Role of the cyclooxygenase pathway in the protection against postischemic stunning in conscious sheep

Nitroglycerin induces late preconditioning against arrhythmias but not stunning in conscious sheep

OBJECTIVES

Nitroglycerin, a nitric oxide donor, induces late preconditioning against stunning by short ischemia-reperfusion periods. The study purpose was to assess similar nitroglycerin protection against stunning and arrhythmias produced by prolonged reversible ischemia.

DESIGN

Four groups of conscious sheep were studied, control: 12 minutes ischemia and 2 hour reperfusion; late preconditioning: six periods of 5 min ischemia-5 min reperfusion 24 h before 12 min ischemia and late preconditioning with 120 microg/kg and 600 microg/kg nitroglycerin administered instead of the ischemia-reperfusion periods.

RESULTS

Although late preconditioning protected against stunning (mean postischemic recovery of wall thickening fraction, control (n=10): 54.8+/-3.2, late preconditioning (n=9): 74.4+/-3.0, p<0.01), nitroglycerin 120 microg/kg (n=6) did not reproduce mechanical protection (50.1+/-3.8), even with a higher concentration of 600 microg/kg (59.1+/-3.7, n=4). However, nitroglycerin decreased arrhythmia severity index (control: 2.3+/-0.6, late preconditioning: 0.5+/-0.4, nitroglycerin 120 microg/kg: 1+/-0.4 and 600 microg/kg: 0.1+/-0.1 (p<0.05 vs. control).

CONCLUSIONS

Nitroglycerin only has a limited late preconditioning protective effect in conscious animals submitted to a reversible prolonged ischemia since it protects against arrhythmias but not against stunning.

The cycloxygenase 2 (COX-2) story: it's time to explain, not inflame

Despite initial promising reports that anti-inflammatory properties of cycloxygenase-2 (COX-2) inhibitors may confer anti-atherosclerosis effects and stabilize the atherosclerotic plaque, subsequent data from long-term clinical trials have shown that selective COX-2 inhibitors are associated with increased risk of cardiovascular events. The commonly cited explanation is that selective inhibition of COX-2 leads to depletion of prostacyclin, whereas the production of pro-thrombotic thromboxane by means of cycloxygenase-1 (COX-1) is unopposed. This hypothesis seems unlikely as the overall explanation, because low-dose aspirin does not decrease the increased risk associated with COX-2 inhibitors. Moreover, the risk associated with nonselective COX inhibitors may be similar to selective COX-2 inhibitors. Alternative hypotheses include (1) elevated blood pressure, (2) abnormal vascular remodeling, (3) inhibition of protective mechanisms against ischemia-reperfusion injury, and (4) inhibition of 15-epi-lipoxin production. Varying results in different experimental models may be related to the fact that COX-2 is involved in numerous cellular functions. Inhibiting COX-2 in inflammatory cells may have favorable effects, whereas in organs such as the heart and brain and/or blood vessels may have deleterious effects. Currently, the "selective COX-2 inhibitors" are not selective in the sense that they inhibit COX-2 in all tissues without predilection to inflammatory cells and, as a result, may summate to increase the risk of cardiovascular events.