Uterine Responses to Exogenous Oxytocin Before and After Pre‐partum Luteolysis in the Cow

Abstract

The aim of this study was to test the functional status of uterine oxytocin receptors in cows in vivo around parturition. The animals received consecutive, intra-arterial injections of 800, 1,600 and 3,200 mU of oxytocin at three different stages: during late gestation (days 260-274), at 12 h and at 24 h after intramuscular injection of a prostaglandin F2alpha analogue at day 275 to induce parturition. Cows (n = 6) had been provided with myometrial electrodes and a catheter had been installed in the aorta and in a branch of the uterine vein (UV). Regular blood samples were obtained from the UV from 5 min before until 45 min after each oxytocin injection to measure plasma levels of prostaglandin F2alpha (PGF2alpha) and oxytocin. Uterine electromyographic (EMG) activity was registered continuously during each experiment. The increase of oxytocin levels in UV plasma after intra-arterial injections was dose dependent (p < 0.02). Pre- and post treatment oxytocin levels at 24 h after induction of parturition were significantly increased (p = 0.0313). Both during late pregnancy and at 12 h after induction of parturition, oxytocin caused a significant increase in EMG activity (p = 0.022). After the 3,200 mU dose the increase was significantly higher than with the other 2 doses (p = 0.004). After each dose, EMG activity returned to baseline levels within some 15 min. At 24 h after induction of parturition, the pre-treatment level of EMG activity had increased. Doses of 800 mU and 1,600 mU of oxytocin produced a significant (p = 0.022) increment of EMG activity, which was of the same magnitude as during the preceding stages; after 3,200 mU of oxytocin the response was significantly higher than before (p = 0.008). No significant increases of PGF2alpha levels in UV plasma could be measured after oxytocin injections at any of the three stages. It is concluded that the myometrium of the pregnant cow responds in vivo to physiological doses of oxytocin. At 24 h after induction of parturition, when luteolysis has occurred and a parturient pattern of parturient myometrial activity has already started to develop, the response is enhanced. Physiological doses of oxytocin did not evoke a spurt release of PGF2alpha in uterine venous blood during the peripartal period.

Validation of pressure measurements and electromyographic results for the uterus of cattle during the early postpartum period

“…In addition to an intrinsic control mechanism that is associated with these contractures in pregnant sheep, 30 oxytocin, prostaglandins, and catecholamines may also affect the pattern of myoelectric activity. 32 Therefore, the characteristics and regulation of contractures in postpartum cows need additional study.…”

Quantitative morphological changes in the interplacentomal wall of the gravid uterine horn of cattle during pregnancy

“…This final step transforms the myometrium into an electrical syncytium capable of synchronized contractions, i.e. , exhibiting a parturient pattern of myometrial activity, and expulsion of the fetus [ 50 , 51 ]. Besides the increase in size of MSMC because of the cytoplasmic accumulation of cytoskeletal and contraction-associated proteins, the synthesis of these proteins and of ECM materials, of receptors for ovarian steroids and other hormones and the secretion of prostaglandins may to varying degrees also contribute to the enlargement of these cells [ 15 , 33 , 37 , 47 , 48 , 52 - 55 ].…”

Effects of the administration of oxytocin or carbetocin to dairy cows at parturition on their subsequent fertility

“…Myometrial biopsies from women undergoing a caesarean section after 12 hours in labour contained 50 times less oxytocin receptor mRNA than biopsies from women in labour for less than 12 hours (Phaneuf and others 1997). The short half‐life of oxytocin and receptor downregulation explain both the short uterotonic effect of the administration of oxytocin on bovine myometrium in vitro (Patil and others 1980) and uterine electromyograph (EMG) activity lasting only 15 minutes in cattle injected with oxytocin (Taverne and others 2001). In cattle, the concentration of oxytocin receptors in the myometrium increases from 50 to 150 days after conception and then plateaus, but their concentration in the endometrium increases throughout pregnancy, rising steeply before the onset of labour; in both tissues the concentrations decrease rapidly in the 24 hours after parturition (Fuchs and others 1992).…”