Methacholine-induced contraction of rabbit pulmonary artery: role of platelet-endothelial transcellular thromboxane synthesis.

Arachidonic acid- and methacholine-induced contractions of rabbit pulmonary arteries are mediated by thromboxane (TX) A2. Although removal of the endothelium abolishes the contractions, endothelial cells isolated from pulmonary arteries do not synthesize TXA2. Further studies described here showed that the expression of TX synthase was evident in platelets and intact pulmonary artery but not in endothelial cells. These studies examined the role of platelet TXA2 production in the vasoconstrictor response to methacholine. Endothelial cells were incubated with platelets in the presence or absence of methacholine. Methacholine caused an increase in TXB2 production. Pretreatment of endothelial cells with aspirin (100 micromol/L) before the addition of platelets did not impair the ability of methacholine to increase TXB2 synthesis. Conversely, if platelets were pretreated with aspirin, methacholine failed to stimulate TXB2. Using endothelial cells with their cellular lipids labeled with [3H]arachidonic acid, methacholine did not stimulate the production of [3H]TXB2. When the endothelial cells were incubated with methacholine and control platelets, [3H]TXB2 was detected. If aspirin-treated platelets were incubated with endothelial cells, methacholine did not increase the production of [3H]TXB2. However, pretreatment of the endothelial cells with aspirin did not affect the ability of methacholine to induce [3H]TXB2 release. This suggests that methacholine stimulated the endothelial cell to release arachidonic acid, which was transferred to the platelets and metabolized to TXA2. To test whether this cell-cell interaction is necessary for methacholine-induced contractions, rabbits were administered aspirin (20 mg/kg) for 2 days. On day 4, methacholine-induced contractions of pulmonary arteries were depressed in aspirin-treated compared with control subjects. Control arteries synthesized 6-keto-prostaglandin F1alpha and TXB2. Aspirin treatment inhibited both pulmonary artery and platelet TXB2 production but had no effect on vessel 6-keto-prostaglandin F1alpha. These studies implicate platelets as a vascular source of TXA2 and indicate that both endothelial cells and platelets may be required for methacholine-induced TXA2 synthesis and vasoconstriction.