Soy isoflavone yeast fermented extract inhibits contraction of rat ileal smooth muscle via opening of K+ channels and activation of intracellular cyclic adenosine monophosphate- and cyclic guanosine monophosphate-related pathways.

This study investigated the effects of soy isoflavone yeast fermented extract (soyF) and soy isoflavone yeast unfermented extract (soyN) on rat ileal smooth muscle contraction. SoyF and soyN inhibited carbachol (CCh)- or KCl-induced contraction in a concentration-dependent manner; however, these effects were stronger for CCh-induced contraction than that for KCl, and the relaxation effect was stronger for soyF than for soyN. SoyF-induced relaxation was attenuated by 4-aminopyridine (4-AP), a Kv channel inhibitor, and iberiotoxin (IbTX), a calcium-activated potassium channel (BK channel) inhibitor. This relaxation was also decreased by 9-(tetrahydrofuran-2-yl)-9h-purin-6-amine (SQ22536) and 1H-[1,2,4]oxadiazolo[4,3-a] quinoxalin-1-one (ODQ). Moreover, simultaneous treatment involving SQ22536 with 4-AP or IbTX further restored soyF-induced inhibition compared with each single treatment, whereas simultaneous treatment with ODQ and 4-AP or IbTX did not restore soyF-induced inhibition. SoyF increased cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) levels in ileal strips. In β-escin-permeabilized muscle, CCh, guanosine 5'-O-(3-thiotriphosphate) (GTPγS), and phorbol-12,13-dibutyrate ( PDBu)-enhanced Ca²⁺-induced contraction, but soyF suppressed this Ca²⁺ sensitization. In summary, yeast-fermented treatment increased soy isoflavone-induced inhibition of ileal smooth muscle contraction. These data suggest that soyF-induced relaxation was due to increasing cAMP and cGMP related to activating adenylate cyclase and guanylate cyclase. Moreover, it was demonstrated that soyF-induced relaxation is due to cell membrane hyperpolarization and inhibition of contractile proteins. Cell membrane hyperpolarization is probably mediated by the activation of Kv and BK channels via cGMP-related signals. Furthermore, the inhibitory effect on the contractile protein is due to Ca²⁺ desensitization, probably through suppression of the Rho kinase/myosin phosphatase targeting subunit (MYPT) and/or the PKC/CPI-17 pathway.