Increased inhibitory effect of phorbol ester on cytosolic Ca2+ level and contraction in rat myometrium after gestation

Role of protein kinase Cbeta in rhythmic contractions of human pregnant myometrium

To assess the role of protein kinase Cbeta (PKCbeta) in human myometrial contractions during pregnancy, we evaluated the effect of a PKCbeta inhibitor (LY333531) on the pregnant and nonpregnant myometrial contractions and compared the level of PKCbeta in the pregnant myometrium with that in the nonpregnant myometrium. The effects of LY333531 on the myometrial contractions were examined by measuring contractile activity (frequency and amplitude). PKCbeta in human myometrium was assessed at mRNA level using real-time PCR method. The characteristics of contractile activity were different between the pregnant and the nonpregnant myometrium. The amplitude of rhythmic contractions in the preterm and term myometrium was increased 2- to 2.5-fold when compared with that in the nonpregnant myometrium, but the frequency of rhythmic contractions was decreased by about half. LY333531 (10(-6) M) reduced the increased amplitude in the preterm and term myometrium by about 50%, and the inhibitory effects of LY333531 in the pregnant myometrium were significantly greater than that in the nonpregnant myometrium (about 50 vs 25%). However, the frequency in the pregnant and nonpregnant myometrium was not influenced by LY333531. Real-time PCR revealed a significant, five- to sevenfold increase in the expression of PKCbeta mRNA in the preterm and term myometrium when compared with the nonpregnant myometrium. These findings suggest that the increased amplitude of human myometrial contractions during pregnancy is related to the increased level of PKCbeta. A PKCbeta inhibitor may reduce preterm uterine contractions and prevent preterm delivery.

Differences in the gestational pattern of mRNA expression of the Rnd family in rat and human myometria

Uterine myometrial contractility remains a poorly characterized area of research in reproductive physiology. Rnd1, a novel member of the GTP-binding Rho protein family, inhibits Ca(2+)-sensitization by specifically interfering with a RhoA/Rho-activated kinases-dependent mechanism in smooth muscle. In addition to Rnd1, there are two other members, Rnd2 and Rnd3, in the Rnd family of Rho proteins. In the present comparative study of myometrial contractility in rats and humans, we found that all three Rnd mRNAs were expressed in nonpregnant rat myometrium and in nonpregnant human myometrial tissues. Although all three mRNA levels increased significantly after gestation in rat myometria, only Rnd1 expression was significantly greater after gestation in human samples. In the ovariectomized rat, administration of estrogen and/or progesterone increased the expression of all Rnd mRNAs. These results suggest that universal Rnd family up-regulation during pregnancy in rats may have an important role for negative-feedback control of uterine contraction during gestation by inhibiting RhoA-mediated increase in Ca(2+) sensitivity of contractile elements. Such increases in Rnd levels may be due to augmented levels of reproductive steroids in rats. Our data also point to gestational differences between rats and humans in Rnd isoform patterns.

Functional role of PKC in contraction of cultured human prostatic stromal cells

The contractile activity of prostatic stromal cells contributes to symptoms of benign prostatic hyperplasia (BPH). However, the mechanisms for this contraction have not yet been fully elucidated. In this study, we investigated the role of protein kinase C (PKC) in prostatic contraction by measuring the isometric tension development of cultured human prostatic stromal cells (CHPSCs) derived from BPH patients. Fresh human BPH tissue was used only in a Western blot analysis. A ring preparation made of CHPSCs and collagen gel could develop an isometric tension during activation with various agonists. Phorbol 12,13 dibutyrate (PDBu), a PKC activator, induced a relaxation. A Western blot analysis revealed the expression of PKC-potentiated protein phosphatase-1 inhibitory protein (CPI-17) in both CHPSCs and fresh human BPH tissue to be much lower than that in the rabbit aorta. When CPI-17 was over-expressed, PDBu induced a large contraction, but the agonist-induced contraction did not become larger than expected. In alpha-toxin permeabilized preparations, PDBu induced a relaxation in control CHPSCs, while it induced a contraction at a constant [Ca2+]i in CPI-17 over-expressing CHPSCs. These results indicated that the activation of PKC in CHPSCs induces a relaxation probably due to low expression level of CPI-17 and also that the PKC-CPI-17 pathway does not appear to play a major role in the agonist-induced contraction even when CPI-17 was over-expressed.

Abnormal response to ryanodine in oesophageal striated muscle of spontaneously hypertensive rats

The effects of ryanodine on twitch contraction and basal tension of oesophageal striated muscle were compared between preparations from stroke-prone spontaneously hypertensive rats (SHRSP) and normotensive Wistar Kyoto rats (WKY). Ryanodine (3 x 10(-7) M) augmented the twitch contraction in WKY preparations, butt attenuated it in SHRSP preparations. Rates of contraction and relaxation of twitch contraction, normalized to developed tension, were slightly decreased by ryanodine in both preparations. The effect of ryanodine was not different between WKY and SHRSP preparations. Ryanodine elevated the basal tension in WKY preparations but not in SHRSP preparations. Ryanodine elevated the intracellular Ca(2+) level in both preparations, but the response was significantly less in SHRSP preparations. Resting and action potentials were not significantly different between WKY and SHRSP preparations, while the duration of the action potential was significantly longer in SHRSP preparations. Ryanodine did not alter the resting and action potentials of either preparation. These results suggest that the Ca(2+) handling properties, including the ryanodine receptor, of the sarcoplasmic reticulum are genetically altered in oesophageal striated muscle of SHRSP.

