Inorganic phosphate promotes relaxation of chemically skinned smooth muscle of guinea-pig Taenia coli.

In the presence of calmodulin and phosphate and an ATP-regenerating system, Triton-treated "skinned fibers' of the Taenia coli could be made to contract and relax by step changes of Ca++ within about 30 sec. In the absence of phosphate, relaxation was slower, and during this slow relaxation tension was not maintained actively. The passive tension could be abolished by phosphate (3-6 mM). Phosphate had little effect on contractile tension but decreased the speed of contraction.