Regulation of baseline Ca2+ sensitivity in permeabilized uterine arteries: effect of pregnancy.

The adaptation of contractile mechanisms of the uterine artery to pregnancy is not fully understood. The present study examined the effect of pregnancy on the uterine artery baseline Ca2+ sensitivity. In beta-escin-permeabilized arterial preparations, Ca2+ -induced concentration-dependent contractions were significantly decreased in uterine arteries from pregnant animals compared with those of nonpregnant animals. Time-course studies showed that Ca2+ increased phosphorylation of 20-kDa myosin light chain (MLC20), which preceded the tension development in vessels from both pregnant and nonpregnant animals. When compared with vessels from nonpregnant animals, there was a significant increase in the protein level of MLC20 and an accordance increase in the level of Ca2+ -induced phosphorylated MLC20 (MLC20-P) in uterine arteries during pregnancy. Simultaneous measurements of MCL20-P levels and contractions stimulated with Ca2+ in the same tissues demonstrated a significant attenuation in the tension-to-MLC20-P ratio in uterine arteries during pregnancy. Activation of PKC with phorbol 12,13-dibutyrate (PDBu) potentiated Ca2+ -induced contractions in uterine arteries from nonpregnant but not pregnant animals. Accordingly, inhibition of PKC attenuated Ca2+ -induced contractions in uterine arteries from nonpregnant but not pregnant animals. PDBu produced contractions in the presence or absence of Ca2+ in the beta-escin-permeabilized arteries, which were significantly decreased in uterine arteries from pregnant compared with nonpregnant animals. The results suggest that pregnancy upregulates the thick-filament regulatory pathway by increasing MLC20 phosphorylation but downregulates the thin-filament regulatory pathway by decreasing the contractile sensitivity of MLC20-P, resulting in attenuated baseline Ca2+ sensitivity in the uterine artery. In addition, PKC plays an important role in the regulation of basal Ca2+ sensitivity, which is downregulated during pregnancy.