Regulation of smooth muscle contraction through remodeling of actin filaments

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1)Mechanism of the inhibitory effects of Phx-1 on the smooth muscle contraction To elucidate the mechanisms of relaxing effect of 2-amino-4,4 a-dihydro-4 a-7-dimethyl-3H-phenoxazine-3-one (Phx-1) on the smooth muscle preparations, we investigated effects of Phx-1 on the contraction of both intact and skinned (cell membrane peremeabilized) preparations from guinea pig tenia cecum. In intact preparations, Phx-1 concentration-dependently suppressed the contraction induced by either acetycholine(ACh) or high-K+ with the IC50 value estimated at around 100 pM. Similar inhibitory actions of Phx-1 on force were observed in intracellular Ca store depleted preparations. In the cell membrane depolarized preparations in the absence of extracellular Ca, however, Phx-1 had little effects on the caffeine-or ACh-induced contraction. In skinned preparations, Phx-1 suppressed the Ca^<2+->induced contraction at concentrations higher than 100 μM. These results suggest that Phx-1 seems to have direct inhib … More itory effect on the actin filaments organization, although inhibitory effect, of the Phx-1 on the Ca^<2+>-influx is much stronger.2)Contractile filaments remodeling by blebbistatin, a potent inhibitor of myosin II We studied the effects of blebbistatin, a potent inhibitor of myosin II on the mechanical and structural properties of beta escin skinned taenia cecum. Blebbistatin inhibited both Ca^<2+> induced contraction with only little effects on the Ca^<2+> sensitivity for the force, and 30 mM Mg^<2+> induced, myosin phosphorylation independent, force development, Also, blebbistatin accelerated relaxation by Ca^<2+> removal. IC_<50> values of the blebbistatin effects presented above were nearly 25 micro M. The present results suggested that blebbistain directly interfare cross-bridge interaction. Furthermore, blebbistatin inhibited organization of actin-and myosin-filaments. Since blebbistatin did not bind to actin directly, disruption of myosin filaments seemed to modulate actin filaments organization. Less