Uterine production of oestrogens and progesterone at parturition in the sheep

Pregnancy modifies the large conductance Ca2+-activated K+ channel and cGMP-dependent signaling pathway in uterine vascular smooth muscle

“…This, however, does not explain why the ␣-subunit is unchanged, whereas UPBF increases another threefold and the ␤ 1 -subunit continues to increase. The latter may be due to increases in placental estrogen synthesis that begin with placentation and continue throughout the remainder of pregnancy (4,5). This is supported by the finding that E 2 selectively increases ␤ 1 expression in uterine arteries from nonpregnant ewes without altering the ␣-subunit (26) and evidence that placental estrogen is directly transferred to the uterine artery during pregnancy (18).…”

“…Placental estrogen synthesis increases during pregnancy, resulting in elevated circulating estrogen levels (4,5). In nonpregnant ewes, daily E 2 increases basal uterine blood flow, vascular synthesis of NO and cGMP, and BK Ca activation (26,43,47).…”

“…However, acute uterine NOS inhibition decreases basal UPBF modestly (24,39). Estrogen is a potent uterine vasodilator in nonpregnant and pregnant sheep (21,36,40) and is markedly elevated in pregnancy (4,5), but its contribution to the rise in UPBF remains unclear. It is possible that each of these modulators contributes to the rise in UPBF, but their importance and the signaling cascades remain to be determined.…”

Selected Factors that Affect the Measurement of Plasma Progesterone Concentrations in Pregnant Ewes

“…Thorburn and Mattner (1971) found that the concentration of progesterone in the peripheral plasma during the luteal phase of the oestrous cycle reflected the differences in secretion rate of progesterone. However, it may not be valid to extrapolate this finding to the pregnant state as Challis et al (1971) found that, in a single ewe, while concentrations of progesterone in uterine venous blood decreased during the last week of pregnancy, the jugular levels did not fall until 48 h before parturition and in fact, at term, the concentrations in uterine and jugular blood were similar. Meschia et al (1967) found that the flow rate through the uteroplacental circuit varied from sheep to sheep over a very wide range (138-492 ml per kilogram per minute) without any systematic relationship to stage of pregnancy over the range 102-139 days.…”

Physio-pathologic studies in induced parturition in the cow (Bos taurus)

“…Plasma estrogens increase in a similar manner in the sow (Molokwu and Wagner, 1973); the ewe (Challis, Harrison and Heap, 1971a) and the goat (Thorburn et a^. , 1972) primates (Diczfalusy, 1964) and in some other mammals (Davies, Ryan and Petro, 1970).…”