Change of Ca2+ requirement for myosin phosphorylation by prostaglandin F2 alpha.

The mechanism of contraction of vascular smooth muscle by prostaglandin F2 alpha (PGF2 alpha) was examined by simultaneous measurement of the intracellular Ca2+ concentration [( Ca2+]i), force, and myosin light-chain (MLC) phosphorylation in ferret aorta. In the presence of 2.5 mM extracellular Ca2+, PGF2 alpha (10(-5)M) produced a tonic contraction with a transient spike in [Ca2+]i, followed by a relatively small sustained increase in [Ca2+]i (from a basal level of 2.32 +/- 0.07 x 10(-7) to 2.72 +/- 0.05 x 10(-7) M). In Ca(2+)-free bathing media, PGF2 alpha also produced a tonic contraction with a small spike in [Ca2+]i, indicating a release of Ca2+ from intracellular store sites, followed by no significant increase in [Ca2+]i. Ca(2+)-force curves were constructed by plotting the calibrated steady-state aequorin light signal against the resulting steady-state force. The curve was significantly shifted to the left by PGF2 alpha. PGF2 alpha also shifted the Ca(2+)-phosphorylation curve to the left. These results suggest that PGF2 alpha causes contraction by both elevating [Ca2+]i and decreasing the Ca2+ requirement for MLC phosphorylation. The data are consistent with a mechanism where there is either an increase in activity of MLC kinase or a decrease in phosphatase activity. Additionally, there was a smaller, but statistically significant, effect to increase force at any one phosphorylation level, pointing to the possibility of regulation of contractile force separate from MLC phosphorylation.