Effects of tyrosine kinase inhibitor, genistein, and phosphotyrosine-phosphatase inhibitor, orthovanadate, on Ca(2+)-free contraction of uterine smooth muscle of the rat

1. The effects of a protein-tyrosine kinase inhibitor, genistein, and a protein-tyrosine phosphatase inhibitor, orthovanadate, were tested on the Ca(2+)-free contraction of the estrogen-dominated rat, which has been proved to be induced mainly via protein kinase C entirely independently of Ca2+. 2. Genistein (30 microM) significantly inhibited the contraction indicating participation of tyrosine kinase activity in the contraction. 3. Orthovanadate caused contraction concentration-dependently and augmented the Ca(2+)-free contraction at concentrations of more than 1 microM. The contraction by orthovanadate was not inhibited so significantly by genistein (30 microM). 4. Possible participation of tyrosine kinase activity in Ca(2+)-free contraction is discussed in addition to the formerly reported participation of protein kinase C.

Ca(2+)-independent contraction of uterine smooth muscle induced by vanadate and its inhibition by Ca2+

Vanadate, 30 microM, contracts uterine smooth muscle of estrogen-dominated non-pregnant rats in Ca(2+)-free medium after preincubation with 3 mM EGTA. In spite of the phosphorylation of the myosin light chain during this contraction, studies with fura-2 suggested that this contraction was not accompanied by an increase in the cytosolic Ca2+ level. Inhibitors of the myosin light chain kinase and protein kinase C partly inhibited this contraction. Vanadate seems to enter the cell through anion channels to inhibit phosphatases, resulting in phosphorylation via basal activities of the myosin light chain kinase and protein kinase C. An increase in the cytosolic free Ca2+ level resulted in relaxation of the contracting muscle in the same manner as in the oxytocin-induced Ca(2+)-free contraction.

Ca(2+)-dependent inhibition of Ca(2+)-independent contraction in uterine smooth muscle

Uterine smooth muscle of the rat shows Ca(2+)-independent contraction in response to oxytocin in Ca(2+)-free medium. Micromolar Ca2+ inhibits this contraction. We now tested whether Ca2+ itself is the cause of this inhibition. The ratio of fura-2 fluorescence, the indicator of the intracellular level of Ca2+, was increased in parallel with the degree of inhibition by Ca2+. When inhibition was elicited by Ca2+, EGTA released the inhibition. Comparison of the dose-response curve for oxytocin in Ca(2+)-free solution and that in the medium with 1 microM Ca2+ showed that the inhibition by Ca2+ is non-competitive. EGTA chelation of the intracellular Ca2+ by loading of EGTA as its acetoxymethylester resulted in diminution of inhibition by Ca2+. EGTA suppressed the Ca(2+)-induced contraction but did not affect Ca(2+)-independent contraction. It is concluded that the inhibition is induced by intracellular Ca2+ itself.

Inhibitory effects of protein kinase inhibitors and cytoskeletal inhibitors on Ca2(+)-free contraction of rat uterus

In rat uterine smooth muscle, sustained Ca2(+)-free contraction was observed by oxytocin in Ca2(+)-free solution. This Ca2(+)-free contraction was effectively inhibited by protein kinase inhibitors and cytoskeletal inhibitors but myosin-light chain kinase (MLCK) inhibitors were not so effective. Simultaneous addition of a protein kinase inhibitor and a cytoskeletal inhibitor caused synergistic inhibition. These results suggest that the mechanism for Ca2(+)-free contraction involves some protein kinase and cytoskeletal elements rather than MLCK.

Ca2+-independent contraction of uterine smooth muscle

Rat uterine smooth muscle shows sustained contraction to oxytocin in Ca2+-free medium with EGTA, that is called "Ca-free contraction"(1). Participation of the rise in cytosolic free Ca2+ in this Ca-free contraction was tested. In Ca-free contraction, the cytosolic free Ca2+ level was not changed at all as measured with fura-2. Further, the chelation of cytosolic free Ca2+ with quin-2 did not at all affect Ca-free contraction. These results strongly suggest that Ca-free contraction is not triggered by Ca2+.