Effects of a P2Y(12) receptor antagonist on the response of equine platelets to ADP. Comparison with human platelets

Pharmacokinetics and competitive pharmacodynamics of ADP-induced platelet activation after oral administration of clopidogrel to horses

OBJECTIVE

To determine pharmacokinetics and pharmacodynamics after oral administration of a single dose of clopidogrel to horses.

ANIMALS

6 healthy adult horses.

PROCEDURES

Blood samples were collected before and at various times up to 24 hours after oral administration of clopidogrel (2 mg/kg). Reactivity of platelets from each blood sample was determined by optical aggregometry and phosphorylation of vasodilator-stimulated phosphoprotein (VASP). Concentrations of clopidogrel and the clopidogrel active metabolite derivative (CAMD) were measured in each blood sample by use of liquid chromatography-tandem mass spectrometry, and pharmacokinetic parameters were determined with a noncompartmental model.

RESULTS

Compared with results for preadministration samples, platelet aggregation in response to 12.5μM ADP decreased significantly within 4 hours after clopidogrel administration for 5 of 6 horses. After 24 hours, platelet aggregation was identical to that measured before administration. Platelet aggregation in response to 25μM ADP was identical between samples obtained before and after administration. Phosphorylation of VASP in response to ADP (20μM) and prostaglandin E₁ (3.3μM) was also unchanged by administration of clopidogrel. Time to maximum concentration of clopidogrel and CAMD was 0.54 and 0.71 hours, respectively, and calculated terminal-phase half-life of clopidogrel and CAMD was 1.81 and 0.97 hours, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

Clopidogrel or CAMD caused competitive inhibition of ADP-induced platelet aggregation during the first 24 hours after clopidogrel administration. Because CAMD was rapidly eliminated from horses, clopidogrel administration may be needed more frequently than in other species in which clopidogrel causes irreversible platelet inhibition. (Am J Vet Res 2019;80:505-512).

Inhibition of platelet function with clopidogrel, as measured with a novel whole blood impedance aggregometer in horses

This study aimed to validate a loading and maintenance clopidogrel dosing scheme for the inhibition of platelet function, measured by whole blood impedance aggregometry in healthy adult horses. Ten Warmblood horses received oral clopidogrel once daily. Doses were based on 50 kg weight categories and resulted in one loading dose of 6-6.5 mg/kg bodyweight and maintenance doses of 1.2-1.4 mg/kg over the next 4 days. Platelet function was measured via whole blood multiple electrode impedance aggregometry prior to (T0) and at 6, 12, 24, 48, 72, 96, 144, 192 and 240 h following the loading dose. Aggregometries for collagen (COLtest), arachidonic acid (ASPItest), adenosine diphosphate (ADPtest) and ADP with prostaglandin E1 (ADPtestHS) were performed. Statistical analyses included one way repeated measures ANOVAs and subsequent Dunnett's tests. Platelet aggregation induced by collagen remained unchanged. There were significant inhibitions in the ASPItest (P <0.01 at 192 h, and P <0.05 at 240 h) and the ADPtest and ADPtestHS (P < 0.01, with the exception of 240 h). The loading dose of clopidogrel induced rapid inhibition of platelet function within hours, and the low dose was suitable for maintaining the inhibition over the 4 days of therapy. Recovery of platelet function was restored 6 days after the cessation of medication, determined with the ADPtest and ADPtestHS, but remained inhibited with the ASPItest. The prolonged effect of clopidogrel may indicate differences in the activation of platelets between horses and humans that were previously unknown.

In vitro effects of three formulations of hydroxyethyl starch solutions on coagulation and platelet function in horses

OBJECTIVE

To investigate the in vitro effects of 3 hydroxyethyl starch (HES) solutions on viscoelastic coagulation testing and platelet function in horses.

SAMPLE

Blood samples collected from 7 healthy adult horses.

PROCEDURES

Blood samples were diluted with various crystalloid and HES solutions to approximate the dilution of blood in vivo that occurs with administration of a 10 and 20 mL/kg fluid bolus to a horse (1:8 and 1:4 dilutions, respectively). Diluted samples were analyzed through optical platelet aggregometry, platelet function analysis, thromboelastography, and dynamic viscoelastic coagulometry. Colloid osmotic pressure and concentrations of von Willebrand factor and factor VIII:C were also determined for each sample.

RESULTS

For all HES products, at both dilutions, the colloid osmotic pressure was significantly higher than that in the respective carrier solutions. At the 1:4 dilution, nearly all HES solutions resulted in significant alterations in platelet function as measured via the platelet function analyzer and dynamic viscoelastic coagulometer. Significant decreases in platelet aggregation and factor concentrations were also evident. Fewer HES-associated changes were identified at the 1:8 dilutions.