Possible role of the protein kinase C/CPI-17 pathway in the augmented contraction of human myometrium after gestation

1. Activation of protein kinase C (PKC) by phorbol 12,13-dibutylate (PDBu, 1 microm) induced sustained contractions with no increase in [Ca2+]i in nonpregnant and pregnant human myometria. The contractile effects of PDBu in pregnant myometrium were much greater than those in nonpregnant myometrium, and the contractions in pregnant myometrium were accompanied by an increase in myosin light chain (MLC) phosphorylation at Ser19. 2. The contraction induced by PDBu in pregnant myometrium was inhibited by the inhibitors of conventional PKC isoforms, bisindolylmaleimides and indolocarbazole, such as Go6976, Go6983, and Go6850 (1 microM). LY333531 (1 microM), a specific inhibitor of PKC beta, also inhibited the PDBu-induced contraction in the pregnant myometrium. 3. In the pregnant myometrium permeabilized with alpha-toxin, PDBu increased the contractions induced at fixed Ca2+ concentration (0.3 microM) both in nonpregnant and pregnant myometria, indicating Ca2+ sensitization of contractile elements. 4. Western immunoblot analysis indicated that pregnant myometrium contained PKC isozymes such as conventional PKC (alpha, beta, gamma), novel PKC (delta, epsilon, theta), and atypical PKC (zeta but not iota and lambda). RT-PCR and real-time RT-PCR analysis indicated that, among the conventional PKC, the levels of mRNA of beta isoform in pregnant human myometrium were greater than those in nonpregnant myometrium. 5. CPI-17 is a substrate for PKC, and the phosphorylated CPI-17 is considered to inhibit myosin phosphatase. The levels of CPI-17 mRNA and protein expression were also greater in the pregnant myometrium. 6. These results suggest that the PKC-mediated contractile mechanism is augmented in human myometrium after gestation, and that this augmentation may be attributable to the increased activity of the beta PKC isoform and CPI-17.

Conventional-type protein kinase C contributes to phorbol ester-induced inhibition of rat myometrial tension

1 Phorbol ester decreases muscle tension in the rat myometrium, and the effect is more potent in late-pregnant myometrium than in nonpregnant myometrium. In the present study, we have examined the contribution of protein kinase C (PKC) isoforms to the phorbol ester-induced inhibition of tension in rat uterine smooth muscle. 2 Thymeleatoxin (THX), a selective activator of conventional-type PKC (cPKC), and 12-deoxyphorbol 13-isobutyrate (DPB), an activator of pan PKC, inhibited the tension induced by high K(+), and inhibitions were significantly increased in pregnant myometrium compared to nonpregnant myometrium. The inhibition by DPB and THX of high K(+)-induced tension was significantly attenuated when PKC was downregulated by long-term pretreatment with THX and inhibited by Go6976, a cPKC inhibitor. 3 Of the cPKCs, PKC alpha is predominantly expressed in the rat myometrium, as detected by Western blot analysis. The expression of PKC alpha gradually increases from the beginning of gestation, reaching a maximum at day 21 of pregnancy. Treatment with DPB induced PKC alpha to translocate from the cytosol to the membrane in the pregnant myometrium. PKC epsilon and PKC zeta, other dominant PKC isoforms in the rat myometrium, decrease during gestation, reaching a minimum in late pregnancy. 4 These results suggest that cPKC may be at least partly involved in the PKC-mediated inhibition of muscle tension in the rat myometrium.

Increased contractility of rat uterine smooth muscle at the end of pregnancy

Upon stimulation with high K+, oxytocin, prostaglandin E2, prostaglandin F2 alpha or carbachol, myometrium isolated from pregnant rats (21 days after pregnancy) developed 2-3 times greater isometric force than that from non-pregnant rats (estrus). High K+ increased the level of myosin light chain (MLC) phosphorylation to a similar extent in these tissues, and therefore pregnant myometrium developed greater contraction than non-pregnant myometrium at a given MLC phosphorylation. In the permeabilized muscle with alpha-toxin, Ca2+ (0.1-10 microM) induced greater contraction in pregnant myometrium than in non-pregnant myometrium. Ca2+ sensitivity was not altered after pregnancy. MLC kinase and phosphatase activities did not differ significantly between pregnant and non-pregnant myometria. Stimulation with 10 microM Ca2+ and 1 microM calyculin-A elicited similar magnitudes of contractions in the permeabilized muscles isolated from non-pregnant and pregnant rats. SDS-PAGE showed that the percentage of the content of MLC was not altered between these preparations, although actin content increased after pregnancy. These results suggest that the stress generating capacity of myometrium is increased after pregnancy without changing the MLC phosphorylation step. The equal capacity of force generation after the maximum phosphorylation by Ca2+ and phosphatase inhibitor suggests that a MLC phosphorylation-independent mechanism is responsible for the development of greater force in the pregnant myometrium.

Role of protein kinase C in endothelin-1-induced contraction of human myometrium

The role of protein kinase C (PKC) in the contraction of human myometrium induced by endothelin-1 was investigated. The PKC inhibitor, calphostin C, reduced the sustained phase of endothelin-1-induced contraction. The expression and subcellular distribution of PKC isoforms were determined in unstimulated myometrium by Western blotting using isoform-specific antisera. At least five PKC isoforms (PKCalpha, PKCbeta1, PKCbeta2, PKCzeta, and trace amounts of PKCepsilon) were detected. Quantitative immunoblotting revealed that all these isoforms were diversely distributed between the cytosolic and particulate fractions. After stimulation with phorbol 12,13-dibutyrate (PDB) and endothelin-1, differential redistribution occurred, suggesting a selective role of these isoforms in the physiological function of the myometrium. Biochemical assay confirmed that PDB as well as endothelin-1 evoked a decrease in cytosolic PKC activity. Taken together, these results suggest that PKC may play a role in endothelin-1-induced contraction of human uterine smooth muscle.