Effects of Pimobendan and Its Active Metabolite, UD-CG 212 Cl, on Ca2+-Activated K+ Channels in Vascular Smooth-Muscle Cells

The aim of this study was to clarify the mechanisms of activation of Ca2+-dependent K+ (K(Ca)) channels by pimobendan, a new cardiotonic and vasodilator agent with phosphodiesterase inhibition properties, and its main metabolite, UD-CG 212 Cl, in vascular smooth-muscle cells from porcine coronary arteries. Both pimobendan and UD-CG 212 Cl induced relaxation of porcine coronary artery strips. However, in the presence of 100 microM Rp-cAMPS (Rp diastereomer of adenosine cyclic 3',5'-phosphorothioate), a membrane-permeable antagonist of cAMP, the effects of pimobendan decreased significantly. Application of 1 microM pimobendan activated K(Ca) channels in cell-attached patches, and this increase in activity was suppressed by 100 nM H-89 (N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinoline-sulfonamide), a cAMP-dependent protein-kinase inhibitor. Pimobendan was ineffective in altering the activity of K(Ca) channels in inside-out patches. In contrast, UD-CG 212 Cl, at a concentration of 1 microM, activated K(Ca) channels not only in cell-attached patches but also in inside-out patches. Application of 100 nM H-89 also inhibited UD-CG 212 Cl-induced K(Ca) channels activity but to a lesser degree than that induced by pimobendan in cell-attached patches. In conclusion, there are two mechanisms of activation of K(Ca) channels by pimobendan and UD-CG 212 Cl. Activation by pimobendan occurs primary through the cAMP pathway, whereas UD-CG 212 Cl activates K(Ca) channels directly as well as through the cyclic AMP pathway.