GTP GAMMA-S-DEPENDENT REGULATION OF SMOOTH-MUSCLE CONTRACTILE ELEMENTS

Guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) increases the sensitivity of the contractile response to activation by Ca2+ in permeabilized tracheal smooth muscle. Increased tension was associated with a proportional increase in myosin light chain phosphorylation. The site of phosphorylation was determined to be serine-19, which corresponds to the site rapidly phosphorylated by myosin light chain kinase. GTP-gamma-S did not affect the contraction induced by the protein phosphatase inhibitor okadaic acid but did enhance contraction produced by Ca2+-independent myosin light chain kinase. In tracheal homogenates Ca2+-dependent myosin light chain kinase activity was not affected by GTP-gamma-S; however, dephosphorylation of P-32-labeled heavy meromyosin by phosphatase was inhibited. Thus GTP-gamma-S may increase the Ca2+ sensitivity of contractile elements in tracheal smooth muscle by inhibition of protein phosphatase activity toward myosin light chain.