg! min-I for 4 h. On e week later the amount

ofmyocardial necrosis was measured. Dipyridamole had a significant effect although

it is difficult to understand the mechanism and arecent publication (Moschos,

Ha ider, Khan, Lyon s & Regan, 1978) has shown that adrenaline-induced experimental myocardial necrosis probabl y doe s not involve platelet thrombus formation.

Further eviden ce that dipyrid amole can have a favourabl e outcome on the course of

acute experimenta l myocardial infarction has recentl y been published (Roberts,

Jacobstein, Cipriano, Alonso, Combes & Ga y, 1980).

Aspirin

Whole anima l models

Aspi rin is active in a vari ety of mod els in which thrombus formation takes place

(Barth, Zimmerman , Zieboll & Lan ge, 1975; Danese el al., 1971; Hladovec, 1975;

EXPERIMENTAL MODELS AND PLATELET REGULATORY DRUGS 219

Petersen & Zucker, 1970) but attempts to show effects in models closely related to

arterial thrombosis have been variable. In the hamster cheek pouch, using the same

technique as was used to demonstrate the activity of sulphinpyrazone (Lewis &

Westwick, 1977) aspirin was shown to inhibit white body formation at 10 mg kg-1

and 30 mg kg-1 but not at higher or lower doses . However, the formation ofplatelet

thrombi and ernbol i in the rabbit cerebraI vasculature were not inhibited by four

bolus injections of aspirin, each of 65 mg kg-1 (Adams & MitchelI, 1979), whereas

Meng & Seuter (1977) showed an effect of I mg kg-1 given orally to rats I h before

producing a thrombus in the carotid artery by a freezing technique. High doses of

aspirin may be ineffective because of complete suppression of PGh and in

experiments designed to demonstrate the point Kelton, Hirsh, Carter & Buchanan

(1978) showed in rabbits that aspirin given at 200 mg kg-I by intravenous infusion

actually augmented the size ofvenous thrombi compared with controls and animals

given a dose of 10 mg kg-1

• At the high dose PGh production was totally inhibited.

Tranylcypromine, an inhibitor of PGb production also augmented thrombus size.

The thrombogenic effect of a high dose of aspirin may explain the bell-shaped dose

response curve obtained by Lewis & Westwiek (1977) and the inability of Adams &

Mitchell (1979) to show an effect in the cerebral vasculature.

Direct activation ofplatelets in vivo

In recent experiments (Butler, Pay, Roberts & White, 1979) aspirin inhibited the

thrombocytopenia of the Arthus reaction in thc guinea pig when given at 10 mg kg-1

(i.v.) I h before the antigenic challenge. No effect was seen against the sub-Iethal

Forssman reaction at this dose or at 100 mg kg-1 (Butler & White, 1980). The

thrombocytopenia caused by the injection of soluble collagen was also inhibited by

aspirin given by infusion at 10 mg kg-1 (Buchanan & Hirsh, 1978).

Models relating to atherosclerosis

Clowes & Karnovsky (1977) could observe no effect of aspirin on myointimal

thickening of the carotid artery of the rat. The endothelium was removed by air

drying and aspirin was administered by stomach tube at 200 mg kg-1 daily. Likewise,

Baumgartner & Studer (1976) could find no suppressive effect of aspirin (30 mg kg-1

daily) on the volume ofthe neo intima during the 14 days after balloon catheterization

ofthe iliac artery ofrabbits.

In a study involving Cynomolgus monkeys fcd an atherogenic diet Pick, Chediak

& Glick (1979) showcd that daily aspirin (approximately 13 mg kg-1 orally) had no

effect on aortic atherosclerosis but did not reduce the number of coronary vessels

with atherosclerotic involvement and the number of coronary vessels narrowed by

20% or more.

Aspirin, like sulphinpyrazone decreased platelet accumulation in the renal arteries

and prevented hyperacute rejection (MacDonald, Busch, Alexander, Pheteplace,

Menzoian & Murray, 1969; Mathew, Hogan, Lewers, Bauer, Maher & Schremer,

1971).

Experiments relating to myocardial infarction

Experimental myocardial necrosis and ischaemia are inhibited by aspirin using the

same techniques as were used for the invcst igation of dipyridamole (Moschos et al.,

1973; Haft et al., 1972).

Conclusion

Unfortunately the gap between the an imal models described above and the aspects of

the human disease states they are supposed to mimic is wide. All ofthe drugs inhibit

220 A. M. WHITE & K. D. BUTLER

platelet function in one or other of the models a nd the more useful of these models

allow proper dose and time-response relationships to be carried out. However, in

none of them are the rate limiting steps of platelet aggregation understood. Deeper

understanding oftheir characteristics would be rewarding in all cases. In the models

ofarrythmia the role ofplatelets is not properly clarified and no control experiments

have been reported in which the animals have been rendered thrombocytopenic.

The atherosclerosis models are time consuming and true dose response relationships are virtually impossible to achieve, particuarly in the larger species. Perhaps a

better understanding of experimental atherosclerosis and arteriosclerosis in smaller

animals would be rewarding in order to tackle the subject more effectivel y.

These comments emphasise that true knowledge concerning the effectiveness of

these drugs in man can only come from the results offurther well-planned and wellconducted clinical trials.

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