Lack of Effect of Postinjury Treatment with Methylprednisolone or Tirilazad Mesylate on the Increase in Eicosanoid Levels in the Acutely Injured Cat Spinal Cord

Methylprednisolone (MP) improves motor recovery in spinal cord-injured patients when administered in a 24 h intensive high dose regimen beginning within 8 h after spinal cord injury (SCI). The rationale for this regimen has been based upon the need for high doses (i.e., 30 mg/kg initial IV dose) to inhibit posttraumatic lipid peroxidation (LP) in the injured spinal segment. However, injury also triggers the immediate calcium-mediated activation of phospholipase A₂ (PLA₂), the release of arachidonic acid, and the enzymatic formation of potentially deleterious prostaglandins (PGF_2α, PGE₂), thromboxane A₂ (TXA₂), and leukotrienes (LTs). Thus, in view of the glucocorticoid receptor-mediated inhibition of PLA₂ that underlies much of MP's antiinflammatory actions, an additional neuroprotective mechanism may relate to an inhibition of eicosanoid formation. Using the cat spinal cord compression model (180g × 5 min at L3; Na pentobarbitol anesthesia), we examined whether 30 min postinjury dosing with MP (30 mg/kg IV) could attenuate spinal tissue eicosanoid levels measured by enzyme immunoassay at 1 h (Experiment 1). Pial blood flow was measured over the dorsal columns at the injury site using laser doppler flowmetry to monitor posttraumatic hyperperfusion as an index of the microvascular pathophysiology of acute SCI. In vehicle treated animals at 1 h postinjury, there was a significant increase in the tissue levels of PGF_2α (+290%), PGE₂ (+260%), TXB₂ (stable analog of TXA₂, +126%), and LTB₄ (+73%) in comparison to sham, uninjured animals. However, 6-keto-PGF_1α (stable analog of prostacyclin or PGI₂) and LTC₄ did not increase. Methylprednisolone did not reduce the increase in eicosanoid production. In the case of LTB₄ and LTC₄, MP actually increased the levels further. In addition, we examined the effects of a double dose MP regimen (30 mg/kg IV at 30 min plus 15 mg/kg IV at 2.5 h postinjury) on spinal cord eicosanoid levels at 4 h postinjury (Experiment 2). At 4 h postinjury, significant increases in PGF_2α, PGE₂, TXB₂, and 6-keto-PGF_1α were observed, and with the exception of PGE₂, no MP attenuation of the increased eicosanoids was seen. These results fail to provide evidence that postinjury administration of high dose MP exerts a significant anti-PLA₂ action. On the other hand, MP effectively inhibited secondary spinal cord pial hyperperfusion, which is believed to be largely mediated by free radical-lipid peroxidative mechanisms. Thus, it seems likely that the protective action of MP on the acute microvascular pathophysiology of SCI is mediated by its well-documented effects on posttraumatic LP. In support of this conclusion, parallel experiments were carried out that demonstrated that 30 min postinjury treatment with the nonglucocorticoid 21-aminosteroid LP inhibitor tirilazad mesylate, which completely lacks any PLA₂-inhibitory action, also eliminated posttraumatic pial hyperperfusion without any effect on the increase in spinal cord eicosanoids at either 1 or 4 h postinjury.

Key words:

spinal cord injury, eicosanoid formation, methylprednisolone, tiriliazad, mesylate; cats