␣1-Adrenoceptor-mediated phosphorylation of MYPT-1 and CPI-17 in the uterine artery: role of ERK/PKC. We previously demonstrated that ERK/PKC signaling pathways play a key role in regulation of Ca 2ϩ sensitivity and contractility of the uterine artery. The present study tested the hypothesis that ERK and PKC differentially regulated myosin light chain phosphatase activity by phosphorylation of myosin phosphatase target protein-1 (MYPT-1) and CPI-17. Agonist-induced contractions and

more »

... on of MYPT-1/Thr 696 , MYPT-1/ Thr 850 , and CPI-17/Thr 38 were measured simultaneously in the same tissues of isolated near-term pregnant ovine uterine arteries. Phenylephrine produced time-dependent concurrent increases in the phosphorylation of ERK 44/42 and MYPT-1/Thr 850 that preceded contractions. In addition, phenylephrine induced phosphorylation of CPI-17/ Thr 38 that was concurrent with the contractions. In contrast, phenylephrine did not induce phosphorylation of MYPT-1/Thr 696 in the uterine artery. PD-098059 inhibited phosphorylation of ERK 44/42 and the initial peak phosphorylation of MYPT-1/Thr 850 but did not affect CPI-17/Thr 38 phosphorylation. Activation of PKC by phorbol 12,13-dibutyrate induced a time-dependent phosphorylation of CPI-17/Thr 38 that preceded contractions of the uterine artery. In addition, phorbol 12,13-dibutyrate activated PKC-␣ and induced a coimmunoprecipitation of PKC-␣ with caldesmon. The results suggest that phosphorylation of MYPT-1/Thr 850 and CPI-17/Thr 38 play important roles in regulation of agonist-mediated Ca 2ϩ sensitivity in the uterine artery, in part by ERK and PKC, respectively. In addition, phosphorylated CPI-17 may regulate Ca 2ϩ sensitivity by interacting with caldesmon and reversing its inhibitory effect on myosin ATPase.