Oxytocin contraction

Oxytocin contracts vascular smooth muscle, which can cause an increase in blood pressure, particularly if administered in a large dose. This is usually preceded by a decrease in pressure. 

Oxytocin, a nonapeptide hormone secreted by the pituitary gland, functions by stimulating uterine contraction and lactation during childbirth. Its sequence was determined from the following evidence ... 

Tocolytics are drugs such as the selective (32-adrenoceptor agonists, e.g. salbutamol or terbutaline, which inhibit both the spontaneous and oxytocin-induced contractions of the pregnant uterus by relaxing... 

OXYTOCIN The patient receiving oxytocin to induce labor may have concern over the use of the drug to produce contractions. When given to induce or stimulate contractions, oxytocin may only be given intravenously (IV). The nurse explains the purpose of the IV infusion and the expected results to the patient. Because the patient receiving oxytocin must be closely supervised, the nurse spends time with the patient and offers encouragement and reassurance to help reduce anxiety. 

When oxytocin is prescribed, the primary health care provider orders the type and amount of IV fluid, the number of units of oxytocin added to the IV solution, and the IV infusion rate An electronic infusion device is used to control the infusion rate. The primary health care provider establishes guidelines for the administration of the oxytocin solution and for increasing or decreasing the flow rate or discontinuing the administration of oxytocin based on standards established by the Association of Women s Health, Obstetric, and Neonatal Nurses (AWHONN). Usually, the flow rate is increased every 20 to 30 minutes, but this may vary according to the patient s response. The strength, frequency, and duration of contractions and the FHR are monitored closely. 

If contractions are frequent, prolonged, or excessive, die infusion is stopped to prevent fetal anoxia or trauma to die uterus. Excessive stimulation of die uterus can cause uterine hypertonicity and possible uterine rupture. The nurse places die patient on her side and provides supplemental oxygen. The effects of die drug diminish rapidly because oxytocin is short acting. 

Oxytocin is well established as an inducer of labour and is frequently used clinically for this purpose. Oxytocin acts by both direct and indirect mechanisms to stimulate uterine contraction. Firstly, it acts directly on the myometrium to cause uterine contractions and secondly, oxytocin acts to stimulate the production of other mediators of uterine contractions, most notably prostaglandins [8]. 

Nearly all the clinical data comes from the use of atosiban (see Peptide Antagonists), a peptide oxytocin antagonist that is licensed in Europe for acute (48 h) treatment of preterm labour. Early clinical studies demonstrated the ability of atosiban to inhibit uterine contractions associated with labour [14]. Following these successful phase II trials, full phase III trials were... 

Oxytocin is required for lactation. Milk is initially secreted into small sacs (alveoli) within the mammary gland. These alveoli are surrounded by smooth muscle, which contract to eject the milk. The oxytocin receptor is the key receptor that mediates this ejection of the milk. 

One compound from this series, (10), has been tested in vitro in human myometrium tissue obtained at term following caesarean section and shown to inhibit contractions induced by oxytocin [44] with a pA2 of 7.6. This is one of the first direct indications that the use of an oxytocin antagonist may be of benefit in the treatment of preterm labour in humans. This compound has been extensively studied in the near-term baboon and has been shown to inhibit nocturnal and near-term contractions following an intravenous bolus injection [45]. Further studies on the effect of oxytocin antagonism in the weeks leading up to delivery in the baboon have also been published [46]. 

Ki of 6 nM against a guinea-pig uterine oxytocin preparation and at least 100-fold selectivity over rat vasopressin tissue preparations. These molecules show a pseudo-irreversible pharmacology, with an extended action against spontaneous contractibility of rat uterus 24 h post partum. 

The posterior pituitary is innervated by direct nervous stimulation from the hypothalamus, resulting in the release of specific hormones. The hypothalamus synthesizes two hormones, oxytocin and vasopressin. These hormones are stored in and released from the posterior pituitary lobe. Oxytocin exerts two actions (1) it promotes uterine contractions during labor, and (2) it contracts the smooth muscles in the breast to stimulate the release of milk from the mammary gland during lactation. Vasopressin is an antidiuretic hormone (ADH) essential for proper fluid and electrolyte balance in the body. Specifically, vasopressin increases the permeability of the distal convoluted tubules and collecting ducts of the nephrons to water. This causes the kidney to excrete less water in the urine. Consequently, the urine becomes more concentrated as water is conserved. 

Many hormones and other blood-borne substances (including drugs) also alter contractile activity of smooth muscle. Some of the more important substances include epinephrine norepinephrine angiotensin II vasopressin oxytocin and histamine. Locally produced substances that may alter contraction in the tissue in which they are synthesized include nitric oxide prostaglandins leukotrienes carbon dioxide and hydrogen ion. 

Kupittayanant S, Luckas MJM, Wray S 2002 Effects of inhibiting the sarcoplasmic reticulum on spontaneous and oxytocin-induced contractions of human myometrium. Br J Obstet Gynaecol 108 1—8... 

Taylor That is certainly what we see with oxytocin on spontaneous contractions in human myometrium. 

Oxytocin, a hormone composed of nine amino acids, which induces labor by stimulating contraction of uterine muscles... 

Oxytocin, a peptide which initiates uterine contractions during labour is identical in structure to ADH except at position 8 where a leucine residue replaces arginine. The close structural similarity but radically different biological functions, illustrate how specific some hormone receptors are in recognising only their own signal . 

By comparing time-resolved and steady-state fluorescence parameters, Ross et alm> have shown that in oxytocin, a lactation and uterine contraction hormone in mammals, the internal disulfide bridge quenches the fluorescence of the single tyrosine by a static mechanism. The quenching complex was attributed to an interaction between one C — tyrosine rotamer and the disulfide bond. Swadesh et al.(()<>> have studied the dithiothreitol quenching of the six tyrosine residues in ribonuclease A. They carefully examined the steady-state criteria that are useful for distinguishing pure static from pure dynamic quenching by consideration of the Smoluchowski equation(70) for the diffusion-controlled bimolecular rate constant k0,... 

Posterior pituitary Two hormones, vasopressin and oxytocin, are synthesised in the hypothalamus and then transported through nerve axons to the posterior pituitary, where they are stored until released. Vasopressin acts on the kidney to conserve water. Its secretion is stimulated by thirst and a decrease in blood pressure. Secretion of oxytocin initiates uterine contraction for parturition. It also stimulates milk ejection from the mammary glands. 

These messengers also play a role in regulating contraction of myometrium, which consists of smooth muscle fibres. Contraction is controlled by increases in the concentration of cytosolic Ca ions. Prostaglandins activate Ca ion channels in the plasma membrane of the fibres oxytocin activates release of Ca from intracellular stores. The increase in concentration of Ca ions leads to activation of myosin light-chain kinase which leads to crossbridge cycling and contraction (as described in Chapter 22 Figure 22.12). 

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