Seasonal differences in progesterone production by luteinized bovine thecal and granulosa cells.

This study examined seasonal differences in progesterone (P4) production by granulosa cells (GC) and thecal cells (TC) that were luteinized in vitro during the winter or the summer; it also compared plasma P4 concentrations of lactating dairy cows in the two seasons. First-wave dominant follicles obtained from Holstein cows were dissected on day 6 of the cycle, GC and TC were separated, enzymatically dispersed, and cultured for 9 days in media containing 1% fetal calf serum, forskolin (10 micromol/mL) and insulin (2 microg/mL), to induce cell luteinization. All experimental procedures were identical and characteristics of the follicles were similar in the two seasons.

Delayed effect of low progesterone concentrations on bovine uterine PGF(2alpha) secretion in the subsequent oestrous cycle.

Low progesterone concentrations during the bovine oestrous cycle induce enhanced responsiveness to oxytocin challenge late in the luteal phase of the same cycle. The delayed effect of low progesterone concentrations during one oestrous cycle on uterine PGF(2alpha) secretion after oxytocin challenge on day 15 or 16 of the subsequent cycle was studied by measuring the concentrations of the major PGF(2alpha) metabolite (13,14-dihydro-15-keto PGF(2alpha); PGFM) in plasma. Two experiments were conducted, differing in the type of progesterone treatment and in the shape of the low progesterone concentration curves.

Delayed effect of heat stress on steroid production in medium-sized and preovulatory bovine follicles.

During the autumn, the conception rate of dairy cattle in warm countries is low although ambient temperatures have decreased and cows are no longer exposed to summer thermal stress, indicating that there may be a delayed effect of heat stress on cattle fertility. Two experiments were conducted to examine possible delayed effects of heat stress on follicular characteristics and steroid production at two distinct stages of follicular growth: medium-sized and preovulatory follicles, 20 and 26 days after heat exposure, respectively. Lactating cows were subjected to heat stress for 12 h a day in an environmental chamber, during days 2-6 of a synchronized oestrous cycle.

Two methods of inducing low plasma progesterone concentrations have different effects on dominant follicles in cows.

The effects of two methods of inducing low progesterone concentrations on the shape of the plasma progesterone curve and on follicular characteristics in lactating cows were studied. A low ascending progesterone curve was elicited by three PGF2alpha injections on d 3 to 4 of the estrous cycle; a low constant curve by induction of corpus luteum regression on d 6 and insertion of two progesterone-containing intravaginal devices from d 6 to 15 of the cycle. Plasma progesterone concentration was highest in the untreated control group, intermediate in low ascending group, and lowest in the low constant group.

Comparison of the steroidogenic capacity of bovine follicular and luteal cells, and corpora lutea originating from dominant follicles of the first or second follicular wave.

This study, compared the endocrine function of dominant follicles of the first and second follicular waves (DF1 and DF2, respectively) and the corpora lutea that were subsequently formed. In the experiments conducted in vitro, ovaries were collected from dairy cows on day 6.1 +/- 0.

Antioxidant capacity is correlated with steroidogenic status of the corpus luteum during the bovine estrous cycle.

The reactions of steroid hormone biosynthesis are accompanied by formation of oxygen radicals. We determined the levels of some antioxidants and antioxidative enzymes at different developmental stages of bovine corpora lutea to examine their correlation with steroidogenic status. Plasma progesterone concentrations of estrous cycle synchronized cows increased until day 16, and then decreased rapidly during luteal regression.

Delayed effect of progesterone on endometrial morphology in dairy cows.

Two experiments were conducted to investigate the effect of vaginal progesterone (P4) administered during the luteal phase, on endometrial morphology during the subsequent oestrous cycle. In experiment 1, lactating Holstein cows were allotted to three groups: (1) Control group in which cows remained untreated; (2) The CIDR group in which cows were treated with two P4-containing controlled intravaginal-drug releasing devices (CIDR) during days (d) 6-12 of the cycle; and (3) The PG + CIDR group of cows that received two prostaglandin F2 alpha (PGF2 alpha) injections on d 6 and 7 of the oestrous cycle, to regress the corpus luteum (CL), and were treated with CIDRs on d 6-12, like the CIDR group. All cows were slaughtered on d 3 of the subsequent oestrous cycle.