Triton skinned smooth muscle activation

In triton skinned and in a-toxin-permeabilized smooth muscle preparations, the Ca + sensitivity of force production is also decreased by cGMP (Pfitzer et al., 1984,1986 Nishimura efflZ., 1992). This may be due to an up-regulation of MLCP (Pfitzer etal., 1986). Clear evidence that modulation of the activity of MLCP would affect Ca + sensitivity of tension development was in fact obtained in triton skinned smooth muscle when it was shown that inhibition of MLCP by the black sponge toxin, okadaic acid, increased Ca + sensitivity (Takai et al., 1987 Bialojan et al., 1988). On the other hand, incubation of triton skinned chicken gizzard fibers with a purified phosphatase decreased Ca + sensitivity (Bialojan et al., 1987). 

Using freeze-dried fibers, Riiegg and coworkers (1984) estimated the fraction of calmodulin available for activation of MLCK to be in the range of 0.3 to 4 yJVl, which is about 10% of the total cellular calmodulin. This pool is readily exchangeable and therefore rapidly equilibrates with the incubation medium (Riiegg et al., 1984). Similar results were obtained in triton skinned tracheal smooth muscle (Tansey et al., 1994). In contrast, calmodulin appears to be retained in P-escin-permeabilized tracheal smooth muscle (Tansey et al., 1994). 

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