Pregnancy attenuates uterine artery pressure-dependent vascular tone: role of PKC/ERK pathway

The mechanisms of adaptation of uterine artery vascular tone to pregnancy are not fully understood. The present study tested the hypothesis that pregnancy decreases the PKC-mediated Ca2+ sensitivity of the contractile process and attenuates myogenic tone in resistance-sized uterine arteries. In pressurized uterine arteries from nonpregnant (NPUA) and near-term pregnant (PUA) sheep, we measured, simultaneously in the same tissue, vascular diameter and vessel wall intracellular Ca2+ concentration ([Ca2+](i)) as a function of intraluminal pressure. In both NPUA and PUA, membrane depolarization with KCl caused a rapid increase in [Ca2+](i) and a decrease in diameter. A pressure increase from 20 to 100 mmHg resulted in a transient increase in diameter that was associated with an increase in [Ca2+](i), followed by myogenic contractions in the absence of further changes in [Ca2+](i). In addition, activation of PKC by phorbol 12,13-dibutyrate induced a decrease in diameter in the absence of changes in [Ca2+]. Pressure-dependent myogenic responses were significantly decreased in PUA compared with NPUA. However, pressure-induced increases in [Ca2+](i) were not significantly different between PUA and NPUA. The ratio of changes in diameter to changes in [Ca2+](i) was significantly greater for pressure-induced contraction of NPUA than that of PUA. Inhibition of PKC by calphostin C significantly attenuated the pressure-induced vascular tone and eliminated the difference of myogenic responses between NPUA and PUA. In contrast, the MAPKK (MEK) inhibitor PD-098059 had no effect on NPUA but significantly enhanced myogenic responses of PUA. In the presence of PD-098059, there was no difference in pressure-induced myogenic responses between NPUA and PUA. The results suggest that pregnancy down-regulates pressure-dependent myogenic tone of the uterine artery, which is partly due to increased MEK/ERK activity and decreased PKC signal pathway leading to a decrease in Ca(2+)sensitivity of myogenic mechanism in the uterine artery during pregnancy.