Effect of inorganic phosphate on the Ca2+ sensitivity in skinned Taenia coli smooth muscle fibers. Comparison of tension, ATPase activity, and phosphorylation of the regulatory myosin light chains.

Inorganic phosphate (Pi) decreases maximal tension in contracted skeletal and heart muscle fibers. We investigated the effects of 10 mM Pi on the force-calcium relationship in Triton X-100-skinned Taenia coli smooth muscle fibers. Isometric force measurements show that the calcium sensitivity of the force depends on the phosphate concentration. Furthermore 10 mM Pi relaxes the fibers more at intermediate than at high calcium ion concentrations: At pCa 4.5 tension decreases in the presence of 10 mM Pi by approximately 12% but it decreases 70% at pCa 6.17. Removal of phosphate partially reverses the relaxation. Simultaneous determination of actomyosin ATPase activity and force (Güth, K., and J. Junge, 1982, Nature (Lond.), 300:775-776) shows that the ATPase activity does not correlate with the changes in force. In the presence of Pi, tension decreases more than the ATPase activity. The level of phosphorylation of the 20,000-D regulatory myosin light chain is not changed in the presence or absence of 10 mM Pi. The results are discussed in terms of slowly or noncycling myosin crossbridges formed at lower calcium concentrations, which contribute to the force development but not to the ATPase activity. These crossbridges are considered to be dissociated in the presence of phosphate.