Responses of rabbit platelets to adrenaline induced by other agonists

Characterization of the aggregation responses of camel platelets

BACKGROUND

Despite evidence of active hemostasis, camel platelets barely respond to common aggregating agents at standard doses used for human platelet aggregation.

OBJECTIVES

The purpose of the study was to find out whether camel platelets can be activated by high doses or combinations of aggregation agonists, and to characterize the receptor that mediates the aggregation response to adenosine diphosphate (ADP), the most potent agonist for camel platelets known so far.

METHODS

Aggregation studies were performed with platelet-rich plasma (PRP) in response to multiple doses or combinations of ADP, epinephrine (EPN), collagen, and arachidonic acid (AA). Aggregation responses to ADP were performed before and after the addition of the ADP receptor (P2Y12) antagonist Clopidogrel.

RESULTS

Camel platelets responded to ADP at doses higher than the standard dose for human platelets, and to combinations of EPN and other agonists, while no aggregation was elicited with EPN or AA alone. Clopidogrel blocked the ADP-induced aggregation responses in a dose-dependent fashion in vitro.

CONCLUSIONS

Camel platelet aggregation can be activated by increasing the dose of some agonists such as ADP, but not AA or EPN. Irreversible aggregation of camel platelets could also be triggered by a combination of EPN and ADP, and collagen and AA. Inhibition with clopidogrel suggests that camel platelets express the ADP receptor, P2Y12. Understanding platelet function in camels will add to the understanding of platelet function in health and disease.

Alpha-adrenoceptors

Major advances have been made in our understanding of the molecular structure and function of the alpha-adrenoceptors. Many new subtypes of the alpha-adrenoceptor have been identified recently through biochemical and pharmacological techniques and several of these receptors have been cloned and expressed in a variety of vector systems. Currently, at least seven subtypes of the alpha-adrenoceptor have been identified and the molecular structure and biochemical functions of these subtypes are beginning to be understood. The alpha-adrenoceptors belong to the super family of receptors that are coupled to guanine nucleotide regulatory proteins (G-proteins). A variety of G-proteins are involved in the coupling of the various alpha-adrenoceptor subtypes to intracellular second messenger systems, which ultimately produce the end-organ response. The mechanisms by which the alpha-adrenoceptor subtypes recognize different G-proteins, as well as the molecular interactions between receptors and G-proteins, are the topics of current research. Furthermore, the physiological and pathophysiological role that alpha-adrenoceptors play in homeostasis and in a variety of disease states is also being elucidated. These major advances made in alpha-adrenoceptor classification, molecular structure, physiologic function, second messenger systems and therapeutic relevance are the subject of this review.

Proaggregatory effect of epinephrine on rabbit platelets inhibited by ticlopidine

Ticlopidine is a potent inhibitor of ADP-induced aggregation of rabbit platelets ex vivo. In vivo, however, multiple agonists play a role in platelet activation. In this study, we examined the effect of epinephrine on the antiplatelet action of ticlopidine in rabbit platelets. Epinephrine reversed the inhibitory effect of drug on ADP-induced platelet aggregation. The potentiating effect of epinephrine was mediated through alpha 2-adrenergic receptors, was reversed by pretreatment with the Na+/H+ exchange inhibitor dimethylamiloride, and was mimicked by agents that increased intracellular sodium or pH. Ticlopidine had no effect on resting intracellular pH, an indication that the effect of epinephrine was not compensating for a drug-induced intracellular acidification. While this potentiation was also found to be inhibited by aspirin, it did not involve enhanced release of thromboxane A2. Our results demonstrate that epinephrine can overcome the inhibitory effect of ticlopidine on ADP-induced aggregation through a mechanism involving activation of Na+/H+ exchange and through an as yet unidentified mechanism sensitive to aspirin.

Effects of ticlopidine and indobufen on platelet aggregation induced by A23187 and adrenaline in the presence of different anticoagulants

As Ca2+ is known to play a fundamental role in platelet function, the effect of combining two platelet aggregating agents (adrenaline and the ionophore A23187) with different effects on Ca2+ was studied at levels subthreshold for aggregation using platelet-rich plasma from eight atherosclerotic patients. Adrenaline lowered the A23187 threshold required to induce aggregation. The effects of treating patients with the antiplatelet agents, indobufen and ticlopidine, on A23187 and adrenaline induced aggregation of platelets prepared in hirudin or sodium citrate was also evaluated. Aggregation was also studied using platelets resuspended in Ca2(+)-free and Ca2(+)-enriched Tyrode solution. Before treatment hirudin treated platelet-rich plasma, which has physiological extraplatelet Ca2+ levels, was more sensitive to A23187 and adrenaline than was citrated platelet-rich plasma, which has suppressed Ca2+ levels. Ticlopidine significantly raised the concentration of A23187 required to induce aggregation in citrated but not hirudin treated platelet-rich plasma. Indobufen did not significantly affect A23187 induced aggregation. Ticlopidine acts by inhibiting the glycoprotein IIb-IIIa complex on the platelet membranes. Low levels of extracellular Ca2+ and ticlopdine may act synergistically to reduce the aggregatory response of stimulated platelets.

