The role of cyclooxygenase–derived metabolites in contractions induced by hyposmotic solution in rat isolated tail arteries

M. Rafferty, A.D. Hughes, S. Wijetunge. Clinical Pharmacology, International Centre for Circulatory Health, NHLI Division, Faculty of Medicine, Imperial College London. London, W2 1NY. UK.

Blood vessels contract in response to hyposmotic solutions. This contraction is largely attributable to calcium influx via L-type calcium channels (Wijetunge & Hughes, 2005) and shows some similarities to myogenic contraction. The myogenic response is modulated by arachidonic acid metabolites of the cyclooxygenase pathway (Zsekeres et al., 2004); therefore the aim of this study was to examine the role of cyclooxygenase derivatives in contractions induced by a hyposmotic solution.

Male Wistar rats (150-200g) were stunned and killed by decapitation, their tails removed, arteries isolated and mounted in a myograph. The myograph contained an isosmotic solution containing (mM) NaCl 100, KCl 5, NaHCO3 20, HEPES 5, MgCL2 1, CaCl2 1.6, glucose 5 and mannitol 50 (300mosm/l). The solution was aerated with 95% O2 / 5% CO2 and maintained at 370C. Hyposmotic solution (PSShypo) consisted of PSS without mannitol (250mosm/l). To examine the effect of cyclooxygenase (COX) and its metabolites, 2 segments of artery were pre-incubated with a selective inhibitor or vehicle control for 30 minutes prior to stimulation with PSShypo. All data are presented as means ± SEM. Statistical comparisons were made using a paired Student’s t test and p < 0.05 was considered statistically significant.

Rat tail arteries contracted on exposure to PSShypo. This contraction was 47.8 ± 0.1 % (n = 34) of the maximum contraction induced by a high potassium (118mM) solution (KPSS) . Two structurally different COX inhibitors, indomethacin (10μM) and flurbiprofen (10μM) reduced the contraction induced by hyposmotic solution by 27.2 ± 0.1% (n=15, p < 0.001) and 39.3 ± 0.7% (n = 12, p < 0.001) respectively . A selective prostaglandin E2 receptor (EP1) antagonist, SC-19220 (100nM), also inhibited contraction to PSShypo by 28.3 ± 0.3 % (n = 8, p < 0.05). In contrast the selective thromboxane (TP) receptor antagonist, IC 192605 (100nM), did not inhibit responses (0.75 ± 1.5%; n = 10), although it inhibited the contractile response to the thromboxane mimetic, U-46119 (400nM). None of the inhibitors affected the contractile response to KPSS (n = 3 - 4).

Hyposmotic contraction of rat tail arteries involves COX-derived vasoactive mediators acting on EP1 but not TP receptors. The identity of the eicosanoid(s) involved remains to be established, but PGE2 is a likely candidate.

Wijetunge S, & Hughes A.D. (2005). J. Vasc. Res. 42, 93–100.
Zsekeres M et al., (2004). J. Cardiovasc. Med. 43, 242-9