Influence of time at which oxytocin is administered during labor on uterine activity and perinatal death in pigs

Oxytocin is extensively used to induce or augment uterine contractions, especially to facilitate the third stage of labor in humans. Administration of oxytocin to parturient sows reduces duration of labor whereas mortality of the offspring may remain unchanged. This study aimed to evaluate whether time of administration of oxytocin during parturition may alter the uterine response and fetal outcomes. Two hundred parturient sows were randomly assigned to intramuscularly receive either saline solution (control group) or oxytocin 0.083 IU/kg immediately after the delivery of the 1st, 4th or 8th piglet (groups O-1, 0-4 and 0-8, respectively). Uterine effects and fetal outcomes were registered in all groups. The duration of labor was 20-40 min shorter (P < 0.0001) and time interval between babies was reduced by 3-5 min (P < 0.0001) in the three groups receiving oxytocin. The duration and intensity of contractions, meconium-stained piglets and intrapartum deaths decreased as time at which oxytocin administered during labor was increased. In group 0-8, we observed approximately 70 percent less meconium-stained piglets and intrapartum deaths than in the control group. In conclusion, oxytocin administered at early phases of parturition to sows may increase duration and intensity of uterine contractions as well as adverse fetal outcomes.

Ca2+ entry, efflux and release in smooth muscle

Control of smooth muscle is vital for health. The major route to contraction is a rise in intracellular [Ca2+], determined by the entry and efflux of Ca2+ and release and re-uptake into the sarcoplasmic reticulum (SR). We review these processes in myometrium, to better understand excitation-contraction coupling and develop strategies for preventing problematic labours. The main mechanism of elevating [Ca2+] is voltage-gated L-type channels, due to pacemaker activity, which can be modulated by agonists. The rise of [Ca2+] produces Ca-calmodulin and activates MLCK. This phosphorylates myosin and force results. Without Ca2+ entry uterine contraction fails. The Na/Ca exchanger (NCX) and plasma membrane Ca-ATPase (PMCA) remove Ca2+, with contributions of 30 percet and 70 percet respectively. Studies with PMCA-4 knockout mice show that it contributes to reducing [Ca2+] and relaxation. The SR contributes to relaxation by vectorially releasing Ca2+ to the efflux pathways, and thereby increasing their rates. Agonists binding produces IP3 which can release Ca from the SR but inhibition of SR Ca2+ release increases contractions and Ca2+ transients. It is suggested that SR Ca2+ targets K+ channels on the surface membrane and thereby feedback to inhibit excitability and contraction.

Modulación de la actividad contráctil uterina por hormonas esteroides sexuales. Revisión / Modulation of uterine contractile activity by sexual steroid hormones

Los esteroides sexuales tienen una función importante en la regulación de la actividad contráctil uterina. Progesterona ejerce un efecto relajante sobre el útero de coneja, rata y huamano. Adicionalmente se ha informado que los metabolitos 5 b-reducidos son más potentes que su precursor en el útero de rata. Con respecto a su mecanismo de acción se sabe que actúan por mecanismo no genómico y no se ha identificado su receptor específico para estos compuestos en las memebranas de las células miometriales. Sin embargo, algunos datos proponen que los esteroides inhiben el influjo de calcio a la célula muscular por bloqueador los canales operados por voltaje y o los canales operados por receptos. La regulación de la contractilidad uterina por hormonas esteroides durante el ciclo sexual y el embarazo sugiere una función fisiológica importante, al inducir una quiescencia uterina requerida para el mantenimiento del embarazo

Influência do fator mecânico sobre a involuçäo do miométrio de ratas após a parturiçäo / The role of the mechanical stimuli on myometrial involution after parturition in the rat

Apresenta-se um estudo ultra-estrutural sobre a influência do fator mecânico na involuçäo do miométrio de ratas após a parturiçäo. Foi realizada uma distensäo em parte do útero de rata após parturiçäo, pela introduçäo de parafina; a outro parte do mesmo corno uterino permaneceu vazia. O estudo das eletromicrografias mostrou, no local que contém parafina, a presença de leiomiócitos e fibroblastos bem desenvolvidos. No local que näo continha parafina observamos leiomiócitos com sinais de atrofia e fibroblastos repletos de vesículas intracitoplasmáticas com fibrilas colágenas em vários estágios de reabsorçäo. Os resultados levam a crer que a remoçäo da distensäo da parede uterina possa ser o principal estímulo para a involuçäo do miométrio