Iloprost activates KCa channels of vascular smooth muscle cells: role of cAMP-dependent protein kinase

The patch-clamp technique was used to investigate the effect of iloprost on activity of calcium-activated potassium (KCa) channels of freshly isolated rat tail artery smooth muscle cells. In the whole cell configuration, outward current, determined largely by KCa channels, was enhanced 1.73 +/- 0.11-fold by 5 x 10(-7) M iloprost, 1.80 +/- 0.12-fold by 10(-4) M 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole-3', 5'-cyclic monophosphothioate (Sp-5,6-DCl-cBIMPS), a specific activator of adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase (PKA), and 2.78 +/- 0.95-fold by 10 U/ml of the catalytic subunit of PKA + 10(-4) M MgATP, whereas the heat-inactivated catalytic subunit of PKA + MgATP was without effect. Iloprost at 5 x 10(-7) M increased this current 1.70 +/- 0.27-fold after pretreatment of cells with 10(-6) M okadaic acid, a specific phosphatase inhibitor, but did not alter this current after pretreatment of cells with 2 x 10(-4) M Rp-8-(4-chlorophenylthio)-adenosine-3',5'-cyclic monophosphorothioate, a specific PKA inhibitor. In the cell-attached configuration, activity of KCa channels was enhanced 2.48 +/- 0.44-fold by 5 x 10(-7) M iloprost and 2.09 +/- 0.07-fold by 10(-4) M Sp-5,6-DCl-cBIMPS. Iloprost at 5 x 10(-7) M did not alter intracellular calcium concentration in these cells measured using indo 1. In the inside-out configuration, activity of KCa channels was increased 87.12 +/- 45.04-fold by 10 U/ml of the catalytic subunit of PKA together with 10(-4) M MgATP, whereas no effect was observed after application of the catalytic subunit of PKA together with its regulatory subunit and MgATP, MgATP, cAMP, or the catalytic subunit of PKA alone also did not change KCa channel activity. Thus these results show that iloprost is able to activate KCa channels of freshly isolated rat tail artery smooth muscle cells and suggest that this effect is mediated by a PKA-induced phosphorylation of the channel.