CONCLUSIONS AND CLINICAL RELEVANCE

Dilution of blood samples with all HES solutions resulted in changes in viscoelastic coagulation and platelet function that did not appear to be attributable to dilution alone. In vivo evaluations are necessary to understand the clinical impact of these in vitro changes.

Protein expression in platelets from six species that differ in their open canalicular system

Platelets contain an invaginated, tubular membranous structure called the surface-connected open canalicular system (SCCS or OCS), which is contiguous with the plasma membrane and serves as a site for granule fusion and as a reservoir of membrane for platelet spreading. According to ultrastructural studies, platelets from some species lack OCS. In an attempt to correlate biochemical and functional attributes with the presence of an OCS, platelets from human, mouse and dog (OCS(+)), and from cow, camel and horse (OCS(-)) were analysed for differential protein expression and aggregation in response to thrombin. Among the 18 different cytoskeletal and regulatory proteins examined, five (Rac1, RhoA, Ras, calmodulin and Src) were expressed at higher levels in OCS(+) platelets (p < 0.05). Given the role of Arf6 in the formation of tubular invaginations in nucleated cells, the levels of Arf6-GTP were analysed in OCS(+) and OCS(-) platelets. There was no significant correlation between the presence of OCS and total Arf6 or Arf6-GTP levels. Comparison of platelet aggregation between different species suggests that OCS(-) platelets have delayed responses. This comparison of platelets from six different species, which differ in their OCS, shows the differential expression of known signaling components and foreshadows future studies focusing on OCS formation and function.

Agonist concentration-dependent differential responsivity of a human platelet purinergic receptor: pharmacological and kinetic studies of aggregation, deaggregation and shape change responses mediated by the purinergic P2Y1 receptor in vitro

Platelet shape change (SC), aggregation and deaggregation responses are integral components of hemostasis that are elicited and modulated in vivo by the simultaneous activation of several membrane receptors. Selective activation of the purinergic P2Y1 receptor in vivo elicits a sustained SC and a small, transient aggregation response that is reversed rapidly by a robust deaggregation response (Platelets 2003; 14: 89). Using a kinetics-based turbidimetric approach to study the modulation of these concurrent components of human platelet responses, we demonstrate that these P2Y1 receptor-related responses and a number of their kinetic and steady-state characteristics are differentially elicited and modulated. P2Y1 receptor agonist concentrations that elicited aggregation (pEC50 for ADP, 2-MeSADP; 5.88, 6.69) were 10-fold greater than those that elicited SC (7.33, 7.67). The magnitude of the aggregation response was agonist concentration-dependent, saturable and was associated with an agonist concentration-dependent deceleration of the deaggregation response. Gi-coupled receptor (alpha 2A-adrenoceptor, EP3 and P2Y12 receptors) agonists also enhanced aggregation through deceleration of the deaggregation response, and an inhibitor of PI3K activity (wortmannin) inhibited aggregation through acceleration of the deaggregation response. Neither treatment affected the extent or the kinetics of the SC response. The aggregation but not the SC response was rapidly desensitized by P2Y1 receptor activation by ADP. The affinity of the presence of a single P2Y1 receptor subtype. The differential characteristics and modulation of the SC and aggregation responses by a single receptor support the idea that different signaling pathways activated at different occupancy states of the same receptor underlie the two responses. P2Y1 receptor-mediated platelet aggregation and SC responses provide a convenient model for studying the phenomenon of agonist-directed signaling by differential occupancy of the same membrane receptor.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Differential effects of phosphodiesterase inhibitors on platelet activating factor (PAF)- and adenosine diphosphate (ADP)-induced equine platelet aggregation

Compounds that activate adenylate cyclase, increasing intracellular cyclic adenosine monophosphate (cAMP), inhibit equine platelet aggregation. Cyclic AMP is broken down by phosphodiesterase (PDE) and, in the present study, the effects of theophylline, a nonselective PDE inhibitor, and selective inhibitors of PDE isoenzymes PDE3, PDE4 and PDE5, on equine platelet aggregation in response to platelet activating factor (PAF) and adenosine diphosphate (ADP) have been examined. Theophylline and the PDE3 inhibitors, trequinsin and quazinone, inhibited both PAF and ADP-induced aggregation in a concentration dependent manner. The inhibition of PAF-induced aggregation was, however, significantly greater than that of the response to ADP. The inhibitory effects of theophylline and the PDE3 inhibitors on ADP- but not PAF-, induced aggregation were reversed by addition of the calcium ionophore, A23187. Rolipram and zaprinast, inhibitors of PDE4 and PDE5, respectively, had no effect on either PAF- or ADP-induced aggregation. These results demonstrate that inhibition of aggregation caused by PAF or ADP can be achieved by selective inhibition of PDE3 but suggest that there may be agonist-specific differences in the intracellular signalling pathways that regulate equine platelet aggregation.