Involvement of phosphoinositide metabolism in potentiation by adrenaline of ADP-induced aggregation of rabbit platelets

Changes in phosphoinositide metabolism were examined in washed rabbit platelets stimulated with 0.5 microM-ADP, 50 microM-adrenaline, or ADP and adrenaline in combination. Adrenaline does not stimulate platelet aggregation when used alone, but does potentiate aggregation stimulated by ADP. In platelets prelabelled with [32P]Pi and [3H]glycerol, adrenaline was found to potentiate the ADP-induced changes in platelet phospholipids, causing larger increases in the amount and labelling of phosphatidylinositol 4-phosphate (PIP) and phosphatidic acid than was observed with ADP alone. The combination of ADP and adrenaline did not produce a greater decrease in phosphatidylinositol 4,5-bisphosphate (PIP2) than was produced by ADP alone. In platelets prelabelled with [3H]inositol, adrenaline potentiated the increases in labelling of inositol phosphate and inositol bisphosphate stimulated by ADP; no increase in inositol trisphosphate labelling was detected with ADP alone or with the combination of ADP and adrenaline. Phentolamine, an alpha-adrenergic-receptor antagonist, blocked potentiation by adrenaline of ADP-induced changes in phosphoinositide metabolism. Propranolol and sotalol, beta-adrenergic-receptor antagonists, augmented the potentiation; this is consistent with the concept that the effect of adrenaline is mediated by beta-adrenergic receptors. The effect of adrenaline on phosphoinositide metabolism appears to be to potentiate the mechanisms by which ADP causes turnover of PIP and possibly degradation of PI, rather than the mechanism by which PIP2 is decreased.

Tight coupling of thrombin-induced acid hydrolase secretion and phosphatidate synthesis to receptor occupancy in human platelets

Human platelets incubated with [32P]Pi and [3H]arachidonate were transferred to a Pi-free Tyrode's solution by gel filtration. The labile phosphoryl groups of ATP and ADP as well as Pi in the metabolic pool of these platelets had equal specific radioactivity which was identical to that of[32P]phosphatidate formed during treatment of the cells with thrombin for 5 min. Therefore, the 32P radioactivity of phosphatidate was a true, relative measure for its mass. The thrombin-induced formation of[32P]-phosphatidate had the same time course and dose-response relationships as the concurrent secretion of acid hydrolases. 125I-alpha-Thrombin bound maximally to the platelets within 13s and was rapidly dissociated from the cells by hirudin; readdition of excess 125I-alpha-thrombin caused rapid rebinding of radioligand. This binding-dissociation-rebinding sequence was paralleled by a concerted start-stop-restart of phosphatidate formation and acid hydrolase secretion. [3H]Phosphatidylinositol disappearance was initiated upon binding but little affected by thrombin dissociation and rebinding. ATP deprivation caused similar changes in the time courses for [32P]-phosphatidate formation and acid hydrolase secretion which were different from those of [3H]phosphatidylinositol disappearance. The metabolic stress did not alter the magnitude (15%) of the initial decrease in phosphatidylinositol-4,5-bis[32P]phosphate, but did abolish the subsequent increase of phosphatidylinositol-4,5-bis[32P]-phosphate in the thrombin-treated platelets. It is concluded that in thrombin-treated platelets (1) phosphatidate synthesis, but not phosphatidylinositol disappearance, is tightly coupled to receptor occupancy and acid hydrolase secretion in platelets, (2) successive phosphorylations to phosphatidylinositol-4,5-bisphosphate is unlikely to be the main mechanism for phosphatidylinositol disappearance, and (3) only a small fraction (15%) of phosphatidylinositol-4,5-bisphosphate is susceptible to hydrolysis.

Agonist-specific refractory states of bovine platelets-disappearance of synergism in aggregation

The specificity of refractory state of bovine platelets developed by various agonists was examined. When bovine unwashed platelets were preincubated with 5-hydroxytryptamine (5HT) (or ADP), they became non responding to that agonist (refractory) but they responded quite normally to collagen, thrombin and ADP (or 5HT). When platelets became refractory to 5HT, the synergistic aggregation normally obtained with this agonist plus ADP was lost, but that with ADP plus thrombin was not changed. Platelets made refractory to ADP (or 5HT) after the aggregation and disaggregation cycle by the agonist responded normally to 5HT (or ADP). These results strongly indicate that platelets become refractory to one agonist without impairing the sensitivity to any other agonists.

Synergistic responses and receptor occupancy in rabbit blood platelets

Several observations indicate that simultaneous receptor occupancy is necessary for generation of a synergistic response of rabbit platelets to two excitatory agonists. First, an aggregatory response to adrenaline induced by prior addition of 5HT reverses rapidly unless stabilised by inhibition of 5HT uptake. The extent of response correlates with the extracellular 5HT concentration. Second, addition of an ADPase following adrenaline enhances the rate of disaggregation if the response to this agonist has been induced by prior addition of ADP. Third, disaggregation is induced, or enhanced, if an antagonist selective to the inducing agonist is added following completion of the induced aggregatory response. These data suggest marked lability in the mechanisms responsible for generation of synergistic responses.