Evidence for a dilator function of 8-iso prostaglandin F2 alpha in rat pulmonary artery

1. 8-Iso prostaglandin F2 alpha (8-iso PGF2 alpha) is one of a series of prostanoids formed independently of the cyclo-oxygenase pathway. It has been shown to be upregulated in many conditions of oxidant stress where its formation is induced by free radical-catalysed actions on arachidonic acid. As 8-iso PGF2 alpha is formed in vivo in diseases in which oxidant stress is high such as septic shock, we have assessed the relative potency and efficacy of this compound in pulmonary arteries from control and lipopolysaccharide (LPS)-treated rats. 2. Several studies have characterized the contractile actions of 8-iso PGF2 alpha on various smooth muscle preparations, but its potential dilator actions have not been addressed. Thus these studies examined both the contractile and dilator actions of 8-iso PGF2 alpha in rat pulmonary artery rings. The thromboxane mimetic U46619, PGE2 sodium nitroprusside (SNP) and acetyl choline (ACh) were used for comparison. Each prostanoid had to be dissolved in ethanol to a maximum concentration of 1 x 10-2 M. At high concentrations, ethanol directly contracted pulmonary vessels. We were therefore limited by the actions of the vehicle such that we were unable to add prostanoids at concentrations higher than 1 x 10-4 M. In some cases this meant that maximum responses were not achieved and in these cases the Emax and pD2 values are apparent estimates. 3. The following rank order of potency was obtained from contractile studies; U46619 > 8-iso PGF2 alpha > PGE2, each prostanoid producing concentration-dependent contractions (10(-10)-3 x 10(-4) M, 10(-9)-10(-4) M, 10(-8)-10(-4) M, respectively). As has been shown previously for other smooth muscle preparations, the thromboxane receptor (TP) antagonist ICI 192605, (1 x 10(-6), 1 x 10(-5) and 1 x 10(-4) M), inhibited the contractions of 8-iso PGF2 alpha in a concentration-dependent fashion. 4. The nitric oxide synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME; 1 x 10(-4) M), enhanced the contractile function of both 8-iso PGF2 alpha and PGE2, but had no effect on that caused by U46619. Similarly, L-NAME inhibited the dilator function of all agents tested except the exogenous nitric oxide (NO) donor SNP indicating that PGE2 and 8-iso PGF2 alpha like ACh, act through the release of NO. The specificity of the effects of L-NAME were confirmed in studies with the inactive enantiomer D-NAME (1 x 10(-4) M), which did not affect the contractile or the dilator actions of 8-iso PGF2 alpha. Furthermore, ICI 192605 enhanced the dilator actions of 8-iso PGF2 alpha, suggesting that the dilator component of 8-iso PGF2 alpha was achieved via activation of a non-TP receptor. 5. Isoprostanes may modulate vascular tone by a direct action on TP receptors to cause contraction and via a distinct receptor leading to the release of NO to cause dilation.