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Activated platelets contribute to stimulation of cardiac afferents during ischaemia in cats: role of 5-HT₃ receptors

Liang-Wu Fu and John C. Longhurst

Myocardial ischaemia activates blood platelets and cardiac sympathetic afferents, which mediate chest pain and cardiovascular reflex responses. We have demonstrated that activated platelets stimulate ischaemically sensitive cardiac sympathetic afferents. Platelets absorb and release 5-hydroxytryptamine (5-HT) when they are activated. In the present study we hypothesized that, by releasing 5-HT, activated platelets stimulate cardiac afferents during ischaemia through a 5-HT₃ receptor mechanism. Platelet-rich plasma (PRP) and platelet-poor plasma (PPP) were obtained from cats. Activation of platelets in PRP was induced by thrombin (5 units ml^-1) or collagen (2 mg kg^-1). Using high-performance liquid chromatography, we observed that the concentration of 5-HT was increased significantly in suspensions of platelets activated with thrombin (PRP+thrombin, 28 ± 1.7 µM) or collagen (PRP+collagen, 27 ± 2.5 µM) compared with suspensions of unactivated platelets (PRP+saline, 2.3 ± 0.8 µM) and PPP. During myocardial ischaemia and reperfusion, tirofiban, a specific inhibitor of platelet glycoprotein (GP) IIb-IIIa receptors (100 µg kg^-1, I.V., followed by 5 µg kg^-1 min^-1), significantly reduced the increase in the concentration of 5-HT in cardiac venous plasma from ischaemic region. Nerve activity of single-unit cardiac afferents was recorded from the left sympathetic chain (T2-T5) in anaesthetized cats. Eighty ischaemically sensitive and seven ischaemically insensitive cardiac afferents were identified. Tirofiban reduced the ischaemia-related increase in activity of seven cardiac sympathetic afferents by 50 %. Injection of 1.5 ml of PRP+collagen or PRP+thrombin into the left atrium (LA) increased activity of 16 cardiac afferents. Tropisetron (300 µg kg^-1, I.V.), a selective 5-HT₃ receptor antagonist, eliminated the afferent's responses to platelets activated with collagen or thrombin. Moreover, LA injection of 5-HT (20-40 µg kg^-1) and PBG (100 µg kg^-1), a 5-HT₃ receptor agonist, stimulated nine ischaemically sensitive cardiac sympathetic afferents, significantly increasing the activity of these afferents. However, injection of -M-5-HT (100 µg kg^-1, LA), a 5-HT₂ receptor agonist, stimulated only two of the nine ischaemically sensitive cardiac afferents, and thus did not significantly alter impulse activity of this group of afferents. Both the 5-HT₁ (5-CT, 100 µg kg^-1, LA) and 5-HT₄ receptor agonists (SC53116, 100 µg kg^-1, LA) did not stimulate any of the nine afferents tested. Tropisetron (300 µg kg^-1, I.V.) also eliminated the response of seven ischaemically sensitive cardiac afferents to exogenous 5-HT and attenuated the ischaemia-related increase in activity of nine cardiac sympathetic afferents by 41 %. Conversely, LA injection of 5-HT (40 µg kg^-1) did not stimulate any of seven ischaemically insensitive cardiac afferents, although this group of afferents consistently responded to bradykinin (3 µg, LA). These data indicate that during myocardial ischaemia the activated platelets stimulate cardiac sympathetic afferents, at least in part, through a 5-HT₃ receptor mechanism.

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