Physiological vascular tone is modulated by various substances released from the endothelial layer including cyclo-oxygenase (COX) products such as prostacyclin (PGI₂). In healthy blood vessels PGI₂ is formed predominately in the endothelium by constitutive isoforms of COX (COX-1, Mitchell et al., 1993). However, under some conditions, such as when exposed to bacterial lipopolysaccharide (LPS), vascular smooth muscle cells can also release PGI₂, a phenomenon associated with expression of the inducible isoform of COX (COX-2, Mitchell and Warner, 1999). The induction of COX-2 in vivo is associated with deleterious responses such as hypotension, plasma extravasation and pain. However, there may be situations in which the induction of COX-2 and the subsequent release of PGI₂ may represent a defence mechanism to compensate for endothelial dysfunction (Bishop-Bailey et al., 1997). The induction of COX-2 has been well described in rodent aorta, however has not been investigated in smaller vessels in vitro. Thus, our aim was to determine the effects within the rat carotid artery of COX-2 expression.

Male Wistar rats (250-280 g) were injected with Escherichia coli LPS (serotype 0127:B8; 6 mg kg^-1, i.p.) left for 4 h and then killed with a lethal dose of pentobarbitone sodium (300 mg kg^-1, i.p.). Both carotid arteries were immediately removed and cleaned of connective tissue. Vessels were then cut into rings of approximately 2-3 mm width. Individual rings were placed in sterile 96-well plates containing 100 µl of DMEM. All tissue incubations were carried out at 37^o C in an atmosphere of 5% CO₂. Vessels were then left to equilibrate for 15 min before the medium was replaced with fresh medium containing drugs or vehicle (0.1% DMSO). One hour later arachidonic acid (10 µg/ml) was added for 15 min and the medium was removed to measure 6-keto-PGF₁ (the breakdown product of PGI₂) by radioimmnuassay (Mitchell et al., 1993).

Treatment of rats with LPS caused a marked increase in the levels of 6-keto-PGF₁ (control, 13.5±1.5 ng/ml, n=3; LPS-treated 27.4±3.6 ng/ml, n=3, p < 0.05). The release of 6-keto-PGF₁ by rat carotid artery rings was significantly diminished by incubation with either indomethacin (10 µM) or DFP (100 µM) (vehicle 100±4%; indomethacin 9±1%; DFP 34±3%; n=3 for all).

We then studied the effect of LPS on vascular function of carotid artery rings using a conventional organ bath system for isometric tension recording. Relaxations of precontracted carotid artery rings to acetylcholine (1 µM) in the presence of L-NAME (1 mM) were inhibited by both indomethacin (10 µM) or DFP (100 µM) (control 46.7±6.4%; indomethacin 4.9±3.1%; DFP 17.1±9.6%; n=6 for all)

These observations show that COX-2 induced within the carotid artery participates in the vasodilator effects of acetylcholine. The induction of COX-2 and subsequent release of prostacyclin may represented an endogenous compensation for endothelial damage

Bishop-Bailey et al., (1997) Br. J. Pharmacol., 121: 125-133.
Mitchell et al., (1993) Proc. Natl. Acad. Sci.,. 90: 11693-11697.
Mitchell and Warner (1999), Br. J. Pharmacol., 128: 1121-1132.

This work was supported by a Fellowship grant from the Spanish Government and grants from the Medical Research Council, the Joint Research Board of St. Bartholomew's Hospital, and The William Harvey Research Foundation.