Endothelium-independent vasorelaxation by the selective alpha estrogen receptor agonist propyl pyrazole triol in rat aortic smooth muscle

Objectives This study investigated the signalling mechanism of the relaxant responses to the estrogen receptor alpha (ER alpha) agonist PPT (propyl pyrazole triol) in endothelium-denuded rat aortic rings. Methods Several compounds, including protein kinase G (PKG) inhibitors and potassium channel inhibitors, were tested against PPT-dependent rat aortic relaxation. Cyclic GMP and cytosolic calcium responses to PPT in isolated aortic smooth muscle were investigated in parallel. Key findings PPT vasorelaxation was largely reduced by the selective ER alpha antagonist methyl-piperidinopyrazole (MPP; -91.6 +/- 2.5%), by the selective PKG inhibitor Rp-8-Br-cGMP (-78.6 +/- 4.9%), by the specific soluble guanylyl cyclase inhibitor ODQ (1H-(1,2,4)-oxadiazolo[4,3-a]quinoxalin-1-one; -85.3 +/- 5.2%) and to a lesser extent by the selective BKCa (large-conductance calcium- and voltage-activated potassium channel) inhibitor iberiotoxin (-59.3%), the selective IKCa (intermediate-conductance calcium-activated potassium channel) inhibitor TRAM-34 (1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole; -50.7%) and the voltage-gated potassium channel inhibitor 4-aminopyridine (-40.8%). In isolated aortic smooth muscle, PPT strongly enhanced the cyclic GMP content (+144%) and Rp-8-Br-cGMP largely reduced the PPT-dependent calcium signal (-80.8%). Conclusions ER alpha receptor stimulation in rat aortic smooth muscle evokes a PKG-signalling pathway, likely triggering relaxation by BKCa and IKCa channel opening.