Apelin induces myometrial contractions through the involvement of protein kinase c mechanism in rat Apelin, is a recently discovered peptide hormone and its receptor were found in the hypothalamus, uterus, ovary and some other peripheral tissues. We have previously demonstrated that apelin-13 has stimulatory effect on myometrial contractility in rats. Morever, this effect turns out in calcium free condition. The purpose of this study was to investigate the possible involvement of protein kinase C (PKC) pathway in stimulation of myometrium contractility by apelin. Additionally we investigated plasma apelin levels in pregnant and lactating rats. Adult Wistar rats were anaesthetized with ketamine (60 mg/kg, intraperitoneally) and myometrium strips were removed following decapitation. Strips placed in a jacked tissue bath containing Krebs solution at 37oC and pH 7.4, constantly bubbled with 95% oxygen and 5% carbon dioxide. They were allowed to contract under 1g tension and isometric contractions were measured by force displacement transducer. After equlibration of spontaneous contractions, calcium free condition was constitute. Silent period (no contraction) was recorded and protein kinase C inhibitor agent, chelerythrine chloride (10µM), added to the organ bath. Apelin-13 at a concentration of 20µM was also applicated 5 mins later. In the other experimental protocol, we collected blood samples via tail vein of Wistar rats in dioestrus and in different stages of pregnancy and lactation under light ether sedation. Plasma apelin-13 concentration was determined by ELISA. Hormone concentrations were calculated as mean±SEM and One Way ANOVA with posthoc Student-Newman-Keuls test was used for statistical analysis. Apelin has stimulatory effect on myometrial contraction in calcium free condition. However, chelerythrine chloride pretreatment blocked the effect of apelin in same status. In the other experimental protocol, plasma apelin levels were 120.2±10.9ng/ml, 101.9±14.5ng/ml, 151.1±31.7ng/ml and 235.8±46.5ng/ml in dioestrus, and 12th, 18th and 21th day of pregnancy groups, respectively. Blood apelin concentration in 21th day of pregnancy was significantly high compared to both dioestrus and first pregnant groups (p<0.05). There was a significant decrease in plasma apelin level in 2th day of lactation group (111.4±19.2ng/ml) compared to 21th day of pregnancy group (p<0.05). The 10th day of lactation group had no different hormone level compared to the others (143.3±13.2ng/ml). The present data indicate that apelin induce myometrium contraction through the possible involvement of PKC mechanism. According to the results of this study and our previous data, apelin can stimulate uterus contractions by inducing Ca2+ release from intracellular stores. Additionally, the elevation of plasma apelin concentration at the end of pregnancy may reveal a facilitatory role for apelin on myometrium contraction during parturition.