3. initiation of partuition Flashcards
what are the phases of parturition
- initiation of myometrial contraction
- expulsion of the fetus
- expulsion of fetal membranes
how is parturition initiated
by hormonal signals from the fetus when the fetus is sufficiently mature to survive the trauma of the experience
- influenced by fetal stress hormones
what hormones are involved in parturition
- glucocorticoids
- fetal cortisol
fetal stress = parturition
which hormone takes dominance leading up to parturition in many species and what initiates this swtich
oestradiol
switch is influenced by fetal cortisol
outline the hormonal profile around parturition in the cow
- increasing fetal cortisol
- removal of progesterone block (progesterone negatively impacts the myometrium, preventing contractile force during pregnancy. parturition = need myometrial contractility and excitability. want propulsive force generated by the uterus. removing progesterone removes negative feedback on the myometrium)
- increased oestrogens
- increased prostaglandins
- = parturition in most species
give an example of the importance of the HPA axis in parturition
- ingestion of veratrum californicum (skunk cabbage) by dam produces a cycloptic lamb
- has prolonged gestation
- absence of fetal hypophysial stalk and pituitary adrenal hypoplasia
- altered fetal steroid production and metabolism
insufficient fetal HPA = delayed labour
what does fetal cortisol do
- reduces progesterone levels (removes block on myometrial contractions and reproductive tract secretions increase)
- stimulates uterine prostaglandin production (cuases uterin contraction, acts on the ovary and causes lysis of corpus luteum, relaxin produced changes structure of cervix (not cow))
outline the sequence of events prompted by fetal stress
ACTH also known as corticotropon, produced an secreted by the anterior pit.
fetal cortisol encourages p4 conversion to e2
e2 stimulates range of aspects that contribute to myometrial contraction to increase pressure
as young moves possition for delivery, stimulates pressure stimulated neurons
how does oxytocin get released in parturition
- increasing pressure on cervix activates pressure sensitive neurons
- relay afferent information to the hypothalamus
- neural input to the posterior pituitary stimulates oxytocin release
- positive feedback loop to stimulate progressive contractions
explain the mechanism behind myometrial contraction
- myometrial contractions are initiated by bursts of propagating action potentials
- Calcium is critical second messenger
- voltage gated calcium channels responsible for mose Ca entry
- myosin light chain kinase is a key regulatory enzyme
- the myoisinlight chain enzymes are important in the mechanism of contraction in muscle
- influx of calcium ions into the muscle from the extracellular space or sarcoplasmic reticulum)
- calcium binds to calmodulin
- activates MLCK which phosphorylates myosin light chains
- enables actin-myosin crossbridge to form
- smooth muscle does not contain a troponin complex for regulating contraction (unlike striate muscles)
what is the role of gap junctions in coordination of contractility of the myometrium
- low resistance pathways between individual myometrial cells allows uterus to act as electrical syncytium
- marked increase in density and size towards parturition (within 24hrs back to preterm levels)
- presence and permeability under steroid and PG control ( also some influence of stretch)
how is myometrial contraction regulated
- oxytocin increases force, duration and frequency of contractions via inositol triphosphate
- prostaglandin isoforms locally produced to act locally (pgf2a = contraction, pgI2 = relaxation, PGE2 important in cervical softening)
- neural input = autonomic NS via pelvic plexus innervates uterus (endo/myometrium) through alpha 1 and 2 and beta 1 and 2 adrenergic receptors. alpha 1 = activation causing contraction, b1 = activation causing relaxation. high progesterone increases number of receptors
what are mediators of contraction
positive feedback system
COX-2 = enzyme
steroids increase enzyme activity and sensitivity to stimulators
how might you terminate a pregnancy
induce luteolysis
- natural ot synthetic prostaglandins
- remove luteotrphic support (prolactin inhibitors in bitch and queen)
- success depends on reliance of CL and species specific time of any luteo-placental shift
remoce progesterone support for pregnancy
- inhibit progesterone synthesis (3BHSD blockers)
- inhibit progesterone receptor actions (PR antagonsits)
induce premature parturition
why might you want to induce parturition
- predetermine parturition and better manage animal care
- industry advantage (group farrowing)
- reduce late calving (seasonal herds) and ensure calving coicides with good pasture
- disedase or illness of dam or offspring
- history of high risk pregnancy
