Blood-vessel wall arachidonate metabolism and its pharmacological modification in a new in vitro assay system

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Abstract
Prostacyclin and thromboxane production was measured in human umbilical cord arteries bathed in clotting human blood and compared with arteries bathed in Krebs buffer or clotting blood without vessels. In comparison with the combined system, vessels in buffer generated only minute amounts of immunoreactive thromboxane B2 while blood alone generated only minute amounts of immunoreactive 6-oxo-PGF1α. Incubation of vessels in blood was associated with an enhanced 6-oxo-PGF1α formation at 1–2 h of incubation, demonstrating an active prostacyclin synthetase and a transfer of the platelet endoperoxide precursor to this enzyme. This new combined system was used to reevaluate the selectivity of cyclooxygenase inhibitors for vascular and platelet derived eicosanoid formation. With respect to 6-oxo-PGF1α accumulation, the IC50 value [μmoles/l] for tiaprofenic acid (8.5 ± 3.0) was significantly higher than that for diclofenac (0.14 ± 0.03) (P < 0.05) while acetylsalicylic acid (18.0 ± 7.0) was less potent than diclofenac and indomethacin (2.4 ± 1.0) (P < 0.05). With respect to thromboxane B2 formation, the IC50 values for diclofenac (0.26 ± 0.04), indomethacin (IC50 0.30 ± 0.05) and tiaprofenic acid (IC50 0.71 ± 0.08) were not significantly different from each other. Acetylsalicylic acid (7.7 ± 1.8) was less potent than either of the other compounds (P < 0.01). While these IC50 values might suggest different potencies for inhibition of vascular and platelet cyclooxygenases by tiaprofenic acid and, possibly, indomethacin, statistical analysis was not possible because of different slopes of the dose-response curves. Moreover, a nearly complete inhibition of both enzymes might be expected at therapeutic antiinflammatory plasma levels of the compounds in vivo. It is concluded that this in vitro system using solely human tissues might be a useful tool to analyse time-dependent alterations in endogenous arachidonate metabolism during blood-vessel wall interactions and its modification by drugs and that this system might be superior to either blood or vessel preparations alone.