Cyclooxygenase Expression and Platelet Function in Healthy Dogs Receiving Low‐Dose Aspirin

Abstract

Background Low dose aspirin is used to prevent thromboembolic complications in dogs, but some animals are non-responsive to the anti-platelet effects of aspirin (‘aspirin resistance’). Hypothesis/Objectives That low dose aspirin would inhibit platelet function, decrease thromboxane synthesis, and alter platelet cyclooxygenase (COX) expression. Animals Twenty-four healthy dogs Methods A repeated measures study. Platelet function (PFA-100® closure time, collagen/epinephrine), platelet COX-1 and COX-2 expression, and urine 11-dehydro-thromboxane B2 (11-dTXB2) was evaluated prior to and during aspirin administration (1 mg/kg Q24 hours PO, 10 days). Based on prolongation of closure times after aspirin administration, dogs were divided into categories according to aspirin responsiveness: responders, non-responders, and inconsistent responders. Results Low dose aspirin increased closure times significantly (62% by Day 10, P<0.001), with an equal distribution among aspirin responsiveness categories, 8 dogs per group. Platelet COX-1 mean fluorescent intensity (MFI) increased significantly during treatment, 13% on Day 3 (range, −29.7%–136.1%) (P=0.047) and 72% on Day 10 (range, −0.37–210.36%) (P<0.001). Platelet COX-2 MFI increased significantly by 34% (range, −29.2–270.4%) on Day 3 (P = 0.003) and 74% (range, −19.7–226.2%) on Day 10 (P<0.001). Urinary 11-dTXB2 concentrations significantly (P=0.005, P<0.001) decreased at both time points. There was no difference between aspirin responsiveness and either platelet COX expression or thromboxane production. Conclusions and Clinical Importance Low dose aspirin consistently inhibits platelet function in approximately one third of healthy dogs, despite decreased thromboxane synthesis and increased platelet COX expression in most dogs. Pre-treatment COX isoform expression did not predict aspirin resistance.

Effects of aspirin dose escalation on platelet function and urinary thromboxane and prostacyclin levels in normal dogs

“…The ideal dose of aspirin that consistently inhibits platelet function in dogs, while minimizing any drug‐associated side effects, is unknown. Currently used standard low or antiplatelet dosages (0.5–1 mg kg −1 day −1 ) of aspirin, however, do not reliably inhibit platelet function (Dudley et al., ; Haines et al., ; Hoh et al., ). Platelet function in dogs will, in contrast, be consistently inhibited by high dosages of aspirin (Thomason et al., ), suggesting that, if dogs are treated with high enough doses of aspirin, there is an increased likelihood of inhibition of platelet function.…”

“…In contrast to high‐dose aspirin, a “low” dosage of aspirin inhibits platelet TXA 2 synthesis in humans, without permanently inhibiting COX function within the vascular endothelium and other cells, thereby allowing prostacyclin synthesis and vasodilation to continue, and reducing the likelihood of gastrointestinal and renal side effects (Patrono et al., ). Unfortunately, low‐dose aspirin (0.5–1 m/kg once daily), unlike high‐dose aspirin, does not consistently inhibit platelet function in dogs (Dudley et al., ; Haines et al., ; Hoh, Smith, McMichael, & Byron, ). Patients that are poorly responsive to the antiplatelet effects of aspirin are termed “aspirin resistant” (Helgason et al., ).…”

“…Patients that are poorly responsive to the antiplatelet effects of aspirin are termed “aspirin resistant” (Helgason et al., ). The incidence of aspirin resistance ranges in humans from 5% to 57% (Eikelboom, Hirsh, Spencer, Baglin, & Weitz, ; Grotemeyer, Scharafinski, & Husstedt, ; Gum et al., ; Helgason et al., ; Pappas, Westengard, & Bull, ; Sane, McKee, Malinin, & Serebruany, ) and, in healthy dogs, our group has previously estimated that the incidence ranges from 19% to 56% depending on the technique used to assess inhibition of platelet function (Dudley et al., ; Haines et al., ).…”

The using of a piglets as a model for evaluating the dipyrone hematological effects

“…The study was performed within 15 min of the collection of blood and platelet activation was measured by quantity of time needed to complete closure of the micro-holes of the cartridge membrane and the cessation of blood flow – the so-called ‘occlusion time’ (CT – closure time). Herein, a shorter CT means a higher degree of platelet activity, whereas an elongated CT indicates their lower activity [19–23]. …”

“…Blood spinning (centrifugation) was carried out at 21 °C at 1500 × g. For calibration the coagulometer used a freeze-dried human standard plasma (Bio-Ksel, Poland). The results for PT and APTT are given in seconds, and the PT and international normalized ratio for prothrombin time and ratio for APTT were calculated [19, 20, 24, 25]. …”