Summary
The present study was designed to evaluate the effectiveness of the ergoline 5HT2 receptor antagonist, LY53857 in a rabbit model of vascular arterial occlusion. LY53857
(1 and 10 εM) inhibited serotonin amplified platelet aggregation responses to threshold
concentrations of ADP in rabbit platelets in vitro. LY53857 (1 εM) not only inhibited
the serotonin component of rabbit platelet aggregation, but also inhibited in vitro
aggregation induced by ADP (48.7 ± 16.7% inhibition), collagen (76.1 ± 15.9% inhibition)
and U46619 (65.2 ± 12.3% inhibition). The effectiveness of this ergoline 5HT2 receptor antagonist in blocking aggregation to ADP, collagen and U46619 may be related
to its ability to inhibit a serotonin component of platelet aggregation since rabbit
platelets possess high concentrations of serotonin that may be released during aggregation
produced by other agents. Based on the effectiveness of LY53857 to inhibit rabbit
platelet aggregation, we explored the ability of LY53857 to extend the time to carotid
artery occlusion in rabbits following electrical stimulation of the artery. Reproducible
carotid artery occlusion was induced in rabbits by moderate stenosis coupled to arterial
cross clamping, followed by electrical stimulation. With this procedure, occlusion
occurred at 47.0 ± 7 min (n = 30) after initiation of the electrical stimulation.
Animals pretreated with LY53857 (50 to 500 εg/kg i.v.) showed a delay in the time
to carotid artery occlusion (at 100 εg/kg i.v. occlusion time extended to 164 ± 16
min). Furthermore, ex vivo platelet aggregation from animals treated with LY53857
(300 εg/kg i.v.) resulted in 40.5% inhibition of platelet aggregation in response
to the combination of ADP (1 εM) and serotonin (1 εM). These studies document the
ability to obtain reproducible arterial occlusion in the rabbit and showed that intravenously
administered LY53857 prolonged the time to carotid artery occlusion. Prolongation
of carotid artery occlusion time was accompanied by inhibition of serotonin-amplified
ADP-induced aggregation in rabbit platelets, an effect observed both in vitro and
ex vivo. Thus, the rabbit is a useful model for studying the effectiveness of 5HT2 receptor antagonists in prolonging vascular occlusion induced by insult of the carotid
artery.